Octal Tweed Princeton

[yaanno]

Hi folks,

I'm experimenting with a modified 5FA-2 circuit using 6SL7 preamp instead of the usual 12AX7. Due to components I have at hand I had to change a few values to keep the voltages and current at a "safe" level.

Power Transformer: Tube-Town TT-5E-PW (320-0-320, 85mA)
Output transformer: Hammond 125CSE (4 ohm tapped at the moment so around 5k at the primary)

Voltages seem to be in the ballpark and everything sounds good. Extra filtering added and dropping resistor upped to 470R to tame the rectified voltage closer to the original design (the stout PT and the "5Y3" elevated the voltage quite a bit. The TAD 5Y3 is a russian 5Ц4С / 5Z4 so the voltage drop there is lower than usual). Now everything seems to be fine, however I have an issue with the amp. At certain point, close to the maxed out volume pot, a loud cracking sound appears and the output tube? shuts off and slowly comes back.

This is probably a rookie question but if you have insight on this I'd greatly appreciate any advice. I was thinking maybe I should add an extra 1M ground reference (leak) resistor to the second grid so when the volume pot is maxed out I'd still have grid reference for the V1B grid. Also a grid stopper there?

Attached the current state of the schematics.

Thanks!

[maxkracht]

I would suspect a damaged pot and/or a bad solder joint. You shouldn't lose the ground reference at max volume if everything is wired as it should. Easy enough to measure, with the amp off, what resistance you get at max volume. Could be an oscillation, but my guess would be a bad connection. You could add a grid stopper, but probably not necessary. Why are you using 820r cathode resistor on the 6v6?

[yaanno]

I would suspect a damaged pot and/or a bad solder joint. You shouldn't lose the ground reference at max volume if everything is wired as it should. Easy enough to measure, with the amp off, what resistance you get at max volume. Could be an oscillation, but my guess would be a bad connection. You could add a grid stopper, but probably not necessary.

Good advice thanks, will look into that!

Why are you using 820r cathode resistor on the 6v6?

Due to the higher output transformer and the low drop 5Y3GT (5Z4 in fact) the voltages are higher than usual. Started with 470R but then the current went over the roof (14W PD at idle). Now with the 820R (and the extra 470R dropping resistor at the first node) I have 361V / 29mA (approx 10W plate dissipation), cathode is at 24V. I'll try to lower the B+ first and try a lower cathode resistor to move the dissipation towards the max.

Thanks mate!

[yaanno]

Not surprisingly it was a bad solder joint on the V1B grid. Also the 500pF mica cap was faulty so I replaced it with a ceramic one (probably will use these in the future instead of the micas). No extra grid stopper or 1M reference resistors added. Might tinker with the stopper tho.

Much better! Everything maxed out gives a nice creamy drive but also my P90 pickups add decent amount of noise Still want to hunt down a few noise issues, otherwise it is nice. That russian 6P6P/6V6 is really cool, the 6N9C/6SL7GT too. Tried the amp with brand new Tung Sols but hey these vintage ones make better noise

The main limitation now is the housing. I used a Marshall Origin 5c combo (that sounded like nothing really), not a great construction in any way, cheapo MDF and hardware, underpowered PT. Got a new WGS 8 inch, sounds a ton better.

Thanks again!

[maxkracht]

6v6 running at 14w isn't unusual for a champ, cathode biased amps are usually close to 100% at idle, somewhere around 90-95% is a good place to shoot for if you are being conservative. I often see stock champs that have been used for years in the 115+% range without issues. This isn't ideal, but some tubes can last a long time in that condition. No worries if you like the sound of it running cooler, but little risk in bumping it up a fair amount.

[yaanno]

Gotcha. I'll keep experimenting with the circuit. Strange thing is I've seen different values on the PT secondary side ranging from 250-0-250 to 360-0-360. Now I'm guessing its all about biasing, still it seems odd. Anyhow 320-0-320 looks like a nice in between level.

Thanks again.

[maxkracht]

Lots of different champs throughout the years and I think the B+ was creeping up higher in the CBS era. Same power transformer was used for princetons, so maybe they were going for a compromise between the two, maybe they were trying to sell tubes so they didn't bother to rebias to compensate for the higher voltage?

[yaanno]

Yeah, it is an interesting question. I'm not sure how this affected the amps and "tone". So far the tubes are not stressed but I'll keep an eye on the 6V6 just to be on the safe side, especially now when these bottles are expensive, even the old russian ones (hot little devils). Funny thing is I never really liked the classic SE amps, or to put it better, just ignored them but now I'm intrigued. There is something about them. Also I'd never thought people would ask for combo amps. I thought everyone is using heads (foolish I know) but even gigging musicians use them extensively for that specific sound like a Champ - Princeton combination, not just for recording. I decided to dig a bit deeper into SE Fenders and the largely ignored Gibson combos. Lesson learned! :)

[dragonbat13]

Any time I see "octal tweed Princeton" I feel I need to feel like I'm wanted.

I am the second owner of a wide panel with a block of plate, and the info on the Internet says its the rarest of them all. I'm not positive on this, I have researched it for years ..

All I did was rebuild the power supply.

The preamp is all original.

I don't know if you care, but I am rather confident that this amp is a mutt of the octal and the nine pin.

I've been out of the amp building game, but I'm coming back. I'll post pics if wanted.