BF Princeton High B+ & PI

[Justin Thomas]

Hello again everyone,

A friend gave me his 66 Princeton to fix after sitting in the closet for 20 years. He said the fuse blew years ago and he just put it in the closet. Put a fuse in it and it worked, but one 6v6 was blown, and volume is low after replacement tubes. Doesn't hum! Voltage check gives 470V B+ with a 5AR4 recto with tubes in, and all downstream okay in proportion. Could I swap out the recto tube (5y3, 5v4, 5r4, any w. 2A current draw) to lower that a bit? Also, there are about 30VDC on the grid of the PI. I'm still learning about that circuit; is this normal for this PI, or do I need to do the full cap test (disconnect one end and check) on the .022 between preamp and PI?

Cleaned pots, and will do a full cap job and three-prong conversion, new power tubes and bias. The sound is in there, just not really coming out. Any other suggestions for safety? Thanks, Justin

[ThermionicScott]

470V sounds right for an amp that expects 117V at the wall. If anything, though, that voltage at the PI grid seems low -- if you look at the schematic, the bottom of the grid-leak resistor measures +49V. To determine leakage, try pulling the PI tube and then measuring both sides of the coupling cap.

http://www.ampwares.com/schematics/p...z34_aa1164.pdf

Wouldn't hurt to test the preamp tubes and replace any that are worn out.

- Scott

[riz]

Have you tried another speaker? The speaker is where the sound "comes out", obviously, but I mention it because often we look for problems that aren't there, while overlooking the simple stuff.

[Justin Thomas]

Thanks guys!

Scott, I'm wondering about the B+ because one, the filter caps are only rated at 450. And my wall is about 125VAC. Guess that's due for an upgrade... Could I still swap a different rectifier in? So that's a grid leak bias resistor... I'll do some reading on that bias scheme. And pull the tube to check that cap.

Riz, yeah, that's an obvious one... I'll try another speaker.

Justin

[ThermionicScott]

Scott, I'm wondering about the B+ because one, the filter caps are only rated at 450. And my wall is about 125VAC. Guess that's due for an upgrade... Could I still swap a different rectifier in? So that's a grid leak bias resistor... I'll do some reading on that bias scheme. And pull the tube to check that cap.

Oh yeah, it's definitely recap time! Don't stop at 475V, I'd get this guy and use the 40uF for the first stage: More Info for item C-EC40-20X3-525

Go ahead and try other rectifiers... some of the BF-era schematics show a 5U4GB rectifier, so I believe you'd be safe trying one. That or a 5R4 is where I'd start.

As for the phase inverter, do some reading on cathodyne/split-load PI's. The cathode is biased about 1V over the grid, like you'd expect in a regular gain stage, but the grid and cathode are then elevated by about 1/4 of the B+ using a big unbypassed resistor that is equal to the plate resistor, so that the whole stage "swings" with the input signal. They're kinda neat.

- Scott

[jmaf]

I don't think those 6V6 plates should be seeing much more than 400 VDC.

You should check if there is a rectifier limiting resistor and if not, add a 150 OHM 10 Watt resistor in series with the rectifier plates. You can experiment with up to 220 ohms.

[tubeswell]

another vote for lowering the B+ if you are hoping to keep using 6V6s in that amp.

[Justin Thomas]

Thanks everyone!

Hey all,

Thanks for all the help. I'll be needing a couple weeks away, but I'll get to it at the end of Easter and post back with the results. I may put 6L6s in it, too... I know those EH 6v6's are supposed to take higher voltages, but I've had bad experiences with them in my Champ...

Justin

[ThermionicScott]

Hey all,

Thanks for all the help. I'll be needing a couple weeks away, but I'll get to it at the end of Easter and post back with the results. I may put 6L6s in it, too... I know those EH 6v6's are supposed to take higher voltages, but I've had bad experiences with them in my Champ...

Justin

What exactly happened in the Champ? I remember years ago there was a lot of fuss over EH 6V6's not handling high screen voltages well, and I haven't heard much about that since. Not sure if they fixed the problem or people aren't complaining as loudly anymore.

I'd use JJ 6V6's and a 5U4 rectifier. Here's the schematic that calls for one: http://www.ampwares.com/schematics/p...erb_aa1164.pdf

I'm not entirely sure what the story is with the multiple AA1164 schematics. Both have CBS stamps on them, but they never changed the date or schematic code or transformer part numbers, so it's hard to say which one came first.

- Scott

[jmaf]

What exactly happened in the Champ? I remember years ago there was a lot of fuss over EH 6V6's not handling high screen voltages well, and I haven't heard much about that since. Not sure if they fixed the problem or people aren't complaining as loudly anymore.

I think they're not complaining as loudly anymore but the issues are still there. I finished a build for a friend a few weeks ago and tried two pairs of EH 6v6 both pairs started arc'ing after a few moments. I thought it was too much of a coincidence to have two faulty pairs. I guess that's the issue. I must have the voltages written down somewhere, but I substituted for JJ's and he was good to go....

[HTH]

I know this is an old thread, but bear in mind that if you want to use 6V6s and your amp has high plate voltages, try JJ 6V6s - they are spec'd for 500v on the plates and 450v on the screens.

[MWJB]

Yes try a lower voltage rectifier, a US NOS 5Y3 should get you down to NOS 6V6 friendly voltages, or as HTH suggests just use EH or JJ 6V6s. Fender use EH in the 65 RI, it runs the same voltages that you have with a GZ34...watch the current though, stay in the low 20's, Myles Rose once recommended 19mA per tube to me.

DO NOT fit 6L6s, you will kill the PT!

Forget a can cap & 450/475v rated caps, fit discrete electrolytic caps, 500v rating, minimum.

[Justin Thomas]

Wow, I forgot about this!

Hey, thanks everyone! I forgot I posted this. I still haven't had a chance to get to it, as I've been house-sitting almost non-stop and playing or out nearly every night. I've got all the parts for it; I found a can cap 30/20/20/20 @525, I'm hoping that'll be enough. I'm also putting JJ 6V6s in it. I may still drop the recto a bit to help lower voltages more. New power cord (DESPERATELY needed!), no more death cap, fuse holder, pots if needed, the usual stuff.

Regarding the EH 6V6s, the grid shorted to the cathode. I also had a JJ 6L6 short something to the heater... I'm starting to wonder about new tubes, but they're still relatively cheap compared to other parts that I could blow up instead... trannies, can caps, NOS tubes...

MWJB, no 6L6s because of the higher heater current, right? Didn't think about that at the time, thanks for the reminder. I'm hoping (yet again) to get this thing up sometime before October.

Justin Thomas