Princeton Reverb red plating

Hello all

I have a blackface/early silver face princeton rvb with GZ34. All the voltages look very close to published specs on the schematic. The power tubes have 407VDC on the plates and 46-49ma of current respectively. They are both showing pink & a touch of red down the center of the plates.

The power supply looks to be normal in terms of voltage as well, there is a little bump in the PI where volts are maybe 215 on the PI tube plate as opposed to the published 200 but that doesn't jump out as being way off.

What I am wondering is what the bias system here is. Both power tubes cathodes are tied directly to ground with no resistor/cap network as per cathode bias. Grid 1 however is tied to the intensity pot wiper through a network of 220K resistors. Where they meet, there is -34volts as specified on the schematic.

Is this a grid bias system? How to adjust to make the power tubes stop red plating? Or is this the proper approach?

Fixed bias.

You have a fixed bias circuit.
The tremolo circuit is varying the bias voltage to the power tubes to manifest its tremolo effect. I know very little about how an oscillator circuit works exactly but something may be wacky with it. Your power tube voltages seem correct though.
I would suggest taking out the tremolo tube but that same tube also contains your phase inverter. Try turning off the tremolo, and turning the intensity all the way down.

Try changing out the tremolo/PI tube?

Your coupling caps may be leaking +DC voltage to the power tube grids. It may be unlikely since yer bias voltage is OK, but worth checking. It doesn't take a lot of leakage to throw off the bias.

Lastly, if the 6V6s are original or old, or just can't handle the voltage, maybe they are just slap wore out. >400v is kinda high for them, even if it was designed that way.

Do you have that -34 volts ?

Bias voltage!! if it is high (closer to 0) then look for leaking caps. Could be the coupling caps between the PI and the 6V6's, the coupling cap between the trem oscillator and the intensity pot, or it could be the electrolytic cap in the bias circuit itself. When was the last cap job?

OK - duh, fixed bias, of course.

Yes, the -34VDC is present. So what should I do first?

If you have negitive bias voltage make sure it's getting all the way to the pin of each tube also look for a dirty or loose tube socket.
If it's getting to the tube you need to get more negative voltage like -38 -40. (I don't know the right number but it will be a higher negative number)
Also I belive that amp calls for a 5u4 rectifer and you have a gz35 that may be it right there.
One other thing those didn't come with a bias pot so you have to change resisters or put a bias pot in it to set bias.

These are good tubes and the amps is serviced and in good working order right ?

+1 with celeste. Does this amp have the original electrolytics in it??? Ime, chasing problems in an old amp that has old electrolytics in it is like a dog chasing its tail....no pint to it, right?
IF the amp has good e.-caps in it then one starts wondering about the other thigns. AS Bobby notes, that amp calls for a 5U4 rectifier. Your -34VDC of bais voltage is 'good' for the schematic, but that current draw is too high. That high draw will bring the plate voltage down. Did you recetnly install new power tubes??

the power supply can looks new; there are 5U4 and GZ34 models of this amp; this one is a 5AR4/GZ34.

I actually got access to an identical model yesterday and had the chance put both chassis on the bench. The 2nd amp was original and not redplating. It has 450 on the plates and the current is 30ma.

It took a little adjustment to get the first amp to these specs. I raised the 100K resistor between the rectifier and the diode to 150K and raised the value of the bias cap from 25uf to 50uf and the bias resistor from 22K to 38K.

I dont really care to see 450VDC at 30ma on a 6V6 - but on the other hand, 6V6 that are cathode bias are routinely pushed beyond the recommended limits.

I put a trim pot in there for adjustable bias and will tell the owner that 5881s would probably last a little longer in the amp, to use a bucking transformer, not to use old/expensive 6V6.

centervolume, you say you have a blackface/early SF Princeton REverb there. Looking at the schematics, thatmap should have a 5U4 rectifier. Teh original circuit for the PRinceton Reverb carried a GZ34. That is the AA964 model, iirc. The AA1164 schematic from two months later shows a 5U4 rectifier. IF that amp is as 'new' as you say but carries an AA964 tube chart, one might think that the tube chart was a carry over.....but the amp probably wants to see the 5U4 rectifier. I am going to doubt that a late BF/earlySF Princeton REverb should have a GZ34 in there. The PT will handle the lower current draw of the GZ34/5AR4 on the heater filament, but the voltages will be high. I would suggest running a 5U4 in there.??? YMMV

I put a 5R4 nos rect. tube in both of my amps. A 1980 silver face Princeton Reverb and a 1968 Bandmaster Reverb... The Band master Reverb had a tube chart that called for a GZ34/5AR4, but I learned on this forum that a 5R4 draws the lesser current like the 5AR4 but drops the voltages to the tubes a little, similar to the 5U4. I love the sound I have now, and there's less stress on the power Transformer. And... the new old stock, [nos], 5R4 is cheap in price. Mine are from yr. 1950, and these should last for decades??? They were made by RCA. There is a guy on this forum that sells these tubes . You could go to the "Amp Central Station" section on this forum and ask who to get these nos. tubes from... It worked out great for me. And my Princeton sounds more soulful and musical with the slight 'sag' in the sound. Sag is good..... I love it.

Wally
thanks for the reply. Both of these amps are first generation drip edge silverface with the thin black vertical lines on the faceplate. For whatever reason, there was a batch of these with a factory tube chart that reads aa764, which of course is the BF champ circuit. So, a typo or whatever, but the tube chart also specifies GZ34 for both of these amps, both with stock Mullards dating late 67.

Here is a selection of links to others with the same amps:

http://vintageamps.com/plexiboard/viewtopic.php?f=8&t=20531

http://acapella.harmony-central.com/showthread.php?1406364-1969-Fender-Princeton

here is one more link with several pics including the factory tube chart showing the GZ34 rectifier tube as well as the aa764 circuit designation

http://www.massachusettssuperads.com/other_electronics_For_Sale/C540A431935P1/1965_Vintage_Princeton_Reverb_Amp_AA764.aspx

Rod- yes, I have some 5R4s and 5V4s to try, as well as 5U4s. While tone is clearly very important, I am trying to find a graceful solution to what I see as a sort of borderline operating condition, in terms of safety, exploding expensive vintage gear etc. I am trying to strike a balance between the original design, sound and safety. I am already starting +8-10 volts above the original design wall voltage (125 here) so I am trying to find as many ways to counter this so I can return the amp to the owner who is probably not going to want a bucking transformer as a new accessory. Alternate rectifier tubes are a possibility, but knowing the owner, I dont think he will want to forsake the aggressive GZ34 responsiveness. I think 5881s are a good solution, if they sound alright.

centervolume, yes the AA964 schematic adn tube chart call for a GZ34 rectiifer. That November of 1964 schematic calls for a 5U4. AFter November of '64, it doesn't matter what tube chart you have in that amp. The PT sepcifications were changed and the recitifer tube should be a 5U4. Teh PT carries the saem model number as before, but the specs are different. AS I noted...adn this is common knowledge....FEnder tube charts are very unreliable in many cases.
Imho, your amp is exhibiting high voltages in part because the rectiifer tube is not 'correct' for that PT. The other reason for the high voltage is the wall voltages tha we usually see today taht are higher than what we woudl have seen back when this amp was built.
The GZ34 will actually place less stress on the PT due to the lower current draw on the rectifier heater filamnet, but that Princeton REverb really doesn't want to see that high B+. IF it works for you, continue with the GZ34....but it is not what the amp came with from the factory...regardless of the tube chart.

The small Reverb transformer is 1965.. If it's original,[ most likely is], the your amp was built in '65..... Use 5U4 if you want, as that is what it calls for , or use the 5R4, since you have one, and see how it sounds. Both of these will lower the B+ voltage a little... That's what you're trying to do, right??? Good luck....

could you clarify this for me?

Teh PT carries the saem model number as before, but the specs are different. ... continue with the GZ34....but it is not what the amp came with from the factory...regardless of the tube chart.

I understand the switch from triad to schumacher was made in part for the latter's use of heavier gauge wire, less blown transformers. But that shift was marked in new mfr codes and part numbers. I just can't seem to make out what "new specs" may have been implemented on the 125P1B? Or do you mean the specs different in the amp circuit/schematic?

I also have difficulty in saying what came factory stock on a given amp since I was not able to verify it as it rolled off the line. That is why my inference was based on date codes on the tubes in comparison with pots, trannies, caps codes.

No doubt Fender's typos extend far beyond charts to the catalogs themselves where 37" tall concert cabs are cited among other oddities like a 2x6L6 brown twin, so I have no problem observing that. But I am surprised that the gZ34 version is more the norm - are the 5U4 ones later silverface or something?

here's another couple- I am having a hard time finding one with a 5U4:

http://www.ebay.com/itm/Fender-Princeton-Reverb-Amp-1966-Original-Speakers-and-Tubes-Orig-Foot-Switch-/220941406071?pt=LH_DefaultDomain_0&hash=item3371224f77

http://www.ebay.com/itm/1966-Fender-Princeton-Reverb-amp-cab-vintage-blackface-Deluxe-cabinet-amplifier-/110809843986?pt=LH_DefaultDomain_0&hash=item19ccc80912

http://www.ebay.com/itm/RARE-VINTAGE-66-GUITAR-AMP-FENDER-PRINCETON-REVERB-AMPIFIER-/250982235607?pt=LH_DefaultDomain_0&hash=item3a6fb4fdd7