PI Balance/Unbalance for Harmonics

[CHIP]

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.

Agreed!

[Max]

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?

AFAIR in the "Ampeg procedure" the test signal is injected at the "power amp in"?

Cheers,

Max

[sergio]

while adjusting P.I. , don't you disconnect the NFB?

[talbany]

Ideally, you would want balanced outputs whether or not you are using the PI as a feedback summing junction or as a second input

Tony

[glasman]

I have used a similar approach the last few years.

Inject a signal into the PI, reference my analyzer to the injected signal and measure the second order harmonic at the 4 ohm tap. Adjust trimmer for minimum second order (this gives the lowest THD). Normally I see a delta between the fundamental and second harnonic of about 80dB or so with a 820/24K PI and about 65 dB with a 510/9K1 PI.

The sweet spot range is VERY small.

Gary

[ayan]

I have used a similar approach the last few years.

Inject a signal into the PI, reference my analyzer to the injected signal and measure the second order harmonic at the 4 ohm tap. Adjust trimmer for minimum second order (this gives the lowest THD). Normally I see a delta between the fundamental and second harnonic of about 80dB or so with a 820/24K PI and about 65 dB with a 510/9K1 PI.

The sweet spot range is VERY small.

Gary

Super tiny range indeed. Having a decent scope, courtesy of Gary, I still find it easier to dial in the trimmer by ear.

Cheers,

Gil

[glasman]

Super tiny range indeed. Having a decent scope, courtesy of Gary, I still find it easier to dial in the trimmer by ear.

Cheers,

Gil

Yup, the range is like nano-gnats a$$

[Structo]

What range of trimmer do you guys like?

I think I have a 10K on my 110K/120K.

Would a 5K make it less touchy?

[brewdude]

I would like to know what the equivalent plate load resistance tends to be after you set the PI trimmer. Does the trimmer widen the difference between the 110/120k plate resisters? Or, does it tend to balance out the values?

I have been playing with the PI plate resister values in my D'Lite, but I don't have a PI trim pot.

[glasman]

What range of trimmer do you guys like?

I think I have a 10K on my 110K/120K.

Would a 5K make it less touchy?

I normally use a 10K pot, why, because that is what i have used for the past 7 years. No other reason. Sure 5K would make it less touchy, but your PI tube had better be well matched otherwise you will run out of travel.

[talbany]

I would like to know what the equivalent plate load resistance tends to be after you set the PI trimmer. Does the trimmer widen the difference between the 110/120k plate resisters? Or, does it tend to balance out the values?

I have been playing with the PI plate resister values in my D'Lite, but I don't have a PI trim pot.

The next best thing is to increase the value of the tail resistor,( but this limits the headroom of the stage). When using both PI inputs, it is preferable to use a larger tail resistor and make both plate resistors equal. This is why Vox type amps that have large tail resistors have equal value plate loads, and Fender and Marshall/Dumble type phase inverters have small tail resistors and unequal plate load values..Also 6L's and 34's require a much larger voltage swing at the grid to drive them to full output (especially in a 100W amp) than El-84's


If you use a smaller value tail resistor, deliberately unbalancing one of the resistors to balance the outputs, one input side will be perfectly balanced, but the other input side will be way out of balance. This is not so important when using global negative feedback and only one of the inputs, but pretty important when using both inputs.

Tony

[dobbhill]

I use multi-turn trimpots.

[groovtubin]

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.

Seems like ALL my builds work when i set the trimmer outa balance, i can do it by ear NO problem, The opposite way is HIFI and cleaner, no brainer....imho!! Just set the way ya EARS like it..., no test equip needed imho..

jimbobampdudeinmoravianfallsnc

[Structo]

It would be really cool if one of you guys that does this successfully to shoot a video while balancing the PI by ear.
I'm not sure if we are all talking about the same thing.

[Max]

Hi,

IMO it all depends on the kind of personal approach concerning Dumble amps:

The "replica approach":

Based on my own experiences with Dumble ODS amps and based on the THD results of the power amp of an ODS 150W I remember (AFAIR around 1% THD at rated output), I have no doubt at all that the "Dumble ODS concept" is to intentionally generate the harmonic content (timbre) of the amp in the preamp section and not in the power amp section.

So if someone with a "replica approach" isn't satisfied with the timbre of his ODS replica, I would recommend to solve the problem in the "timbre generator" of his amp (preamp) and not to compensate for potential preamp problems by intentionally trying to generate power amp distortion of a certain kind (2nd order harmonics or whatever).

In the context of a "replica approach" I would recommend to look at the power amp of an ODS as the white canvas upon which the preamp can paint its colorful pictures. And to leave the "canvas" as white and clear as possible I would recommend to adjust the "dynamic balance" pot for lowest THD values of the power amp, AFAIR just as recommended by Alexander Dumble himself (SVT procedure).

(BTW: Once I really met some kind of "2nd order harmonics generator" in a Dumble amp, but not in its power amp section, but in its preamp section.)

The "inspiration approach":

If you shouldn't have in mind to build a replica of a Dumble ODS amp, but just to study the "Dumble ODS concept" as one of many valid technical concepts to create a certain kind of timbre of an electric guitar, I would recommend to do whatever your own inspiration tells you. So if you like your built better after adjusting the "dynamic balance" pot for an intentional unbalance -why not?

Cheers,

Max