Marshall JTM60 Problem - Advice please

[Steve Robinson]

I've just been playing thru my Marshall JTM610. I was aware of a louder than
normal hum coming from it, then the hum went away and so did the sound of my
guitar. :(

Normal (low) background hum is still present so I think that the power stage
is okay, and the problem is with the preamp stages. Nothing seems overly
hot - no burning smells or anything.

I changed the EL84s a little while back but not the preamp valves. I think a
good bet be to replace the preamp valves?

My initial thoughts are to buy high-gain Harmas from Watford Valves but I am
open to suggestions as to which 12AX7s to go for.

Cheers,
Steve.

[Al]

Post by Steve Robinson
I've just been playing thru my Marshall JTM610. I was aware of a
louder than normal hum coming from it

Possibly as one of the power supply caps started to fail big time ...

Post by Steve Robinson
then the hum went away and so
did the sound of my guitar. :(

Possibly as the rectifier went phut because the cap had failed. Or
perhaps the cap only supplied part of the amp.

I don't seem to have a schematic to hand for this amp, so I don't know if
this is SS or valve recto. I guess its SS though. It's quite possible
that an overstressed (through voltage or temperature) smoothing cap has
failed and taken out the rectifier. But ...

Post by Steve Robinson
Normal (low) background hum is still present

Hmmm (or hum). So there's still power somewhere. If the power supply is
the usual arrangement with 2 or 3 smoothing stages for preamp, driver and
power with current limiting resistors between each it could be the one of
the former two. So it could just be the preamp cap failed - Look for a
blown vent, and/or stuff coming out of it.

Post by Steve Robinson
so I think that the power
stage is okay

Can you plug in via the FX loop (if it has one of course!). That would
prove if the preamp stage was dead.

Post by Steve Robinson
no burning smells or anything.

Shame. The flames and blackened bits are always a good clue ;-)

Post by Steve Robinson
think a good bet be to replace the preamp valves?

Possibly. Try swapping them around, or plug in a known good valve into
each position (one at a time, it's unlikely that more than one valve will
fail at a time).

Where are you geographically? I can take a quick peek if you're local to
me, and I have plenty of spare valve to hand.

Post by Steve Robinson
My initial thoughts are to buy high-gain Harmas from Watford Valves
but I am open to suggestions as to which 12AX7s to go for.

Don't get me started - Lot's of BS about valves. But I wouldn't worry
about whch to choose yet as we don't know what the problem is!

Al.

[Steve Robinson]

Post by Al
I don't seem to have a schematic to hand for this amp, so I don't know if
this is SS or valve recto. I guess its SS though.

Correct - schematics are here:
http://www.marshallschematics.com/

Post by Al
Can you plug in via the FX loop (if it has one of course!). That would
prove if the preamp stage was dead.

I tried that but no noise.

Post by Al
Possibly. Try swapping them around, or plug in a known good valve into
each position (one at a time, it's unlikely that more than one valve will
fail at a time).

Still no noise

Post by Al
Where are you geographically? I can take a quick peek if you're local to
me, and I have plenty of spare valve to hand.

Thanks for all your suggestions, I'm in South Manchester - and you?

Steve.

[Ivar Arnesen]

Post by Steve Robinson

Post by Al
I don't seem to have a schematic to hand for this amp, so I don't know if
this is SS or valve recto. I guess its SS though.

http://www.marshallschematics.com/

Post by Al
Can you plug in via the FX loop (if it has one of course!). That would
prove if the preamp stage was dead.

I tried that but no noise.

Post by Al
Possibly. Try swapping them around, or plug in a known good valve into
each position (one at a time, it's unlikely that more than one valve will
fail at a time).

Still no noise

Post by Al
Where are you geographically? I can take a quick peek if you're local to
me, and I have plenty of spare valve to hand.

Thanks for all your suggestions, I'm in South Manchester - and you?
Steve.

This amp has a history of going down like this, mine did the exact same
thing, luckily while still under guarantee.
Can't remember what it was though..

Manchester-aera? Trev's your man!

http://www.tipton-amps.co.uk/

Ivar

[Steve Cobham]

On Thu, 13 Jul 2006 22:52:46 +0200, "Ivar Arnesen"

Post by Ivar Arnesen
This amp has a history of going down like this

Ah, that must be the Tara Palmer Tompkinson sig model!

IGMC

Steve.

[Trevor Ridney]

Post by Steve Robinson
I've just been playing thru my Marshall JTM610. I was aware of a louder
than normal hum coming from it, then the hum went away and so did the
sound of my guitar. :(
Normal (low) background hum is still present so I think that the power
stage is okay, and the problem is with the preamp stages. Nothing seems
overly hot - no burning smells or anything.

Hi Steve,

I'd lay even money the output valves are toast, mate.

What you hear through the speakers is slight AC 'ripple' from the B+ (anode)
voltage applied to the output transformer primary centre tap

Unlikely to be a power supply capacitor as this would have popped the HT
fuse and you'd hear nary a squeak out of it.

I'd be inclined to try some old but working output valves in there, assuming
you have a couple to hand.

If that brings the amp back to life, get some new valves and have the amp's
bias checked and if necessary adjusted. If in doubt, get thee to a tech.
Valve amps work at voltages that can kill.

regards,

Trev

[zorro]

Post by Steve Robinson
I've just been playing thru my Marshall JTM610. I was aware of a louder than
normal hum coming from it, then the hum went away and so did the sound of my
guitar. :(
Normal (low) background hum is still present so I think that the power stage
is okay, and the problem is with the preamp stages. Nothing seems overly
hot - no burning smells or anything.
I changed the EL84s a little while back but not the preamp valves. I think a
good bet be to replace the preamp valves?
My initial thoughts are to buy high-gain Harmas from Watford Valves but I am
open to suggestions as to which 12AX7s to go for.
Cheers,
Steve.

Hi,
I had quite the same problem with a
JTM30, bad cold solder joint in a
power valve...
Regards
Rob