Crackling noise from cap when powering off

[Hardtailed]

I've never had that before, but this new amp I built does that.
When I turn it off there is some "clicking" noises coming from the main filter cap (or at least it seems to be coming from there).
The main filter cap is the only one that is outside the amp (an F&T 16/16 can)
It does that pretty much right after switching off (whether the amp was on the standby or not).

It's also the first amp I do with a tube rectifier (GZ34) which is just next to the cap can.

I noticed the cap discharges pretty quickly (perhaps cause it's such a low value) through the tubes. A few seconds and it reads 0V.
If I switch to Standby first, it takes a little more time (still discharges rather quickly through the preamp tubes and the 288Kohms bleeder circuit).

Could it be the quick discharge that does that?

My Marshall clone (with a JJ paralleled 50/50 uF main filter cap and SS recto) does not do that.

There is no sound at all coming through the speaker, it's really just coming from the amp itself. I'm assuming it's harmless... but just making sure.

[Kazooman]

Better Check The Cap

You should test the cap. There are a lot of fake electrolytic caps floating around. Often made by taking recycled electrolytics and rehousing them in an additional shell. Often the capacitance and/or voltages are not what the outer shell claims.

Check out this thread (and one other about capacitors) at the Amp Garage:

The Amp Garage :: View topic - Inexpensive Electrolytic Capacitors

Some amazing pictures of caps that have been cut open. Sometimes there is an old radial cap inside a new axial shell that is then put inside yet another axial shell.

BTW, on page 5 of the thread there is a list of some test results someone ran. The F&Ts did not fare well.

[Hardtailed]

Yeah I've read that very thread recently.

This is however an F&T from a reputable seller (NOT Weber )
I'm not willing to cut it open and my DMM doesn't do capacitance

Works perfectly fine while the amp is on though, it's just when I turn it off (and it's not something that comes through the speaker)

[Kazooman]

Working Voltage?

Good that you are aware of the problem. I was concerned that you might have a bogus cap made from junk with a lower working voltage than what you require. That would really stress the cap and it might make some bad noises when powered down and discharging. If that is the case it is destined for failure and you don't want to have other parts go with it.

You could solder in a replacement (or even two 16's temporarily) and see if you get the same result.

[Kazooman]

Cap Meter

I bought one of these capacitance meter kits and it works great.

Capacitance Meter Kit

Or I should say that it seems to work great since I have no gold standard to compare it to for calibration. However, when I tested numerous caps that I had that I believe were of high quality the meter reading and the cap value were spot on.

[Hardtailed]

Good that you are aware of the problem. I was concerned that you might have a bogus cap made from junk with a lower working voltage than what you require. That would really stress the cap and it might make some bad noises when powered down and discharging. If that is the case it is destined for failure and you don't want to have other parts go with it.

You could solder in a replacement (or even two 16's temporarily) and see if you get the same result.

Yeah, those bogus caps freak me out... I can't see making much profit on doing that too. I try to avoid components from asia as much as possible now...
I do have some Sprague Atoms in there (small preamp filter caps on board, and cathode-bypass caps) and apparently those are not what they used to be. It,s not the big fat blue ones though, it's the small black ones.

Good thing is all caps are rated for 450V or 500V and the B+ is barely 335V.

I do have a dual 32uF JJ can I could try... but the amp is getting crowded in the area (I hate lay-down PTs) and replacing it is something I wished to avoid. So I was kind of hoping someone would say "Yeah, they do that sometimes, don't worry" Hahahaha.

Thanks buddy... might have to do it for peace of mind

[Hardtailed]

Hmmm, upon further investigation, doesn't seem to be precisely located to the cap. It's actually hard to pinpoint it but it seems to be coming from all around, mostly the tubes (there are 8 of them). I guess I'm not used to the behaviour of an amp that is so hard on the tube (14.5W at idle per EL84, 16W if you include screens... typical AC30!). They dissipate a lot of heat for their size, so it's probably just typical metal contraction/dilatation from the heat cycling.

[R.G.]

I try to avoid components from asia as much as possible now...

There's no good way to do that. I guess more importantly, there's no reason to do that.

I've dealt with Asian suppliers for years. They do what USA and European businesses do - they sell their customers what the customers demand.

Right now, the customers that matter to them, that being the ones who buy 10K to 100K or more parts at a time, are demanding lowest price that barely meets the specs. Asian suppliers would probably LOVE to sell high quality stuff at a higher price. But their big customers won't buy them.

The demand for lower price by the customers in the USA and the rest of the world has caused a race to the bottom in cost, and that necessarily caused a race to the very edge of the cliff in meeting specs. You can't really blame a supplier for giving you what you demanded. Interestingly, the Chinese are busy off-shoring some of their lowest price orders to lower labor cost places themselves. How's that for a twist?

You may think "Butbutbut I'm not demanding a super cheap cap! I'd pay for better quality." Right and I would too. But you don't go ask a cap supplier for 100K caps next year; in fact, you probably don't even email your preferences to people who do make caps. That makes us invisible. We may want it but they don't know.

As I said, I've worked with Asian suppliers. I spend a fair amount of my time in activity that can all be lumped under the heading of demanding the highest quality that's available at a reasonable price instead of simply demanding the lowest price. It takes work defining where that is, but our suppliers are happy to provide it when they know what we want.

"Asian" isn't necessarily low quality. But like all suppliers in a open market, they work to "you ask for it, you got it". And there are shoddy suppliers everywhere - yes?

[Enzo]

Are you sure it is not the rectifier tube? Tubes are metal things inside the glass, and as they heat and cool they expand and contract. This can cause clicking noises. Like may car engines that creak as they cool off.

[Gtr_tech]

Heh....I always use the car engine creaking analogy when people question why I tell them to leave their amp on (off stby) for the whole night (when gigging out) and shut it off when its time to quit. I still end up attempting to explain thermo-mechanical stress for 10 mins but the engine reference gives average Joe something to bite into.

[Hardtailed]

Heh....I always use the car engine creaking analogy when people question why I tell them to leave their amp on (off stby) for the whole night (when gigging out) and shut it off when its time to quit. I still end up attempting to explain thermo-mechanical stress for 10 mins but the engine reference gives average Joe something to bite into.

Yep, I leave my amps on all night!
Plus, my main amp has the standby switch after the first filter cap (ala Marshall) and the no load condition makes it rise above 500V on standby, so I try to avoid that.

The standby scheme I used on this new one lifts the cathodes on the power tubes only, it's really just a mute. The caps stay charged, the rectifier and preamp tubes keep conducting.
Matter of fact, it does crackle a little when I go to standby (60W of dissipation through the power tubes suddenly dropping to 0...)