When to replace caps in old amps?

[kgreene]

Looking for comments on assessing when to replace signal capacitors in old amps. For example, can you just measure value and replace when out of tolerance, or do you think it is necessary to substitute a new cap and listen, etc. I'd like to hear anyone share their thoughts on this. Thanks!

[Structo]

It depends on the amp and caps.

Also how much heat is generated.

Most E caps have 20% + - tolerance.

Power caps will last longer when used regularly.

I believe what happens is, as the electrolyte (paste) dries out, the ESR value increases causing it to heat, hum or explode.
Not necessarily in that order!

I recently bought a Blue ESR meter that can measure ESR in circuit. (discharge caps first).

Looks like the price went up. I paid $99 for mine.

http://www.ebay.com/itm/AnaTek-Blue-ESR ... 2a2ea6ec7a

[martin manning]

Looking for comments on assessing when to replace signal capacitors in old amps...

They are not so likely to change in value as they are to leak, and that is probably the only reason to replace them. A telltale sign is DC on the end that goes to grids or pots. Old paper-in-oil types can leak oil, and you would either have to see residue or measure them.

[kgreene]

Thanks, but your comments would apply to electrolytic caps in the power supply? How about coupling caps? I have a bunch of older Fender amps, and am loath to replace components willy-nilly, but find it hard to evaluate these critters without removing them, and trying something else - curious what system others use.

[kgreene]

Looking for comments on assessing when to replace signal capacitors in old amps...

They are not so likely to change in value as they are to leak, and that is probably the only reason to replace them. A telltale sign is DC on the end that goes to grids or pots.

So, you only replace leaky ones... do you check them in circuit, or out?

[billyz]

If they are the blue Ajax caps or later they are very seldom bad. Replace the power supply caps and bias supply cap. Good tubes, bias.
, play it. If it sounds good leave it be.

[Structo]

Sorry I misread your first post concerning signal caps.

Signal caps are the coupling caps and tone stack caps.
Usually a film cap of some sort.

You measure the DC leakage when the amp is powered up.

Set your meter to DC.
Clip your black lead to the chassis and touch the red probe on the end away from the power supply.

The meter must be one that can measure small amounts of voltage.

[martin manning]

To see if you can check for leakage in-circuit, examine the schematic. In most cases there is high voltage DC on one end and a path to ground through a relatively large resistance on the other. These you can check in-circuit. If one end is grounded, then you will have to lift it.

[kgreene]

Sorry I misread your first post concerning signal caps.

Signal caps are the coupling caps and tone stack caps.
Usually a film cap of some sort.

You measure the DC leakage when the amp is powered up.

Set your meter to DC.
Clip your black lead to the chassis and touch the red probe on the end away from the power supply.

The meter must be one that can measure small amounts of voltage.

Thanks for the comments... I understand the basics, as I've done some amp tech work in the 80's when I was gigging full time. But what I have here are old tweed amps from the 50s and some very early 60s. I just wondered if the tone response changed, slope, etc.? Things that are not so easily evaluated as leakage.

[kgreene]

If they are the blue Ajax caps or later they are very seldom bad. Replace the power supply caps and bias supply cap. Good tubes, bias.
, play it. If it sounds good leave it be.

Amen... I generally leave most parts alone if possible, other than the obvious things that get old or worn out.

[kgreene]

To see if you can check for leakage in-circuit, examine the schematic. In most cases there is high voltage DC on one end and a path to ground through a relatively large resistance on the other. These you can check in-circuit. If one end is grounded, then you will have to lift it.

Yep, that's about what I've always done.... I appreciate the comment. I guess I'm also wondering if being loaded in circuit could potentially influence the tendency for leakage.

[kgreene]

Sorry, I don't mean to get all OCD on you guys.

I do appreciate the chance to ask a few questions that are probably obvious to many on this forum.

[rfgordon]

I work on a lot of old (50s & 60s) amps. It's amazing how well some of those old filters still work. I've got a 59/60 tweed Harvard that has a really strong filter set--all original, and no leaky signal caps.

My rules are:
1. Filters: Replace if failed, hummy or leaking goop
2. Signal caps: clean pots first & check DC. Replace only if absolutely needed.
3. Tremolo oscillator caps: Check tube first, then switch, then replace caps if still not working.