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This statement was made by someone on headfi. Is this (at least somewhat) correct. Why or why not?? Thanks, Tweaker
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This is very dependent on the amp in question.
With PP amps it is sometimes nice to have both fixed (adjustable) bias and balance pots. In PP and SET amps paralleled tubes per channel can be problematic with either type of bias system, if one starts to go before the other in a single channel.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
You want tubes to be dynamically matched in a push pull pair. Just because they bias the same, does not mean they conduct the same with signal. Mutual conductance testing is just as important.
Chip, Can you explain what dynamically matched means? Thanks,Tweaker
Emission tests measure the ability of the cathode to emit electrons. Transconductance measures the gain of the tube, more specifically dVg/dVp, or the ratio of the effectiveness of grid voltage at controlling the plate current to the effectiveness of the plate voltage at controlling the plate current under conditions of some reasonable bias conditions and constant plate current.
Emission is useful for testing the life remaining in the tube, and transconductance is useful for gain matching. In an amp that has an "ac balance" control in the power amp transconductance matching is unimportant. If dc balance is available emission matching is unimportant except to test "health" of the tube.
When you buy matched tubes, be careful to ask how they were matched and be aware of what kind of matching is important for your amp. There are lots of claims of matched tubes, and sometimes that can mean as little as same manufacture and date code!
Most tube testers measure emission OR transconductance, but not both. A tube curve tracer will give you both sets of information.
Most seem to match for gm and or Ip.
are really required for amps that don't offer output tube specific bias controls, be they manual or automatic. It's nice for them to be close, but not critical when you have individual bias control.
Many Audio Research amps, for example, use a single bias point for either a pair or a quad of tubes where matching is critical.
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