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previous: Peter S Re: Drift in matching of O/P valves? -- 8/7/2000 2:42 PM View Thread

Re: Drift in matching and balance fix?

8/7/2000 3:41 PM
Re: Drift in matching and balance fix?
Another way to solve the imbalance of the output is to add a second pot in the bias circuit, not to provide two individual levels for the tubes, but to trim one up as the other is decreased. Leave the level control intact, and feed it to the wiper of a pot whose two legs go to the grid bias resistors. This is a balance control and is fairly common in hifi amps.  
How to use it:  
This is what I do, and I wonder what others think of this, I know its crude and I would like to know if there is problem I haven't thought of.  
I "balance" the output by setting the symmetry pot so that the height of the waveform of the output (feeding the sine generator into the speaker and set at the level I play at) is the same above and below the no-signal line. I figure this produces matching, (or mismatching, if that what you prefer,) that takes into account the tube effeciencies, the windings of the OPT, load resistor changes of the PI, etc.  
In the Williamson Amp, this sort of arrangment is used with the grid resistors on a balance pot to ground, thereby effecting balance adjustment in a cathode biased output stage.  
I've had success with a 50K lin pot and 75 K resistors.  
If the tubes won't balance out within the extreme of the pot, try reversing the tubes. If they still won't balance, better put the tubes on the tester.  
The only way I can think of this really going south is if you were to dial in so much more from one tube that you start to run some appreciable current through one winding of the OPT and not the other. The use of a 50 K pot keeps you from that extreme, in my experience.  
I know that the better way to set balance is with a distortion meter, but they're expensive and I don't have one.  

R.G. Bias/balance is indeed one way to d... -- 8/7/2000 4:08 PM