Steve Deckert
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The tubes are biased with a resistor on the cathode that connects to the meter which is connected to ground. So the meters are showing the actual current of the installed tube.
Some of the earlier SE34I.3's (white ones) were wired with a single cathode resistor serving both output tubes. That means the cathodes were connected to the meters then to the resistor which was connected to ground. In this scenario, if a tube on one channel changes, the other channel changes in the opposite direction causing the actual difference between the two tubes to double. This makes matching the tubes more important than ever. Of course everything that goes out of here is precision matched, so I didn't notice how pronounced this effect was until someone sent an amp in for upgrades that had the customer's own (mismatched) tubes with it.
To reduce this effect, I started making the amps with a separate bias resistor for each tube so that what one tube does has no effect on the other tube. Most white ones were this way, all black ones are this way. To find out if your amp is this way or the "old" way, simply remove one of the power tubes and see if the meter on the other tube changes. It should remain where it was to within a couple mA. If it jumps way up or down then you're welcome to send the amp to us for the upgrade.
Against my original findings when the amp was designed, I found the new way also sounds better. Reason for the change in perception was the coupling caps are better now and made the change easier to hear.
-Steve
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