Bluesmaster PI trimmer

[greiswig]

This is how I do it. Got the method from David Manley of Manley Labs/ Vacuum Tube Logic.

The trimpot is there for AC Balance of the output section. Someone please correct me if there are errors:

How to set AC balance using the trimmer between the PI plates:
Hook up a suitable dummy load to the amp (or get out your earplugs!).
Apply a sine wave signal to the amplifier input that gives a reading of about 20 volts AC measured at the point where the bias voltage is applied to the output tube grids. I like using a 400Hz tone, but others may do. Some people use pink noise. I stay away from higher tones, as they may have more harmonic content, and 400Hz is within the guitar's range.
Adjust the trimmer until the voltage is equal on both output tube grids. I like 20volts or a little less because my Fluke gives 2 decimal points up to 20volts.
You have just set the AC balance.
I have heard from people who hook up the output of the transformer to a scope and adjust the balance until the positive and negative halves of the waveform are equal in voltage. That method looks at the PI, output tubes, and transformer as a complete system. Overkill, IMHO, but some think AC balance controls are overkill.
This insures that the output tubes are both being driven by the same voltage (symmetrical drive).
D

D

Interesting and logical method. I tried it. Even more interestingly, the voltage at the lower plate resistor on the PI fell below that of its counterpart when I did this, although only by 2volts. The amp definitely did sound more "sterile" than before when adjusted this way.

[tele_player]

My D'Lite doesn't have a PI balance trimmer, so I can't check this... but balancing the ac this way, let's say at 20vac, it might not be balanced at other signal levels. I'd check it at a few signal levels to verify.

[greiswig]

My D'Lite doesn't have a PI balance trimmer, so I can't check this... but balancing the ac this way, let's say at 20vac, it might not be balanced at other signal levels. I'd check it at a few signal levels to verify.

What I found was that if you balanced it at one, it basically was balanced at all the levels that I measured down to about a volt. I didn't measure below that.

[markmalin]

Nope. Not at all. Only a scope.

Just set to a 12-13K difference as a baseline. Then adjust trimmer a little each way while hitting the low E string. Set to taste. I use a balanced PI tube since they will give me repeatable results and are screened better.

So do you look at the plate voltage to see how balanced the PI is?

You mentioned striking the E string and setting the trimmer "to taste". Do you mean set it to where it sounds best to you? If so, where does the scope come in?

Mark.

[Fischerman]

I think what he meant was; if you don't have a scope then use this method. And yes...set it to whatever sounds best.

[markmalin]

I installed the Bluesmaster PI trimmer and 100k/110k resistors. I have a scope but no signal generator or dummy load so I tried the "pluck the low E string" way, but I have to admit, I really don't hear any difference. Yes, after 32 years of playing guitar there is some hearing loss , but I notice no change in tone or responsiveness. What exactly should I be listening for?

Maybe I should borrow a signal generator and a big wirewound resistor and do it the scientific way....?

Mark.

[heisthl]

Try playing a first position "G" chord and listen for background overtones/harmonics. Strum at the same level for each minute change of the trimpot. They will be slightly more pronounced at the sweet spot. You may not be close enough voltage wise to be getting it - make sure one extreme of the trim adjustment gets to 4 or 5 volts difference and goes up from there. Different PI tubes will change the voltage difference, or worst case you may have to change a plate resistor to a different value.

[markmalin]

Try playing a first position "G" chord and listen for background overtones/harmonics. Strum at the same level for each minute change of the trimpot. They will be slightly more pronounced at the sweet spot. You may not be close enough voltage wise to be getting it - make sure one extreme of the trim adjustment gets to 4 or 5 volts difference and goes up from there. Different PI tubes will change the voltage difference, or worst case you may have to change a plate resistor to a different value.

Thanks for the explanation. I was thinking I wanted to get rid of the overtones, so this is a big help

Mark.

[markmalin]

...and listen for background overtones/harmonics...

Quick question. This is in the clean setting, right? Not OD?

[heisthl]

Clean

[ampgeek]

Which trimmer (P/N, spec, manuf., etc..) is preferred in the PI balancing application?

I scanned through the Phier offerings at Mouser but didn't see anything rated for the 300V range. Maybe I missed it?

TIA,
Dave O.

[heisthl]

There is less than 25 volts across this trimmer

[ampgeek]

Thanks H!

Yes. I realize that the voltage drop is in the 10'ish VDC/leg range (assuming 10 K Ohm end to end and set at mid point) and the power dissipation roughs out to ~0.04W total.

However, the Piher datasheet for the 6mm carbon pots states a 100VDC maximum. I sorta-kinda *ass*umed that meant absolute voltage. Like the way that a cap has a max voltage rating. Granted, totally different item but I thought that the rating "system" was the same.

Since the data sheet provides a max allowable power dissipation, one might think that the mere mention of a max allowable voltage has another meaning.

But....I am a lowly chemical engineer who still tries to look at all of this like fluids flowing through pipes, valves and filters!!

What do you typically use in this spot?

Thanks again,
Dave O.

[heisthl]

http://www.mouser.com/Search/ProductDet ... -PT10V-10K

[ampgeek]

Mucho gracias!!
Dave O.