‘81 Princeton Reverb Bias Question

[Perkinsman]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

Actually there is still a little too much distortion when over 3-4 on volume. Is 19ma the correct bias?’

[Leo_Gnardo]

Actually there is still a little too much distortion when over 3-4 on volume. Is 19ma the correct bias?’

19 mA is primo - park the bias right there & worry no more.

You may be hearing clipping distortion - not unusual on most Fenders with volume control on 3-4 and most any guitar played full volume into the #1 jack. If you had a scope you would see the waveform hitting the rail - "flat-topping" and for that matter flat bottoming too. I find healthy Princetons turning out anywhere from 13 to 22 watts just below the clipping point.

[Perkinsman]

This is a friends amp & it never distorted like this at 3-4 volume, it wouldn’t break up until about 6 before. I think there may be another problem in addition to the bad bias cap. I don’t have any hum & the old 20uf caps measure at about 24uf at full 450 voltage so I think they’re fine. Other ideas?

[The Dude]

The best way to find out if there is indeed a problem would be to actually measure/calculate max output power. The number on the dial is somewhat meaningless. In fact, I've had customers with the same complaint and all it was was the set screw on the knob came loose so pointer orientation was incorrect.

Also, is B+ still low? That will certainly effect output.

[Perkinsman]

I wish this was a set screw issue, I know this amp & it’s not sounding right. B+ is within spec now, resistors measured within spec. How do I measure power output?

[Enzo]

Is 19ma the correct bias?’

This implies there is incorrect bias. It is just a guitar amp. Too cold and you get a lot of ugly crossover distortion. Too hot and your power tubes start to red plate and melt. ANything between those is OK with the amp. it is a matter of personal preference where along that range you like to be.

Another very common lesson is what happens when you rebuild an amp. it will not sound the same. People expect an amp they love the sound of but needs caps will come back with bright shiny new caps but still sound the same. it won't. It used to take until 6 to break up and now it breaks up at 4? That could just be the nature of the beast.

[g1]

Have you checked all the DC voltages on the preamp tubes? They are all listed on the schematic Enzo posted.
It would not be easy to check the power output without a scope as you would have to decide by ear where the output clipping begins. At least you would need a load resistor.

[Perkinsman]

The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

Problems with either power supply COULD cause distortions.

Please explain in detail exactly how you measured the tube current.

I guess the bias cap is leaking. But also check the 100k and 22K voltage divider resistors.

Well, the amp has hardly been rebuilt, just a new pair of power tubes & a bias cap that brought the bias current to 70% of a 6v6 max dissipation. Something else is wrong with the amp. I watched an Uncle Doug Video on how to measure power output so I’ll work on doing that this weekend to see how much undistorted rms power its putting out @1k hertz. Since I replaced the bias cap I haven’t remeasured the preamp tubes so I’ll have to do that first.

[Perkinsman]

Here’s the results of the 4 preamp tubes:
V1a)145 b)142 V2a)287 b)288 V3a)104 b)107 V4a)jumpy 190-350 b)160. As you can see, V3a&b are lower by over 30%. V4 off 40%.

[Helmholtz]

Please also post supply voltages at points A,B,D.

[Enzo]

Half of V4 is the trem circuit, that is why it is jumping around.

V3 PLATES read low? Then do the cathodes read high?

[Perkinsman]

Voltages at A=408, B= 285, D=205. V3 cathode=1.6 I just noticed that my schematic is a little different than the one Enzo posted above. Mine has an 18K resistor between A&B, the one above is a 4.7K and mine doesn't even list an A. The schematic I've been using is for a B1270 Silverface Princeton Reverb. Is there a difference? Should I just replace the 18K with a 4.7K and check voltages again?

[Helmholtz]

Should I just replace the 18K with a 4.7K and check voltages again?

No.

Posting your schematic might avoid some guessing. And remember ALL voltages in a Fender schematic have some meaning and use for troubleshooting. So please also check and post cathode voltages as Enzo proposed.

[Perkinsman]

I changed the resistor to a 4.7K and the voltages are now B=366 & D= 262, much closer to spec. Would the lower voltages have been enough to cause distortion at lower (3-4)volumes?

[Perkinsman]

Oops, too late, I changed the resistor but here's the B1270 schematic. Enzo requested V3 cathode, which I listed earlier as 1.6