What's the best method to set the PI balance trim?

[heisthl]

I never see any ouptut change as far as amplitude or distortion on the scope when I adjust this trim pot with the output at maiximum clean - so far, depending on the build, I inject the audio generator into the pwr amp in point and adjust for minimum transformer squeal or I put audio generator into the input, turn up the clean and master volumes into major clipping and adjust for uniform crossover notches. These two methods both leave the trim pot at a completely different point, yet I can't hear the difference. What am I missing? What hz should be injected to adjust this pot?

[chris_sanford]

I never see any ouptut change as far as amplitude or distortion on the scope when I adjust this trim pot with the output at maiximum clean - so far, depending on the build, I inject the audio generator into the pwr amp in point and adjust for minimum transformer squeal or I put audio generator into the input, turn up the clean and master volumes into major clipping and adjust for uniform crossover notches. These two methods both leave the trim pot at a completely different point, yet I can't hear the difference. What am I missing? What hz should be injected to adjust this pot?

I've heard (through one of those friend of a friend of a friend deals... so take it with the appropriate grain of salt) that HAD uses a distortion analyzer for setting the pi trim. Now what is he adjusting for? Minimum thd? I have no idea.

chris

[Tonegeek]

Henry,
I asked this same question a while back. (I can't hear the difference either) Check this out:

https://tubeamparchive.com/viewtopic.ph ... highlight=

whit

[VacuumVoodoo]

Adjusting PI symmetry on its own is not enough. You need to work with bias balance adjustment first.

Without any signal adjust bias balance for minimum hum output. You can do this by ear or use a scope. This is to compensate for unequal DC resistance in OT primary windings.

With sine signal adjust level so you can observe onset of clipping on the output signal. It will most probably not be symmetrical. Adjust PI symmetry so that both positive and negative halves of the sine begin to clip equally.

This procedure will give you power amp that produces odd harmonics.You can skew the PI symmetry to introduce some even harmonics.

This is audible first when you play att levels that will drive the power amp into cliiping. Makes no or little difference at bedroom levels.

[heisthl]

With sine signal adjust level so you can observe onset of clipping on the output signal. It will most probably not be symmetrical. Adjust PI symmetry so that both positive and negative halves of the sine begin to clip equally.

That is my frustration at a 1 to 2k test frequency using 91k and 110K plate resistors I can never get one peak to clip even slightly before the other. Is there a favorite test frequency you use for this adjustment?

This is audible first when you play att levels that will drive the power amp into cliiping. Makes no or little difference at bedroom levels.

This might explain why I haven't noticed it sonically- I generally don't test over more than moderate volume.

['67_Plexi]

Dump the trimmer and use a decent set of calibrated ears.

['67_Plexi]

Seriously now..

Using a reasonably, but not necessarily perfectly balanced AX7 without a trimmer is totally fine.

Reducing the use of trimmers throughout an amp is sensible anyway because they are basically the crapiest form of resistors you can find (unless you spend big $$$).
You spend all this time and effort putting the correct material resistors in the correct places, then you place a turd of a trimmer right in there, this is especially true for any trimmers that are directly in the signal chain.

Forget trying to use use an oscilloscope or distortion meter or whatever other pieces of instrumentation you deem potentially effective and just have a supply of good tubes, keep trying them and keep listening. It's a way more effective use of your time.

Alan.

[heisthl]

Henry,
I asked this same question a while back. (I can't hear the difference either) Check this out:

https://tubeamparchive.com/viewtopic.ph ... highlight=

whit

I had forgotten that thread - I think I'll try this method:

Method Two - Inject signal at loop return. Display both PI outputs on the scope and use the scopes ADD function. I then adjust for MINIMUM signal.

I find that sonically, I prefer method two (at least this week). It seems to add more harmonic complexity with the overdrive enabled.

If i don't notice anything better afterwords I'll dust off my old Heathkit distortion analyzer and see what it tells me.

[BobW]

Is there a favorite test frequency you use for this adjustment?

I used 600 and 1k. As you know, The 1k can get pretty annoying w/o a dummy load, but for this particular test I used a speaker load. You can see a slight difference if you adjust the tone controls for a good sinewave output.
Although Gary's method to ADD the signals seems to make good sense, and would probably be easier to see even with your tone controls set to your typical settings.

[skyboltone]

Seems to me that Brandon had a post long past wherein he described a balancing menthod and then skewed the trimmer anyway because he liked the sound better unbalanced. Maybe him, maybe my imagination.

Just seems like a VTVM with equal output from both sides ought to do it but I'm not there yet so we'll see.

[makrisp]

Dump the trimmer and use a decent set of calibrated ears.

Alan,

Your dead on!