Silver Face Princeton B+ voltage

[AmberB]

Do all silver face Princeton amps have high B+ voltage, or just mine?
My amp is putting about 475 volts onto the plates of the 6V6s with the GZ-34 tube rectifier, with about 17-18 milliwatts of cathode current.  I could adjust the bias higher, but I think that would be hard on these poor NOS RCA 6V6s at this B+ voltage.
I have the "Jim Jones" bias chart that I got off the internet years ago, but of course, it only goes up to 400 volts for the 6V6, so I'm making a bias guess based on what I see on that chart.
As far as I can tell, the power transformer is original to the amp, but I suppose I could be wrong.  The chassis code indicates that it was built in 1970.  The codes on the pots are all from 1969.  The amp is the non reverb version.  The only schematic I have for the non reverb Princeton is for the AA964.

[Esquirefreak]

I can't remember exactly what year, but I think later Princetons used a 5U4 recto. That would knock the B+ down some. But make sure that the PT can handle the fil. current draw. Compare schematics with the numbers on your PT.

/Max

[tubeswell]

So its not an export model (with a mains voltage selector)?

[Latole]

IMO these amp are built for 110 volts wall outlets . Today we use 120 to 125 volts. That is why you may read higher B+

I use Zeners stack to lower B+ on few of old amps

[Latole]

Some more ;

Schematic ; I'll use the AA964

http://www.thevintagesound.com/ffg/

[sluckey]

Crank the bias up to about 24mA cathode current and see where the plate voltage lands. Consider using JJ 6V6s if this makes you nervous. You could probably benefit by using a bucking transformer to lower voltages. See page 2 of my Amp Scrapbook.

[pdf64]

I second trying a 5U4 as per the B1270 drawings.
And a mains bucker, if the loaded heater voltages are more than 5% above nominal.
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_bf_ab1270.pdf

[HotBluePlates]

Do all silver face Princeton amps have high B+ voltage, or just mine?
My amp is putting about 475 volts onto the plates of the 6V6s with the GZ-34 tube rectifier ...

IMO these amp are built for 110 volts wall outlets . Today we use 120 to 125 volts. That is why you may read higher B+

I tested several vintage Fender amps, looking for the wall voltage required to obtain 6.3vac on the heater wiring with all tubes installed.  I found that by the 1964 blackface amps, 120vac from the wall yielded correct voltages inside the amp (Champ/Vibro Champ is an exception because it used the too-big Princeton power transformer).

In both schematics and direct observation, Fender kept cranking up the supply voltage as they moved form the tweed era through the brownface, to/through blackface, into the silverface amps.  I've also found cases where more than 1 power transformer spec had been used within a single era (like 3 different power transformers for a brownface model, another 3 or 4 power transformers for the blackface version of the same model, etc).

... I think later Princetons used a 5U4 recto. ...

I second trying a 5U4 as per the B1270 drawings.
And a mains bucker, if the loaded heater voltages are more than 5% above nominal.
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_bf_ab1270.pdf

+1 to using a 5U4.

I owned a 1967 blackface Princeton Reverb that used the 5U4 (while some others of the same year might use a GZ34), so it's not unheard of to see those early.

Also, even though the AB1270 schematic shows "420v" I've found in my testing that the actual voltage in the amp varies.  While I haven't had a silverface Princeton in a long while to be able to test, checking my own amps & polling others for their measurements confirmed the actual amps rarely end on "schematic voltage."

I suspect you might ultimately have 430v to 450v with right-wall-voltage, and that might be "normal" for your amp.

[AmberB]

What's left of the tube chart inside of the cabinet calls for a GZ-34 rectifier tube, so I'm a bit nervous about using a 5U4 with it's extra amp of heater power draw.
I noticed that the tube chart lists 117 volts AC as being what it was designed for, so it's pretty close in that regard.  Of course, the part of the paper with the model or chassis design is long since gone, so all I have to go by is the date code inside the chassis for the design, and I don't know exactly which chassis design Fender used on the Princeton non reverb amps in 1970.

I was reading something on the internet last night in regard to using a diode rectifier adapter on amps with a rectifier tube.  One of the things they talked about was adding a resistor in the adapter between the diodes and pin 8 of the adapter to lower the voltage in a manner similar to the tube rectifier.  They suggested at least a 10 watt resistor, and there was a range of resistance depending on the type of tube rectifier being replaced.  The article also suggested the alternative of putting a power resistor between the secondary center tap of the power transformer and ground.  That would obviously also have to be at least 10 watts and probably bigger to keep it from getting hot.  Of course, I forgot to bookmark the site, so I'll have to find it again.  That might be a solution to the extra high B+ voltage of this amp.
I'll have to check out those JJ 6V6s...

Of course, when I crank up the bias on the power tubes, it's going to mess with the tremolo response of the amp, which I just got working decently...

[AmberB]

If I put the numbers of the power transformer on here, can someone tell me what the specs are on it?

[tubeswell]

If I put the numbers of the power transformer on here, can someone tell me what the specs are on it?

Who knows unless you put the numbers on here?

[HotBluePlates]

What's left of the tube chart inside of the cabinet calls for a GZ-34 rectifier tube, so I'm a bit nervous about using a 5U4 with it's extra amp of heater power draw. ...

That's a fair concern.  By some point in the early 1970s, the tube chart changed to indicate a 5U4GB rectifier.

... I noticed that the tube chart lists 117 volts AC as being what it was designed for, so it's pretty close in that regard.  ...

If I put the numbers of the power transformer on here, can someone tell me what the specs are on it?

The best way to obtain specs is to use a variac yourself:

   Install all tubes.
   Measure the 6.3vac wiring and adjust the variac voltage until you get right on 6.3vac.
   Check the 5v winding to see what voltage you have; if higher than 5v when using GZ34, you possibly have a winding that will support 5U4.
   Measure the high voltage AC output (one leg to center-tap).
   Measure the DC Volts output of the rectifier (make a note of the rectifier the used).
   Turn amp off and measure the resistance of the high voltage winding.

I followed the procedure above, and while my 1964 Deluxe Reverb's tube chart says "117 volts" it delivered 6.3v to the heaters when 120v was applied to the PT primary.


I wish I had the foresight 22-30 years ago to measure & record the PT characteristics of the different vintage amps that went through my hands.  I had around 8 total blackface & silverface Princeton & Princeton Reverb amps back in that timeframe.

[mresistor]

I have the "Jim Jones" bias chart that I got off the internet years ago,

You could use this bias calculator  http://www.electrotubes.com/bias.html