Bias current sweetspot for 1975 Princeton?

[mike_mccue]

Hi,
I am working with a 1975 Fender Princeton.

I have 6 sets of fairly well matched NOS JAN Phillips 6V6GT tubes and today I have been swapping them out and playing/testing/listening while measuring with my CompuBias meter.

The tube sets are running along a range from 32mA through 39mA with about 396-400vDC on the plates.

Is there a sweet spot for the bias current for these tubes? I am wondering if I should plug and play a set running at 33mA or elect to replace the resistor?

best regards,
mike


edit to add:

The tubes that came with the amp are labeled Fender 6V6GTA. The have a very short plastic base. The amp came with the original bill of sale. I think the existing tubes are "original".
They are measuring a lower vDC at 392 and the idle current is 34-36mA

[Euthymia]

SF Princetons get bias pots around here. With the type of trem circuit they have, it's even more important to be able to dial it in.

All of your sets are running way hotter than I would ever let a Princeton get out of my shop. You'll get weak trem and short tube life with that.

[mike_mccue]

Thanks for your comments.

Is it possible that this is related to increased wall outlet voltage?

I ask because I also have a 1969 Princeton and a 1975 Princeton Reverb here and they seem to be running just as hot.

All seem to work well and have for many years.

I am working with the 1975 Pricenton mentioned above because it has been underused for a while and had seemed to need some service.

After taking some to to play, and test... it seems to have come back to a point where I am reluctant to fix what doesn't seem to be broken.

I guess I could consider the bias adjust mod.

best regards,
mike

[MWJB]

Yes, it is because wall voltage has risen, however, your particular Princeton does not seem to be running particularly high voltages for this model. Low plate voltages may be a sign that your filter caps are past their best.

good NOS 6V6 should take that 32mA, but if you install the bias adjust pot, rebias to the current that keeps you to 425vdc on the plates, or a shade under (probably late 20's to 30mA?). Modern production 6V6 like EH & JJ will take the voltages that Princetons can throw at them, even at cooler plate currents (low to mid 20's). You could see whether you like the tone that a 5Y3GT rectifier gives, if so, this will give you more leeway with plate current/voltage.

Remember to switch the tremolo off (not just turn it down) when setting idle current.

[mike_mccue]

Can you clarify a question for me; Is it the case that the higher current is caused by the screen bias but the plate voltage is brought down when the current flows?

Should I replace the caps and resistors in the bias supply and add a pot?

Or should I try to get the plate voltage up by replacing the filter capacitor can?


edit to add: I just used a CompuBias meter to test the '75 Princeton Reverb. the Plate voltage is 404vDC and current is 27mA

I'm going to try the other Princeton next.

thanks,
mike

[mike_mccue]

FWIW, I just tested a 1969 Fender Princeton.

Plate Voltage 362vDC and 30mA cathode current. That amp uses the GZ34 recitifier while the other two use a 5U4GB.

This is my favorite sounding Princeton of the lot.

The Princeton I mentioned in the OP is still up for some improvement.

best regards,
mike

[MWJB]

The high current is caused by having non adjustable fixed bias and a circuit thet was designed to see less wall AC. Even with a bias pot, as you increase plate current, plate voltage will drop & as you decrease plate current, plate voltage will rise.

362vdc @ 30mA seems low for a GZ34 equipped Princeton with original PT.

Yes, I would suggest replacing filter caps, cathode bypass caps & bias supply electrolytic & add a pot to adjust bias (replace rounded resistor in bias cicut with a trim pot/trim pot in series wih fixed resistor to ground (see artice at EL34world.com).

If after doing this your plate voltage increases you can always reduce it again by using a lower voltage rectifier, or a reverse polarity zener diode. My preference would be to ensure that the amp is working properly, then tweak B+ via controlled methods, rather than rely on a possible malfunction to give the voltages that you like.

I would normally expect a tolex Princeton with TMB controls to run in the low-mid 400's at moderate bias currents (mid 20s?), dropping to 420-425 at 30mA or so. Note that the Fender 65 Princeton RI schem indicates 440vdc on the 6V6 plates at 23mA per tube.

[WholeToneMusic]

The plate voltages you specified 396-400vDC is high for NOS 6v6s. The RCA tube manual lists 315vDC for a 6V6 for maximum rating. Like MWJB was saying the JJ tube can handle more Plate voltage. The JJ spec sheet on their 6V6 lists 500 vDC for max plate voltage. The JJ 6V6 can also handle more plate current than older 6V6s.

[MWJB]

WholeTone - you might be interested to check the factory voltages for the original BF Princeton & Deluxes...nearly all 6V6 amps run their tubes way in excess of the data sheet ratings (as do most popular EL84/6V6/6L6/5881 guitar amps).

[mike_mccue]

Last night I looked at schematics for the Silverface era Princetons and Princeton Reverbs.

The schematics suggest a plate voltage of 410 on the Princeton Reverb and 420vDC on the Princetons.

There was no difference specified when the rectifier tubes were changed to 5u4GB from GZ34. I thought I had read there was a small change, but the Fender schematics do not document it.

I also have a '74 Deluxe Reverb... I think that thing runs a real hot plate voltage.

I have two questions,

1) Is it likely that the electrolytic caps are leaking some DC to ground? is that the general idea? I've replaced caps that weren't bad... I've replaced caps that were falling apart... I never thought much about why. :-) Are there any affordable meters that can measure in circuit? Last night I was reading about ESR meters and they seemed like they might be helpful.

2) Is it possible the rectifier tubes will wear out? Should I try a fresh rectifier?


These amps use the FP style caps. I have read mixed reviews about the new supply of FP cans... and I'm reluctant to replace the caps with some other form factor. So, I'm not sure what my next step will be.

Thanks to everyone for helping with these questions.

best regards,
mike

[MWJB]

Fender may well have hiked up the B+ rating of the PT when swiching from GZ34 to 5U4 so that the amps ran the same voltage at the end of the day...they often hiked up voltage anyway, even on models where rectifiers weren't changed.

If the filter caps are original then thet really need changing..even if there is no visual clue. I would replace them with 500v rated discrete caps, mount main & screen caps over where the old cap can is situated (you can cut the tags & leave the old can in place) ground to a PT bolt, mount preamp filter at the circuit/inoput end of the board & ground to input jack ground. A Fender that actually runs less voltage than the schematic, assuming it still has the original PT, is ALWAYS suspicious to me.

[mike_mccue]

Thank you for the detailed reply.

I know this is a simple question... but I am continually learning about amps and have lots to learn still.

Is the general idea that the electrolytic caps once blocked the DC effectively while grounding the AC ripple out but are now leaking some DC to ground as well?

I appreciate your thoughts about the discrete cap and the tip to place the third item near the input.

I think I may try a new FP can first and then use discrete components if the example I buy gives me trouble.

Thanks again,
mike

[MWJB]

Yes, but also the dialetric breaks down with age, they could be a little leaky, or they can blow completely (smelly, lots of smoke), even if you don't hear hum & they don't blow, old electrolytics sound mushy.

As your amp is showing low voltage, replacing the power supply caps is one of the first things to do in terms of ensuring proper performance.

[WholeToneMusic]

Yes, The Deluxes are notorious for high plate voltages. Shortened tube life. I have noticed alot of the Boutique amps I have serviced with EL84's due the same. An amp always sounds it's best right before it blows up

[mike_mccue]

I forget the details but I have replaced the caps in my Deluxe Reverb. :-)

Thanks again to both of you.