pot in the P.I. plates.

chocopower · Post by **chocopower** » Wed Nov 21, 2007 9:37 am

Hi...

what´s the reason for the 20k trimpot in serie with the 82k resistor in the P.I.?

My fender clon has the usual 100k/82k pair.

Thanks

gearhead · Post by **gearhead** » Wed Nov 21, 2007 12:39 pm

Ah, Dumble schematic.

chocopower · Post by **chocopower** » Wed Nov 21, 2007 1:53 pm

opppssss...

this tread shuold be in "dumble discussion" board.

pease, move it...

this is the layout i told:

http://ampgarage.com/forum/viewtopic.php?t=1473

Bob-I · Post by **Bob-I** » Wed Nov 21, 2007 4:34 pm

I just noticed this here...

The trimmer is to balance the PI, which makes a difference in the harmonic content (to my ears). The more common values used are 91K/110K with a 10K trimmer between them. Set the trimmer so that the voltage is equal on both sides of the plates, then adjust by ear. Every PI tube is slightly different.

Post by **HeeBGB** » Wed Nov 21, 2007 4:44 pm

I have done my first MODly deed at the Ampgarage! Thread moved to the correct discussion area.

Bob-I · Post by **Bob-I** » Wed Nov 21, 2007 4:55 pm

HeeBGB wrote:I have done my first MODly deed at the Ampgarage! Thread moved to the correct discussion area.

You da MAN....

See you Sunday

heisthl · Post by **heisthl** » Wed Nov 21, 2007 5:10 pm

Bob-I wrote:I just noticed this here...

The trimmer is to balance the PI, which makes a difference in the harmonic content (to my ears). The more common values used are 91K/110K with a 10K trimmer between them. Set the trimmer so that the voltage is equal on both sides of the plates, then adjust by ear. Every PI tube is slightly different.

My experience has led me to use 100k and 110k plates when I use a 10K trimmer. I think having a 5 volt difference as a starting point is critical and the "Bliss Point" is somewhere between 5 and 10 volts apart (using 12AX7LPS) as a splitter tube.
Make sure when you get tubes for a new build that the tube you choose for the PI is of the long plate variety.

tomrasdf · Post by **tomrasdf** » Wed Nov 21, 2007 5:53 pm

How do you set the amp when you adjust by ear? Clean and cranked? Do you want the PI to be overdriving when you adjust it? I ask because in the past my ears have failed to recognize the difference with adjustments. I figure if I set the amp so that the odds are most heavily tilted in my favor, I might be enlightened.

Bob-I · Post by **Bob-I** » Wed Nov 21, 2007 6:05 pm

tomrasdf wrote:How do you set the amp when you adjust by ear? Clean and cranked? Do you want the PI to be overdriving when you adjust it? I ask because in the past my ears have failed to recognize the difference with adjustments. I figure if I set the amp so that the odds are most heavily tilted in my favor, I might be enlightened.

I set it while in OD mode but not cranked. IMHO these amps don't need to be cranked to get a great OD tone. I also find that I hear the difference better if I put some distance between me and the speaker. It's quite subtle so if you can't hear it, go for the 5V difference.

dogears · Post by **dogears** » Wed Nov 21, 2007 6:07 pm

DC balance is not very significant IMO. You have the output tubes and tranny upstream. They have a measureable effect to. I'd measure AC balance with a scope.... What volts are on the tube is not of much consequence.

heisthl · Post by **heisthl** » Wed Nov 21, 2007 9:43 pm

This is assuming a matched long plate PI tube. I set it clean at a 1/3 of total volume before distortion (usually preamp on 10 o'clock master on 9 o'clock with tones midway). And on most builds I can hear more or less "fullness" at the 10k trim extremes. I play a chord like a robot and listen to the decay. I make a minute adjustment and try and play the same chord at the same pick position and attack. Somewhere along the way of minute changes I hear more overtones that appeal to me. Generally this has been in the 5 to 10 volts difference range. But there are definitely exceptions I've had amps with 20 volt differences as the "sweetspot". I've tried doing this with a scope for least crossover notch and I've tried using a dummy load and adjusting for minimum OT squeal but the audible method seems to give me an adjustment point that sounds better.

BobW · Post by **BobW** » Wed Nov 21, 2007 10:04 pm

I agree, setting the "sweetspot" using a scope is not the best approach. Just Trust your ears.

I don't measure the voltage difference either, but it's interesting that you've found generally a 5-10 volt difference is in the zone. Thanks Henry.

JimiB · Post by **JimiB** » Wed Nov 21, 2007 10:39 pm

is it considered "bad form" to put metal film resistors in here?

Normster · Post by **Normster** » Wed Nov 21, 2007 10:40 pm

Hopefully Gary will chime in here. If I recall correctly, he sets the trimmer for equal onset of clipping using a scope. I've never done it so I could have the whole concept wrong. (I don't use trimmers in my builds.)

dogears · Post by **dogears** » Wed Nov 21, 2007 11:04 pm

Correct, if using a scope, the only way is to measure the output after the tubes and tranny. Merely setting DC balance on the PI is meaningless.

Andy Fuchs originally showed me how this is done with a dummy load.

The Amp Garage

pot in the P.I. plates.

pot in the P.I. plates.

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Re: pot in the P.I. plates.