1964 Fender Princeton: B+ voltage too high, need fix

[nigelwackenfrets]

Hi amp gurus: I have two princetons that were produced in May 1964, supposedly to the 6G2 circuit. One has the original PT and the other has just had a Weber replacement PT installed. Here is a pic of the first resistor down from the B+ voltage (first amp had a 10K/2watt resistor from Fender).

here is the second amp, configured like the original Fender:
http://www.dilbertcelart.com/dale/princetonresistor.jpg

Circuit says this should be a 1K resistor. But here is the rub: the B+ voltage should be +315V. However the original transformer (with correct stamp on housing) reads B+ = 400V. The Weber PT reads a bit higher, 415V.

I took the first amp to a tech who installed a 1K/10W resistor (per the schematic) and said it was OK. When I got it home it blew the 6V6GT tubes because the plate voltages exceeded their limits.

The 10K/2 watt resistor doesn't blow tubes, but the amp has no warmth and something is obviously amiss.

I ask: The old Brown Princetons had B+ =315V and I think that is the objective. So what is proper way to get the B+ voltage down from 400-415V so the 1K resistor can be used?

Please no dramas about my amp tech, just need to know how to fix the problem.

[nigelwackenfrets]

Can screen resistors be added?

Can screen resistors be added to reduce voltage with 1K resistor installed?

What values and where should these go on schematic?

[theunrulychef]

Absolutely not an amp guru here, but I'll give it a shot. The original was made to run on lower line voltage than we have nowdays, so it's putting out a higher voltage on the secondaries. My 72 Princeton Reverb does the same thing. Usually, the Weber stuff has multiple taps as well, so you can try to run it off the 125v input into the lower output to get in the range you want (depending on your model PT). It might also just be the wrong PT for the application.

The tubes are biased correct, right? The power supply caps are fresh? I see in the pic, the cap in the bias supply is an oldie.

As for the voltage fix, you've got a couple options that I know of. Raising the value of the resistors in the power supply line is usually seen as less than ideal because it can cause other issues down the line - sag etc. Are you running solid state rectifiers? They would likely be too efficient & not drop enough voltage, so I'd stick with tubes. You can also use zener diodes on the PT center tap (sometimes a string of them) to drop the voltage it's putting out.

If you can't beat 'em --- JJ 6V6 tubes can take some serious abuse. You might try a set in there & see how they do after checking the bias. If my memory serves me correctly, I think mine run something crazy like almost 500v on the plates & the amp sounds absolutely killer, all stinking day long. I've still never bothered to put screen resistors in there, though I know it's a good idea & recommended.

Hopefully a real guru will step in and debunk or confirm what I've said.


EDIT - looks like you've already got the screen resistors. 1K5 on the schematic & they're in your pic too. They must've pulled those off to save a couple cents for the Blackface versions.

[MWJB]

"I took the first amp to a tech who installed a 1K/10W resistor (per the schematic) and said it was OK. When I got it home it blew the 6V6GT tubes because the plate voltages exceeded their limits." No, they blew for another reason.

I would expect a BF 6G2 with a GZ34 to run the voltages that you have. Even an earlier Brown 6G2 with a 5Y3 will run 370vdc -ish. You don't need to change B+, or increase the value of the 1K power supply dropping resistor. Your screens may run a little over the plate, but they do this in many tolex Princetons

Those 1K5 resistors are grid stoppers feeding pin 5, not screen grid resistors.

As Unrulychef points out, the amps could do with biasing & that 45yr old bias cap needs replacing (as do all the electrolytic caps).

[zedhed]

"... When I got it home it blew the 6V6GT tubes because the plate voltages exceeded their limits." No, they blew for another reason.

...As Unrulychef points out, the amps could do with biasing & that 45yr old bias cap needs replacing (as do all the electrolytic caps).

New guy here, no expert but a few years futzing with a '65 DR with high plate voltages among other (real) problems.

I bought my Deluxe Reverb in 1977, also a dual 6V6 amp. Per schematic the plate voltages should run at 415 volts. In 1977 they were at 430V and recently I measured just over 450V (wall voltages do keep going up). I decided 450 MIGHT be starting to approach the edge (though these voltages have never caused a problem in 30+ years of hard use) so I put a 'softer' rectifier tube in there, in my case a 5R4, which dropped the voltage back to 430V and also has the advantage (my preference) of responding more slowly, creating sag, which causes a bit of natural compression and starts growling a bit sooner than it did with that unforgiving GZ34 in there. The only rectifier that can hold more voltage than a GZ34 (in that amp) is solid state. The Tube Amp Book's recommendations on switching to a SS rectifier should be ignored by most vintage amp owners IMHO.

So there is a way to drop your voltage if you really want to, but I'm with MWJB and Unrulychef; the tubes didn't blow because the amp design is causing high plate voltages. I have replaced my filter caps and bias cap 3 or 4 times over the last 30 years just to be safe, and power tube bias is a critical setting. An under biased amp will turn those tubes ON; a couple of volts too positive and they'll glow, sound bad, an burn up quickly.

Nothing new here. Just wanted to say Hi! I'll be looking for some advice soon too. I'm a learner so if anything above is inaccurate, anyone please correct.

Zed