Finally got a Compu Bias... need help interpreting data for Fender Princeton

[mike_mccue]

Hi,
It's been a year or so since many here recommended the CompuBias to me.

I finally bought one and today I'm trying it out.

I have a friend's 1975 Fender Princeton at the house because it was physically dropping it's output tubes. I've tensioned the sockets and it seems ok now.
Just for fun I hooked up the CompuBias.

I'm unfamiliar with how the results relate to what I might expect on a Princeton. Can anyone please advise?

Original Issue RCA wafer base Fender labeled U.S.A. 6V6GTA tubes

400vDC

IK 34 W 13.4
IK 38 W 14.9

The Plate voltage and the W figures were pretty wiggly... I had to concentrate to realize the hi and low points. edit to update: I figured out that the Vibrato circuit was causing the readings to fluctuate.

The disparity seemed to follow the tubes. When I switched the tube/probe assemblies in the chassis sockets the results suggested the differences were in the tubes (or the probes). I also switched the tubes in the probe sockets and noticed a slight difference from the original readings... is it possible the probes aren't matched? I attribute some of the noted difference between the first and last readings to the fact that the power caps finally stabilized the plate voltage at 396vDC +/- 1vDC.

I'll suggest to the amp's owner that he buy a pair of NOS Phillips tubes.

Other than that, I'm wondering if anyone thinks the results suggest a need for further maintenance or is this a practical state of affairs? :-)

The amp sounds pretty nice.

best regards,
mike

[Enzo]

The amp sounds pretty nice.

That right there is what matters.


SO the cathode current averaged 36ma. One tube was 2ma light and the other was 2ma heavy. I wouldn't worry about that. The difference followed the tubes.

If one socket consistently read different from the other, two things come two mind. First, power down and measure the resistance of the OT primary from center tap to each plate lead. ANy difference in resistance will affect the current through the tubes. And second, any difference in grid bias voltage will affect currents.

I don;t quite follow what the probe difference was, so try this. Pick one probe and insert it under one of the tubes and take a reading with the amp fully warmed and stable. Take a reading. Now put the other probe under the same tube in the same socket and take a reading. ANy difference will be from the probes themselves. You can see how close the probes track.

[billyz]

Plate current sounds very high to me. For a 6V6gta , assuming 14watts Plate dissipation at 400VDC , I would think you would want around 21-25ma plate current. For a 12watt 6v6 a little less, maybe 21ma max for 70% dissipation.

I bet the tremolo is not working too well either at that high plate current.

[mike_mccue]

The tremolo is fairly weak sounding on that amp.

I'm going to speak to him about fresh tubes and then go from there. What's the practical way to actually set the bias on a Princeton?

On another note, I personally own a 1976 Fender Princeton Reverb that I purchased a few months ago. It is very clean and stock but it came with new Sovtek tubes which only took about 3 weeks to short out and fry.

I've been running some older RCAs in it for the time being but just purchased a few sets of the NOS 6V6GTA JAN Phillips/ECG tubes you see around.

They were supplied as matched pairs... and they actually seemed to be matched. :-)

1976 Fender Princeton Reverb.

NOS 6V6GTA JAN Phillips/ECG

Plate voltage: 413vDC

Ik Cathode Current: 28mA
Ik Cathode Current: 28mA

W: 11.8
W: 11.8

I've got the Vibro and Reverb switched off (and turned down to "1") but after 30 minutes on in there's still a bit of flutter in the readouts.

I've not heard this amp with the new tubes... I'm about to... but I general its sounded great once I put those old parts bin RCAs in it. It is much stronger sounding than the 1975 Princeton I mentioned above.

thanks for the advice,
mike

[billyz]

It has been awhile since I did one but I am pretty sure you'll need to replace a resistor. So while your at it you could add a trim pot to make it more user friendly in the future. There is a 22K resistor right after the bias circuit diode, going to ground. That is your bias adjustment resistor. Make it bigger will get you more neg VDC to lower the current draw of your power tubes. Replace it with 6.8K resistor in series with a 22-25K linear trimmer to ground. That will give you plenty of range to make adjustments.

I know these amps sound great with the higher current but I would not run one that way for long. Hard on the tubes and hard on the Transformers. And the Tremolo (while it is weaker on the Princeton) will be stronger at the better operating condition.

NOS 6v6's are definitely the way to go, I do like the JAN GE's too. of course RCA and Tungsols are even better. Unfortunately, most of the dealer matched tubes are NOT.