Reeves Custom 50PS non-polarized electrolytic replacement?

[Beyer160]

I have a Reeves Custom 50PS with a blown electrolytic cap. I couldn't find this cap on any of the Hiwatt schematics I could find (the Custom 50 is a Hiwatt DR504 derivative with power scaling and an FX loop), but I have contacted Reeves for a schematic. Neither of the resistors immediately connected to the blown cap look damaged, but I'm going to test them outside the circuit. Visually everything else looks OK and the fuses didn't blow when the cap did, which I'm taking as a good sign.

The cap in question is a 10uf, 160v non-polarized electrolytic and I can't seem to find a properly rated replacement. Given that the cap is non-polarized, can I just replace it with a properly rated film cap?

Also, anyone have an idea what this cap does? I'm kinda in the dark about what may have caused the cap to fail or what else might have been damaged since I don't know what it is. One leg goes to ground, the other to the London Power SB power scaling circuit.

Thanks for any/all guidance.


Bad cap is in the top right of the photo.

[Helmholtz]

The cap in question is a 10uf, 160v non-polarized electrolytic and I can't seem to find a properly rated replacement. Given that the cap is non-polarized, can I just replace it with a properly rated film cap?

Yes, but a film cap will be BIG.

Did you also search for bipolar electrolytic?

[R.G.]

The cap in question is a 10uf, 160v non-polarized electrolytic and I can't seem to find a properly rated replacement. Given that the cap is non-polarized, can I just replace it with a properly rated film cap?

Also, anyone have an idea what this cap does? I'm kinda in the dark about what may have caused the cap to fail or what else might have been damaged since I don't know what it is.

Yeah, what that cap does is the central question. If it carries a moderate amount of AC current, "properly rated" becomes an issue with film caps.

You might need to reverse-engineer/trace out what's connected to the cap. Big things I'd look for include: is one end grounded? Are there resistors in series with it, as well as parallel with it? Is it the cathode bypass cap for a tube, maybe even for the output tubes? I'm trying to eagle-eye the picture, and it seems like it may have vented. That would indicate that it was overheated by high-ish AC currents causing I-squared-ESR heating.

Do you have access to 160V NP caps in smaller sizes? For instance, five 2.2uF, or even ten 1uF?

[Beyer160]

Yeah, what that cap does is the central question. If it carries a moderate amount of AC current, "properly rated" becomes an issue with film caps.

You might need to reverse-engineer/trace out what's connected to the cap. Big things I'd look for include: is one end grounded? Are there resistors in series with it, as well as parallel with it? Is it the cathode bypass cap for a tube, maybe even for the output tubes? I'm trying to eagle-eye the picture, and it seems like it may have vented. That would indicate that it was overheated by high-ish AC currents causing I-squared-ESR heating.

Do you have access to 160V NP caps in smaller sizes? For instance, five 2.2uF, or even ten 1uF?

The cap blew it's stack with an impressive pop and a cloud of smoke.

The cap has a 100k resistor across it. One end is grounded, the other end goes to a terminal that splits to a 330k resistor and then to the power supply dropping resistor, and also the London Power SB power scaling circuit board. At the time it failed I had the PS all the way up, or out of circuit.

[Beyer160]

Yes, but a film cap will be BIG.

Did you also search for bipolar electrolytic?

I searched for bipolar and nonpolar electrolytics at Mouser, Digikey and Newark but the highest power rating was 100v, and this one's rated 160v. I found a bunch of film caps that fit the bill, though.

[Helmholtz]

If it carries a moderate amount of AC current, "properly rated" becomes an issue with film caps.

Don't understand what you mean here.
I'd expect a 10µ/160V film cap to stand a higher AC current than a corresponding ecap.
AC current capability is found from the dV/dt rating.
Also there shouldn't be AC amperes flowing in a tube amp (except for the heaters).

[R.G.]

Don't understand what you mean here.
I'd expect a 10µ/160V film cap to stand a higher AC current than a corresponding ecap.

I was trying to be extra cautious. I have burned films types before. The real issue is two-fold: what is the amount of heat produced from i-squared-ESR, and how fast the heat can get out of the cap. Electro caps have higher ESR pretty often, but their construction makes it somewhat easier for the heat to get out. Not knowing what was going on in the circuit is the real issue. I was fishing for the OP to tell me that there was a resistor in series with the cap that would let me reason that the current was limited outside the cap; this would let me feel good about the cap simply having a random hot spot failure.

AC current capability is found from the dV/dt rating.
Also there shouldn't be AC amperes flowing in a tube amp (except for the heaters).

Yes, I = C* dv/dt if the capacitance is what limits current and DC isn't involved; but the heating is from I-squared times the ESR, and this was a heating failure at some point. A parasitic RF oscillation can over-dissipate an otherwise fine electro cap from inaudible AC.

It also matters whether the thing has DC across it (as I was subtly asking for with "is one end ground?") or is pure AC. With DC on it, and not much current limit from an external resistor, a runaway leakage hot spot would boil the electrolyte locally, and produce an eventual failure without it being ESR heating. In that case, films would be just fine, no problems.

[R.G.]

I searched for bipolar and nonpolar electrolytics at Mouser, Digikey and Newark but the highest power rating was 100v, and this one's rated 160v. I found a bunch of film caps that fit the bill, though.

Go for the film then.

[Helmholtz]

The 160V must be a DCV rating.
DCV won't change with the cap removed. So can be measured in the amp.

[g1]

Have you located anywhere a filter cap for the (-) bias voltage, or is that done on the London Power SB board? (and what does the SB stand for?)

[x-pro]

There's another topic.
https://music-electronics-forum.com/...buzz-in-output

[x-pro]

If you're talking about this point, with a 10k resistor and 470 ohms, a capacitor in parallel to the 10k is a feedback improvement at low frequencies. And it's quite possible to put a film capacitor here. To prevent it from exploding, put it at 250V.
https://hiwatt.org/Layouts/PowerAmpLayout50.pdf
https://hiwatt.org/pix.php?p=DR504_75_pwrL

[Helmholtz]

Looks more like the 160µ/63V bias cap to me.

[x-pro]

Looks more like the 160µ/63V bias cap to me.

That's what I thought at first, too.

[Helmholtz]

That's what I thought at first, too.

Why no longer?