PI trimmer and tube mismatch

[greiswig]

Okay, then two things come to mind as I puff my cigar:

1. The paper implies no particular direction of mismatch...in other words, it didn't seem to say that balancing in one side over or under the other was preferable, or that they'd tried it in any direction in particular. But I would think that if small imbalance were all there was to it, you could find two sweet spots...with the FB side 1db higher and with it 1db lower.

2. If 1db is really the point to shoot for, then is there a way to correlate AC voltage to db, so you can set them with a voltmeter for roughly 1db difference, rather than just the balance point?

Gotta go watch the weather report now...

[talbany]

.

If 1db is really the point to shoot for, then is there a way to correlate AC voltage to db, so you can set them with a voltmeter for roughly 1db difference, rather than just the balance point?

They used the various db points to prove the inbalance theory..Keep in mind that was with that particular amp (cathodyne style phase splitter). there are too many factors tube balance, transformer NFB amp to amp so your amp might not perform the same at the 1 db difference..My point in posting the paper is that perhaps when we adjust the AC by ear we are not perfectly balancing the system but causing a small in-balance.. I know this doesn't help you with your analysis problem and may have confused you and others further..I am learning about this right along with you..Here are some scope shots of the experiment..

Enjoy the Weather Women and cigar!!

Tony

[greiswig]

Keep in mind that was with that particular amp (cathodyne style phase splitter). there are too many factors tube balance, transformer NFB amp to amp so your amp might not perform the same at the 1 db difference..

Ah! Yup, that makes sense. Wish I had whatever scope they're using to do the analysis...I like dedicated equipment like that, instead of trying to make my PC work like it.

Okay, I think I'll shuttup and go play my guitar.

[glasman]

Keep in mind that was with that particular amp (cathodyne style phase splitter). there are too many factors tube balance, transformer NFB amp to amp so your amp might not perform the same at the 1 db difference..

Ah! Yup, that makes sense. Wish I had whatever scope they're using to do the analysis...I like dedicated equipment like that, instead of trying to make my PC work like it.

Okay, I think I'll shuttup and go play my guitar.

That looks like an older HP FFT analyzer. They can be had on ebay. Not cheap. The company I work for threw a few of them away last year. I didn't feel like dumpster diving .

I use a Ono Sokki CF-5220, they come up from time to time on ebay, be prepared to shell out about $2500.00 or more.

[stelligan]

And so could the traffic girl from one of our local channels.

[img720]http://1.bp.blogspot.com/_oxXwk8UJ7wQ/S ... drolen.jpg[/img]

She keeps me company while I have coffee before leaving for work in the morning.

Gil

Looks like she can really stop traffic

[talbany]

As Intresting and fun as this discussion was (more so with the News chicks) I think the moral of the story is find the tube with the best range and tone.. Adjust it the best you can with the things on the side of your head don't obcess over it..close it up and Rock the House!!..
Thanks to all those who chipped in..
Tony

[stelligan]

As Intresting and fun as this discussion was I think the moral of the story is find the tube with the best range and tone.. Adjust it the best you can with the things on the side of your head don't obcess over it..close it up and Rock the House!!..

Tony

Amen. Turn it up loud enough and it will bloom.......

[Dr d]

Hope this doesnt sound too dumb, but if we are looking for a slight imbalance, why is it so important the tha PI tube be balanced? Surely the trimmer can make the necessary adjustment?

[stelligan]

In my experience, which is limited, some tubes are so far off the trimmer can't get you there. It just eliminates a lot of tube swapping -knowing the triodes are close together in the first place.

[erwin_ve]

I've been with my head in the ampeg svt balancing procedure. Am I right this method uses cathode AC balance at the power amp. (instead of the PI pins or powertube grids?)
In that case the cathode bias points could be used for measuring AC balance.

[Structo]

That is how Max said HAD advised to adjust it.

This is better resolution of the circuit.

http://www.el34world.com/charts/Schemat ... T_RevA.pdf

[erwin_ve]

That is how Max said HAD advised to adjust it.

Yes it makes sense, because you can take the imbalance of the power amp in consideration.

[markusw]

I've been with my head in the ampeg svt balancing procedure. Am I right this method uses cathode AC balance at the power amp. (instead of the PI pins or powertube grids?)
In that case the cathode bias points could be used for measuring AC balance.

First I tried with AC balance between the bias points. turning the PI trim didn't change AC. On a second look at the SVT procedure I noticed that it says +/- 10mV on a "DC Volt Meter".
Interestingly, when I set the PI pot to minimal DC between the bias points it also resulted in lowest OPT whine.

Peace,

Markus

[talbany]

Guy's If you use the AC scale on your meter I recommend you make up two DMM test leads with a 0.1uf 100 volt or higher rated capacitor in series with each lead. The leads plug into your DMM, and the caps block DC. This allows you to do a more accurate reading of AC balance on the meter's AC ranges.Using a standard interconnect, apply a 100 Hz test signal (at about .30 volt is good) from your function or signal generator..Alot of stereo guys use this method..This method doesn't take into account OPT transformer balance but does take into account tube balance so in this respect is more accurate than measuring at the PI or input grid resistors...

Good Luck!!

Tony

[odourboy]

I've been with my head in the ampeg svt balancing procedure. Am I right this method uses cathode AC balance at the power amp. (instead of the PI pins or powertube grids?)
In that case the cathode bias points could be used for measuring AC balance.

This is the method I've been using also since it was brought to my attention on this forum.

Note that in a 100W build, some builders are using separate resistors on each cathode (I do), which is different than the SVT. In this case, the procedure will not work unless you jumper the cathode test points each pair of power tubes together.