Power amp essentials?

[Mark]

https://tubeamparchive.com/viewtopic.php?t=13811

Firstly an apology, I started the thread and submitted the thread so nothing would be lost, then the damned computer locked up!

Okay, I started the initial thread with a view of getting good advice regarding setting up the trimmer on the phase inverter.

There does appear to be conflicting information and perhaps with everything in one spot I might find the advice that seems correct to me.

Here is Structo's advice:

I was told to turn the amp up quite loud on the clean channel and lightly brush the strings. I was told a single coil neck pickup would work the best.
As you are brushing the strings lightly you are listening for an increase in overtones and bloom, that feels like it's about to feedback.
When you turn the trimmer pot you want to do very small adjustments then brush the strings a few times.

There is a bit of a lag for the adjustment to be heard probably because of the coupling caps.
This was what I missed the first time I tried it. I expected immediate returns on the adjustment.
But no, you have to give it time for the adjustment to be heard.

So brush the strings, listen, turn the trimmer a tiny bit, brush the strings, listen.
Repeat until you get the best bloom into harmonics and good overtones.

Once you find the right spot it will be apparent.
You can also measure the plate voltage on the PI resistors.
The voltage on pin 1 will be around 5-6v higher than pin 6 when adjusted properly.


https://tubeamparchive.com/viewtopic.ph ... ht=trimmer

Iceracer says this:

Phase input trimmer. Located closest to the power tubes. Used to balance the two sides of the phase inverter tube.


https://tubeamparchive.com/viewtopic.ph ... ht=trimmer

Got to go, my 2 year old is breaking stuff!

https://tubeamparchive.com/viewtopic.ph ... ht=trimmer

To be continued.

https://tubeamparchive.com/viewtopic.ph ... ight=ampeg

Fischerman wrote:

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.

Gil Ayan wrote:

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.

Max wrote:

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.

Martin Manning wrote:

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?

Andy Fuchs wrote:

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!


https://tubeamparchive.com/viewtopic.ph ... c&start=15

Brandon wrote:

Plug the amp into an appropriate dummy load, Put a 1Khz signal into the front end of the amp, clean setting, all tone controls dimed, switches set to the most on position, ie mid on, rock mode, deep off, brite on etc. take your scope reading off the output. turn the trimmer until the positive and negative sides of your sine wave are equal.

At that point you can turn the volume up until the sine wave bareley flattens out at the top and take a measurement of the AC output, take that number and square it, then divide by your output impedance(dummy load should be pureley resistive) and you'll have your RMS output wattage.

Then adjust the trimmer by ear till it sounds good

Andy Fuchs wrote:

Ac balance (signal level) and DC balance (tube plate voltage) are two different measurements. The more important of the two is AC balance. I use a Leader Stereo AC volt meter, and measure for the same voltage from both halves of the PI. Another way is to use a well calibrated dual channel scope, set one channel out of phase with the other, and adjust for '0' volts. The output transformer (it seems) is less important. I had set this trimmer at clipping, but no longer do. I set the amp for 5-Watts or so output, and measure right at the PI plates. You can swing the AC voltage about 3db back and forth with 100-K plate loads and a 20-K pot, which is significant IMHO. Some amp builders have posted that "the trimmer is useless and not required", I strongly disagree. Clearly affects OD and clean tones quite a bit imho.

Anyway: I wanted to suggest you use the loop input (or if you have a passive loop, or the ability to enter the PI with your signal generator directly). It's really easy to overload and otherwise mess up the ac waveform going through the front panel input. If you are careful, bypass the tone controls (use a gain boost), watch the incoming levels etc, you can do it. BUT, entering the PI directly is easier, and may yield better results, again IMHO.

Odourboy wrote:

I set my PI trimmer balance using Andy's scope method for a symetrical wave (using a 500Hz sine wave test tone). I then hooked up a real-time spectrum analyzer (a cheapy free-trial software thing) across the dummy load to see what i could see. The analyzer showed a nice abundance of 2nd harmonics. By turning the trimmer, I could get an easily observable drop in 2nd harmonics. I also found that scope setting was near the optimal point, and with a small deviation from the 'scope balanced' point, I was able to get just a hint more harmonics (which leads me to believe that the my 'scope baclanced. position, my not have been the actual point of perfect balance since it's tough to do perfectly with a scope).

[Mark]

Okay, I think I have covered most of the various suggestions.

It would seem to me that there are two trains of thought, one is that it is a tone control of sorts and the other is to ensure maximum headroom within the amp.

My experience with the pot is limited to putting a 1Khz signal into the amp, putting a 8 ohm load on the output of the amp, then adjusting the amp till I can hear (via the load) the tone of amp change due to distortion, I then adjust the pot till the distortion was minimal.

The results were that the amp sounded okay, but I can't say there was night and day difference in tone.

I suppose the issue I have with the various comments is that I wonder how long this state of bliss can last?
Surely as months go by the output tubes are getting further out of balance and the P.I. trimmer becomes less relevant?

[Structo]

I still think the best way to set it is by ear.

Turn the amp up quite loud and then make small 1/8 turn adjustments allowing the new setting to take hold and listen to the amp.

At a certain point the amp will want to bloom into harmonics.

I believe that is the sweet spot.

But, there are several things that can affect this.

#1 the 12ax7 in the PI spot. It may just not be a tube that will work good for the phase inverter and no matter how you set the trimmer it won't bloom.
Time to try a different tube.

#2 The guitar may not put out enough signal for this to work.
Try a humbucker guitar and see if that makes a difference.

#3 Perhaps the speakers also play a role. I use the EVM 12L's and they seem to like it when the amp is balanced and will bloom easily.

#4 Amp and cab placement may play a crucial role in how this works.
My amp and vertical 2x12 cab are situated in a corner of my room.
Not real close to the corner because there is other stuff there, but just the same, a corner placement is going to produce more bass.

[Mark]

Dear Tom

Thanks for your reply. The amp would have to be loud enough for the PI stage to distort surely. If I were to put a sine wave into the amp, and the amp isn't distorting there shouldn't be any harmonics produced.

I concede I may be somewhat naive about the operation of this circuit, but I need a starting point.

I'll give your method a try.

[butwhatif]

I put a 1kz sine wave in, set to approximate normal playing volume and adjust the scope trace to be equal on both sides, done, 5 minute job. i don't see how it could be done by ear,

[dobbhill]

What I do:
Inject a sine wave signal ~ 400Hz (I use the power amp input) to the PI and measure the AC voltage at the power tube grids. I set it about 20vac and use the trim to set each grid the same. Don't forget the dummy load on the OT. This method only balances the PI tube.

The scope method may be better because it balances the entire output section, including the output tubes, in theory.....

As always, YMMV and proceed at your own risk.
D

[dogears]

For me, the most important element is the 12AX7 in the PI slot. Changing tubes has a HUGE difference. Adjusting them to a lesser degree.

I always use matched and balanced tubes for both MU and TC. This way I am comparing apples to apples to the best of my ability. I adjust one to get maximum feel and harmonic depth (similar to Gils post). Then I swap the tubes. Every one is totally different. Even same brand! Then a quick readjust to ensure the selected tube is optimized.

I usually wind up with either Telefunkens, Mullards, or Bugle Boys in that slot. Importantly, tubes with excessive TC don't seem to work as well for me. Strong is good, but not high gain varients.

[greiswig]

FYI, Henry answered me when I asked this same question awhile back, and his is the only method I've been able to use that actually gets me to where I can hear something change. Tele on middle position, clean channel, run loud. Gently brush strings, listen...move the trimmer a hair (less than an 8th of a turn makes a difference when you find the sweet spot!), listen again. What I listen for is a change in the way the guitar is inclined to feed back or start ringing.

HTH,

[Alexo]

Just to be the random jerk asking irrelevant questions:

Aren't even order harmonics asymmetrical? How does maximizing waveform symmetry increase them? Why doesn't this have the opposite effect?

..and more irrelevantly:

All this fuss over getting the best performance out of an LTP... I know you good folks are building Dumbles, but doesn't this imply that for other builds, a better performing Pi where this was a non-issue would be superior? Like a cathodine? Indeed, the descriptions of the optimized LTP's seem in line with what people (myself included) like about the tone of cathodine pi's.

[bluesfendermanblues]

What I do:
Inject a sine wave signal ~ 400Hz (I use the power amp input) to the PI and measure the AC voltage at the power tube grids. I set it about 20vac and use the trim to set each grid the same. Don't forget the dummy load on the OT. This method only balances the PI.
D

I do just like dobbhill

1kHz test signal at input, measuring around 30 VAC on PI out caps.
I adjust the trimmer untill I get 15 VAC between each cap and gound.

Its quite easy, but more left brainish and not as marshmellow as the feedback procedure above

[greiswig]

What I do:
Inject a sine wave signal ~ 400Hz (I use the power amp input) to the PI and measure the AC voltage at the power tube grids. I set it about 20vac and use the trim to set each grid the same. Don't forget the dummy load on the OT. This method only balances the PI.
D

I do just like dobbhill

1kHz test signal at input, measuring around 30 VAC on PI out caps.
I adjust the trimmer untill I get 15 VAC between each cap and gound.

Its quite easy, but more left brainish and not as marshmellow as the feedback procedure above

I love foolproof methods like this. Here's what I've never understood firmly, though: would this accomplish the same thing as the marshmallowish method? I think that's related to another question from earlier...if I balance it perfectly, is that going to get me the most bloom and feedback, or is it that it needs to be just off ever so slightly, or...??

[Mark]

Again thanks for all the replies and procedures.

With regard to these procedures am I to assume that the output tubes are matched?

How are the output tubes biased hot or cold (what percentage)?

Is this bloom/harmonics a result of the P.I. and output stage or the phase inverter exclusively?

I haven't tried anything yet, (remember the two year old ), but I think before I try anything, I would use the CRO to balance the P.I., put individual bias controls into the amp to try and balance the output stage. I know there have been comments about setting the bias and the transconductance/gain of the tubes. I read Merlin Blencowe's book last night and concluded that transconductance, gain, plate resistance are effected by the bias setting. After all a 820 ohm cathode resistor gives more gain than a 10K cathode resistor.

I think this would be a good starting point for experimentation. I also think that this pot would have to be adjusted several times during the life of the tubes as the gain drifts/degenerates.

If you are experimenting with this pot it probably would be a good thing to quantify what is going on with a CRO.

Again thanks for all your replies and assistance. I appreciate all your help.

[Structo]

Good luck Mark.

If you stumble on an easy, foolproof method please keep us informed.

I don't own nor have access to an oscilloscope so I can't use that.

[talbany]

A couple good questions..I'll take a stab at it..

How does maximizing waveform symmetry increase them?

Your not maximizing waveform symmetry your balancing them..

A "phase inverter balance pot" or AC balance pot has several effects... 1st It allows you to balance the output of the phase inverter which will minimize the distortion. Adjusting the typical AC balance pot changes the voltages at the inverter tube, resulting in different amounts of gain...

2nd this in turn means different drive levels to the output tubes, and by adjusting the AC balance we get equal "push" and "pull" from the output.(See Below)
A perfectly balanced push-pull amp will cancel 100% of the even-order harmonic distortion (2nd, 4th, 6th, 8th, etc.) but will not really affect the odd order distortion (3rd, 5th, 7th, etc.). Also, while it is theoretically possible to perfectly balance a push- pull amp, it's a practically impossible to achieve it w/every 12AX you pop in... So the adjustment is a way to minimize - not eliminate - distortion IMO..

Is this bloom/harmonics a result of the P.I. and output stage or the phase inverter exclusively?

A Balanced push pull amp
If you have two tubes fed from a properly balanced phase inverter, one is fed a positive going signal and the other grid gets a negative going signal from the phase inverter, and every half cycle, the PI switches which one is getting the positive going signal at any instant. In class AB, the tubes are biased almost off for the no-signal condition. When one of them gets a positive going grid signal, it pulls current in the OT, and the other one gets a negative going signal that turns it more off; at some point its plate current hits zero, and stays zero until its grid voltage comes back. It doesn't matter HOW off it gets turned, it stays off.

So the active tube plate pulls down on its half of the OT primary. The active, current carrying side gets pulled down and the inactive, non-carrying side gets pulled UP above B+ by an amount equal to the other side being pulled below B+. It's a little like kids playing on a see-saw, but only able to pull their side down with a rope, never push it up.

The pull down causes the flux in the transformer iron to be increased in one direction on one tube's pull, and then in the other direction when the opposite tube pulls. The magnetic field in the transformer iron is where the tube signals mix. (This is where the harmonic balancing takes place and the bloom effect is generated).. The secondary can see nothing of what the tubes do, only what they cause the magnetic flux in the iron to do.

ll this fuss over getting the best performance out of an LTP... I know you good folks are building Dumbles, but doesn't this imply that for other builds, a better performing Pi where this was a non-issue would be superior? Like a cathodine? Indeed, the descriptions of the optimized LTP's seem in line with what people (myself included) like about the tone of cathodine pi's.

An LTP has the advantage that it is less prone to crapping out under overdrive conditions than the cathodyne, but with a little care both circuits can be made immune to this.

Tony

[briane]

I'll add....Turning that trimpot makes a huge difference in the sound I hear.

Just strum the low e, turn the pot till the amp blooms!

I think its very obvious by ear where the amp gets louder, blooms out, and 'sounds right'.

Simple as PI (3.141519......)