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AB165 bias conversion

MarcD

Member
Messages
23
Apologies if this has been done to death, but the threads I found were either too old to resurrect or had drifter far afield...

I have a 1966 Bassman AB165 circuit and would like to convert the original "bias balance" pot (that only adjusts one output tube) to a true bias adjust. I've found a few sites that give general suggestions to convert to earlier Fender wiring and/or not specific to the Bassman.

Question: it seems that if the wire to pin 1 (then on to pin 5 via the 1.5 resistor) was removed (and insulated) from the non-adjustable tube socket, then a jumper added to connect both output tube pins 1, that would give equal bias voltage to both tubes with no further need of modification.

I must be missing something glaringly obvious...it's too simple!

Thoughts?
 

Jeff Gehring

Gold Supporting Member
Messages
5,898
Pin 5, the ultimate destination of the bias voltage (via the 1.5k and pin 1) is the control grid of the power tube. As such there are TWO different voltages of note impressed on the control grid. One is the negative DC bias voltage, used to control the amount of conduction through the tube. If the story stopped there, you could do what you say, and wire both control grids together--both tubes would then necessarily receive the exact same bias voltage.

BUT the other signal that is impressed on the control grid (in fact the whole "raisin de eater--scuse my French" for the power tubes' even being there), is AUDIO. The audio signal on pin 1 of one power tube is NOT the same as the audio signal impressed on the grid of the other power tube. In fact, one is the inverse of the other, the two signals are identical but of opposite polarity.

So if you did what you said and tied both grids together, you would have very little output at all, since the two power tube outputs would nearly cancel each other. If you've got two guys working a two man saw, and both guys push (or pull) at the same time, not much sawdust gets made...
 
Messages
4,887
For me, the best and easiest thing to do with these bias balancing circuits is to add a trim pot prior to that balancing pot so as to be able to adjust the bias voltage that the balancing circuit sees and also use the balancing act to match unmatched tubes or to slightly mismatch matched tubes.
 

MarcD

Member
Messages
23
Ah. So the 220k resistors in each leg of the bias voltage keeps the two sides push-pull signal from the 12AT7 phase inverter from canceling each other out as the negative bias enters, correct?

I just compared the AB165 and AA864 schematics and layouts relative to the bias circuit and that's how it appears.

The discussions I've seen regarding restoring the AB165 bias circuit to the AA864 suddenly makes sense!

Thanks, Jeff and Steppin' Wolfe!
 

robrob

Member
Messages
408
What Steppin' Wolfe says is your best option. You get adjustable bias and balance too.
 

Jeff Gehring

Gold Supporting Member
Messages
5,898
So the 220k resistors in each leg of the bias voltage keeps the two sides push-pull signal from the 12AT7 phase inverter from canceling each other out as the negative bias enters, correct?
Yes, the 220k resistors provide isolation for the two grid feeds, both from each other, and also isolating the audio from the low AC impedance of the bias supply filter cap.
 

scott1568

Member
Messages
408
For me, the best and easiest thing to do with these bias balancing circuits is to add a trim pot prior to that balancing pot so as to be able to adjust the bias voltage that the balancing circuit sees and also use the balancing act to match unmatched tubes or to slightly mismatch matched tubes.
That's what I did (my tech anyway..)
 

sickboy79

Member
Messages
12,913
I have two BF Bassman heads and both have been converted to the AA864 bias section. Easy mod for a good tech and well worth it, IMO.
 




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