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TheGimp 17th September 2012 03:31 AM

Bassman 50 red plate and bias
I'm working on a friends Fender BASSMAN 50 (Schematic here : and it is running what I believe is a bit hot.

B+ is 465V
Grid bias is -45V

There is no bias adjustment, but there is a balance pot.

I can adjust it so both tubes are just slightly red at the seam of the fin (best seen with the lights out).

I'm considering increasing the value of the 15K resistor from the wiper of the balance pot to ground. This will increase the bias and reduce the cathode current.

So my basic question is "Is this how guitar amps run (which contributes to short tube life)? Or is the amp running too hot.?"

20to20 17th September 2012 04:01 AM


Are you sure the schematic is for that amp? If it is then the bias is adjustable at that pot. It's not a "balance" pot. Are you getting the full -45v? Can you not adjust it higher than that? May have a bad filter.

Is the bias rectifier a diode or is it a selenium type?

HollowState 17th September 2012 04:07 AM

Well Mr. Gimp, you're throwing us a curve. If the schematic provided is correct, there is a bias adjustment but no balance pot. And the 15K resistor is not from the center of the pot, but from the low end.

With the plate voltage stated, and the negative grid voltage given, a 6L6GC should not red plate. If they do, I would guess that they are out of specification. What kind of tubes are they and how do they test? With today's poor quality tubes one can expect "OOT". So yes, raise the 15K to 20K or so and see if that helps. And no, I don't think amplifiers are designed to run their tubes excessively hot.

HJWeedon 17th September 2012 05:31 AM

Amplifiers and aging capacitors.

I studied the schematic, and if it appears as if the adjustment potentiometer in the grid circuit is a "balance pot" there is something wrong in the circuit. I suggest that one or both coupling capacitors to the output stage are leaking so that the grid voltages of the output stage are not what you think they are, but something much more positive.

In aging amplifiers the first thing to check are those old coupling capacitors if they leak, which they often do, replace them with new and more modern caps. I suggest a 630V rating, same value, maybe a 1000V rating to be on the safe side.

Hans J Weedon

kevinkr 17th September 2012 06:55 AM

:cop: All guitar amp threads belong in Instruments and Amplifiers so I'll move it there. Plenty of troubleshooting horse power over there too.. :D

TheGimp 17th September 2012 12:36 PM

Oops, sorry. Wrong version of the schematic (amp is from 1962). Try this one:

The 15K resistor goes from the wiper on the pot to the pot body, so it is to gnd.

Rectifier appears to be silicon not selenium (given that all selenium I've e ver seen were stack type).

I guess one option is to change to the other bias adjustment circuit, although I hate to make more changes than necessary.

Tubes are 6P3S (multiple tubes trying to select a pair), 6P3S-EB and TAD 6L6GC. That the 6P3S red-plated is not surprising. I was surprised that the 6P3S-EB and TAD tubes did the same.

All tubes are new as the tubes that were in the amp were damaged (broken bases) and I did not trust using them. When the amp was purchased, it was turned on to demo it and smoke came out.

I suspect the smoke was the AL electrolytic caps venting, as three had done so. I replaced all caps before powering up the unit (light bulb test).

TheGimp 18th September 2012 03:10 AM

Man, is this amp overbiased!

I added a 10K pot in series with the 15K resistor to gnd.

I dug out my bias adjustment socket and even at the minimum setting (-45V) it is still over 50mA per tube!

I'll crank it down below 40mA/Tube and let him try it and see how he likes it. (40mA is as low as I can go without changing something else.)

HJWeedon 18th September 2012 05:13 AM

Hi with the new and corrected schematic, some parts must be missing there is over 100% negative feedback because there is not an attenuator to gnd in the feedback path. If the amplifier is really built like the schematic shows, I believe the reason for the "over-biased" condition is that the output stage is oscillating at supersonic frequencies. An oscillating amplifier does what it wants to do relatively independently of adjustments. Please either investigate what is wrong with the schematic, or correct the circuit so that the amplifier stops oscillating, and then adjust the balance or the bias condition.

I will be glad to estimate a bias-point based on circuit parameters for you. The proper way to bias a high quality amplifier is to set the bias at about 75% of maximum spec-sheet dissipation in the anode when the amplifier is idling. This is easily determined from the manufactures data sheet. Overbiasing the output stage of Push-Pull amplifiers does not buy you anything except repeated trips to the tube-store for new tubes. Personally I like to bias tubes like the 6L6, EL34, KT66, KT88, or 6550 at 1/2 the spec sheet maximum allowable dissipation. The bias point has very little to do with the amplifier performance in a push-pull amplifier. In a single ended output stage, however, you gain significant dynamic range by biasing the tubes close to maximum.

Hans J Weedon

TheGimp 18th September 2012 09:38 PM

Thanks Hans,

I'll check it out.

I was trying to adjust to 38mA which should be about 75% of where it was with the plates starting to glow.

The RCA datasheet shows 50W out in class AB1 at 450V B+ (close) with 95mA per pair (also close), but at -30V bias. So the bias certainly does look like it is off.

Ill check for the potential oscillation.

HJWeedon 18th September 2012 09:57 PM


While you are checking: The plate current in a pentode is more a function of the potentials on the two grids rather than the plate voltage. The mu of the screen-grid to the control grid is about 8. That means that to a first approximation if you divide the screen grid voltage by 8 and apply that negative voltage to the control grid that should be the voltage that should cut the anode current to zero. Since the tube is somewhat non linear the anode current will only drop to like 10% of max, about 5 to 10mA or so.

Good luck.

Hans J Weedon

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