Fender Concert 6g12-A?

[J Luth]

Just a quick question.
Do these amps really need 600V filter caps?

Could you use 2 - 40uF x 350V or 2 - 70uF x 350V in series for 20uF or 35uF at 700V in the first section, then 22uF x 500V for the rest?

Or are the voltages that high that 500 isn't good enough?
600V caps ridiculously expensive from what I have seen.

[g1]

schematic attached. They show two 20's in parallel for the first stage, that equals 40. So you could use two 80's (350V) in series (with balancing resistors).

[J Luth]

Thanks g1
DUH, should have looked at what it had there. Stupid mistake.
Figure 22uf by 500 should be OK for the rest.

[Chuck H]

Thanks g1
DUH, should have looked at what it had there. Stupid mistake.
Figure 22uf by 500 should be OK for the rest.

I was schooled on this some time back. I was instructed that peak, unloaded voltages are present on ALL the filter caps before the tubes start drawing current. Makes sense. This risk only aaplies if the standby switch is in the play position when the amp is turned on. But of course that can happen. That said, there are a lot of amps with similar (or higher) loaded voltages using 500V caps throughout. And Fender used 500V caps for all nodes following the main filter in most later models that had the same tube compliment and higher voltages. So I think 500V caps for filters other than the main should be fine.

[J Luth]

Thanks g1 and Chuck H
Next time I'll look at the schematic before I type a question. Mental Pause

[g1]

I just added the schematic to aid the discussion as you had not posted it. I think your questions were valid concerns.

[Chuck H]

I just added the schematic to aid the discussion as you had not posted it. I think your questions were valid concerns.

Yes they were. And they were for me once upon a time when I asked about a similar issue. On that particular build I ended up using 700V totem poles throughout! Which was a calculated choice considering the cost of E caps over 500V (as is the case here). My particular build had 470Vp so the unloaded voltage would be well over a 500V cap spec. And I had to deliver this to a "client" which means there's no counting on the use of the standby switch. So I played it safe. The cost was the same for 600V single caps or 750V totems and I already trusted the brand I used to build the totems. So I gained an extra 150V spec with a trusted product for the same price as using whatever I could get in a 600V cap I'd never used. Of course the build looks a little over done but so what.?. Anyway...

Yes, this is a real issue and I think it's a demonstration of awareness on J Luth's part that it came up here. There are no stupid questions.

[J Luth]

Chuck H, what are the 700V totem poles?

The amp is here now. It's a cut down aka just a head brown concert. It appears to be closer to 6G12 not a 6G12-A.
Haven't checked everything but it has not been touched much other than a terrible 3 prong cord install with tapped wire nuts, maybe a couple of resistors, one for sure and the cabinet cut.
Most all original parts are still there.
One thing is the schematic for both the 6G12 and 6G12A call for the second dual cap in the preamp to be 25uF 25V. In this amp it's a 50uF 25V dual brown cardboard covered Astron, noted on attachment, it looks factory. All the filer caps are original.

Did some quick voltage testing at my current 122V incoming line volts and at 115 through the variac.

At 122V turning the power and standby switch at the same time, the res caps reach 526 then settles at 499 the 3rd filter cap also does the same. All others are 450 and below.

At 115V and same procedure, res and 3rd caps reach 505 then settle at 451.

Might do some more testing in a day or so.

Yikes, the 20uiF 600V are pricey.

[Chuck H]

Chuck H, what are the 700V totem poles?.

Sorry... Often when two electrolytic caps are put in series (typically with parallel resistors for balancing voltage across the caps) the arrangement is called a "totem pole". So two 350V caps in series (with the balancing resistors) makes a 700V totem pole, as it were.

[The Dude]

This is a typical "totem pole" arrangement taken from a Fender Twin schematic.

[J Luth]

Thanks Chuck H and The Dude. Never heard that one before.

[J Luth]

Got some of the parts in, waiting on the rest for the amp.
Didn't do any more voltage testing.

Anyone have any idea why the second dual cathode bypass cap would be a 50uF 25V when both of the schematics call for 25uF 25V? It looks factory.
And or what effect the 50uF would have as apposed to a 25uf

[Chuck H]

The 50uf as opposed to 25uf should have no audible effect. It if it's factory it wasn't put there for any tonal or operating condition reason. It would have have simply been what they had on hand and sufficient to the task. 25uf easily qualifies full cathode bypass for the audible response of a guitar amplifier and the speakers used therein. I'm fairly certain that if you swapped it for a 25uf you wouldn't be able to measure or hear any difference. So if it IS factory, and functional, it's best left in place as one of those oddities that happened back in the era. Representitive of the genre and unique to that particular amplifier as it came from the factory. Never "fix" what isn't broken.

[g1]

I seem to recall there was talk that larger bypass caps may have been used to help with heater hum?

[J Luth]

Thanks Chuck H and g1.
Going to replace it with 2- 50uF. It has probably been in there since it was built. Time to go.