I’ve read several of the threads on here, and understand the different procedures…
Good - Balancing AC signal at PI plates/power tube grids, to compensate for PI tube imbalance
Better - Balancing DCmV at power tube cathodes, to compensate for any power tube mismatch
Best - Measuring speaker output for THD/minimal 2nd harmonic, adjusting trimmer accordingly
My first question, is the general feeling that each procedure takes the desired result up a notch? Does each “level” make the previous approach more or less irrelevant?
Also, i assume everyone is keeping their sine wave clean thru output… is it best to get maximum output before clipping for this purpose, or just a healthy level overall? I’m injecting at PI entrance FYI
(I know the SVT method that HAD recommended states 40hz sine, 20Vrms at output… I’ve been playing around with 400hz at about 23VAC at speaker out before clipping, as it displays best on my analyzer for THD/2nd harmonic method)
PI trimmer question
Moderators: pompeiisneaks, Colossal
Re: PI trimmer question
This is the one I use. (along with listening) Quick and easy, Others can of course recommend their preferred method
https://tubeamparchive.com/viewtopic.ph ... 78#p216778
Tony
https://tubeamparchive.com/viewtopic.ph ... 78#p216778
Tony
" The psychics on my bench is the same as Dumble'"
Re: PI trimmer question
First of all, the ears decide what sounds good, no matter what imbalances there might be
Good-Better-Best is not describing what happens as you have at least three major sources of imbalance that may need independent adjustment:
- PI imbalance
adjusting a trimmer to balance the AC output makes sense, in simpler amps achieved (or tried) by using 100k / 82k plate resistors, a trimmer is more accurate
what sounds better is a different question (perfect balance or a certain imbalance)
- Balancing DCmV at power tube cathodes
this is not better than PI balancing, it is an additional source of potential imbalance. So both should be considered. I recently bought a quad of (expensive) "matched" KT-88 tubes. Quiescent currents were so badly off, it was a shame. I needed to bias the tubes individually
- Output transformer imbalance
this is nothing you can address by any trimming device. The OT is as it is. live with it or get one which is balanced better. 100% symmetry is not achievable
Measuring speaker output THD won't necessarily give you any hint where to change settings. It is a trial and error method. To get an idea where to make changes, you need to follow steps 1 and 2 in this order anyway
Then I usually hook up a scope and check the input signal (sine) vs the signals after the PI, the power tubes and at the speaker having them as clean as possible. Be aware that a DMM will have problems with low frequencies to read accurate results.
Other imbalances may occur due to component tolerances...
Good-Better-Best is not describing what happens as you have at least three major sources of imbalance that may need independent adjustment:
- PI imbalance
adjusting a trimmer to balance the AC output makes sense, in simpler amps achieved (or tried) by using 100k / 82k plate resistors, a trimmer is more accurate
what sounds better is a different question (perfect balance or a certain imbalance)
- Balancing DCmV at power tube cathodes
this is not better than PI balancing, it is an additional source of potential imbalance. So both should be considered. I recently bought a quad of (expensive) "matched" KT-88 tubes. Quiescent currents were so badly off, it was a shame. I needed to bias the tubes individually
- Output transformer imbalance
this is nothing you can address by any trimming device. The OT is as it is. live with it or get one which is balanced better. 100% symmetry is not achievable
Measuring speaker output THD won't necessarily give you any hint where to change settings. It is a trial and error method. To get an idea where to make changes, you need to follow steps 1 and 2 in this order anyway
Then I usually hook up a scope and check the input signal (sine) vs the signals after the PI, the power tubes and at the speaker having them as clean as possible. Be aware that a DMM will have problems with low frequencies to read accurate results.
Other imbalances may occur due to component tolerances...