PI trimmer and tube mismatch

[greiswig]

I'm still not sure I understand exactly what you're adjusting when you adjust the PI trimmer, so this may be a dumb question, but...

On tubes whose triodes are definitely NOT matched, you can easily run out of adjustment on a 10k trimmer before you get the 5-10V difference that is typically recommended here. And I know the sweet spot may not be in that range, but the voltage just serves as a guideline.

So...is it maybe better to have a 25k trimmer in there, with more adjustment room, or is a tube that out of balance simply never going to be suitable in the first place?

[dogears]

Thing is that if the tube halves conduct much differently, then the observed plate voltage means nothing! If I use a fully matched and balanced tube, then these generalities hold more true. IMO.

[greiswig]

Thing is that if the tube halves conduct much differently, then the observed plate voltage means nothing! If I use a fully matched and balanced tube, then these generalities hold more true. IMO.

Thanks, Scott. But which generalities are you talking about? The 5-10V guideline, or the idea that (no matter what the voltage difference is) you'll be able to find the sweet spot even with a larger trimmer? (Or both)

PS - I ordered a matched/balanced Sovtek LPS, and it's noisy. Turns out they test for everything but whether it actually generates a squeal. That's why I'm back to seeing if I can make one of my current tubes "magic" using the trimmer.

[bluesfendermanblues]

Why dó you keep talking about DC adjustment and DC voltage difference???
The PI trimmer is for AC adjustment, so get out your tonegenerators and measure The AC at PI outs

[67plexi]

The PI trimmers that I have seen in photos of real Dumbles used a 5k trimmer.
Part # CTS- 201xr502b. Way easy to dial in less than thirty seconds.
For the PI V-3 I use Seimens 6681/E83CC early 1960’s works like no other.
Well worth the extra $$$.

[greiswig]

Why dó you keep talking about DC adjustment and DC voltage difference???
The PI trimmer is for AC adjustment, so get out your tonegenerators and measure The AC at PI outs

Two reasons:

- The one time I tried, I couldn't figure out how. I've tried with a scope and with a voltmeter. I don't end up seeing a change in the AC as I adjust the trimmer
- I have yet to hear anything that says that the audible "sweet spot" corresponds precisely to the point of balance. Some people seem to think that the sweet spot is a little outside perfect balance.

Love to hear a better description of your method, though.

[bluesfendermanblues]

Inject a sine wave signal ~ 400Hz -1000Hz at the amp input and measure the AC voltage at the power tube grids. I set it about 20-30 v AC -between the grids - and use the trim to set each grid the same -measured from grid to ground. Don't forget the dummy load on the speaker out. I use a 10 watt 8 ohm resistor. It gets a little hot, but thats not an issue since the whole procedure takes about 2-5minutes.

This gives you the correct balance, with max signal possible out off the PI. And it'll look great on the scope.

Mark the trimmer position with a pen and check out the feedback-with-telecaster-between-pickups-position-procedure described by others on this forum, and slowly try adjusting the trim around the point you marked.

[FUCHSAUDIO]

Remember kids: AC balance and DC balance are different. You need to set for AC balance, and not worry so much about DC plate voltage. WHile you may find a tube with dc and ac balance happening at the same point, the main concern is AC, as that's the signal. If the tube is mismatched, use it somewhere else (loop, reverb, garbage can etc..)

The lat post about how to set it is very simple and effective. I do it similarly, and it works nicely.

[greiswig]

Okay, thanks for your patience. So you're looking to measure between pins 4 and 5, after getting enough signal in that you can see roughly 30VAC difference between those two?

[talbany]

Inject a sine wave signal ~ 400Hz -1000Hz at the amp input and measure the AC voltage at the power tube grids. I set it about 20-30 v AC -between the grids - and use the trim to set each grid the same -measured from grid to ground. Don't forget the dummy load on the speaker out. I use a 10 watt 8 ohm resistor. It gets a little hot, but thats not an issue since the whole procedure takes about 2-5minutes.

This gives you the correct balance, with max signal possible out off the PI. And it'll look great on the scope.

Mark the trimmer position with a pen and check out the feedback-with-telecaster-between-pickups-position-procedure described by others on this forum, and slowly try adjusting the trim around the point you marked.

I have heard that this is how many of the HiFi guy's adjust the balance using Blues method...Just like to add that they use 2 meters one for each grid (So Grids see the same load) then adjust for equal voltage balance..Thought I'd mention if have a couple meters..

So you're looking to measure between pins 4 and 5,

Pin 5 to ground.. AC
Tony

[FUCHSAUDIO]

Put an AC voltmeter from 1 to 6 on the PI and set for zero volts AC....

[angelodp]

What is an alternative method to injecting the signal if you do not have a signal generator?

Ange

[FUCHSAUDIO]

Get them on the internet.

http://www.audiocheck.net/

[greiswig]

Pin 5 to ground.. AC
Tony

and..

Put an AC voltmeter from 1 to 6 on the PI and set for zero volts AC....

Okay, I just tried both of these, using the AC setting on my Fluke 29. 1kHz sine wave from my mixer. First off, I don't know that I could have gotten to 30VAC measured from Pin 5 at the power tubes: MV was up high, as was the input volume, on the clean setting. At about 3:00 on the MV, I was getting about half that voltage.

Secondly, neither measurement seemed to result in any change in AC voltage from adjusting the trimmer. It's a little hard to read the AC, since for some reason the voltage fluctuates a bit anyway. Should I expect that to be rock solid if the input signal is?

As far as I can tell, I have the trimmer set up properly: voltage to the wiper, then each leg of the trimmer attached to the 86k/100k resistor pair.

Not enough impedance on my DVM? Maybe this is revealing something about why I can't hear the sweet spot, either.

[da Geezer]

Here's a 1000hz from youtube>

http://www.youtube.com/watch?v=R7D1f6U6 ... re=related