Old Fender Circuits .05uF 600V Ripple Dropping Cap

[David Root]

Is this considered safe these days? Between rectifier pin 8 out and standby switch ahead of first filter cap. For example, in the 5E8A circuit.

[ToneMerc]

David, I'm copying what Marshall did for my my JTM50 clone. In the context of modern AC mains wiring and fusing, I'm not too concerned.


See that Hunts .05/1000V resting on the transformer bracket.

[David Root]

OK, I'll go to 1000V too. That is a very nice looking build BTW!

[ToneMerc]

OK, I'll go to 1000V too. That is a very nice looking build BTW!

It should be, it's an original from 1967

S/N 10469

TM

[David Root]

No wonder it looked so good! I was looking for the pink wire!

[Cliff Schecht]

David, FWIW I do consider this safe and haven't had a problem with this on any of my amps that do this. I've actually never heard of this cap failing because I don't think it sees very much abuse. I think it's there so the rectifier has SOME capacitance before the standby switch and maybe to help squash transients when the standby switch is flipped.

Also ToneMerc, I would NEVER trust one of those Hunts caps. Even if it's a 1000V part. I have more than a handful of these that are literally crumbling and cracking apart. The ones I had are from a hifi unit from the late 50's I think (Arkay something or another, don't matter cuz it's gutted now) and were stored in a perfect environment (no heat, low humidity) for probably 40+ years and they were still shit. Did Marshall use these originally?

[ToneMerc]

Also ToneMerc, I would NEVER trust one of those Hunts caps. Even if it's a 1000V part. I have more than a handful of these that are literally crumbling and cracking apart. The ones I had are from a hifi unit from the late 50's I think (Arkay something or another, don't matter cuz it's gutted now) and were stored in a perfect environment (no heat, low humidity) for probably 40+ years and they were still shit. Did Marshall use these originally?

Cliff, not all Hunts/Erie capacitors were known to be fail nor all of them were made of the same composition or materials type. I bet the ones you have are the MouldSeal types from the 50's and 60's, which are known to behave exactly as you described.

Yes, see the above picture.

TM

[David Root]

Cliff, that's interesting. I assumed the cap was there to send some ripple to ground before it hit the first filter cap, so they could use a smaller cheaper first filter cap.

Maybe we're both right. Someone out there knows for sure!

[jjman]

Maybe it reduces the "pop" from throwing the standby switch.

[Cliff Schecht]

That's what I meant by squishing transients.

I think it also helps the GZ34 by giving it a small amount of standing current. When you flip the standby switch it won't have to start from nothing which might help reduce surge currents as seen by the rectifier. Just a thought, although I don't know how much work the rectifier is actually doing with just this cap there. All it has to fight against is the ESR of the 0.05uF cap to ground (the only thing discharging the cap really) which is minimal in most all film and ceramic capacitors.

[David Root]

So in that scenario it should help the 5U4G even more, because the 5U4 doesn't have the 11 second time delay built in to the GZ34.

[Cliff Schecht]

Yah I would think it would help reduce the inrush current an appreciable amount. When you flip the switch all of the energy in that 0.05uF cap is going to dump into the downstream e-caps which would take at least a bit of work from the rectifier.

[martin manning]

Is it possible that the 0.05uF is located on that side of the standby just because that was a convenient place to put it? The 16uF e-cap after the standby is over 300 times as big, energy storage-wise. I'm more inclined to think its purpose is a low-esr bypass.

[martin manning]

double