Runaway current draw from single 6L6 tube

[HaroldBrooks]

I guess a 1948 PT would be intended for 110V; feeding it 125V will probably result in all secondaries running too high, resulting in limiting values being exceeded. eg what are the heater voltages?
Bear in mind that the amp was designed around early 6L6, which had much lower voltage and dissipation limits on plate and screen grids.

6.8 vac, again a bit high, and I think you are correct about the wall voltage being a part of the problem. I do have an inexpensive surge protector that automatically limits wall voltage to 114vac from the normal 125vdc I see here, and I'm not sure how this get's done in the unit, but it's consistent and the unit can handle 3k watts. I don't use it because it weighs a lot ! and I don't want to be tied to lugging that around when I use this amp. I was just thinking of employing some Zener diodes I have, as I used those in the past with another amp, and they worked well.

QUESTION : the B+ on this amp peaked last night to 462vdc, am I running any real risk in using 450vdc rated brand new filter caps exposed to 462vdc ? I would think it's close, but I would like to here from the voice of experience as to what the potential risk is !

[HaroldBrooks]

Drawing and posting at least the section of the circuit where these devices were, showing them in circuit (identified with a question mark?) will help us answer those questions. Really, take a half-hour and sketch it out on a piece of paper. Don't worry about how bad it looks, it will jump start your understanding of this amp tremendously.

Till this point I haven't had the confidence to draw any circuit, but I think your advice is good, and I will attempt it as part of my learning and initiation. Probably have something later today. Verbally now I can say that each one of those Blue/Violet assumed resistors were connected to the screens #4 pin of the 6L6 tubes, and someone here mentioned as a voltage divider. Here's the weird thing for me though, one was connected to pin #2 of the rectifier, and the other just to ground. Again I may not see the obvious implications of this.

[Chuck H]

I think it's probably that in your first observation there was a tube that wasn't conducting. 200°F isn't indicative of a tube that conducting, but 538°F might be about right for a hot bias on the surface of a metal can tube. Possibly one tube was not conducting due to the bad screen grid resistor/s. After replacing those there was some mismatch in the current between the power tubes. The RCA's ARE old tubes, so there may be a problem with one or both. And I wouldn't expect the JJ to match up with them at all. I think you need a pair of good tubes in there for more evaluation and before any further circuit adjustment.

[Helmholtz]

Here's the weird thing for me though, one was connected to pin #2 of the rectifier, and the other just to ground. Again I may not see the obvious implications of this.

As pin 2 of the rectifier is B+, the 2 resistors constitute a typical voltage divider if screens are jumpered (looks as if they are). Screen voltage being R1/(R1+R2) times B+, with R1 to ground, R2 to B+.

[mozz]

Could you post the tube lineup again, i may have the schematic on my old Rider PA. manual. Does it have a interstage transformer?

[HaroldBrooks]

Could you post the tube lineup again, i may have the schematic on my old Rider PA. manual. Does it have a interstage transformer?

Only a power and an output transformer, no chokes. The tubes are 6J7, 6KS7, 6SN7, and two 6L6s.

Thanks for any help !

[g1]

[B]QUESTION : the B+ on this amp peaked last night to 462vdc, am I running any real risk in using 450vdc rated brand new filter caps exposed to 462vdc ?

As pin 2 of the rectifier is B+, the 2 resistors constitute a typical voltage divider if screens are jumpered (looks as if they are). Screen voltage being R1/(R1+R2) times B+, with R1 to ground, R2 to B+.

Yes, that is how the screen supply was configured. In the other thread, I had mentioned that judging by the size (power) of those resistors they were probably pulling the whole B+ down somewhat and it may put the main filter caps & power tubes at risk to remove them.
That seems to be the case, or at least it is a contributor along with the higher modern line voltage.

Harold, was the 462V just during power up, or while running?

[HaroldBrooks]

Yes, that is how the screen supply was configured. In the other thread, I had mentioned that judging by the size (power) of those resistors they were probably pulling the whole B+ down somewhat and it may put the main filter caps & power tubes at risk to remove them.
That seems to be the case, or at least it is a contributor along with the higher modern line voltage.

Harold, was the 462V just during power up, or while running?

426vdc is While running, and as you know there is a brief exposure to higher voltage (peaks briefly at 540vdc) on startup. I will order a few 600v 47uf caps. It's the first one off the B+ that will take the beating, as the others are reduced I believe by the resistance later in the chain.

Thanks for solving the mystery of the big resistors. I think at this point they will stay out as the amp sounds good with the current voltages.

Update :

I found another JJ 6L6 lurking in one of my forgotten single ended amps ! So I popped it in, and the balance is restored ! one tube is biased now at 21 watts for just the plate dissipation, and the other is 24 watts, so all is well I believe. Today I added a bunch of my usual safeguards, i.e. Varistors, Diodes for the rectifier. I also put the amp through the paces with a few different type speakers, and it sounds best playing through my twin 10" Celestion Greenbacks (So Far). I don't think this amp is producing all that many more watts than my 6V6 amps, but It handles the bass transients a whole lot better and cleaner. I don't think the power tubes are being driven into distortion all that much, and that may account for it.

So I still haven't drawn a "modified and current state" schematic, but I may get to that later tonight. I abandoned using the 6SK7 tube in the amp, as it was wired in concert with the 6J7 in a way that I don't understand, and I saw little advantage at this point (Famous last words !), so I just hooked up the 6J7 to the 6SN7 through using a .01uf coupling cap off the plate to the grid (pin #1) of the 6SN7. That works rather well ! Also this amp runs a higher plate voltage for the 6J7 then I've seen in the past, right around 250vdc, but using a 1meg screen resistor and as a result it's only running a modest screen voltage of 77vdc. In any event, the gain is very good, but not too much to overload the 6SN7 too much it seems. I also reduced the size of the coupling caps to the 6L6s, from .1uf to .047uf 630v metalized Polyester caps. This seems to have sharpened up the sound a bit. Still there is a lot of bass in the very low region (below 100hz) and I filter that out up front with my EQ, but I probably should nock it out in the amp itself with some changes to the bass pot, or perhaps an even smaller coupling cap from the 6J7 to the PI tube.

I boosted the heck out of the amp and it sounds superb even with punishing levels of gain !

Great for 80's rock (think AC/DC, Scorpions, etc..) Fat and big sound, and clear as a bell when playing 7th and even on a 7th#9 chords, the ones that are normally chaotic. This amp for me is a winner so far !

Thanks for everyone's help so far. I will try to make a "schematic" the best I know how, and get further input from you guys. There is always room for improvement !