Hi,

I've just turned on my amp and it's not working (was working fine the
last time it was turned on!).

The light in the power switch is coming on so there's power but no
output.

Looking in the back the valves aren't glowing as I'd expect them to
be. I suspect there might be a fuse blown somewhere but never having
worked on a valve amp before (though I'm well aware of the dangers) I
thought I'd ask here to see if anyone has any advice before digging
into the chassis!

Any ideas?

Thanks

Phil

"Phil Kett"  wrote in message 
news:pikhc51n933nf911l95e57lqggghpgolju@4ax.com...

> Any ideas?

Don't let any smoke come out of it. Once the smoke leaves they no longer 
work.

icarusi
--

remove the 00 to reply

icarusi wrote:

> "Phil Kett"  wrote in message
> news:pikhc51n933nf911l95e57lqggghpgolju@4ax.com...
> 
>> Any ideas?
> 
> Don't let any smoke come out of it. Once the smoke leaves they no longer
> work.
> 
> icarusi

Hehe - well, it hasn't started smoking yet! 

--

Phil

Phil Kett wrote:

> Hi,
> 
> I've just turned on my amp and it's not working (was working fine the
> last time it was turned on!).
> 
> The light in the power switch is coming on so there's power but no
> output.
> 
> Looking in the back the valves aren't glowing as I'd expect them to
> be. I suspect there might be a fuse blown somewhere but never having
> worked on a valve amp before (though I'm well aware of the dangers) I
> thought I'd ask here to see if anyone has any advice before digging
> into the chassis!

Bad form to reply to my own post I know but I thought I'd post an update.

I pulled the chassis out last night, there are three fuses on the circuit
board and they are all fine.

At this point I'm a bit lost - I just don't know enough about tube amps to
diagnose the fault. If what I've read is accurate though, I'd guess that
it's the heater circuit that's not working. As the fuses are ok does that
mean it's likely to be the transformer that's died?

I have a copy of the schematic if anyone is willing to help.

Thanks

--

Phil

Phil Kett  wrote in
news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com: 

> Phil Kett wrote:
> 
>> Hi,
>> 
>> I've just turned on my amp and it's not working (was working fine the
>> last time it was turned on!).
>> 
>> The light in the power switch is coming on so there's power but no
>> output.
>> 
>> Looking in the back the valves aren't glowing as I'd expect them to
>> be. I suspect there might be a fuse blown somewhere but never having
>> worked on a valve amp before (though I'm well aware of the dangers) I
>> thought I'd ask here to see if anyone has any advice before digging
>> into the chassis!
> 
> Bad form to reply to my own post I know but I thought I'd post an
> update. 
> 
> I pulled the chassis out last night, there are three fuses on the
> circuit board and they are all fine.
> 
> At this point I'm a bit lost - I just don't know enough about tube
> amps to diagnose the fault. If what I've read is accurate though, I'd
> guess that it's the heater circuit that's not working. As the fuses
> are ok does that mean it's likely to be the transformer that's died?

Possible, but unlikely. A more probable cause is a cold or dry solder 
joint (or even a break) at the start of the heater chain, for example at 
the point where the wires from the transformer connect to the PCB, if the 
valve bases are PCB mounted. Be wary of any charge stored in the 
reservoir capacitors after the amp is switched off - it can take several 
minutes to reduce to a safe level when the valves are not conducting.


> 
> I have a copy of the schematic if anyone is willing to help.
> 
> Thanks
> 
> --
> 
> Phil
>

Richard Kaulfuss wrote:

> Phil Kett  wrote in
> news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com:
> 
>> Phil Kett wrote:
>> 
>>> Hi,
>>> 
>>> I've just turned on my amp and it's not working (was working fine the
>>> last time it was turned on!).
>>> 
>>> The light in the power switch is coming on so there's power but no
>>> output.
>>> 
>>> Looking in the back the valves aren't glowing as I'd expect them to
>>> be. I suspect there might be a fuse blown somewhere but never having
>>> worked on a valve amp before (though I'm well aware of the dangers) I
>>> thought I'd ask here to see if anyone has any advice before digging
>>> into the chassis!
>> 
>> Bad form to reply to my own post I know but I thought I'd post an
>> update.
>> 
>> I pulled the chassis out last night, there are three fuses on the
>> circuit board and they are all fine.
>> 
>> At this point I'm a bit lost - I just don't know enough about tube
>> amps to diagnose the fault. If what I've read is accurate though, I'd
>> guess that it's the heater circuit that's not working. As the fuses
>> are ok does that mean it's likely to be the transformer that's died?
> 
> Possible, but unlikely. A more probable cause is a cold or dry solder
> joint (or even a break) at the start of the heater chain, for example at
> the point where the wires from the transformer connect to the PCB, if the
> valve bases are PCB mounted. Be wary of any charge stored in the
> reservoir capacitors after the amp is switched off - it can take several
> minutes to reduce to a safe level when the valves are not conducting.

Thanks!

From what I've read about the build quality of these amps a dry joint
wouldn't surprise me. 

I'll have a look tonight and let you know how I get on.

-- 
--

Phil

Had a very similar problem with my Laney VC50 a few weeks back - the
heater circuit failed to light up any of the valves.  Re-flowed the
solder joint on one of the output valve bases and normal service was
resumed - happiness!

Best of luch

Tim



On Oct 6, 10:49 am, Phil Kett  wrote:
> Phil Kett wrote:
> > Hi,
>
> > I've just turned on my amp and it's not working (was working fine the
> > last time it was turned on!).
>
> > The light in the power switch is coming on so there's power but no
> > output.
>
> > Looking in the back the valves aren't glowing as I'd expect them to
> > be. I suspect there might be a fuse blown somewhere but never having
> > worked on a valve amp before (though I'm well aware of the dangers) I
> > thought I'd ask here to see if anyone has any advice before digging
> > into the chassis!
>
> Bad form to reply to my own post I know but I thought I'd post an update.
>
> I pulled the chassis out last night, there are three fuses on the circuit
> board and they are all fine.
>
> At this point I'm a bit lost - I just don't know enough about tube amps to
> diagnose the fault. If what I've read is accurate though, I'd guess that
> it's the heater circuit that's not working. As the fuses are ok does that
> mean it's likely to be the transformer that's died?
>
> I have a copy of the schematic if anyone is willing to help.
>
> Thanks
>
> --
>
> Phil

On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
 wrote:

>Phil Kett  wrote in
>news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com: 
>
>> Phil Kett wrote:
>> 
>>> Hi,
>>> 
>>> I've just turned on my amp and it's not working (was working fine the
>>> last time it was turned on!).
>>> 
>>> The light in the power switch is coming on so there's power but no
>>> output.
>>> 
>>> Looking in the back the valves aren't glowing as I'd expect them to
>>> be. I suspect there might be a fuse blown somewhere but never having
>>> worked on a valve amp before (though I'm well aware of the dangers) I
>>> thought I'd ask here to see if anyone has any advice before digging
>>> into the chassis!
>> 
>> Bad form to reply to my own post I know but I thought I'd post an
>> update. 
>> 
>> I pulled the chassis out last night, there are three fuses on the
>> circuit board and they are all fine.
>> 
>> At this point I'm a bit lost - I just don't know enough about tube
>> amps to diagnose the fault. If what I've read is accurate though, I'd
>> guess that it's the heater circuit that's not working. As the fuses
>> are ok does that mean it's likely to be the transformer that's died?
>
>Possible, but unlikely. A more probable cause is a cold or dry solder 
>joint (or even a break) at the start of the heater chain, for example at 
>the point where the wires from the transformer connect to the PCB, if the 
>valve bases are PCB mounted. Be wary of any charge stored in the 
>reservoir capacitors after the amp is switched off - it can take several 
>minutes to reduce to a safe level when the valves are not conducting.

Well, I've just checked the output from the transformer and got 0.1V -
not good. Checked continuity on those wires and there's no resistance.
Looks like that coil on the power transformer has fused.

There is a brown 'disk' (I suspect a capacitor) in series with these
outputs (where they connect to the circuit board) that looks like it's
slightly burnt. The only markings on it are 'CHANGE' - it's not shown
on the circuit diagram I have.

I guess the next question is whether or not it's worth trying to
source a replacement transformer for it.

Thanks for your advice and help!

Phil

Phil Kett  wrote in
news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com: 

> On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
>  wrote:
> 
>>Phil Kett  wrote in
>>news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com: 
>>
>>> Phil Kett wrote:
>>> 
>>>> Hi,
>>>> 
>>>> I've just turned on my amp and it's not working (was working fine
>>>> the last time it was turned on!).
>>>> 
>>>> The light in the power switch is coming on so there's power but no
>>>> output.
>>>> 
>>>> Looking in the back the valves aren't glowing as I'd expect them to
>>>> be. I suspect there might be a fuse blown somewhere but never
>>>> having worked on a valve amp before (though I'm well aware of the
>>>> dangers) I thought I'd ask here to see if anyone has any advice
>>>> before digging into the chassis!
>>> 
>>> Bad form to reply to my own post I know but I thought I'd post an
>>> update. 
>>> 
>>> I pulled the chassis out last night, there are three fuses on the
>>> circuit board and they are all fine.
>>> 
>>> At this point I'm a bit lost - I just don't know enough about tube
>>> amps to diagnose the fault. If what I've read is accurate though,
>>> I'd guess that it's the heater circuit that's not working. As the
>>> fuses are ok does that mean it's likely to be the transformer that's
>>> died? 
>>
>>Possible, but unlikely. A more probable cause is a cold or dry solder 
>>joint (or even a break) at the start of the heater chain, for example
>>at the point where the wires from the transformer connect to the PCB,
>>if the valve bases are PCB mounted. Be wary of any charge stored in
>>the reservoir capacitors after the amp is switched off - it can take
>>several minutes to reduce to a safe level when the valves are not
>>conducting. 
> 
> Well, I've just checked the output from the transformer and got 0.1V -
> not good.

I have to agree :-(

> Checked continuity on those wires and there's no resistance.
> Looks like that coil on the power transformer has fused.

By 'no resistance', do you mean open-circuit? If you probe gently where 
the wires come out of the bobbin, you might be lucky enough to find a 
break there.

 
> There is a brown 'disk' (I suspect a capacitor) in series with these
> outputs (where they connect to the circuit board) that looks like it's
> slightly burnt. The only markings on it are 'CHANGE' - it's not shown
> on the circuit diagram I have.

It's probably a thermistor to limit the inrush current. A rather unusual 
measure IME, and possibly indicates that the transformer was marginally 
specified. If the transformer voltage had been normal, I might have 
suspected that as the culprit. 


> 
> I guess the next question is whether or not it's worth trying to
> source a replacement transformer for it.
 
There's no harm in looking. If you can find out the spec for the HT 
winding (I'm assuming there aren't any others), you can probably source a 
beefier transformer for less than the cost of an OEM replacement.


> Thanks for your advice and help!
> 
> Phil
>

Richard Kaulfuss wrote:
> Phil Kett  wrote in
> news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com: 
> 
>> On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
>>  wrote:
>>
>>> Phil Kett  wrote in
>>> news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com: 
>>>
>>>> Phil Kett wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I've just turned on my amp and it's not working (was working fine
>>>>> the last time it was turned on!).
>>>>>
>>>>> The light in the power switch is coming on so there's power but no
>>>>> output.
>>>>>
>>>>> Looking in the back the valves aren't glowing as I'd expect them to
>>>>> be. I suspect there might be a fuse blown somewhere but never
>>>>> having worked on a valve amp before (though I'm well aware of the
>>>>> dangers) I thought I'd ask here to see if anyone has any advice
>>>>> before digging into the chassis!
>>>> Bad form to reply to my own post I know but I thought I'd post an
>>>> update. 
>>>>
>>>> I pulled the chassis out last night, there are three fuses on the
>>>> circuit board and they are all fine.
>>>>
>>>> At this point I'm a bit lost - I just don't know enough about tube
>>>> amps to diagnose the fault. If what I've read is accurate though,
>>>> I'd guess that it's the heater circuit that's not working. As the
>>>> fuses are ok does that mean it's likely to be the transformer that's
>>>> died? 
>>> Possible, but unlikely. A more probable cause is a cold or dry solder 
>>> joint (or even a break) at the start of the heater chain, for example
>>> at the point where the wires from the transformer connect to the PCB,
>>> if the valve bases are PCB mounted. Be wary of any charge stored in
>>> the reservoir capacitors after the amp is switched off - it can take
>>> several minutes to reduce to a safe level when the valves are not
>>> conducting. 
>> Well, I've just checked the output from the transformer and got 0.1V -
>> not good.
> 
> I have to agree :-(
> 
>> Checked continuity on those wires and there's no resistance.
>> Looks like that coil on the power transformer has fused.
> 
> By 'no resistance', do you mean open-circuit? If you probe gently where 
> the wires come out of the bobbin, you might be lucky enough to find a 
> break there.
> 

By no resistance I mean that my meter is showing 0 ohms, or at least 
close enough to set the buzzer of the continuity meter off. I wouldn't 
have expected that from a transformer winding.

>  
>> There is a brown 'disk' (I suspect a capacitor) in series with these
>> outputs (where they connect to the circuit board) that looks like it's
>> slightly burnt. The only markings on it are 'CHANGE' - it's not shown
>> on the circuit diagram I have.
> 
> It's probably a thermistor to limit the inrush current. A rather unusual 
> measure IME, and possibly indicates that the transformer was marginally 
> specified. If the transformer voltage had been normal, I might have 
> suspected that as the culprit. 

Looking at the circuit diagram there isn't a fuse on that part of the 
circuit - is that normal? I would have thought that'd mean that if 
anything went wrong the first thing that's going to take the brunt of it 
would be the transformer...

> 
> 
>> I guess the next question is whether or not it's worth trying to
>> source a replacement transformer for it.
>  
> There's no harm in looking. If you can find out the spec for the HT 
> winding (I'm assuming there aren't any others), you can probably source a 
> beefier transformer for less than the cost of an OEM replacement.
> 

There are a total of three windings on the transformer - there's the HT 
and a 12V supply for the DSP - looking closer at the diagram I can see 
that the heater circuit needs 6.3v, I'm wondering if I could perhaps 
find another transformer (or even a wall wart as a test) to see if the 
rest of it still works? There's probably enough room on the chassis to 
mount another transformer for the heater circuit as a 'kludge'

> 
>> Thanks for your advice and help!
>>
>> Phil
>>
>

On Oct 7, 12:32 am, Phil Kett  wrote:
> Richard Kaulfuss wrote:
> > Phil Kett  wrote in
> >news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com:
>
> >> On Tue, 6 Oct 2009 13:36:36 퍍 (UTC), Richard Kaulfuss
> >>  wrote:
>
> >>> Phil Kett  wrote in
> >>>news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com:
>
> >>>> Phil Kett wrote:
>
> >>>>> Hi,
>
> >>>>> I've just turned on my amp and it's not working (was working fine
> >>>>> the last time it was turned on!).
>
> >>>>> The light in the power switch is coming on so there's power but no
> >>>>> output.
>
> >>>>> Looking in the back the valves aren't glowing as I'd expect them to
> >>>>> be. I suspect there might be a fuse blown somewhere but never
> >>>>> having worked on a valve amp before (though I'm well aware of the
> >>>>> dangers) I thought I'd ask here to see if anyone has any advice
> >>>>> before digging into the chassis!
> >>>> Bad form to reply to my own post I know but I thought I'd post an
> >>>> update.
>
> >>>> I pulled the chassis out last night, there are three fuses on the
> >>>> circuit board and they are all fine.
>
> >>>> At this point I'm a bit lost - I just don't know enough about tube
> >>>> amps to diagnose the fault. If what I've read is accurate though,
> >>>> I'd guess that it's the heater circuit that's not working. As the
> >>>> fuses are ok does that mean it's likely to be the transformer that's
> >>>> died?
> >>> Possible, but unlikely. A more probable cause is a cold or dry solder
> >>> joint (or even a break) at the start of the heater chain, for example
> >>> at the point where the wires from the transformer connect to the PCB,
> >>> if the valve bases are PCB mounted. Be wary of any charge stored in
> >>> the reservoir capacitors after the amp is switched off - it can take
> >>> several minutes to reduce to a safe level when the valves are not
> >>> conducting.
> >> Well, I've just checked the output from the transformer and got 0.1V -
> >> not good.
>
> > I have to agree :-(
>
> >> Checked continuity on those wires and there's no resistance.
> >> Looks like that coil on the power transformer has fused.
>
> > By 'no resistance', do you mean open-circuit? If you probe gently where
> > the wires come out of the bobbin, you might be lucky enough to find a
> > break there.
>
> By no resistance I mean that my meter is showing 0 ohms, or at least
> close enough to set the buzzer of the continuity meter off. I wouldn't
> have expected that from a transformer winding.
>
>
>
> >> There is a brown 'disk' (I suspect a capacitor) in series with these
> >> outputs (where they connect to the circuit board) that looks like it's
> >> slightly burnt. The only markings on it are 'CHANGE' - it's not shown
> >> on the circuit diagram I have.
>
> > It's probably a thermistor to limit the inrush current. A rather unusual
> > measure IME, and possibly indicates that the transformer was marginally
> > specified. If the transformer voltage had been normal, I might have
> > suspected that as the culprit.
>
> Looking at the circuit diagram there isn't a fuse on that part of the
> circuit - is that normal? I would have thought that'd mean that if
> anything went wrong the first thing that's going to take the brunt of it
> would be the transformer...
>
>
>
> >> I guess the next question is whether or not it's worth trying to
> >> source a replacement transformer for it.
>
> > There's no harm in looking. If you can find out the spec for the HT
> > winding (I'm assuming there aren't any others), you can probably source a
> > beefier transformer for less than the cost of an OEM replacement.
>
> There are a total of three windings on the transformer - there's the HT
> and a 12V supply for the DSP - looking closer at the diagram I can see
> that the heater circuit needs 6.3v, I'm wondering if I could perhaps
> find another transformer (or even a wall wart as a test) to see if the
> rest of it still works? There's probably enough room on the chassis to
> mount another transformer for the heater circuit as a 'kludge'
>
>
>
> >> Thanks for your advice and help!
>
> >> Phil

If it is indeed the transformer, you should not have that much of a
problem finding a replacement.

There are several firms that provide "upgrade" kits for the Epi which
include a new speaker and a transformer.

In fact, a quick google revealed that the transformer dying seems to
be a common problem, and various better-quality replacement units can
be had.

Sjfdix wrote:

> On Oct 7, 12:32 am, Phil Kett  wrote:
>> Richard Kaulfuss wrote:
>> > Phil Kett  wrote in
>> >news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com:
>>
>> >> On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
>> >>  wrote:
>>
>> >>> Phil Kett  wrote in
>> >>>news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com:
>>
>> >>>> Phil Kett wrote:
>>
>> >>>>> Hi,
>>
>> >>>>> I've just turned on my amp and it's not working (was working fine
>> >>>>> the last time it was turned on!).
>>
>> >>>>> The light in the power switch is coming on so there's power but no
>> >>>>> output.
>>
>> >>>>> Looking in the back the valves aren't glowing as I'd expect them to
>> >>>>> be. I suspect there might be a fuse blown somewhere but never
>> >>>>> having worked on a valve amp before (though I'm well aware of the
>> >>>>> dangers) I thought I'd ask here to see if anyone has any advice
>> >>>>> before digging into the chassis!
>> >>>> Bad form to reply to my own post I know but I thought I'd post an
>> >>>> update.
>>
>> >>>> I pulled the chassis out last night, there are three fuses on the
>> >>>> circuit board and they are all fine.
>>
>> >>>> At this point I'm a bit lost - I just don't know enough about tube
>> >>>> amps to diagnose the fault. If what I've read is accurate though,
>> >>>> I'd guess that it's the heater circuit that's not working. As the
>> >>>> fuses are ok does that mean it's likely to be the transformer that's
>> >>>> died?
>> >>> Possible, but unlikely. A more probable cause is a cold or dry solder
>> >>> joint (or even a break) at the start of the heater chain, for example
>> >>> at the point where the wires from the transformer connect to the PCB,
>> >>> if the valve bases are PCB mounted. Be wary of any charge stored in
>> >>> the reservoir capacitors after the amp is switched off - it can take
>> >>> several minutes to reduce to a safe level when the valves are not
>> >>> conducting.
>> >> Well, I've just checked the output from the transformer and got 0.1V -
>> >> not good.
>>
>> > I have to agree :-(
>>
>> >> Checked continuity on those wires and there's no resistance.
>> >> Looks like that coil on the power transformer has fused.
>>
>> > By 'no resistance', do you mean open-circuit? If you probe gently where
>> > the wires come out of the bobbin, you might be lucky enough to find a
>> > break there.
>>
>> By no resistance I mean that my meter is showing 0 ohms, or at least
>> close enough to set the buzzer of the continuity meter off. I wouldn't
>> have expected that from a transformer winding.
>>
>>
>>
>> >> There is a brown 'disk' (I suspect a capacitor) in series with these
>> >> outputs (where they connect to the circuit board) that looks like it's
>> >> slightly burnt. The only markings on it are 'CHANGE' - it's not shown
>> >> on the circuit diagram I have.
>>
>> > It's probably a thermistor to limit the inrush current. A rather
>> > unusual measure IME, and possibly indicates that the transformer was
>> > marginally specified. If the transformer voltage had been normal, I
>> > might have suspected that as the culprit.
>>
>> Looking at the circuit diagram there isn't a fuse on that part of the
>> circuit - is that normal? I would have thought that'd mean that if
>> anything went wrong the first thing that's going to take the brunt of it
>> would be the transformer...
>>
>>
>>
>> >> I guess the next question is whether or not it's worth trying to
>> >> source a replacement transformer for it.
>>
>> > There's no harm in looking. If you can find out the spec for the HT
>> > winding (I'm assuming there aren't any others), you can probably source
>> > a beefier transformer for less than the cost of an OEM replacement.
>>
>> There are a total of three windings on the transformer - there's the HT
>> and a 12V supply for the DSP - looking closer at the diagram I can see
>> that the heater circuit needs 6.3v, I'm wondering if I could perhaps
>> find another transformer (or even a wall wart as a test) to see if the
>> rest of it still works? There's probably enough room on the chassis to
>> mount another transformer for the heater circuit as a 'kludge'
>>
>>
>>
>> >> Thanks for your advice and help!
>>
>> >> Phil
> 
> If it is indeed the transformer, you should not have that much of a
> problem finding a replacement.
> 
> There are several firms that provide "upgrade" kits for the Epi which
> include a new speaker and a transformer.
> 
> In fact, a quick google revealed that the transformer dying seems to
> be a common problem, and various better-quality replacement units can
> be had.

I'm fairly convinced now that it is indeed the transformer. As you say, it
seems to be a fairly common problem with the Epi Valve Standard.

I haven't managed to find anywhere in the UK that I can buy a replacement
(upgrade or original) - there are loads of upgrade kits for the valve
junior but not for the standard. Do you have any links?

Thanks

Phil

--

Phil

Sjfdix wrote:

> On Oct 7, 12:32 am, Phil Kett  wrote:
>> Richard Kaulfuss wrote:
>> > Phil Kett  wrote in
>> >news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com:
>>
>> >> On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
>> >>  wrote:
>>
>> >>> Phil Kett  wrote in
>> >>>news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com:
>>
>> >>>> Phil Kett wrote:
>>
>> >>>>> Hi,
>>
>> >>>>> I've just turned on my amp and it's not working (was working fine
>> >>>>> the last time it was turned on!).
>>
>> >>>>> The light in the power switch is coming on so there's power but no
>> >>>>> output.
>>
>> >>>>> Looking in the back the valves aren't glowing as I'd expect them to
>> >>>>> be. I suspect there might be a fuse blown somewhere but never
>> >>>>> having worked on a valve amp before (though I'm well aware of the
>> >>>>> dangers) I thought I'd ask here to see if anyone has any advice
>> >>>>> before digging into the chassis!
>> >>>> Bad form to reply to my own post I know but I thought I'd post an
>> >>>> update.
>>
>> >>>> I pulled the chassis out last night, there are three fuses on the
>> >>>> circuit board and they are all fine.
>>
>> >>>> At this point I'm a bit lost - I just don't know enough about tube
>> >>>> amps to diagnose the fault. If what I've read is accurate though,
>> >>>> I'd guess that it's the heater circuit that's not working. As the
>> >>>> fuses are ok does that mean it's likely to be the transformer that's
>> >>>> died?
>> >>> Possible, but unlikely. A more probable cause is a cold or dry solder
>> >>> joint (or even a break) at the start of the heater chain, for example
>> >>> at the point where the wires from the transformer connect to the PCB,
>> >>> if the valve bases are PCB mounted. Be wary of any charge stored in
>> >>> the reservoir capacitors after the amp is switched off - it can take
>> >>> several minutes to reduce to a safe level when the valves are not
>> >>> conducting.
>> >> Well, I've just checked the output from the transformer and got 0.1V -
>> >> not good.
>>
>> > I have to agree :-(
>>
>> >> Checked continuity on those wires and there's no resistance.
>> >> Looks like that coil on the power transformer has fused.
>>
>> > By 'no resistance', do you mean open-circuit? If you probe gently where
>> > the wires come out of the bobbin, you might be lucky enough to find a
>> > break there.
>>
>> By no resistance I mean that my meter is showing 0 ohms, or at least
>> close enough to set the buzzer of the continuity meter off. I wouldn't
>> have expected that from a transformer winding.
>>
>>
>>
>> >> There is a brown 'disk' (I suspect a capacitor) in series with these
>> >> outputs (where they connect to the circuit board) that looks like it's
>> >> slightly burnt. The only markings on it are 'CHANGE' - it's not shown
>> >> on the circuit diagram I have.
>>
>> > It's probably a thermistor to limit the inrush current. A rather
>> > unusual measure IME, and possibly indicates that the transformer was
>> > marginally specified. If the transformer voltage had been normal, I
>> > might have suspected that as the culprit.
>>
>> Looking at the circuit diagram there isn't a fuse on that part of the
>> circuit - is that normal? I would have thought that'd mean that if
>> anything went wrong the first thing that's going to take the brunt of it
>> would be the transformer...
>>
>>
>>
>> >> I guess the next question is whether or not it's worth trying to
>> >> source a replacement transformer for it.
>>
>> > There's no harm in looking. If you can find out the spec for the HT
>> > winding (I'm assuming there aren't any others), you can probably source
>> > a beefier transformer for less than the cost of an OEM replacement.
>>
>> There are a total of three windings on the transformer - there's the HT
>> and a 12V supply for the DSP - looking closer at the diagram I can see
>> that the heater circuit needs 6.3v, I'm wondering if I could perhaps
>> find another transformer (or even a wall wart as a test) to see if the
>> rest of it still works? There's probably enough room on the chassis to
>> mount another transformer for the heater circuit as a 'kludge'
>>
>>
>>
>> >> Thanks for your advice and help!
>>
>> >> Phil
> 
> If it is indeed the transformer, you should not have that much of a
> problem finding a replacement.
> 
> There are several firms that provide "upgrade" kits for the Epi which
> include a new speaker and a transformer.
> 
> In fact, a quick google revealed that the transformer dying seems to
> be a common problem, and various better-quality replacement units can
> be had.

Would this work as a replacement for the heater circuit?

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0504561

Thanks

Phil

--

Phil

Phil Kett  wrote in
news:WeSdnSnp_cf-WFbXnZ2dnUVZ8lmdnZ2d@supernews.com: 

> 
> 
> Richard Kaulfuss wrote:
>> Phil Kett  wrote in
>> news:985nc5trdrt1n19qkqf488mhh9ac27mrso@4ax.com: 
>> 
>>> On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
>>>  wrote:
>>>
>>>> Phil Kett  wrote in
>>>> news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@supernews.com: 
>>>>
>>>>> Phil Kett wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I've just turned on my amp and it's not working (was working fine
>>>>>> the last time it was turned on!).
>>>>>>
>>>>>> The light in the power switch is coming on so there's power but
>>>>>> no output.
>>>>>>
>>>>>> Looking in the back the valves aren't glowing as I'd expect them
>>>>>> to be. I suspect there might be a fuse blown somewhere but never
>>>>>> having worked on a valve amp before (though I'm well aware of the
>>>>>> dangers) I thought I'd ask here to see if anyone has any advice
>>>>>> before digging into the chassis!
>>>>> Bad form to reply to my own post I know but I thought I'd post an
>>>>> update. 
>>>>>
>>>>> I pulled the chassis out last night, there are three fuses on the
>>>>> circuit board and they are all fine.
>>>>>
>>>>> At this point I'm a bit lost - I just don't know enough about tube
>>>>> amps to diagnose the fault. If what I've read is accurate though,
>>>>> I'd guess that it's the heater circuit that's not working. As the
>>>>> fuses are ok does that mean it's likely to be the transformer
>>>>> that's died? 
>>>> Possible, but unlikely. A more probable cause is a cold or dry
>>>> solder joint (or even a break) at the start of the heater chain,
>>>> for example at the point where the wires from the transformer
>>>> connect to the PCB, if the valve bases are PCB mounted. Be wary of
>>>> any charge stored in the reservoir capacitors after the amp is
>>>> switched off - it can take several minutes to reduce to a safe
>>>> level when the valves are not conducting. 
>>> Well, I've just checked the output from the transformer and got 0.1V
>>> - not good.
>> 
>> I have to agree :-(
>> 
>>> Checked continuity on those wires and there's no resistance.
>>> Looks like that coil on the power transformer has fused.
>> 
>> By 'no resistance', do you mean open-circuit? If you probe gently
>> where the wires come out of the bobbin, you might be lucky enough to
>> find a break there.
>> 
> 
> By no resistance I mean that my meter is showing 0 ohms, or at least 
> close enough to set the buzzer of the continuity meter off. I wouldn't
> have expected that from a transformer winding.

It's fairly normal for a heater winding, which consists of a relatively 
low number of turns of relatively heavy gauge wire. Unfortunately, a 
resistance measurement alone will not distinguish a healthy winding from 
one with a shorted turn. You can confirm that the transformer is faulty 
by pulling out all the valves and checking the voltage again. You could 
also check the other windings at the same time (Be careful with the HT!)

> 
>>  
>>> There is a brown 'disk' (I suspect a capacitor) in series with these
>>> outputs (where they connect to the circuit board) that looks like
>>> it's slightly burnt. The only markings on it are 'CHANGE' - it's not
>>> shown on the circuit diagram I have.
>> 
>> It's probably a thermistor to limit the inrush current. A rather
>> unusual measure IME, and possibly indicates that the transformer was
>> marginally specified. If the transformer voltage had been normal, I
>> might have suspected that as the culprit. 
> 
> Looking at the circuit diagram there isn't a fuse on that part of the 
> circuit - is that normal? I would have thought that'd mean that if 
> anything went wrong the first thing that's going to take the brunt of
> it would be the transformer...

It's not usually considered necessary, as there's so little to go wrong 
and a short circuit (which is all the fuse protects against) is the least 
likely occurrence.

> 
>> 
>> 
>>> I guess the next question is whether or not it's worth trying to
>>> source a replacement transformer for it.
>>  
>> There's no harm in looking. If you can find out the spec for the HT 
>> winding (I'm assuming there aren't any others), you can probably
>> source a beefier transformer for less than the cost of an OEM
>> replacement. 
>> 
> 
> There are a total of three windings on the transformer - there's the
> HT and a 12V supply for the DSP - looking closer at the diagram I can
> see that the heater circuit needs 6.3v, I'm wondering if I could
> perhaps find another transformer (or even a wall wart as a test) to
> see if the rest of it still works? There's probably enough room on the
> chassis to mount another transformer for the heater circuit as a
> 'kludge' 

Probably not a good idea. If the heater winding is shorted, as it likely 
is, there will already be an abnormally high current flowing in the 
primary winding (I'm a little surprised it hasn't caused the mains fuse 
to blow) which might ultimately present a fire hazard if the fuse doesn't 
blow once the amp starts producing power. A safer option would be to 
source separate (HT + heater) and 12 V transformers if you can't find a 
single unit to do the job.

Richard Kaulfuss wrote:


> 
> It's fairly normal for a heater winding, which consists of a relatively
> low number of turns of relatively heavy gauge wire. Unfortunately, a
> resistance measurement alone will not distinguish a healthy winding from
> one with a shorted turn. You can confirm that the transformer is faulty
> by pulling out all the valves and checking the voltage again. You could
> also check the other windings at the same time (Be careful with the HT!)
> 

Ok, I checked the transformer properly last night - the heater winding is
definitely toast. The 12V is fine though, as is the HT (270V).

> 
> It's not usually considered necessary, as there's so little to go wrong
> and a short circuit (which is all the fuse protects against) is the least
> likely occurrence.
> 

That makes sense - it is a pretty simple circuit after all.

> 
> Probably not a good idea. If the heater winding is shorted, as it likely
> is, there will already be an abnormally high current flowing in the
> primary winding (I'm a little surprised it hasn't caused the mains fuse
> to blow) which might ultimately present a fire hazard if the fuse doesn't
> blow once the amp starts producing power. A safer option would be to
> source separate (HT + heater) and 12 V transformers if you can't find a
> single unit to do the job.

The only reason I suggested this was that I'd read somewhere about someone
doing just that - replacing the heater winding with an additional
transformer and keeping the rest. I don't know whether that was a
preventative measure though or if the heater winding had already blown.

Yesterday, I contacted Gibson to see if it was possible to obtain a
replacement power transformer - their only response was 'We don't provide
parts - take it to a tech.'

I can't find anywere in the UK that's selling either an upgrade transformer
for this amp or a replacement. I also don't know of any amp techs in my
area (Woodford, Essex).

Can anyone recommend anywhere to either get it fixed or get a replacement
transformer?

Thanks

Phil

 
--

Phil

On 8 Oct, 10:32, Phil Kett  wrote:
> Richard Kaulfuss wrote:
>
> > It's fairly normal for a heater winding, which consists of a relatively
> > low number of turns of relatively heavy gauge wire. Unfortunately, a
> > resistance measurement alone will not distinguish a healthy winding from
> > one with a shorted turn. You can confirm that the transformer is faulty
> > by pulling out all the valves and checking the voltage again. You could
> > also check the other windings at the same time (Be careful with the HT!)
>
> Ok, I checked the transformer properly last night - the heater winding is
> definitely toast. The 12V is fine though, as is the HT (270V).
>
>
>
> > It's not usually considered necessary, as there's so little to go wrong
> > and a short circuit (which is all the fuse protects against) is the least
> > likely occurrence.
>
> That makes sense - it is a pretty simple circuit after all.
>
>
>
> > Probably not a good idea. If the heater winding is shorted, as it likely
> > is, there will already be an abnormally high current flowing in the
> > primary winding (I'm a little surprised it hasn't caused the mains fuse
> > to blow) which might ultimately present a fire hazard if the fuse doesn't
> > blow once the amp starts producing power. A safer option would be to
> > source separate (HT + heater) and 12 V transformers if you can't find a
> > single unit to do the job.
>
> The only reason I suggested this was that I'd read somewhere about someone
> doing just that - replacing the heater winding with an additional
> transformer and keeping the rest. I don't know whether that was a
> preventative measure though or if the heater winding had already blown.
>
> Yesterday, I contacted Gibson to see if it was possible to obtain a
> replacement power transformer - their only response was 'We don't provide
> parts - take it to a tech.'
>
> I can't find anywere in the UK that's selling either an upgrade transformer
> for this amp or a replacement. I also don't know of any amp techs in my
> area (Woodford, Essex).
>
> Can anyone recommend anywhere to either get it fixed or get a replacement
> transformer?
>
> Thanks
>
> Phil
>

You might like to try Mike @ Living In The Past (http://
livinginthepast-audioweb.co.uk/valveaudio.html) - he's a reseller of
Danbury Electronics transformers and was pretty helpful to me when I
was looking to build a 5W amp a while back...

...but I succumbed to temptation and bought a Blackstar instead :-)


Ross.
--
www.rossedwards.net

Phil Kett  wrote in
news:NaGdnR77DYAkLFDXnZ2dnUVZ_vKdnZ2d@supernews.com: 

> Richard Kaulfuss wrote:
> 
> 
>> 
>> It's fairly normal for a heater winding, which consists of a
>> relatively low number of turns of relatively heavy gauge wire.
>> Unfortunately, a resistance measurement alone will not distinguish a
>> healthy winding from one with a shorted turn. You can confirm that
>> the transformer is faulty by pulling out all the valves and checking
>> the voltage again. You could also check the other windings at the
>> same time (Be careful with the HT!) 
>> 
> 
> Ok, I checked the transformer properly last night - the heater winding
> is definitely toast. The 12V is fine though, as is the HT (270V).
> 
>> 
>> It's not usually considered necessary, as there's so little to go
>> wrong and a short circuit (which is all the fuse protects against) is
>> the least likely occurrence.
>> 
> 
> That makes sense - it is a pretty simple circuit after all.
> 
>> 
>> Probably not a good idea. If the heater winding is shorted, as it
>> likely is, there will already be an abnormally high current flowing
>> in the primary winding (I'm a little surprised it hasn't caused the
>> mains fuse to blow) which might ultimately present a fire hazard if
>> the fuse doesn't blow once the amp starts producing power. A safer
>> option would be to source separate (HT + heater) and 12 V
>> transformers if you can't find a single unit to do the job.
> 
> The only reason I suggested this was that I'd read somewhere about
> someone doing just that - replacing the heater winding with an
> additional transformer and keeping the rest. I don't know whether that
> was a preventative measure though or if the heater winding had already
> blown. 

It's a perfectly reasonable solution if the transformer has failed open-
circuit but because yours appears to have a shorted winding, there could 
be all sorts of problems, such as overheating, core saturation and poor 
regulation, if you tried to run it normally.

> 
> Yesterday, I contacted Gibson to see if it was possible to obtain a
> replacement power transformer - their only response was 'We don't
> provide parts - take it to a tech.'
> 
> I can't find anywere in the UK that's selling either an upgrade
> transformer for this amp or a replacement. I also don't know of any
> amp techs in my area (Woodford, Essex).
> 
> Can anyone recommend anywhere to either get it fixed or get a
> replacement transformer?

Can't help with any specifics there. The 12 V transformer should be easy 
to source. One of the audiophile sites might be able to supply a suitable 
HT+heater transformer. Make sure they have adequate current ratings.

Richard Kaulfuss wrote:
> Phil Kett  wrote in
> news:NaGdnR77DYAkLFDXnZ2dnUVZ_vKdnZ2d@supernews.com: 
> 
>> Richard Kaulfuss wrote:
>>
>>
>>> It's fairly normal for a heater winding, which consists of a
>>> relatively low number of turns of relatively heavy gauge wire.
>>> Unfortunately, a resistance measurement alone will not distinguish a
>>> healthy winding from one with a shorted turn. You can confirm that
>>> the transformer is faulty by pulling out all the valves and checking
>>> the voltage again. You could also check the other windings at the
>>> same time (Be careful with the HT!) 
>>>
>> Ok, I checked the transformer properly last night - the heater winding
>> is definitely toast. The 12V is fine though, as is the HT (270V).
>>
>>> It's not usually considered necessary, as there's so little to go
>>> wrong and a short circuit (which is all the fuse protects against) is
>>> the least likely occurrence.
>>>
>> That makes sense - it is a pretty simple circuit after all.
>>
>>> Probably not a good idea. If the heater winding is shorted, as it
>>> likely is, there will already be an abnormally high current flowing
>>> in the primary winding (I'm a little surprised it hasn't caused the
>>> mains fuse to blow) which might ultimately present a fire hazard if
>>> the fuse doesn't blow once the amp starts producing power. A safer
>>> option would be to source separate (HT + heater) and 12 V
>>> transformers if you can't find a single unit to do the job.
>> The only reason I suggested this was that I'd read somewhere about
>> someone doing just that - replacing the heater winding with an
>> additional transformer and keeping the rest. I don't know whether that
>> was a preventative measure though or if the heater winding had already
>> blown. 
> 
> It's a perfectly reasonable solution if the transformer has failed open-
> circuit but because yours appears to have a shorted winding, there could 
> be all sorts of problems, such as overheating, core saturation and poor 
> regulation, if you tried to run it normally.

I've just done a little more extensive testing.

With the two wires connected to the circuit but no valves in place, I 
get virtual nothing registering on the volt meter, however if I 
completely remove the two wires from the circuit and test I get the 
voltage I'd expect (A bit more in fact 6.9V).

I've checked continuity between the power connectors and the power pins 
on the valve sockets - everything checks out OK. I reflowed the solder 
on those sockets anyway just to be sure.

I don't know what to think now...

Phil

Phil Kett  wrote:

> With the two wires connected to the circuit but no valves in place, I
> get virtual nothing registering on the volt meter, however if I 
> completely remove the two wires from the circuit and test I get the 
> voltage I'd expect (A bit more in fact 6.9V).
> 
> I've checked continuity between the power connectors and the power pins
> on the valve sockets - everything checks out OK. I reflowed the solder
> on those sockets anyway just to be sure.
> 
> I don't know what to think now...

What is the static resistance (with no power and no valves) to the place
where the heater transformer goes in? ie, actual resistance, not on buzz
mode. I haven't seen the valve standard circuit (and couldn't be
bothered to register on a forum to get it), but the other epiphones
generally have a couple of 100r to ground (so 200 to each other), unless
it is rectified, then it will give different figures.

If that seems ok, you could connect one of the wires back to the
circuit, and connect the other one through your meter set to amps.
Without the valves in place there should be very little current flowing
(if it is just the 200r resistance I would guess about 30mA). If more
current is flowing something is shorting out, if it is less then your
tranformer isn't capable of producing anything like enough current so it
has gone byebye.

-- 
Woody

www.alienrat.com

Woody wrote:
> Phil Kett  wrote:
> 
>> With the two wires connected to the circuit but no valves in place, I
>> get virtual nothing registering on the volt meter, however if I 
>> completely remove the two wires from the circuit and test I get the 
>> voltage I'd expect (A bit more in fact 6.9V).
>>
>> I've checked continuity between the power connectors and the power pins
>> on the valve sockets - everything checks out OK. I reflowed the solder
>> on those sockets anyway just to be sure.
>>
>> I don't know what to think now...
> 
> What is the static resistance (with no power and no valves) to the place
> where the heater transformer goes in? ie, actual resistance, not on buzz
> mode. I haven't seen the valve standard circuit (and couldn't be
> bothered to register on a forum to get it), but the other epiphones
> generally have a couple of 100r to ground (so 200 to each other), unless
> it is rectified, then it will give different figures.
> 

The circuit is exactly as you describe and the two resisters are fine, I 
get 200 exactly across the circuit.

> If that seems ok, you could connect one of the wires back to the
> circuit, and connect the other one through your meter set to amps.
> Without the valves in place there should be very little current flowing
> (if it is just the 200r resistance I would guess about 30mA). If more
> current is flowing something is shorting out, if it is less then your
> tranformer isn't capable of producing anything like enough current so it
> has gone byebye.
> 

This pretty much confirms my suspicions, with no valves in place the 
current through the circuit is miniscule, I can only get any kind of 
reading with my meter set to 2mA - even then it's only showing 0.65.

I'm going to stop messing with it now - there's obviously nothing to be 
done unless I can get a replacement power transformer.

Thanks for all your help guys!

Phil

Phil Kett  wrote in 
news:qOednYwahd9pjFLXnZ2dnUVZ8hudnZ2d@supernews.com:

> 
> 
> Woody wrote:
>> Phil Kett  wrote:
>> 
>>> With the two wires connected to the circuit but no valves in place, I
>>> get virtual nothing registering on the volt meter, however if I 
>>> completely remove the two wires from the circuit and test I get the 
>>> voltage I'd expect (A bit more in fact 6.9V).
>>>
>>> I've checked continuity between the power connectors and the power 
pins
>>> on the valve sockets - everything checks out OK. I reflowed the 
solder
>>> on those sockets anyway just to be sure.
>>>
>>> I don't know what to think now...
>> 
>> What is the static resistance (with no power and no valves) to the 
place
>> where the heater transformer goes in? ie, actual resistance, not on 
buzz
>> mode. I haven't seen the valve standard circuit (and couldn't be
>> bothered to register on a forum to get it), but the other epiphones
>> generally have a couple of 100r to ground (so 200 to each other), 
unless
>> it is rectified, then it will give different figures.
>> 
> 
> The circuit is exactly as you describe and the two resisters are fine, 
I 
> get 200 exactly across the circuit.
> 
>> If that seems ok, you could connect one of the wires back to the
>> circuit, and connect the other one through your meter set to amps.
>> Without the valves in place there should be very little current 
flowing
>> (if it is just the 200r resistance I would guess about 30mA). If more
>> current is flowing something is shorting out, if it is less then your
>> tranformer isn't capable of producing anything like enough current so 
it
>> has gone byebye.
>> 
> 
> This pretty much confirms my suspicions, with no valves in place the 
> current through the circuit is miniscule, I can only get any kind of 
> reading with my meter set to 2mA - even then it's only showing 0.65.
> 
> I'm going to stop messing with it now - there's obviously nothing to be 
> done unless I can get a replacement power transformer.

As a final comment, I would add that since the transformer is producing a 
normal off-load voltage when disconnected (I had assumed that was how you 
were measuring it in the first place), the fault is probably due to a 
high resistance somewhere in the path, not a shorted winding, so it would 
be safe to proceed with your original idea of using a separate 
transformer to supply just the heaters.

Richard Kaulfuss wrote:

> As a final comment, I would add that since the transformer is producing a 
> normal off-load voltage when disconnected (I had assumed that was how you 
> were measuring it in the first place), the fault is probably due to a 
> high resistance somewhere in the path, not a shorted winding, so it would 
> be safe to proceed with your original idea of using a separate 
> transformer to supply just the heaters.
> 

Thanks for that Richard, what sort of current loading can I expect on 
the heater circuit (the amp has 5 tubes 2 x EL84 and 3 x 12AX7) - the 
original circuit diagram I've got (which I've uploaded here 
http://www.remosync.org/amps/StdSch.2005.10.21-2.pdf) states 6.3VAC/1.6A 
but I've been advised elsewhere that that's way too low.

Thanks

Phil

Phil Kett  wrote in
news:jqWdnU0hX8Mf8E3XnZ2dnUVZ8rFi4p2d@supernews.com: 

> 
> 
> Richard Kaulfuss wrote:
> 
>> As a final comment, I would add that since the transformer is
>> producing a normal off-load voltage when disconnected (I had assumed
>> that was how you were measuring it in the first place), the fault is
>> probably due to a high resistance somewhere in the path, not a
>> shorted winding, so it would be safe to proceed with your original
>> idea of using a separate transformer to supply just the heaters.
>> 
> 
> Thanks for that Richard, what sort of current loading can I expect on 
> the heater circuit (the amp has 5 tubes 2 x EL84 and 3 x 12AX7) - the 
> original circuit diagram I've got (which I've uploaded here 
> http://www.remosync.org/amps/StdSch.2005.10.21-2.pdf) states
> 6.3VAC/1.6A but I've been advised elsewhere that that's way too low.
> 

My data book states 300 mA for a 12AX7 and 760 mA for an EL84. That makes 
2.42 A total, so you would need a 2.5 or 3 A transformer. Using a 
transformer rated at 1.6 A was optimistic, to say the least.

Richard Kaulfuss wrote:
> Phil Kett  wrote in
> news:jqWdnU0hX8Mf8E3XnZ2dnUVZ8rFi4p2d@supernews.com: 
> 
>>
>> Richard Kaulfuss wrote:
>>
>>> As a final comment, I would add that since the transformer is
>>> producing a normal off-load voltage when disconnected (I had assumed
>>> that was how you were measuring it in the first place), the fault is
>>> probably due to a high resistance somewhere in the path, not a
>>> shorted winding, so it would be safe to proceed with your original
>>> idea of using a separate transformer to supply just the heaters.
>>>
>> Thanks for that Richard, what sort of current loading can I expect on 
>> the heater circuit (the amp has 5 tubes 2 x EL84 and 3 x 12AX7) - the 
>> original circuit diagram I've got (which I've uploaded here 
>> http://www.remosync.org/amps/StdSch.2005.10.21-2.pdf) states
>> 6.3VAC/1.6A but I've been advised elsewhere that that's way too low.
>>
> 
> My data book states 300 mA for a 12AX7 and 760 mA for an EL84. That makes 
> 2.42 A total, so you would need a 2.5 or 3 A transformer. Using a 
> transformer rated at 1.6 A was optimistic, to say the least.

Just thought I'd let you know that I got hold of a replacement 
transformer for the amp.

I'm only using the 6.3v winding at the moment (it also has a 270V 
winding) but the amp is working again.

Many many thanks for all your help!

Phil