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Re: Citation II AC balance

Posted by Jim McShane (D) on April 6, 2007 at 19:01:07

In Reply to: Citation II AC balance posted by PJADDEO on April 6, 2007 at 17:30:33:

The problem is simply the inexpensive meter in the Cit II can't respond to the waveform it sees at the tube cathodes. When you turn the switch to one of the AC Balance setting positions you are connecting one channel's output tube cathodes to the meter terminals. The waveform the meter sees is not at all sine-wave like, so the meter does not respond well. The waveform definitely changes as you move the AC balance pots, but the meter can't respond.

You have two choices. First, the AC balance adjustment is certainly non-critical. You won't blow anything up if it's adjusted wrong. You can set it to the midpoint of it's rotation then turn it clockwise about 20 degrees and you'll be darn near right on in most amps.

Or second, if you have a true RMS AC voltmeter you can use it to set the balance. Make up a couple test lead adapters with .1 uf 100 volt or higher capacitors in line to block the DC, and hook the adapters between the meter terminals and the leads of a true RMS voltmeter. Adjust for minimum AC voltage.

Better yet, a dual channel scope with two probes can be hooked to the meter terminals. Set up your scope with both channels identical, and with the inputs grounded adjust the traces on the scope so they overlap as precisely as you can. Then set the inputs to AC coupling. Flip the input selector to dual so the traces from both probes can be added together on the screen, then adjust for the most symmetrical waveform you can get.

With the stock meter, what you are seeing is normal.

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