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Re: what did u do with X100B Re: why poeple alway talking about dynaco ST-70 ?

Cisco,

I understood you OK, maybe I just didn't address your concern completely.

Firsst, aside from the need for closely matched tubes, the system is really a good one. Many folks who have heard the same amp set up to run both ways prefer the common setup.

It is good for the manufacturers because they use fewer components, esp. bias pots which are more expnedive than resistors and caps.

But it does mean that it is not possible to get fine bias adjustment of all 4 tubes. Guess what? Find bias adjusment isn't really so important unless they are grossly different. There is no adcustment whatsoever for AC balance on most commercial amps, and that is more important!!! Problem? Well, DC bias is easy to check with just a voltmeter. It takes some gear to set AC balance. I wouldn't worry about a 2V spread acroiss all 4 tubes. Put the pairs that match closest in the same channel and you'll be just fine.

OK, just say you can't stand to have them out of DC balance that much. Put a 50 Ohm or smaller WW pot in series in one cathode. Set the pot to zero. Now, put the tube that measures highest bias in that socket. Put the tube that measures lowest in the other socket in that channel. Now lower the bias via way of the pot until the two match, no matter how low that is. Now adjust the common pot so that the two pairs are averaged above and below the nominal.

It is a lot of work for little benefit, but you may feel better. And, you've done it without having 4 pots to muck around with.

cheers/Fader
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