PI Balance/Unbalance for Harmonics

[David Root]

This afternoon I did some tube rolling in my '70s 50W ODS. I should preface my remarks by saying that this amp uses a 12AT7 in the PI, not a 12AX7, and 47K/51K plate resistors with a (HAD not approved) 5K trimpot on the B+.

I ended up with an old Mazda in the PI, and proceeded to do the harmonics optimization procedure, brushing the strings and listening, moving the trimpot around, to find maximum harmonics. After I had found it, I measured the plate voltages and they were identical at 293.4V, ie this is perfect DC voltage balance.

Usually we find maximum harmonics several volts out of DC balance. This intrigued me, because other tubes I have tried always ended up that way at best harmonics content.

So I put a 400Hz sinewave into the amp and measured the AC balance on the plates, after the coupling caps so as to eliminate PS ripple. 5% spread between the triodes, and there are the harmonics.

Anyone else had this happen?

The DC balance of this tube was average, not very close like I usually try to get, and I only have one of them.

[Max]

Hi David,

around 20 years ago I got a first hand information from Alexander regarding the question how to adjust the internal or external "dynamic balance" control of his amps.

His advice:

Do it in exactly the same way as you find it explained in a service manual for an Amgeg SVT or V9. But of course at different output voltages that can be easily calculated by any tech that is able to interpret the output voltages given in the Ampeg SVT/V9 manual and knows the differences between an Ampeg SVT/V9 and a 50W, 100W or 150W Dumble amp. When you adjust the "dynamic balance" control of a 300SL, you can use exactly the voltages given in the Ampeg SVT/V9 manual. But for the 50W, 100W and 150W Dumble amps you will have to calculate the correct voltages. Best is to use the "lowest THD" method as explained in the SVT/V9 service manual. Without a THD meter at hand the other method can be used with very good results, too.

The "Calibration Procedure" for an Ampeg SVT/V9 you find as an example at the pages 344 and 345 of the 4th edition of Aspen Pittman’s "The Tube Amp Book".

I personally did never have any kind of problem regarding the harmonic content of any of the Dumble amps that I had the opportunity to play, that had been set up in the way Alexander recommended. But this judgement is based on my personal taste and your personal taste may be different of course.

Some questions regarding your new build:

What do you refer to in your post as "my 70s 50W ODS"? Is this a replica of a 1st, a 2nd, a 3rd, or a transition generation ODS? Or some kind of "mixture" like in Tony's (talbany) layout? What kind of tone stack did you use as an example?

Have a nice weekend,

Max

[lovetone]

This is very interesting, can any body point me in the right direction in the old threads where I can find full details on the best way to adjust the PI trim pot, or give the details on how it’s done.

Thanks

[Max]

This is very interesting, can any body point me in the right direction in the old threads where I can find full details on the best way to adjust the PI trim pot, or give the details on how it’s done.

Thanks

Hi lovetone,

here you find the description of the "CALIBRATION PROCEDURE" for an Ampeg SVT/V9 power amp.

"3 - PHASE INVERTER BALANCE CONTROL ADJUSTMENT": http://www.freeinfosociety.com/electron ... php?id=830

This is the general procedure Alexander Dumble recommended regarding the adjustment of the "dynamic balance" control. An experienced tech will know which kind of test signal and output voltages you should use to set up your amp in a similar procedure.

Have a nice sunday,

Max

[Drumslinger]

nice info, thanks Max.

[bluesfendermanblues]

This is very interesting, can any body point me in the right direction in the old threads where I can find full details on the best way to adjust the PI trim pot, or give the details on how it’s done.

Thanks

Hi lovetone,

here you find the description of the "CALIBRATION PROCEDURE" for an Ampeg SVT/V9 power amp.

"3 - PHASE INVERTER BALANCE CONTROL ADJUSTMENT": http://www.freeinfosociety.com/electron ... php?id=830

This is the general procedure Alexander Dumble recommended regarding the adjustment of the "dynamic balance" control. An experienced tech will know which kind of test signal and output voltages you should use to set up your amp in a similar procedure.

Have a nice sunday,

Max

Max, you always go that extra mile for fellow forumites. Always a pleasure to read you posts.

[lovetone]

Hi Max

Thanks for the info lets see how i get on !!!

Geoff

[David Root]

Max wrote[What do you refer to in your post as "my 70s 50W ODS"? Is this a replica of a 1st, a 2nd, a 3rd, or a transition generation ODS? Or some kind of "mixture" like in Tony's (talbany) layout? What kind of tone stack did you use as an example?]

Max, it is very similar to Tony's except mine is 50W, his was 100W. Like Tony's, mine is a mix of ODS#13 (w/out reverb) and #40, with 12AT7 PI and in my case a 5K PI trimmer.

Tone stack is #13, TMB 1M, 250K, 1M, 100K slope, .05 on mids wiper, James caps on Bass pot, .002 from input to wiper, .02 from wiper to 10K to ground. 500 pF coupling cap to treble pot.

Carbon comp resistors in the OD entrance and V2 grid resistors, also the OD interstage resistor is 150K CC, (#13) as are the 180K/1M output dividers on V2B.

Other than that it's per #40 with an added buffered loop (12AT7). I also increased the dropping resistor to V1A from 2K2 to 4K7 in order to get the plate voltages into the low 180s, same as Tony did. All built in a knock-off Princeton Reverb chassis using '67 Bassman iron.[/quote]

[David Root]

Sorry, double post.

[LPSGME]

This is very interesting, can any body point me in the right direction in the old threads where I can find full details on the best way to adjust the PI trim pot, or give the details on how it’s done.

Thanks

Hi lovetone,

here you find the description of the "CALIBRATION PROCEDURE" for an Ampeg SVT/V9 power amp.

"3 - PHASE INVERTER BALANCE CONTROL ADJUSTMENT": http://www.freeinfosociety.com/electron ... php?id=830

This is the general procedure Alexander Dumble recommended regarding the adjustment of the "dynamic balance" control. An experienced tech will know which kind of test signal and output voltages you should use to set up your amp in a similar procedure.

Have a nice sunday,

Max

Does the Voltmeter method also (as described in the distortion meter method) require driving the amp with a signal?

Unless I'm not reading it right, it sounds like the PI trim (or is it the Presence) is merely adjusted for a zero volt reading between the output tube cathodes... or is it some other trim - what is the dynamic balance control - is it the Presence?

[lovetone]

Hi LPSGME

I read it that you adjust the output for the voltmeter method the same as you would for the distortion meter method, set it to 25V RMS @ 40hHZ into a 4R load. Then adjust the PI pot for 0V across K1 and K2

Hope that helps.

[Structo]

I'm not real sure that method will work for the Dumble amp.
That is not a long tail pair phase inverter.
Plus it is using a 12BH7 to drive the power tubes.

How is it that you would balance the PI through the power tubes?
What if the power tubes are not closely matched themselves.

Not doubting the method entirely, I just don't understand it very well.

What it says is to inject a 40Hz signal to 25v at the output.
That's a pretty loud, low freq signal, so you would have to use a dummy load for sure.
Then adjust V3 for zero volts at K1 and K2.

Is it really the same on such a different amplifier?

Where would you inject the 40Hz signal? At the PI or first stage input?

[lovetone]

The signal would be injected at the input, I would set all controls a 12 oclock. Set signal generator at about 40mv pk to pk and adjust the master volume until you had 25v RMS into the dummy load. You could measure this using a true RMS meter like the Keithley 175. at 40HZ you would get a good reading. I would monitor the output with a scope so I could see what was going on. W= 25 x 25 / 4 = 156 watts. I would say without seeing the SVT manual that this would be 50% output. So for a 50 watt amp you would set the output at 10v RMS that would give 50% output.

Any Body any thoughts?

[talbany]

HH Scott has something similar.. Half way down AC-DC Balance..

Tony

[martin manning]

50% power is exactly what I got from it. When this was discussed before the balance (using a voltmeter) was checked at the PI output, which would leave the dynamic balance of the power stage uncertain. Taking the reading at the power tube cathodes solves that. Most people here are using one 1-Ohm resistor per tube, which is nice to monitor the individual currents in a quad. In that case, though, it would be necessary to temporarily short the cathodes on each side together to do the balance check.