How old is "old" for modern filter caps?

[Rich_S]

My first-and-only amp build, and 18 Watter clone is seven years old. My, how time flies... now my baby's all grown up. I built it on a Bogen CHB-20A donor chassis in 2004. The Bogen used a voltage-doubler power supply, and in order to re-use its PT, I had to use the same setup. Schematic:



The amp has started sounding weird - there's a farty, low frequency overtone. It seems to reinforce the open low E string, but in most cases, the overtone bucks the guitar notes, adding up to a loss of volume and gain, and an overall muted sound.

I'm thinking "filter caps", but I replaced them in '04. C19 now comprises three sections of a JJ 40/20/20/20 500V multi-section, and C18 is an Atom axial-lead 40 uF.

I thought the common wisdom of filter caps was that twenty was too old, so change 'em at ten years just to be sure. Hell, the original early-70s caps that were in the Bogen worked fine when I first fired it up in 2004.

So, do they just not make 'em like they used to? Do I indeed need to replace the filter caps, even though they've only seen seven years of very occasional use?

'Nuther question: I turns out I have spares of both of these caps, I bought them back in '04 for another build I never got around to. They've just been sitting in the parts drawer for 6 or 7 years. Are they going to be any better than the ones in the amp, or should I chuck 'em all and order new ones?

[Keyser Soze]

Seven years sure sounds like a short period of time, but not every device fails on schedule.

You could try swapping out for the 'fresh' (if not new) ones. If it solves the problem you have your answer.

If it doesn't solve the problem...

How sure are you that the transformer is rated for the amount of current the amp demands?

[Rich_S]

I don't think I'm exceeding the PTs current capability - it's the same tube complement as original and dual triode and a pair of ECL86s.

[andyfromdenver]

Just curious, have you plugged in to a different speaker cab?

[Rich_S]

No, but I don't have another to try. There is very little chance my 13 Watter blew up a Celestion Classic Lead 80, though.

[telex76]

Your electrolytic caps from 2004 that are in your amp are probably in better shape than the ones that haven't been used.
The unused ones would need to be reformed slowly before hitting them with full power.

[BobbyZ]

Was just talking about new caps going bad. About a 10 year old Princeton Reverb my brother built is acting up. He was going to mess with other stuff first just to make sure but we're both thinking it's a cap.
I only work on old stuf so I just recap um at the first sign of trouble.

[telex76]

I see alot of people saying electrolytic caps should be changed every 8-10 years.
The filter caps in my Deluxe Reverb went Kaput after 43 years.

[jefrs]

I've got electrolytic smoothing capacitors of over 30yo still running very happily thankyou.

By all means test them, but if it ain't bust, don't fix it.

[JBennett]

What is the process for testing caps?

[Robster]

Oldest working filter caps I had were in a 1954 Valco built Silvertone. They were still OK after 55 years when I sold it.

[jefrs]

Put capacitors (disconnected and discharged) across a capacitance test meter. Some DVMM have them or it's a more specialised meter, but they usually also do resistors, diodes and maybe transistors too.

As in, "oh dear, my 50+50µF is 30yo, I'd better change it", only to find it is 47+48µF - good as new and well within spec and the amp is running sweet as a nut.

Using the amp fairly regularly does preserve these capacitors. The actual 50+50µF cap that was swapped-out in the early 80s and not used since now measures 34+34µF

[Keyser Soze]

Originally Posted by Rich_S

I don't think I'm exceeding the PTs current capability - it's the same tube complement as original and dual triode and a pair of ECL86s.

Did the original circuit also use a voltage doubler?

Remember Power = Voltage * Current

[Rich_S]

Yes, I had to carry over the voltage-doubler topology from the original, because the PT was designed to work in that circuit. Then I had to knock down the B+ with zeners because Bogen wanted 20 watts out of those poor little tubes, and the only way to get it was to use a B+ of about 450. Yikes.