Quinn SDO-Reverb 183/102

[Cliff Schecht]

So why not work up a circuit that automatically corrects for AC and/or dc imbalance? This sort of servo circuit is relatively common on both the hifi world and in IC design..

[boldaslove6789]

In my expieriences with every D-style amp I've been around, fine tuning the best and most musical amount of harmonic feedback content has always been executed by this method:

Measure both sides of the PI trim, adjust to 7-9 AC volts apart, pwr amp being the prodominemt side.

Make slight adjustment if needed when the amp is loud enough to feedback.Use your ear to determine the amps best and most musical harmonic feedback. Using either a Tele or Les Paul middle position (a bridge and neck pup combo (If it will feedback in the middle position it will do it mucho more in the bridge position.)

Is this just me that does it that way?

[bluesfendermanblues]

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 20VAC 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.

[martin manning]

very good and succinct description Fenderman! One comment I have is that the reference dot on the trimmer is only good for the particular PI tube installed and at its immediate state of deterioration. Maybe the dot should be made with a non-permanent marking device?

[talbany]

So why not work up a circuit that automatically corrects for AC and/or dc imbalance? This sort of servo circuit is relatively common on both the hifi world and in IC design..

In the Hifi world a slight imbalance is actually preferred!

I came across a most intriguing article called The Ultimate, by Rickard Berglund of Hoor Sweden, in Sound Practices magazine, Winter 94/95.(sorry out of print and not on the web) Mr. Berglund claims that by reducing the AC drive to one output tubes in a push-pull output stage by approximately 2.3dB, while maintaining DC balance, you get the distortion spectrum of a single ended amp. More specifically, you get back the even harmonics, with the highly desirable second harmonic being significantly higher than the third. You then have a better and more natural sounding harmonic distortion spectrum, and most of the efficiency of the push-pull topology...(note)What is most interesting about the article is Berglung talks about the slight imbalance.. Clearly the HIfi world is more concerned with the proper imbalancing and a dominant 2nd order harmonic spectrum..
So the 10k question being, what is it that actually promotes the bloom (whatever) effect?..Is it the perfect balancing (conduction) of a push pull OPT section or is it the slight imbalance or balance of 2nd to 3rd order harmonics ratio

I vote harmonics!!..

As far as DC Balance in guitar amps>> I believe a small amount of core saturation is a good thing..I suppose this could be argued..

I hope this makes some sense!!

Tony

[greiswig]

Measure both sides of the PI trim, adjust to 7-9 AC volts apart, pwr amp being the prodominemt side.

We're really hijacking this thread in a way, as there are other threads that discuss the PI adjustment in detail. But at the risk of adding to the hijack...

As I was doing the method that I described earlier in this thread (measuring and labeling tubes for the PI), I noticed that the DC imbalance varies greatly from tube to tube. See https://tubeamparchive.com/viewtopic.ph ... mmer+volts

[John_P_WI]

So why not work up a circuit that automatically corrects for AC and/or dc imbalance? This sort of servo circuit is relatively common on both the hifi world and in IC design..

In the Hifi world a slight imbalance is actually preferred!

I came across a most intriguing article called The Ultimate, by Rickard Berglund of Hoor Sweden, in Sound Practices magazine, Winter 94/95.(sorry out of print and not on the web) Mr. Berglund claims that by reducing the AC drive to one output tubes in a push-pull output stage..... Tony

KOC has promoted this for years as a "Body" control (although not specific in terms of db "down"), in which Peavey copied and subsequently patented....

[Max]

Hi,

here's what Alexander Dumble recommended concerning the adjustment of the "dynamic balance" (as he calls this) controls of many of his amps:

https://tubeamparchive.com/viewtopic.ph ... 221#134221

Cheers,

Max

[vibratoking]

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 20VAC after the PI).
e) make a small dot on the trimmer with a sharpie pen.

That's what I am doing, but I only get about 6.5VAC after the caps? Do you really measure 20VAC? I've looked at it with a scope and DMM, same readings. I've also rolled many tubes and get approximately the same AC voltage.

[Max]

Hi,

here's a schematic of an Ampeg SVT with a description of the kind of calibration procedure that Alexander Dumble once recommended to use when adjusting the "dynamic balance" pots of his amps. You'll find the description of the "CALIBRATION PROCEDURE" on the right side of the schematic. And in the schematic you'll see where exactly in the circuit the voltages given in this description have to be measured.

According to what Alexander once recommended, this general kind of procedure can be used for the adjustment of all his amps with an internal or external "dynamic balance" pot.

For adjusting a Dumbleland 300SL you can use exactly this procedure. For adjusting other Dumble amps, of course you'll have to calculate the appropriate voltages.

Cheers,

Max

[Structo]

This print is a bit easier to read.

[vibratoking]

I answered my own question from above. I was not getting ~20VAC as mentioned above for two reasons that should have been obvious to me.

1 - I was putting a 1V peak to peak signal into the PI not 1VAC RMS.
2 - I am guessing that my output tubes are not biased as hot as the ones previously mentioned.

With a 1VAC RMS signal into the PI, I get about 16.5VAC measured after the PI coupling caps. I still cannot dial in the PI to my expectations. I feel like the amp requires too much volume to get to a decent level of feedback. I could be wrong here, but that is what I am working toward. On problem is that my 6L6 Winged Cs are not balanced well enough. I did put in a 20k multiturn pot. The multiturn is the right type pot IMO to reduce the sensitivity of the knob and get repeatable results. I also looked at the frequency spectrum at the speaker output into a dummy load using a digital scope with FFT capabilities. The results are interesting but not conclusive, yet. My next move is to find a better set of output tubes and bring home a network analyzer to make some investigations. I will post some plots and conclusions when I get to that point.

[Luthierwnc]

Hi All,

I was in Lowes the other day and the 40-watt bulb went on over my head when I saw this. It is an inline switch that you would attach to a lamp cord. Usually they are quite small but this unit looked like it had some room to work inside. I had to remove a supporting plastic node along the lower channel to cram everything in but it worked out fine. When reassembled the ends pinch the cables secure.

I soldered a .0022 cap to the signal wire. The other end of the cap is screwed to the switch contact. On the opposite side, the ground is screwed to the other contact. A Sharpie dot on top tells me whether it's on or off.

Just a thought, Skip