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

[embotone]

How does one figure out where to adjust the 5K trimmer feeding the PI plate resistors? Is it a voltage reading? A resistance? Something I need to use a scope to measure?

[Pete]

some info in a recent thread here: https://tubeamparchive.com/viewtopic.php?p=35765

How does one figure out where to adjust the 5K trimmer feeding the PI plate resistors? Is it a voltage reading? A resistance? Something I need to use a scope to measure?

[Fischerman]

I don't think I'll be putting this trimmer in any more of my amps. It just doesn't make that much of a difference unless you go to extremes. I know some people say it affects this or it affects that...then they'll go on to say that you're supposed to set it for equal DC voltage on the PI plates. Look how many times this question has come up over the years and at least half the people say to set it for DC operation while the other half are setting it for AC...and half of those are looking at the PI plates while the other half are looking at the power tube plates.

To me this just means that most folks can't really hear any difference and they want someone to tell them the correct setting (i.e. the one that sounds best). I mean...we all tweeked this before we asked the question...and then still had to ask the question. Doesn't that imply that most of us can't hear any difference between say 108k/100k and 112k/100k? So just stick a 110k/100k combo in there and be done with it.

IMO of course.

[dogears]

There is no way to tell a correct setting. I can say that for me, and when using a nicely balanced PI tube, it is usually about 12K or 13K between the plates. I usually wind up with 5-8V more on the first plate.

I can hear and feel a big difference as I sweep the trimmer. Loud and with OD on. In the bass especially. YMMV.

[ayan]

There is no way to tell a correct setting. I can say that for me, and when using a nicely balanced PI tube, it is usually about 12K or 13K between the plates. I usually wind up with 5-8V more on the first plate.

I can hear and feel a big difference as I sweep the trimmer. Loud and with OD on. In the bass especially. YMMV.

My findigns concur with Scott's stament above, there is just not way to tell for sure. I have an amp with a tube in the PI slot where the best setting made the votage on the 2nd plate be higher...

As for me, the way I test is as follows:

1. Mark a spot on the floor where you will stand.

2. I put the amp in clean + PAB mode and turn it up loud. Don't put anything in the loop.

3. Start upping the preamp gain volume until you get some hint of a feedback to happen.

4. Go to the amp and start on one end of the trimmer settings.

5. Go back to the spot marked on the floor and brush the strings, listening for feedback. You may not be able to produce any.

6. Turn the trimmer a little bit and repeat #5 above until you've covered the entire range of the trimmer.

The key is to be consistent about where one stands, and just brush the strings to where the amp will generate those harmonics that feel like they want to take off by themselves. If your experience is like mine you may find that: (a) Each tube is completely different and some PI tubes will not give up the goods no matter where the trimmer is set; (b) Turning the trimmer, in some cases, will cause the harmonics to appear or go away, and may allow you to dial in "pretty" (2nd order, i.e., and octave above the fundamental) harmonics; (c) In some cases, the harmonics that are generated are ugly sounding ones (odd order), and dialing the trimmer can allow you to tune those (the ones that take off on the wrong note when you pluck a note) out.

I used to be a snob about this for the longest time, could never hear a difference per se. To me it's most important to have the amp be set with only as much gain as needed to produce "clean" feedback. If you dial in some overdrive, then there will be nothing subtle about the feedback, etc., and it's harder for me to tell what's going on.

Gil

[Fischerman]

Any difference in your experiences with the regular PI and the BM? Is one easier to hear the differences or easier to set?

[ayan]

Any difference in your experiences with the regular PI and the BM? Is one easier to hear the differences or easier to set?

Funny you should ask, I should have mentioned that I am mostly a non BM player. To me, the BM is a different animal and is really not a clean sustain, singing type of amp, so I haven't spent much time tweaking the PI in mine.

* Ramble On *
The BM does offer a trick that I like to use and that is, mismatch the impedance (8 ohm load with 4 ohm OT tap) and open up the presence control. Somehow the mids fill in that way and, coupled with the Lead Master set to 6-7 (any other Lead Master setting sound substandard to me, and that is regardless of everything else in the amp, period) 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!

* Ramble Off*

Anyway, considering that + the fact that the amp doesn't have as much negative feedback as the conventional Dumble power section, I wonder if the BM would be as sensitive to trimmer settings.

Gil

[FUCHSAUDIO]

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 !

[odourboy]

Another way to be 'scientific' about it is to hook up an audio spectrum analyzer across the output (I like to use a 8 ohm dummy load for this) and use a sine wave test tone (say 400 Hz) on the input. (I currently have two software spectrum analyzer apps that run on my laptop, one works with my USB soundcard and the other application came with a USB 'Scope.)

When you get the PI trimmer in the sweet spot, you'll see the second harmonic shoot up. My experience is the sweet spot is quite narrow and corresponds to the point AC voltage balance point that Andy describes.

[Structo]

Very interesting topic with the different approaches.

Some say set it with a clean tone, others OD tone.

I find it interesting that some of you say to balance the voltage while others say there should be a difference of voltage between the plates.

For those of you with the trimmer on your PI plates, do you use the same plate load resistor value for both plates or are you using the standard 110k/100k, 120k/110k, etc.?

Or both plates at 100k?

[FUCHSAUDIO]

I'm using 100-100. We've toyed with 100-82, but I use a 20-K trimmer, and can usually match up a PI without much trouble. Sometimes (rarely) I have to switch out a tube or two, but not usually.

[Max]

double

[heisthl]

I'm using 100-100. We've toyed with 100-82, but I use a 20-K trimmer, and can usually match up a PI without much trouble. Sometimes (rarely) I have to switch out a tube or two, but not usually.

I like to use a 20K when not using matched triodes else 10k. I've been using matched Sovtek 12AX7LPs with good results but just had my first non-dead failure, it measured around 320 volts on both plates in a standard config PI. I adjusted it for a 5 volt difference to start the adjustment process and it sounded kind of ok until you tried to get some "pop" out of the low strings and then it had these horrid non musical harmonics. The replacement tube measured around 296 volts initially and everything was fine.
On the subject of PI adjustment has anyone played around with 12AT7 PI trimming? I don't find it to be nearly as rewarding but would like to hear other opinions, also did any real amps have a 12AT7 PI?

[Max]

How does one figure out where to adjust the 5K trimmer feeding the PI plate resistors? Is it a voltage reading? A resistance? Something I need to use a scope to measure?

Hi embotone,

This is the info I got from HAD how to adjust the "Dynamic balance" trim-pots of most of his his 6550A amps (at least all the "blackface" DL/SSS/DL300SL/ODS150 had this pot):

"Calibration procedure just like Ampeg SVT/V9."

The calibration procedure for an Ampeg SVT you can read on page 345 of

"The Tube Amp Book 4th Edition" by Aspen Pittmann".

Of course the voltage values are different for the calibration of an ODS 100 watt. So you will have to calculate the output voltage at load resistor (around half the maximum undistorted output as far as I remeber, but better calculate) for the THD measurements following the same general principle as written there.

As far as I know this trim pot in an ODS 100 you refer to has the same technical function as the "Dynamic Balance" pot in the Dumble 6550A amps. So the recommendation of HAD may perhaps be valid for the ODS too.

As the first step of the calibration procedure before the adjustment of the "Dynamic balance" pot is the setting of the bias:

The bias value given by HAD for 6550A is around 60ma for one tube (look on the backside pictures of ODS 150W). As this is a very high value for a 6550A I would think that he would recommend high bias values for 6L6 too (but I never asked concerning 6L6).

As far as I understood what a tech once told me concerning the setting of the "Dynamic Balance" pot , it's technical function is to assure, that the symmetry of the power amp is as good as possible in all its working range with the given tubes and the given bias setting.

Cheers

Max

[Fischerman]

If your experience is like mine you may find that: (a) Each tube is completely different and some PI tubes will not give up the goods no matter where the trimmer is set; (b) Turning the trimmer, in some cases, will cause the harmonics to appear or go away, and may allow you to dial in "pretty" (2nd order, i.e., and octave above the fundamental) harmonics; (c) In some cases, the harmonics that are generated are ugly sounding ones (odd order), and dialing the trimmer can allow you to tune those (the ones that take off on the wrong note when you pluck a note) out.

Your experience might illustrate why everyone is all over the place on this issue. Different tubes will behave differently...one tube might (just coincidentally) be perceived to sound best with the plate voltages exactly equal. So that person might be led to believe that's the way to do it. But the next guy just so happens to have a tube that he perceives to sound best when he matches up the AC output at the PI plates (this guy also just so happens to have power tubes that are well matched AC-wise). But then the next guy has power tubes that aren't well matched AC-wise and he perceives the best tone when the PI plate outputs are NOT matched AC-wise but rather when the power tube plate outputs are matched AC-wise.

If I'm reading this correctly then is it correct that Andy sets it for equal PI plate output (and I'm assuming he's using well matched power tubes) and dogears/Gil set it by ear...but Andy/dogears usually reach about the same setting? Do you guys do any special screening of your PI tube? Matched triodes?

So just stick a 110k/100k combo in there and be done with it.

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.