When to change an amps filter caps?

Collings

I've just brought a Marshall that is almost 30yrs old. After changing all the valves and setting the bias it sounds great.

However i was wondering given its age if i should consider changing the electrolytic caps.

As i said it sounds great and its is not noisy in terms of hum.

So how do you know when its time to swap the caps?

Modulus_Amps

They dry out regardless of use, you can visually inspect them to see what shape they are in, if they are bulging or leaking then they need changed, being 30 years old they may have been changed already.

If they are the originals I would change them as they can make a huge mess inside the amp.

ecc83

All rather depends upon the quality of the original capacitors (ICBM will surely know) and the life the amp has had but you can do a visual inspection. If the contact ends of the caps are clean, free of any powdery deposit and not bulged in any way I would leave them.

Dave.

jpfamps

I always change the bias filter caps in Marshalls of this age.

If these caps go leaky the amp can go into thermal runaway.

This is more likely in 50W Marshalls as this bias supply is fed from a high value resistor so even a small amount of leakage causes the bias to drift towards to ground, which biases the power valves hotter.

100W Marshalls are less prone to this problems but I've seen bias failure in these too.

Bias failure can and does cause damage to output and mains transformers, so can be expensive.

It's rare to find filter caps that need changing in Marshall amps of this age, and I would not routinely change the caps in an amp this age.

https://jpfamps.com/marshall-jmp-amplifier-repair/

Collings

Thanks for the input.

Visually all the caps look ok.

ICBM

Check the bias voltage, and if it's even slightly low - the symptom is needing to set the trimmer full up to get the bias into the right range - replace the two bias filter caps... they fail much more often than the main filter caps, surprisingly. I wouldn't normally replace the main caps in a JCM800 unless there's definite evidence of leakage - by the 80s, cap quality had improved and they tend to last longer.

ecc83

Ah! Bias caps, yes I would change those anyway. There is a cheap way to derive a bias supply which involves a foil capacitor and a high value resistor, around 100k. Both components stand changing in an old amplifier in my limited experience.

Dave.

ICBM

A trap for the unwary on old Marshalls (and others) is to find the bias voltage is too low, and bodge it by paralleling or replacing the feed resistor to lower its value and give more bias voltage - but it's always the caps that are the problem, and bodging the resistor just increases the leakage rate so it quickly makes the issue worse. This 'fix' was actually recommended in an old book about amp repair, clearly by someone who hadn't really thought about the reason!

Collings

Ok sounds like it might be worth changing the bias caps then.

Not too difficult a job luckily.

RiftAmps

Collings said:

Not too difficult a job luckily.

That depends on whether yours is turret board or PCB construction (probably the latter at 30 years old)

The correct (and fiddly) way is to remove the board - have fun with that - flip it over and unsolder the old caps from the underside of the board where the PCB traces are. Fit new caps, check orientation, and resolder, reassemble.

The bodge way is to snip the cap legs above the board and attach the new caps to the old legs - not recommended unless you're in a pinch and can do the correct repair later.

Modulus_Amps

I have done some recaps on 30 year old Marshalls by request and the end users have almost always come back saying the amp sounds better, of course that is subjective, however if you are doing the work yourself it is not too expensive to carry this out.

....or you can just wait until they really do need changed.

ICBM

RiftAmps said:

The correct (and fiddly) way is to remove the board - have fun with that - flip it over and unsolder the old caps from the underside of the board where the PCB traces are. Fit new caps, check orientation, and resolder, reassemble.

It's not too bad if you undo all the pots and jacks - the tricky bit is the two wires leading to the preamp filter cap, but I usually just cut those and replace with longer ones, since it makes doing it in the 'wrong' order (Marshall fitted the cap first of course) easier.

RiftAmps said:

The bodge way is to snip the cap legs above the board and attach the new caps to the old legs - not recommended unless you're in a pinch and can do the correct repair later.

I do admit to that if I'm just replacing the bias caps and not the main filters - I don't think it's too much of a bodge if you do it neatly, and there's enough length of wire from the old caps that there's very little danger of overheating the joints under the board. The logic is that if you later replace the main filters you can re-do the bias caps properly at the same time, if you want.

Modulus_Amps said:

I have done some recaps on 30 year old Marshalls by request and the end users have almost always come back saying the amp sounds better, of course that is subjective, however if you are doing the work yourself it is not too expensive to carry this out.

....or you can just wait until they really do need changed.

Everyone seems to worry about it 'ruining the tone' - not helped by a lot of what you read online - but I have never, not once, heard an old Marshall that sounded better before a recap than afterwards or had any complaint about it from the owner.

Collings

RiftAmps said:

Collings said:

Not too difficult a job luckily.

That depends on whether yours is turret board or PCB construction (probably the latter at 30 years old)

The correct (and fiddly) way is to remove the board - have fun with that - flip it over and unsolder the old caps from the underside of the board where the PCB traces are. Fit new caps, check orientation, and resolder, reassemble.

The bodge way is to snip the cap legs above the board and attach the new caps to the old legs - not recommended unless you're in a pinch and can do the correct repair later.

Its an old Jubilee and I think I should be able to lift the board enough after undoing the posts and jacks.

clarkefan

I changed both filter and bias caps on a 70s Marshall once, sounded exactly the same afterwards.

Are you supposed to "bring up the voltage slowly" when you change filter caps? I didn't, just turned it on, and there were no problems.

ICBM

Collings said:

Its an old Jubilee and I think I should be able to lift the board enough after undoing the posts and jacks.

Yes, they're easy - the board is mounted via them.

clarkefan said:

Are you supposed to "bring up the voltage slowly" when you change filter caps? I didn't, just turned it on, and there were no problems.

It doesn't matter with new caps.

The only time it's really a good idea is when powering up an old amp that hasn't been used for several years or more.

DJH83004

The bodge way is to snip the cap legs above the board and attach the new caps to the old legs - not recommended unless you're in a pinch and can do the correct repair later.

I actually don't think is a bodge way to be honest, component side repairs are sometimes the right way to go, especially as when lifting the PCB on a Marshall, you put the hideously short wiring under an awful lot of stress to get sufficient clearance to de-solder the caps, which can prove problematic in the future. The trick is to leave sufficient lead length on the old caps and be smartish with the soldering iron. You also have the option of hot-gluing the caps, which I know can look messy if not done properly, but does support the joints.