PI trimmer and tube mismatch

[dogears]

Guys,

I have dozens of matched and balanced vintage tubes, both gain and conductance, here.

Every tube sounds and feels totally different in the amp. I cannot stress the importance of trying tubes in the PI. Mine are all balanced so I am comparing apples to apples and in many cases I am comparing same brand and model tubes to one another!

They all sound different. I have had two Mullards that measure the same sound and feel totally different. One amazing and one like poo.

Moral of story..... If the amp is not blooming or the trimmer seems to not yield results, swap the tube. Swap anyway. Try at least four or five tubes....

[greiswig]

I think this is one of those situations where once you have done it by ear, it is easy. For those of us who have yet to do it by ear, it sounds like voodoo. And who do that voodoo like you do, Scott? Nobody!

Anyway, part of what I've learned from this discussion is that there IS a more objective way of getting a good deal of the way there: you can adjust for AC balance. Cool. Now I at least know I'm in the ballpark. An

But as I understand it, that's not quite the sweet spot in most cases. And Gary discussed a measurable, objective way to find that spot...but using a spectrum analyzer which most of us don't have.

So this becomes applied theory to at least some extent, at least for me, until I've been apprenticed in the voodoo art or stumble across it using my own methods. Even if I have "the right tube," or a couple of them, it would be nice to have a more direct, objective way of getting "there" to compare them and see if they'll sound good when operating at or near this ideal spot.

So part of the theory that's missing for me (and at least a few others, based on the PM's I keep getting) is whether the sweet spot is on one side only, or is there one on either side of AC balance. And does it vary according to the tube? When Gil posts that one of his tubes sounded best with the non-feedback side at higher voltage, he was talking DC voltage, not AC balance. So please be patient with those of us who are struggling with two things: understanding the theoretical (for the sake of understanding) and trying to apply it to milk the best tone from their amps.

[ayan]

I've been following this thread and spent some time playing with my amps this weekend -- I had been on the wagon for over a year! Out of curiosity, I got my "golden" amp out of its cabinet and measured the AC on the PI. By the way, that it can be a pain in the ass to do that. Using my scope in scope mode was all but useless, even playing with the sensitivity settings, variations are very small to jump out at you. Using the scope in meter mode, I was able to measure the AC at the PI's plates and, not surprisingly, it was balanced to within a couple of cents of a volt RMS. I got my not-as-golden amp out and measured -- now much more quickly -- the AC voltage and same deal, balanced. I had tuned both amps by ear using clean channel + PAB + enough volume to produce some feedback when brushing the strings. As I said before, I listened for pretty pretty feedback and dialed out ugly feedback (i.e., non-even harmonics). I had said it before and Scott said it as well, try different tubes... not all tubes will be able to deliver the goods. What I don't know is whether those "dud" tubes were capable of achieving AC balance, because I never measured them.

Next time I have to replace a PI tube I will do the "by ear" method first (since it never fails) and then see whether the tube is at AC voltage balance and if not, I will find the AC balance spot and see what it sounds like. You may want to take breaks when doing this test, and mark the spot on the floor where you will stand with you guitar. Also, you need a guitar that will sound good and bloom and feedback, which not guitars all are capable of doing.

Cheers,

Gil

I think this is one of those situations where once you have done it by ear, it is easy. For those of us who have yet to do it by ear, it sounds like voodoo. And who do that voodoo like you do, Scott? Nobody!

Anyway, part of what I've learned from this discussion is that there IS a more objective way of getting a good deal of the way there: you can adjust for AC balance. Cool. Now I at least know I'm in the ballpark. An

But as I understand it, that's not quite the sweet spot in most cases. And Gary discussed a measurable, objective way to find that spot...but using a spectrum analyzer which most of us don't have.

So this becomes applied theory to at least some extent, at least for me, until I've been apprenticed in the voodoo art or stumble across it using my own methods. Even if I have "the right tube," or a couple of them, it would be nice to have a more direct, objective way of getting "there" to compare them and see if they'll sound good when operating at or near this ideal spot.

So part of the theory that's missing for me (and at least a few others, based on the PM's I keep getting) is whether the sweet spot is on one side only, or is there one on either side of AC balance. And does it vary according to the tube? When Gil posts that one of his tubes sounded best with the non-feedback side at higher voltage, he was talking DC voltage, not AC balance. So please be patient with those of us who are struggling with two things: understanding the theoretical (for the sake of understanding) and trying to apply it to milk the best tone from their amps.

[greiswig]

So this seems to answer a question I'd asked earlier, but now feel I'm getting somewhat contradictory answers for. The question is whether the AC balance point is the sweet spot or not?

Yours seems to indicate that it is, or at least really close. (And with that tube, that amp, etc.) The spot I landed on (but it was far from an "Ah HAH!" moment...pretty subtle) was within 2/100 of a volt of AC balance for a couple of tubes that seem well balanced.

But then Gary seems to say something like "Okay, now you've found AC balance, but not the sweet spot. It'll probably be close by." And the rationale seems sound: the AC balance point on the PI does not factor in variances downstream, such as the OPT, power tubes, and so forth. Even measuring at the power tube grid doesn't factor in everything.

Anyway, thanks for tinkering a bit and reporting here, Gil!

[glasman]

So this seems to answer a question I'd asked earlier, but now feel I'm getting somewhat contradictory answers for. The question is whether the AC balance point is the sweet spot or not?

Yours seems to indicate that it is, or at least really close. (And with that tube, that amp, etc.) The spot I landed on (but it was far from an "Ah HAH!" moment...pretty subtle) was within 2/100 of a volt of AC balance for a couple of tubes that seem well balanced.

But then Gary seems to say something like "Okay, now you've found AC balance, but not the sweet spot. It'll probably be close by." And the rationale seems sound: the AC balance point on the PI does not factor in variances downstream, such as the OPT, power tubes, and so forth. Even measuring at the power tube grid doesn't factor in everything.

Anyway, thanks for tinkering a bit and reporting here, Gil!

What I meant was adjusting for AC balance at the PI plates may or may not be the absolute best position as the OPT and power tubes have not been taken into consideration. The odds are you will be within a 1/16 of a turn on the trimmer.

And I agree with Gil and Scott, trying different tubes in the PI slot makes a lot of difference.

[BobW]

And I agree with Gil and Scott, trying different tubes in the PI slot makes a lot of difference.

...and the next most logical thing to try if the tech can't find the sweet spot.

[glasman]

And I agree with Gil and Scott, trying different tubes in the PI slot makes a lot of difference.

...and the next most logical thing to try if the tech can't find the sweet spot.

I got bit a while back. I had bought a large group of tubes that were "matched / balanced". Took about 3 minutes to find a few of them weren't so "matched / balanced".

[talbany]

I find the NOS tubes, Amphrex,Mullards Funkins,RCA's to name a few (balanced of coarse) seem to be able to hone in on the sweet spot by ear a bit easier than some of the newer stuff..Anyone else notice this..They are in most cases noticeably smoother as well but I miss the girth and throat that some of the newer long plates have.. It never ends..

Tony

[scotto]

Once trimmer is set optimally with an appropriate tube, does the amp have to still be cranked up fairly well (say, above 100db from 20 feet away) to get the feedback going? Also struggling to understand the role of the guitar in this equation. Is the bloom more a function of the signal coming out of the guitar or the guitar's tendancy to couple with the sound waves coming from the cranked speaker such that hollow or semi hollow bodies would work better?

[ayan]

Once trimmer is set optimally with an appropriate tube, does the amp have to still be cranked up fairly well (say, above 100db from 20 feet away) to get the feedback going? Also struggling to understand the role of the guitar in this equation. Is the bloom more a function of the signal coming out of the guitar or the guitar's tendancy to couple with the sound waves coming from the cranked speaker such that hollow or semi hollow bodies would work better?

Getting feedback depends on a number of things. Feedback starts with "an acoustic phenomenon" and if your amp is not loud to the level it puts out enough acoustic waves that will make the guitar strings vibrate sympathetically, feedback will not occur. So, you will have to have some volume happening, and just how much depends on the room you're playing, how much "gain" you have dialed in, the guitar you're using, whether the amp is on the verge on instability and is prone to start feeding back all by itself, etc. Some tubes can make an amp sing, others can make the same amp much less lively.

I don't know the proper answer to your second question. The guitar certainly plays a role in the bloom thing, as does the way in which one gets a note out of a guitar. The guitar will certainly play a huge role in achieving feedback. My 335 can feedback at will because the top of it can vibrate so easily, my Les Paul not as much although it has similar output pickups. My Strats are harder to get feedback out of.

Gil

[heisthl]

I find the NOS tubes, Amphrex,Mullards Funkins,RCA's to name a few (balanced of coarse) seem to be able to hone in on the sweet spot by ear a bit easier than some of the newer stuff..Anyone else notice this..They are in most cases noticeably smoother as well but I miss the girth and throat that some of the newer long plates have.. It never ends..

Tony

The SovTek LP and LPS work pretty well with maybe 10 out of 100 that won't adjust, and to get rid of some of those just do builds with the PI socket rotated 180 degrees (if you're like me and your OCD requires that the the input side DC plate is higher).

[greiswig]

Personally, I'd still really like to hear if someone knows the answer to this question. With the silence, I can't tell if it's just that nobody knows or what:

Can anyone address this?

, and I'd be curious as to your response:

1. Does this sweet spot lie on one side of the AC balance point only or the other? In other words, do you want to start adjusting away from AC balance by making the feedback side go higher? Or is it that you'll find a spot going in either direction? (guess that's what I'd expect) Or is it that you don't know which side it will be on, because it is dependent on the OPT, but once you find it you'll know for that amp which way you need to adjust away from AC balance?

Knowing the answer to this might help a guy know when to stop looking for the sweet spot if he's on the wrong side of the AC balance point. Unless there's a sweet spot on both sides...you get the drift.

Thanks again!

Are there two sweet spots, or just one? If one, which side of AC balance?

[glasman]

Personally, I'd still really like to hear if someone knows the answer to this question. With the silence, I can't tell if it's just that nobody knows or what:

Can anyone address this?

, and I'd be curious as to your response:
Knowing the answer to this might help a guy know when to stop looking for the sweet spot if he's on the wrong side of the AC balance point. Unless there's a sweet spot on both sides...you get the drift.

Thanks again!

Are there two sweet spots, or just one? If one, which side of AC balance?

There is one. It falls where THD is minimum.

Adjusting for AC balance there is no way to say that the best position will be on the high side, low side or at AC balance of the PI. You are missing the output transformers effect as well as the output tubes. They all enter into the equation.

My recommendation, if you can only measure AC balance, is to set to best AC balance and then use the ear method over a range of say 5% of the trimmer range.

Gary

[greiswig]

Thanks, Gary!

[talbany]

Not to confuse this subject further but I came across a most intriguing article worthy of this post called The Ultimate, by Rickard Berglund of Hoor Sweden, in Sound Practices magazine, Winter 94/95. 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..
The Expierment
We first measured the Dyna Stereo 70 as is, and found that the 2nd harmonic was already higher than the third, probably due most likely to the inherent imbalance of the cathodyne style phase splitter. We pulled out the 7199 front end tube and capacitively coupled into the output stage with a Tektronix AA501 precise differential drive sinewave to see what would be the typical balanced topology distortion spectrum with the greatly attenuated even harmonics. Then we took data and photographs with the balance shifted by 0.2dB, 1dB, 3dB and 6dB. Even at 0.2dB, there was a significant improvement in the spectrum. We had a hard time telling the difference in spectrum between the 3dB and 6dB differential. At one dB, it was most of the way to acting like a single ended topology, while maintaining most of the efficiency of the push-pull topology.

Conclusion:

Low amounts of negative feedback means less extension in frequency of harmonic distortion product generation. This means less likelihood of slewing related distortions, and less "noise modulation", especially in the high frequencies. Balanced topologies don't generate much if any even harmonic distortion product, so there's no 2nd harmonic, which is very arguably the most desirable of the distortion products. Slight AC imbalance in the push-pull output stage means you get a 2nd harmonic distortion product that dominates over the 3rd. (This is what we are hearing when we are adjusting by ear)The ideal would seem to be low or no feedback, and avoiding a perfectly balanced topology.

Although I haven't measured it yet, I believe that the intermodulation distortion or THD (sum and difference frequency generation) goes up as you unbalance the push-pull stage, so it's a tradeoff, and you definitely don't want to go too far with the imbalance. These experiments showed that you only need to cause a 1 or 2 dB impbalance to get most of what you want in harmonic products.(2nd order harmonics) As tubes age, a given designed imbalance will drift. This is another issue to look at if you're a perfectionist.

Judging by his conclusions this would imply when we adjust the AC balance by ear we are we are creating somewhat of a slight inbalance (1db) producing the more desirable 2nd order harmonics since this would explain the increased low end content or fuller/singing quality (explained below) and the more likely chance of feedback generated off the fundamental at the sweet spot..Like the expierment suggests closer to single ended operation

SIGNIFICANCE OF MUSICAL HARMONICS

Having divided amplifiers into three groups of distortion characteristics,the next step is to determine how the harmonics relate to hearing. There is a close parallel here between electronic distortion and musical tone coloration that is the real key to why tubes and transistors sound different. Perhaps the most knowledgeable authorities in this area are the craftsmen who build organs and musical instruments [8] [9]. Through many years of careful experimentation these artisans have determined how various harmonics relate to the coloration of an instrument's tonal quality. The primary color characteristic of an instrument is determined by the strength of the first few harmonics.

Each of the lower harmonics produces its own characteristic effect when it is dominant or it can modify the effect of another dominant harmonic if it is prominent. In the simplest classification, the lower harmonics are divided into two tonal groups. The odd harmonics (third and fifth) produce a "stopped" or "covered" sound. The even harmonics (second, fourth, and sixth) produce "choral" or "singing"sounds. The second and third harmonics are the most important from the viewpoint of the electronic distortion graphs in the previous section. Musically the second is an octave above the fundamental and is almost inaudible; yet it adds body to the sound, making it fuller. The third is termed a quint or musical twelfth. It produces a sound many musicians refer to as "blanketed." Instead of making the tone fuller, a strong third actually makes the tone seem softer. Adding a fifth to a strong third gives the sound a metallic quality that gets annoying in character as its amplitude increases. A strong second with a strong third tends to open the "covered"effect. Adding the fourth and the fifth to this changes the sound to an"open horn" like character. The higher harmonics, above the seventh, give the tone "edge" or "bite". Provided the edge is balanced to the basic musical tone, it tends to reinforce the fundamental, giving the sound a sharp attack quality. Many of the edge harmonics are musically unrelated pitches such as the seventh, ninth, and eleventh. Therefore. too much edge can produce a raspy dissonant quality. Since the ear seems very sensitive to the edge harmonics, controlling their amplitude is of paramount importance. The previously mentioned study of the trumpet tone [6] shows that the edge effect is directly related to the loudness of the tone. Playing the same trumpet note loud or soft makes little difference in the amplitude of the fundamental and the lower harmonics. However. harmonics above the sixth increase and decrease in amplitude in a1most direct proportion to the loudness. This edge balance is a critically important loudness signal for the human ear.

Note: When comparing different amplifiers some of which are tube type, don't forget that the different amplifier output impedances will un-calibrate any passive crossover network in a speaker system leaving frequency response and lobing anomolies. Plus, the resonance of the speaker motor will cause a tube output stage with low feedback to have an increase of gain at the frequency of resonance, which will likely exaggerate the resonance problem rather than damping it. This is likely to be a significant part of the difference you are hearing.

THE AUTHOR Russell 0. Hamm received his engineering training at the University of NewHampshire. He worked for Vidcom Electronics and later the Fine Recording division of that Company, designing and supervising the installation of their extensive sixteen-track recording facilities While with Fine Recordings, Mr. Hamm did a great deal of experimentation in stereophonic and quadraphonic sound for records and motion pictures which, in conjunction with Peter Scheiber, formed the basis for the development of the present matrix-quad record. (there was more)

Interesting article.. Anybody?

Tony