70's Silverface Fender Champ AA763 overhaul - voltage issues

[Chuck H]

You didn't mention your power tube plate voltage. It's common for these amps to run higher than schematic voltages in modern AC outlets, but yours seems quite high. The resistors going to pins 1 and 6 should be 100k ohms, not 100 ohms. If you've tried the amp and it's working fine then maybe that's just a typo? Otherwise, if the amp is not working fine 100 ohm plate resistor values would explain the high voltage and perhaps part of what's wrong.?. If the plate resistors are actually 100k, do you have a Sovtek 5y3? There was a long run of those tubes that had a much lower voltage drop than should be expected and those tubes have been a problem for a lot of people. If you have another 5y3 (not Sovtek or relatively new Sovtek) you could plug that in and see if those voltages come down.

Regarding the 566 ohm cathode resistor. A lot of users increase to value of that resistor to compensate tube bias for the higher plate voltages due to modern higher AC mains voltage. 566 was probably fine. But then, it was a badly drifting resistor. So probably better to get rid of it.

I don't think the 40/20/20 can or the 5u cathode bypass cap would be stock. Though it's hard to say. A lot of Fender amps left the factory with whatever components they had that would get the orders filled. But I don't imagine they would have had 40/20/20 cans and 5uf caps on hand. More likely someone did a "cap job" on the amp some time in history and those are just the parts they used. Honestly, don't sweat the 40uf for the 5y3. It'll work fine and the amp will have better ripple filtering.

So, what is the power tube plate voltage? Is the 5y3 a Sovtek? Are the 12ax7 plate resistors 100 ohm or 100k? And is the amp working or broken?

[Chuck H]

Which version of the Champ? (Schematic?)

Considering the schematic shows a 20/20/20 can and 205V on the 12ax7 plates I'll assume it's the aa764.

[Helmholtz]

t's common for these amps to run higher than schematic voltages in modern AC outlets

Increased heater voltage would be a quantitative indicator for this.

[Justin Thomas]

Chuck,

My 79 had a 40/20/20/20. Stock. Still has it. Amp had no issues, saw no need to fix it.

Jusrin

[Chuck H]

Chuck,

My 79 had a 40/20/20/20. Stock. Still has it. Amp had no issues, saw no need to fix it.

Jusrin

Can you report on where the 40uf section was used?

[Helmholtz]

The original cap only had 3 sections.

[Enzo]

still using TUBE RECTIFIERS in 1980

I got news for you, they still use rectifier tobes right now in 2020. They never went away. Fender made many amps with solid state rectifiers, but they also made many with tube rectifiers. The two methods perform differently.

You can also buy a plug-in solid state rectifier to plug right into a tube rectifier socket.

[Justin Thomas]

First. Main Filter. Right on the 5Y3. And that RCA 5Y3GTA was happy the whole decade I owned it. The only tube it ever blew was that EH 6V6. Which is why I don't use those anymore.

Justin

[Justin Thomas]

The original cap only had 3 sections.

True enough. So it was a 40/20/20 with the 40 section first.

And LOTS of Champs came like that.

Justin

[TelRay]

What's wrong with the amp?

Hehehehehe... great first question
my friend said: "the amp doesn't break up as it used to"
i found the 1 K Ohm to be burnt and the 10 K Ohm to measure 16 K Ohm and the 470 Ohm resistor to measure 566 Ohm. so I made those changes, we tried it. He said it sounded better when he took it home (we did the "repairs' at the office).

Which version of the Champ? (Schematic?)\

I hijacked (continued) this post because I thought it was an AA764, however reading forward the CBS schematics match the voltages I am getting now

What is B+? What is heater voltage?

B+ right between the yellow cable, 40 uF cap and 1 K Ohm resistor is +413 VDC

heater voltage was about 7.0 VAC

You didn't mention your power tube plate voltage.

Pin 3 of the 6V6 is +400VDC

The resistors going to pins 1 and 6 should be 100k ohms, not 100 ohms.

Stupid corrector wouldn't let me write KOHM

Regarding the 566 ohm cathode resistor.

it seemed original (as everything else in the amp) in any case the color code was 470 Ohm, i guess it just degraded with time

I don't think the 40/20/20 can or the 5u cathode bypass cap would be stock. Though it's hard to say. A lot of Fender amps left the factory with whatever components they had that would get the orders filled. But I don't imagine they would have had 40/20/20 cans and 5uf caps on hand. More likely someone did a "cap job" on the amp some time in history and those are just the parts they used. Honestly, don't sweat the 40uf for the 5y3. It'll work fine and the amp will have better ripple filtering.

need to check when i have the amp in front of me again. it seemed "unmolested" for what i can tell

The original cap only had 3 sections.

yes, three sections. in the schematic all 20 uF... in the amp 40/20/20

I got news for you, they still use rectifier tobes right now in 2020. They never went away. Fender made many amps with solid state rectifiers, but they also made many with tube rectifiers. The two methods perform differently.
You can also buy a plug-in solid state rectifier to plug right into a tube rectifier socket.

I did not know

[Helmholtz]

heater voltage was about 7.0 VAC

That indicates that most amp voltages will be about 10% higher than originally specified.

(Some of the quotations above are wrongly assigned to me and actually originate from Chuck's posts.)

I prefer to relate to an agreed schematic, so we all know what Rxy and Cxz means.

[Chuck H]

The only tube it ever blew was that EH 6V6. Which is why I don't use those anymore.

Justin

That's interesting. The first Champ I ever worked on was a VC and I wasn't aware of the bogus Sovtek 5y3's at the time. Sho nuff it had one. 423Vp. Blew up the original RCA 6v6 and another 'used but tests good' one I had in my stash. The new EH 6v6 I plugged in survived fine. It was still in the amp the next time I saw it some six years later.

[Chuck H]

(Some of the quotations above are wrongly assigned to me and actually originate from Chuck's posts.)

Corrected... You wouldn't want to be associated with any of MY info

[Helmholtz]

Corrected... You wouldn't want to be associated with any of MY info

Nothing wrong with your valuable info, but I don't want to adorn myself with your feathers .