Question on Champ AA7 64

[iccaros]

My friend has a AA7 64 Champ, no vibrato

I modded it for him by placing the NFB on a 1meg pot so he could adjust how much NFB he got. This replaces out the 2.7k going from the speaker out to pin 8 of the 12ax7

I noticed lots of gain at full resistance (at 1 meg it should look like there no NFB right?) at no resistance it haves the output which is also nice..

but watching the 6v6 I noticed that it glowed more, not quit to the point of red plating, but enough to make me think about it.

Should I worry or should we run it for a few weeks and see what happens?

for history this amp had a bad bypass cap on the 6v6 cathode causing it to red plate before. So we replaced all of the electrolytic caps (to be sure no others just failed from old age) and replaced the sockets as the 6v6 would just fall out when he bought the amp (good buy for $50), so I am not sure if this tube is changing glow because of previous damage or that 12ax7 is driving it too hard..

thanks for any input..

[MWJB]

Tolex Champs run their 6V6 way over data sheet max dissipation, usually around the 20W mark. If the tube isn't redplating and the amp isn't fading during reasonable use, then you might not have anything to worry about. If you are concerned about plate dissipation, try a 800ohm 5W cathode resistor...go with which value sounds best.

"I noticed lots of gain at full resistance (at 1 meg it should look like there no NFB right?) at no resistance it haves the output which is also nice.. " At 1Meg NFB ratio is over 2000:1, so yes there will be no appreciable NFB, why does the pot got to "no resistance"? Surely the pot is in series with the original 2.7K value? In this sceanario I would favour a SPST switch as at basically an setting other than fully off you are drastically cutting NFB. E.g. even at 9'oclock you may have 30-40K on that 1M pot and a NFB ratio of 600:1 or 900:1!

[iccaros]

The 2.7K is not in series anymore, as it acts like a power break by giving a 1:1 ratio on NFB and really cleans up the amp, where he get little to no distortion on 8 for volume. While before at 3 he could not get a clean sound with a les paul. While this cost in output power its nice for practice with his young baby in the house. But I guess I could just put in LPAD to quite it, I was not sure when I started if I would like that feedback ratio, but it is really nice sounding.. Almost like a... don't hit me.... a unity gain opamp ... I should put some value in there, and putting the 2.7k back in line would be a simple single wire movement, as I left it on the board, in case bypassing caused issues.

I was looking at a switch, but could not decide where he liked it most, so I told him to play it for a week and we would figure out the setting he liked most and put in a switch, I'll look at the cathode on resistor, that's a 5 min check. I am concerned about over voltage as with the new rectifier tube he is getting 45 volts between cathode and ground. The original cap is only a 25v cap, which is why it died when he bough the amp.

[MWJB]

"I am concerned about over voltage as with the new rectifier tube he is getting 45 volts between cathode and ground. The original cap is only a 25v cap, which is why it died when he bough the amp." 45v from cathode to ground sounds very wrong with a 470ohm cathode resistor. What is the "new" rectifier tube, only use NOS 5Y3 if you want classic champ voltages. Most tolex champs run the cathode bypass cap at 25v or more, so use a 50-100v cap here.

[iccaros]

replaced the cap with a 120v one I had on hand, I told him to buy NOS 5y3, the one that he got with the amp is a Sovtek, which drops less voltage. I need to pull the cathode resistor and check it, It may be drifting.. Thanks for the suggestion

edit I have two on bid at ebay, GE 5y3, will see what the voltages are If I win...

[ThermionicScott]

What are more of your voltages now? IMO, Fender didn't accomplish much on the BF/SF Champs by jacking up the screen voltage apart from raising the bias point and sending higher voltages to the preamp. (If you look at the 6V6 curves at 285V screen and 350V plate, biased at 14W with a 7K loadline, there's nothing you can do to get more swing if the bias point is kept at 14W!)

So I would consider not just the NOS 5Y3, but changing the first dropping resistor from 1K to 4.7K/1W or something in that ballpark. By reducing the screen voltage, you wouldn't have to go quite as extreme in modifying the cathode resistor value. The second dropping resistor could easily be changed if you want to bring the preamp voltages back up.

- Scott

[iccaros]

Very good point Thermionic, I was going to ask if I should change out the dropping resistors to this tube more in line with the datasheet. The Amp cost him $50 and since I am not charging him for any of this I have free rain to do what I want, with in reason.. I'll do some readings and report what I find.

[MWJB]

"(If you look at the 6V6 curves at 285V screen and 350V plate, biased at 14W with a 7K loadline, there's nothing you can do to get more swing if the bias point is kept at 14W!)" But these kinds of voltages & dissipations have nothing to do with a properly functioning SF Champ. You normally don't NEED to change the cathode resistor value at all, Fender jacked up the screen voltages as much by running higher B+ voltages (usually 390vdc to 420vdc-ish). It would be a good idea to add a discrete screen grid resistor, from the B+ rail to pin 4, say 1K to 2K @ 3W if you want to drop the screen voltage to below the plate voltage?

[ThermionicScott]

"(If you look at the 6V6 curves at 285V screen and 350V plate, biased at 14W with a 7K loadline, there's nothing you can do to get more swing if the bias point is kept at 14W!)" But these kinds of voltages & dissipations have nothing to do with a properly functioning SF Champ. You normally don't NEED to change the cathode resistor value at all, Fender jacked up the screen voltages as much by running higher B+ voltages (usually 390vdc to 420vdc-ish). It would be a good idea to add a discrete screen grid resistor, from the B+ rail to pin 4, say 1K to 2K @ 3W if you want to drop the screen voltage to below the plate voltage?

Yeah, no Champ in history ran its screens at 285V, I bet -- it's the highest screen-voltage curve I can find for the 6V6 but it illustrates why the screen supply voltage doesn't need to be all that high to deliver the output we desire (and so could be reduced for the sake of tube life.) Slapping in big screen resistors works, but it's always struck me as a bandaid fix for an irresponsible design.

Of course, the other side of the "design" argument is that the tubes of the day were inexpensive, put up with the abuse, and the amps didn't fail outright very often, so they got away with it.

When I get an amp to fix or maintain, I like doing sensible things to make sure the amp will keep rocking for as long as possible. Reducing voltage/dissipation to sane levels is one of those.

- Scott

[J M Fahey]

As km6z posted, now he's living in Russia and keeps meeting these wonderful musicians who are still using their 10/20/30 years old amps with their original tubes, working perfectly ... but they were all built following the datasheet suggestions.
No Leo Fenders there slamming 400+ Volts on 300 something V recommended tubes.