how to adjust trimmer on PI plates?(#124 build)

[Fischerman]

...and the amp puts out the best Plexi tone out of any of my clones. I love that sound! RF-like? Heavens no, it's Clapton/Blackmore-like!

Careful...some folks might take that as a 'Politician-tone challenge'...and they never end up pretty.

[ayan]

I think I'm just too lazy for this so that's why I say this. To go through that entire process (of setting by ear that is) and that's just the first tube you tried. If the whole point is to wring that last 1% of tone out of the amp then we can't just stop at trying out one PI tube...or two or three. Now you gotta put in your next tube to try out and do the whole thing over again...and then compare it to a tone you heard at least 10 minutes ago. Then the third tube...then the fourth...etc. I think I'd just rather put 100k/110k (or whatever ratio you choose) in there and then just swap tubes until I found a good match. I always measure the PI plates output signals and choose the plate resistors such that they are equal (or close) with what I deem to be an 'average/typical' tube...I've been doing that for many years...way before I ever messed with D-style amps. Then I just swap tubes and chose the one I perceive to sound best.

I hear you, and I used to feel exactly the same way you do. Then, one fine day I had absolutely nothing better to do and decided to custom match the 12AX7 tubes to one of my amps. I did spend about 3 hours or so doing it, but the end result was that the amp sounded fabulous afterwards. I would have never thought that changing tubes and adjusting the trimmer could actually make or break a sound, but, it can. Unfortunately, I have to confess that the exercise is guitar dependent... Just the other day I had that experience with someone else's amp, where swaping V1 and V2 would work better for a Les Paul, but going back to the original arrangement would work better with a Strat...

Gil

[ayan]

...and the amp puts out the best Plexi tone out of any of my clones. I love that sound! RF-like? Heavens no, it's Clapton/Blackmore-like!

Careful...some folks might take that as a 'Politician-tone challenge'...and they never end up pretty.

Oh, no sweat. I know too well that people cannot overcome their DNA when playing, such that clips by anyone with a definitive style will overcome equipment every single time. Maybe sitting down in front of an amp and plucking one string at a time might reveal differences, but someone playing on a recording... and with distortion and the whole thing is good for fun but nothing else.

Gil

[embotone]

well, for what its worth, I did some reading in TUT 1 regarding the long tail inverter, and, as some here have mentioned, the output of that inverter circuit is inherently unbalanced. This is corrected by applying different voltages to the plates, giving a balanced signal output from both sides. Tweeking the trimmer lets you dial that voltage imbalance in.

[martin manning]

Interesting discussion on the sonic effects of PI balance.

Given this unusual feature, I wonder why HAD did not also include separate bias adjustments for the two sides of the power stage?

MPM

[Max]

Interesting discussion on the sonic effects of PI balance.

Given this unusual feature, I wonder why HAD did not also include separate bias adjustments for the two sides of the power stage?

MPM

Hi Martin,

in most of the 6550A amps he did include separate bias adjustments (see my post in this thread).

But as the characteristic curve of each tube may be different, their "matching" (by selection or individual bias voltages) at a given bias-voltage does not mean that they will work in symmetry at the whole length of their curve. To achieve that (at leat some more steps in this direction) you need to adjust the "Dynamic Balance" in the way I described in my post.

First step: Chose a set of power tubes that is not only matched for bias but for transconductance too.

Second step: adjust bias to your taste (better on the "hot" side according to HAD, but of course not above the possible plate dissipation values for the given tube).

Third step: adjust "Dynamic Balance" in the way HAD recommends (see my post).

Cheers

Max

[FUCHSAUDIO]

I think lots of guitar amp designers choose to ignore this. When I first began discussing circuitry ideas with a old friend (Dennis Kager of Ampeg and Sundown fame), he laughed and said " separate bias pots are a waste of time and money"...."Ya don't need 'em"

Well, most audiophile amps had them, as well as DC preamp tube filaments, AC Balance controls, regulation of certain voltages etc. I just decided I would do it my way, and I think it makes ton's of sense. Besides not having to look for matched tubes, you can set things exactly where you want. I know there were reputedly a few 50-W Dumble amps with a pot for each tube, but in point to point wired amps, it can be a hassle.

Worth doing imho.

[Structo]

Yes I installed dual bias pots on my D'Lite 44.

I'm not certain I hear any difference but when I was searching for NOS tubes Moss suggested I do the dual bias because then I could buy a tube here and a tube there and match them up better.

I understand it probably doesn't do a thing for matching transconductance but it makes me feel better.

BTW, I use Winged=C= 6L6's.

[martin manning]

It seems like balancing the idle currents to zero out the net current through the OT primary (the static part) would always be a good thing, and then balancing the "effective" Gm on each side with the PI trimmer (the dynamic part) would be a way to get a symmetrical output (or un-symmetrical in a controlled way, if that sounds better).

In any of the testing that you guys have done have you looked at the dynamic symmetry of the output after the trimmer is adjusted for the best tone?

MPM

[Fischerman]

There was something in (I think) a TUT book saying that having the current draw equal on both sides of the OT was important i.e. DC balance.

So it appears that theoretically at least we would want to match/balance the DC idle current through the power tubes and then also tweek/balance the PI AC output.

EDIT: Martin, I think that's what Andy was saying earlier...he sets it for AC balance but dogears sets it by ear...and yet they achieve similar settings.

[glasman]

Shame on me, but I actually measure the PI. I use a stereo Leader AC Voltmeter. I set the volume to read about ten volts or so (at 1-Khz) on the plates, and set the trimmer to measure '0' difference between the signal at the plates. I suppose an AC digital voltmeter might do the same, just set for the null point with the meter across the plate pins.

Since AC and DC balance occur at different points, setting both plate volts to be equal is meaningless for tone.

Oddly, when Scott uses his method, and I measure with mine, we usually get the trimmer to the same place. Dogears indeed !

I used to use this same approach. Used a dual trace scope in difference mode and adjusted for a null against a 1Khz signal. I now use the 2nd order harmonic approach on a 400 hertz fundamental measured at the 4 ohm tap.

Another friiend uses an old HP 304 distortion analyzer and tweaks for minimum indicated distortion. Lot of ways to get to the same final place .