Princeton Reverb - the immortal hum

[Sir Cuitous]

Greetings forum folks. I have a nice, old Princeton Reverb that a friend asked me to look at. It hums loudly with the power on. Volume control does not affect the hum. The Intensity control reduces the hum somewhat when it is turned all the way to 10.

I have a scope but I am not experienced with diagnostics. On the scope I can see the hum at the bias supply diode. It appears to come from the PT through the bias diode. I measured -30v DC at the diode, but I also measured 16v AC. Should I be seeing AC from the bias supply?

[Enzo]

Of course not, the bias needs to be smooth DC. Measure the bias at the tube socket. you already report AC on it, does it also measure low on DC? SOunds like bad bias filter cqaps from here.

[Sir Cuitous]

I didn't think so, but I like to post with a question rather than a statement.

OK, I measure -30vdc at the tube sockets. At the junction of the 220k resistors I get -30vdc and +6vac. Oddly only one of the tube sockets measures +6vac.

Right after the diode: -30vdc and +16vac.

[TD_Madden]

I'd just replace the power-supply and bias-supply caps.....they've probably never been replaced.

[pila]

YUP!!

[Don Moose]

Remember - the bias filter cap have (+) to GND for a reason!

Man, those smell bad when you forget.

[pila]

[txstrat]

I had one of them literally exploding in one of my first builds. I'll never forget the bang and probably never forget to check the polarity of the bias supply filter cap.

[Enzo]

Famous last words. I just very carefully and neatly wired one in backwards just last week. BAM!!!

[Sir Cuitous]

Thanks all - comedy and good advice all in one thread

OK, I did replace the bias components and eliminated the AC. I am now getting -39vdc, but the tubes are now biased at 16ma. They were at 32ma before when the bias voltage was -30v. How do I get the tubes back to 32ma? I guess the question could be restated how do I get the bias voltage back to -30v ???

[Mars Amp Repair]

Princeton reverb bias trimmer

Well,
The bias was probably high owing to the fact that the tubes were amplifying the hum which would draw current. Also, having AC on the bias signal would effectively reduce the average DC there to do the biasing thereby causing the tubes to draw more current.

to increase the bias current, you could decrease the value of the divider resistor in the bias ckt that goes to ground, thereby reducing the amount of bias voltage to the tube. this is done best by trial & error.

Merely bridge the resistor with another. I ususally start with one that is about 5 times higher in resistance than the one that is in the ckt. Monitor the bias current & keep reducing the value until you reach the desired value of bias current.

You could also build a little divider network with a pot & resistor to ground to replace the exisiting resistor to ground in the bias divider. That way it is just a matter of adjusting the pot.

A 25K pot & resistor that is about 1/2 of the existing divider resistor should work fine. The Princeton Reverb Schemo I'm looking at looks like it uses a 27K to ground, so a 15K in series with a 25K pot should do the trick.

I don't believe this would in any way effect the Tremolo, which in this version modulated that bias voltage to achieve the tremolo effect.

I'm sure others will have ideas, too.

glen

[Sir Cuitous]

Let me make sure I understand it. I need to lower the value of the 27K resistor in order to increase the bias current over the tubes.

I effectively need to double the tube bias current from 16ma to 32ma or adjust the bias voltage from -39vdc to -30vdc. Can I parallel a 100K pot (about 4 times higher to start) with the 27K resistor, set the bias - then measure the pot and do the math to get the resistance?

[bnwitt]

Let me see if I can clarify this for you. Bias current is not the proper term for what you are trying to adjust. It is quiescent or idle current of the power tubes at zero signal. Bias voltage at pin 5 of the power tubes is what sets that quantity. The higher the negative bias voltage supplied to the power tubes the lower the current will be. This is why the -39 volts DC is allowing less idle current than the -30 volts did. The BFPR did not have an adjustable fixed bias supply. If it were me, I would convert it to adjustable so you can dial in whatever idle current you want whenever changing power tubes, rather than being at the mercy of the non-adjustable setup. Power tube pairs have vastly differing current flows from set to set and a non adjustable fixed bias supply gives you no way to adjust for that. Attached is a schematic of the Hoffman style BFPR which has an adjustable bias supply added. If you study it closely you should be able to convert your amp to adjustable fixed bias. You just need to add a 10K linear pot to the existing circuit as shown and change the fixed resistor to 22k. Now if that doesn't give you a low enough bottom range bias voltage you can further reduce the fixed resistor value until you get what you want.

Now all of that being said, you have not mentioned what your plate voltage is on your power tubes and without that reading, the milliamps of the tubes means nothing. You also haven't said if the ma reading you are taking is for each tube or both tubes. If I assume your plate voltage is 400 VDC as shown on the BFPR schematic and that you are setting idle current for 70% of maximum dissipation (14 watts per tube for 6V6GTA) then I would expect to set the 70% idle current level for each tube to 24.5 milliamps. It would probably be set for about 27ma using a bias probe socket as the screen current would also be present across the 1 ohm cathode resistor. 70% of 14 watts is is 9.8 watts. 400 volts times 0.0245 amps (24.5 milliamps) equals 9.8 watts for one tube.

Since Lord Valve did such a great job of explaining the non-adjustable to adjustable fixed bias conversion in his bias document I thought I would include his take on it:

"Many amps which use "fixed" (negative grid) bias have provisions for adjusting the negative grid voltage upward or downward. Making the grids LESS negative will cause MORE current to flow through the tubes. Some amplifiers don't have a bias-adjusting control (pot) but instead use a fixed resistor to set the voltage. If you encounter one with a fixed resistor, the best thing to do is convert it to an adjustable type. Most of the time, the fixed resistor will be in parallel with the bias capacitor; the lower this resistor's value is, the lower the bias voltage will be. If you can locate and identify this resistor, you can replace it with a simple network consisting of a (lower value) resistor in series with a potentiometer. What you'll be shooting for is a range of adjustment that goes from LESS voltage to MORE voltage than is set by the (existing) fixed resistor. Take the value of the fixed resistor and divide by two; pick the closest standard value to your result, and put it in series with a pot which is as close to the original resistor's value as you can find. Example: the existing resistor is 33K; use a 15K resistor in series with a 25K pot to replace it. The original resistor was 33K; you now have the ability to adjust the value from 15K to 40K. This should provide you with sufficient adjustment range to set any plate current you wish. If not, use a different value pot or resistor."

Remember the fixed resistor is the resistance in the pathway that bleeds the negative bias voltage to ground. The lower the resistance value there, the more voltage will go to ground and as such, the lower the voltage present at the tubes will be.

[Sir Cuitous]

bnwitt, thanks for the details. 32ma is probably a little too high. I do like adjustable fixed bias personally, but I know the owner of this amp and he will not want anything that was not in the original amp design. My preference would be an adjustment pot for each tube.

I will see this evening about getting a good value for that resistor following the method outlined by Mars Amp Repair in this thread.

[Sir Cuitous]

OK, so I used a 250k pot with clips and clipped it to each side of the 27K resistor then I dialled back the resistance until my bias probe socket read 27ma. I unclipped the pot and then clipped it to another 27K resistor on my bench and measured the overall resistance at 20.85k. I realize my 27K resistors may be slightly different, but I believe I am in the ballpark with a 21k resistor. Since I just want to replace the single resistor and not add adjustable bias, it seems like a valid approach.