Fender Pro Reverb Bias Issue - 1970s

[misterc57]

I am restoring a Pro Reverb. I installed all new filter caps and bias caps. The original bias caps were 80uF, I installed 100uF.

Schematic does not seem to match what I am seeing. The bias pot I believe to actually be a balancing pot.

The bias is very high, around 95 mA with the pot centered.

The 15K R at pot and the 3.3K R to the pot read accurate.

I read -56 V before the 3.3K and -46 V at the pot.

From the pot are two paths, each to a 100K R followed by the 1.5K bias R.

I have 334 V at the 6L6 plate.

Schematic attached.

I would like to get the bias current down to 35 mA. Advice greatly welcomed.

Thank you, MarkO





Fender_pro_reverb_aa165_schem.pdf

[misterc57]

Looks like I have the wrong schematic. It is probably this one pictrued below

[misterc57]

And now I notice a different value R before the bias diode. I have 470R and the AA1069 shows 1.8K.

Seems I have a blending of components from different schematics.

[g1]

I think maybe someone modified that circuit to bias hotter. I would not assume those are stock value components.

[Enzo]

Unless I am having a senior moment, your 35ma at 334v yields about 11 watts at idle. Is that what your goal is? Unless that 35ma is for the pair.

And you 95ma, is that for EACH tube? Or is that for the pair? In which case that is 47.5ma per tube, and at 334v that is more like 15 watts Certainly not hot.

May I suggest: instead of trying to find a printed schematic that is just like yours, just draw up the schematic for what you have. The drawing will be similar to the factory drawing, but the parts will reflect YOUR amp.

[misterc57]

I am reading mA using a bias adapter that inserts into one 6L6 tube socket. It then displays the mA as mV on my meter.

So, now I am not clear if when reading 95 mA, is it for the one tube or both?

[The Dude]

If your bias probe inserts into a single tube socket, then the reading is for one tube only and 95mA is ridiculously high. Do both tubes give similar readings (to rule out a defective tube)? The schematic shows -44V at the output tubes matching potentiometer, so your -46 isn't really out of line according to the schematic. Has someone modified the circuit- maybe to make it a bias pot?

Edit: Looking at your picture in post 1, it doesn't look like the circuit is modified. Dead meter battery? Bad bias probe? Defective tube? Leaky PI coupling cap(s)? Open tubes matching pot? Check bias voltage right at the tube pins to be sure it's getting all the way to the sockets. Also, I'd just move that bias filter capacitor leg to the closer ground instead of extending the lead so far.

[Enzo]

If the adaptor is under one tube, then it is for that tube only

They yeah, 95ma is a bit too hot...

[g1]

335V B+ is very low, although it may be that the 95mA tube is pulling it down.
Remove the power tubes, check voltage at pins 5 and 3 of power tube sockets (power tubes removed).
Check that the bias cap connected directly to the pot has it's (+) end to ground.

This Bassman 100 schematic has the same bias circuit values as yours.

[misterc57]

Power tubes removed...

V7 pin 1 -46
V7 pin 3 475
V7 pin 4 474
V7 pin 5 -46
V7 pin 6 474


V8 pin 1 -46
V8 pin 3 475
V8 pin 4 474
V8 pin 5 -46
V8 pin 6 474


Power tubes inserted...

V7 350V plate at 93 mA
V8 344V plate at 82 mA


Tubes tested good on my tube tester. Tried a different 6L6 tube and had the same results.

Bias cap is properly oriented with + to ground

From Dude....
Dead meter battery? - No, other readings are working fine.
Bad bias probe? - I do not think so, considering plate V has over 100V drop.
Defective tube? - Probably not, see above.
Leaky PI coupling cap(s)? - Have not checked yet. I believe all coupling caps may be original.
Open tubes matching pot? - Not sure what you are asking????????
Check bias voltage right at the tube pins to be sure it's getting all the way to the sockets. - Done, see above
Also, I'd just move that bias filter capacitor leg to the closer ground instead of extending the lead so far. - Done

Thank you.

[Helmholtz]

Increase negative grid bias to maybe -50V. You don't want to run the tubes at 100% plate dissipation.
It's not good for the tubes and not good for the PT, which has to deliver almost 100% power all the time without a chance to cool down.

For best advice we need a drawing of your bias circuit.

[pdf64]

Maybe the amp’s oscillating. What happens to the voltages if the 12AT7 valve in V6 is removed?

[g1]

Increase negative grid bias to maybe -50V. You don't want to run the tubes at 100% plate dissipation.

This one does not have adjustable bias, just 'matching'. The description and photo matches the bias circuit of the bassman 100 (schem in post #9).
To adjust bias would require changing resistor from pot wiper to ground.

misterc57 , if pulling V6 tube makes no difference, does pin5 voltage remain -46 when power tubes are installed?
If so, I think the raw bias voltage is weak, as the bias voltage shown for bassman 100 is more like -70 into 3k3, -57 at pot.

[Jon Snell]

Is the output stage in Ultra Linear mode ... if it is look here;
No bias pot just balance pot.
Unplug V5 before checking the bias voltage. The trem level will change the bias voltage!

[Helmholtz]

This one does not have adjustable bias, just 'matching'. The description and photo matches the bias circuit of the bassman 100 (schem in post #9).
To adjust bias would require changing resistor from pot wiper to ground.

Whatever it takes - the amp shouldn't be left like that.
Plate dissipation might exceed 130% at medium output.