For anyone wanting to set the PI Trim pot

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boldaslove6789
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For anyone wanting to set the PI Trim pot

Post by boldaslove6789 »

Thought I'd post my method for setting the PI trimmer.

(Note, a 12ax7 with matched triode's works best for the PI)

Please be safe when doing this, there is dangerous Lethal Voltages inside a tube amp. Only do this unless you have knowlegde of how tube amps work and the methods of being safe when inside an amp when its ON.

Thanx and be Safe!

Here ya go:

You'll need at least a digital Multimeter to measure DC Volts .

Here's my tried and true method for setting the PI trimmer for Max Harmonic Feedback.

Set your rig up how you normally use it (effects in loop etc.)

Make sure the amp is hooked up to a Load (your Speaker cab), and the Ohm setting is right for the speakers you're using.

Turn the amp ON, Standby ON.

Put the Black Multimeter probe (ground) into the Black Banana Bias Test socket under the chassis, or if you don't have a Bias test socket, use an Alligator clip and clip it right on the Chassis so that the Multimeter is grounded properly.

Set your Meter to Volts (usually the highest Volt setting on your meter).

Use the Following settings on your amp:

Preamp Vol: 12:00
Overdrive Level (Gain): 11:00
Overdrive Master Vol (or Ratio): 12:00
Clean Master Vol: 11:00
Presence: 12:00

If you're using a Dumbleator:

Send:12:00
Return: 12:00
(Bright switches Off)

Turn the amps Preamp Boost ON, and the Overdrive ON.

The amp should be quite loud.

There should be 6 trim pots total inside your amp if it is HRM, and 3 if its a NON-HRM. The PI trimmer is the trim pot that is in the center of the amp, on the preamp board, towards the end of the board closest to the Power amps Filter caps.

Using the RED Multimeter Probe, Measure each leg of the trim pot. (the side of the trim pot with the 2 legs pointing towards the back of the amp)

Set the trimmer so that there is 7v Volts difference on each side, the trimmer leg towards the rightmost side of the board should have more Voltage of the two.

The amp should be quite loud.

Then use a Les Paul or Tele on the bridge Pickup setting and hold single notes that normally feedback. Face the guitars pickup to the speaker cab.

Adjust the trimmer till you get the maximum Harmonic Feedback on single notes & Pwr chords.

Take a Marker and mark the setting you like the best.
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Last edited by boldaslove6789 on Wed May 09, 2012 6:18 pm, edited 6 times in total.
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alvarezh
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Re: For anyone wanting to set the PI Trim pot

Post by alvarezh »

Good morning Greg, please clarify:

"Set the trimmer so that there is 7v mA difference on each side. The Power amp being the predominant side (more mV of the two)"

You want us to read volts, milli-volts or milli-amps? all three units are called for, it's confusing (at least to me).

Also, what do you mean by "the Power amp being the predominant side"?

Anyhow, thank you for the procedure and thanks in advance for the clarification.

All the best.
Horacio

Play in tune and B#!
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boldaslove6789
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Re: For anyone wanting to set the PI Trim pot

Post by boldaslove6789 »

Sorry, I corrected it.
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alvarezh
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Re: For anyone wanting to set the PI Trim pot

Post by alvarezh »

:wink:

Cheers!
Horacio

Play in tune and B#!
vibratoking
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Re: For anyone wanting to set the PI Trim pot

Post by vibratoking »

There have been quite a few long threads about PI balancing. I think making this one method a sticky may be misleading to many. There are quite a few methods to accomplish PI balance. Many of them are much more scientific than the one posted. I think you should include all the methods if you are going to make a sticky on this topic.
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boldaslove6789
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Re: For anyone wanting to set the PI Trim pot

Post by boldaslove6789 »

Sure, feel free to post your prefered method, above is mine. Its not defitive which method works best.
bluesfendermanblues
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Re: For anyone wanting to set the PI Trim pot

Post by bluesfendermanblues »

I like it simpler.....:-)

I use both the technical/analytic approach + hereafter fine tuning by ear.

1) Technical/analytic
a) Put a dummy on 'speaker out'
b) put a 1000 hz tone on power amp input (the 'return' jack)
c) measure AC at PI out (after 100n or 22n caps)/PWR tube input grids
d) adjust PI trimmer to the same voltage at each cap (with 1v ac in, I get appx 2 x 20V AC after the PI).
e) make a small dot on the trimmer with a sharpie pen.

Balancing the PI trimmer perfectly in regards to measured AC voltage, will only take you part of the way to 'feedback nirvana', since output tubes are never fully matched/balanced and neither are OT's. Therefore,
you need to fine tune by ear.

2) Fine tune by ear
I use my guitar at a 'bright' setting, just like boldaslove and use very, very small increments around the dot on the trimmer to get max high end - its very easy to finde the sweet spot, once you have a base (the small dot on the trimmer) to work from.

I find that above mentioned approach takes the voodoo out of PI adjusting.

BTW DC measurements are not relevant for setting the PI! :wink:
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erwin_ve
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Re: For anyone wanting to set the PI Trim pot

Post by erwin_ve »

bluesfendermanblues wrote:I like it simpler.....:-)

I use both the technical/analytic approach + hereafter fine tuning by ear.

1) Technical/analytic
a) Put a dummy on 'speaker out'
b) put a 1000 hz tone on power amp input (the 'return' jack)
c) measure AC at PI out (after 100n or 22n caps)/PWR tube input grids
d) adjust PI trimmer to the same voltage at each cap (with 1v ac in, I get appx 2 x 20V AC after the PI).
e) make a small dot on the trimmer with a sharpie pen.

Balancing the PI trimmer perfectly in regards to measured AC voltage, will only take you part of the way to 'feedback nirvana', since output tubes are never fully matched/balanced and neither are OT's. Therefore,
you need to fine tune by ear.

2) Fine tune by ear
I use my guitar at a 'bright' setting, just like boldaslove and use very, very small increments around the dot on the trimmer to get max high end - its very easy to finde the sweet spot, once you have a base (the small dot on the trimmer) to work from.

I find that above mentioned approach takes the voodoo out of PI adjusting.

BTW DC measurements are not relevant for setting the PI! :wink:
+1 more or less the same way I do. De-voodoo-ing :wink:
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dobbhill
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Re: For anyone wanting to set the PI Trim pot

Post by dobbhill »

A suggestion:

A ten turn pot comes in really handy for obsessive types....

D
There are no stupid questions, just stupid people.......
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Structo
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Re: For anyone wanting to set the PI Trim pot

Post by Structo »

Good stuff.

As a safety measure, it's a good idea to use a non-conducting adjustment tool.

You can buy plastic ones or make your own.

Not that anybody else has done it but it can be quite a surprise to slip off while adjusting voltages.
Tom

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heisthl
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Re: For anyone wanting to set the PI Trim pot

Post by heisthl »

Another method: set the trimmer for a 6.5 volt difference (I like the input side as the higher plate). Set the amp clean (no boost , no mid boost, no overdrive, no bright) with all the appropriate controls at noon. Use a neutral sounding guitar on both pickups with it's volume(s) and tone(s) on 10. Play these single notes on the 3rd,4th,5th frets of the B string - a D, E flat E. Let each ring out before playing the next one (no finger vibrato please). In an ideal world they should all bloom equally. If the E blooms but not the D move the trimmer to make the input side lower by a half a volt. Fine tune to taste by ear. Most of the time the 6.5V is near perfect, have seen some amps do this better at a 3 volt difference but it's less common in my experience.
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ayan
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Re: For anyone wanting to set the PI Trim pot

Post by ayan »

After having tried the "Ampeg" method on many amps with great results, it has become my favorite method for setting the PI trimmer. You should have 1-ohm cathode resistors at the power tubes, otherwise this method will not readily work. You will need a signal generator, a dummy load, and a digital multimeter. This is my adaptation of it.

1. Tie together the cathodes of the power tubes that are in phase with each other (i.e., the tubes that have their plates connected to the same B+ wire coming off the OT's primary).

2. Connect a digital multimeter set to DC Volts (yes, DC!) to each pair of cathodes, and set the scale to milivolts. You will be measuring the voltage difference between the two pairs of cathodes.

3. Inject a 1 Vrms 100 Hz signal into the amp. I keep the amp in clean mode with the preamp volume turned down low and the tone controls where I normally set them (it will not matter much where you set them anyway, all I try to do is to not overdrive anything).

4. With a load connected to the speaker jack, set the output level (master volume) to the point where the amp dissipates about as much power as you think it dissipates when you normally use the amp. My rule of thumb is that at gigs I may be running at 50W tops, so into an 8-ohm load I will have a 20 Vrms output (P = V x V/R, so V = SQRT (PxR) = SQRT (50 x 8.) = 20). If you play your amp at lower volumes you may want to shoot for say 20W, which translates into 12.7V, etc. The output level will change the results/readings.

5. The multimeter's reading should be on the order of a few milivots. Turn the PI trimmer until the reading is 0 mV; 0 may go by quickly, so look for the reading to shift from + to - or vice versa. When 0 is reached, each pair of cathodes is at the same voltage with respect to ground and the two sides are therefore balanced. If the reading doesn't cross over, try another tube in the PI slot; it could also mean that your power tubes are not matched.

This method does not take into account any further imbalances introduced by the OT but has, per my experience, consistently yielded excellent results. I hope it works for you too.

Gil
Last edited by ayan on Tue Jan 29, 2013 5:33 pm, edited 2 times in total.
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martin manning
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Re: For anyone wanting to set the PI Trim pot

Post by martin manning »

Excellent Gil, thanks for posting this. I have looked at the balance by putting the cathodes on a scope and looking at matching peak voltage (really current, via the 1R), thinking that this would indicate that both sides are being driven equally. Any comment on this idea? How does an AC voltage measurement compare to DC, and why did you decide that measuring DC is better? Sound?
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ayan
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Re: For anyone wanting to set the PI Trim pot

Post by ayan »

martin manning wrote:Excellent Gil, thanks for posting this. I have looked at the balance by putting the cathodes on a scope and looking at matching peak voltage (really current, via the 1R), thinking that this would indicate that both sides are being driven equally. Any comment on this idea? How does an AC voltage measurement compare to DC, and why did you decide that measuring DC is better? Sound?
A couple of thoughts:

1. You're method is equivalent to the Ampeg method as far as I'm concerned. You need the use of a scope, however, while the Ampeg method allows you to get away with using a digital multimeter.

2. The thing is that most multimeters will only measure AC well for the case of a pure sine wave, otherwise they "get confused" when in AC mode. In the case of an amp, the AC output is riding on top of the DC bias current of the tubes, so the output is actually pulsating DC. I think most digital multimeters will give you some kind of "average" in DC mode value when measuring pulsating DC. The Ampeg method is the one that says to look at DC, so I didn't come up with anything innovative here. :)

Cheers,

Gil
Max
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Re: For anyone wanting to set the PI Trim pot

Post by Max »

ayan wrote:...the "Ampeg" method...
Here I posted what Alexander Dumble once recommended concerning the adjustment of the "dynamic balance" trim pots of his amps: http://ampgarage.com/forum/viewtopic.ph ... 221#134221

And here http://ampgarage.com/forum/viewtopic.ph ... 723#156723 Tom (Structo) posted this link http://www.el34world.com/charts/Schemat ... T_RevA.pdf to the schematic of an Ampeg SVT with a description of the "Calibration Procedure" in good resolution.

What Gil describes is his adaptation of the version B ("VOLTAGE METER METHOD") of the "PHASE INVERTER BALANCE CONTROL ADJUSTMENT" described in this schematic - but for the voltages present in an ODS style amp at some different playing volumes. What he describes is IMO a great way to do this adjustment if a THD meter - that would be needed for the version A ("HARMONIC DISTORTION METER METHOD") - isn’t at hand. The advantage of version A might be that any unbalance introduced by the OT will be taken into account, too. Perhaps power tubes that are matched for transconductance might be helpful to adjust a power amp for a good dynamic balance and lowest THD at different volume levels.

I've attached a picture of the back of ODS 150W #0121 with external controls to adjust the bias of each pair of the push/pull output tubes and the dynamic balance - and with external test points that enable metering current as voltage because of the internal high precision 1 ohm resistors Gil's talking about and that you'll find in the SVT schematic, too. Picture source: http://www.roblivesey.com/dumble/

BTW: AFAIU the "CALIBRATION PROCEDURE" as described in this SVT schematic, the test signal is connected to the input of the SVT power amp and not to the input of the preamp.

Cheers,

Max
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