Adding Balance and Bias adjustments

multichamp12

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Hello, I am working on a pair of Stromberg AP-50 amplifiers, and they have no bias or balance adjustments, which I would like to implement. I have modified the original schematic and posted it here. Im seeking advice on what value potentiometers and resistors to use, and if I am heading in the right direction.

For the balance setup, I was planning to use a 100k pot, with two 100k resistors to ground, and changing the value of R21+R22 from 100k to 47k. Will this work okay?

For the bias I am not sure how to do it correctly, I did make a revision on the schematic that includes a bias pot and resistor to ground, but dont know if this is correct, or what values I should use.

The revisions on the modified schematic are in red.

Modified schematic: https://s33.postimg.cc/b6d6akd5b/Stromberg_AP-50_Schematic_Modification_2.jpg

Original schematic: https://s33.postimg.cc/y93p9fotr/st50.jpg

Thanks!
 
Crispy has given a good circuit to copy. You might also want to check the EICO ST70 which is also fixed bias and the 7591 tube is very similar to the 6l6. That circuit is also pretty much what i use in a PP amp.
In your schematic the 100k pots are probably too big and the usual value for the balance pot is around 25k. I think the adjustment pot is 50k
 
I used a 10k linear pot in my Fairchild EL34, with 47k to grids and 2 x 100K to ground off the pot.
Worked like a charm.
 
Whoops I meant 10K pot for the balance, that has been corrected.

Thanks for the feedback! How does this setup look for the Bias?

Stromberg_AP-50_Schematic_Modification_3.jpg
 
keep the grid resistors at 100K, otherwise its going to appear to the phase inverter as a much lower impedance load than it would have originally.
 
Should be OK, but you may need to bump the value on the 3.3K if it won't drop enough, or increase the value on your added 15K fixed resistor if it won't go high enough.

This is actually back biased, not really fixed bias. Any changes in tubes will mess with how much current flows, which will affect bias voltage. Just keep that in mind if you change tubes. Big tubes will change things a lot more than small tubes, but it will all have at least some effect.
 
Okay thanks, I figured the values might have to change.

When you say change tubes, you mean using something other than a 6L6? Or if I replace the 6L6 tubes just make sure to balance and bias again?
 
Both. This generates voltage from the total of all tubes drawing current, even the small tubes. Honestly the small ones are not liable to change things enough to matter, but if you delete one for whatever reason it may shift the bias slightly. Definitely re-check it if you change an output tube though.
 
Understood, thanks for your help!
If you check the tube data for the 6l6,
(one should always check the tube data sheets if modifying a circuit,)
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf
this is for the GE 6l6gc, in the section on 'Maximum Ratings' , grid number 1 circuit resistance...fixed bias....... is 0.1M ohm
So, all the resistances added up on the grid of the 6l6 should not or can not exceed .1M. Since the fixed bias circuit is also part of the G1 its values also are part of what the 6l6 grid "sees".
 
If I do the math right, it ends up right near 108K to DC ground, and about 105K to AC ground. The cap on the bias supply is near enough to zero ohms impedance at audio frequencies. In stock form it would have been about 103K to DC ground and 100K to AC ground. Honestly thats probably not enough to bother anything, especially considering typical component tolerances of the day. A 92K grid resistor would put you right to datasheet spec if you wanted to be precise though.
 
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