How to adjust summetry?

[tvrgeek]

OK, next step, play with the symmetry of the splitter. Does this sound valid?

Hook up the analyzer and generator, adjust pot for lowest 2nd harmonic.
Use 4K or so.

[Poindexter]

Yes, that will do it. You might also want to test your generator first, to verify that the splitter isn't preserving asymmetry in the original signal.

A better tool for this, actually is a DVM, since it's dificult to read the 'scope to a hundredth of a volt. I seldom use my 'scope for basic stuff like this; it takes a while to set up and calibrate, and takes up a lot of space on the bench.

Aloha,

Poinz
AudioTropic

[tvrgeek]

That's why I was thinking of using the spectrum analyzer. Just look at the output and trim for lowest second order harmonic. ( TrueRTA on my PC) Gad, these tools we could never afford when I started playing with stuff. I guess the procedyure you reccomend is to measure the AC on the resistors feeding the output grids. I'll do it both ways and see what happens. Somehow playing with tubes seems right while watching the snow melt.

[Johan Potgieter]

My apology for perhaps unnecessary advice, but just keep in mind the effect of the probe itself, especially if a 'X 1' is used. A few tens of pF will make a difference at high audio frequencies.

[tvrgeek]

Ah, forgot about that. Just one more reason to build a buffered high Z interface so I can use my scope probes on my computer. Tubes do have a lot higher impedance. I am more used to solid state where it is not as much a factor. Seen any good designs? I was thinking a tube input.......

[tvrgeek]

Did the following:
With DVM,. 4K sine in, measured AC on the grid of the two output tubes. Whatever the input is, it is 2.7 V out on the transformer into 7 Ohms resistance so I am adjusting at a known power of about a Watt.

Adjusted the symmetry for identical readings ( around a volt) on the tubes.

Measured the THD. Second order was worse by a couple of DB, third was slightly lower.

Just using the RTA on the output, adjusted for minimum 2nd order. Gained about 2 dB reduction. But the third order went worse by a couple of dB. ( we are in the .5% range here). Is this expected? So, it would mean a balance, or my guess, a preference to lower third?

[tvrgeek]

Does this tell me my outputs are grossly miss-matched and I am compensating for them with the symmetry?

I went and did it all again, this time same trend, but being a lot more aggressive, found I was in a small peak, but went further and reduced the second harmonic from about .43% to about .14% Third holding steady.

Also notice, one channel had twice the 4th and 5th as the other, where second and third are almost identical.

Any explanations would be nice. Kind of fun. I am going to try and build some shielding for the transformers, as they have none and worse, bolted to a steel chassis. Filaments run about 6.05V, so guess that is OK. B+ is up just a tad at 264 rather than 258. Good as it gives me some margin to think about a regulator, or an LC filter. But gad, are chokes expensive.

[tvrgeek]

Let's see if I understand the theory here. In the Meng, there is no adjustment for bias on the outputs. So, with miss-matched tubes ( what da' ya expect for a hundred clams), I can match the splitter symmetry buy adjusting exactly the voltage on the output grids, but the end result is way off. By miss-adjusting symmetry, I am actually compensating for the output tube variance. Do I understand?

This would be why each tube has it's own bias adjustment in the higher end amps. otherwise one needs perfectly matched tubes.

Details. Is there a "standard" or preferred frequency for adjustment? A standard level? Half power, full power? I did this at 1K about 1V, picked by convenience of what my sound card was happy putting out and measuring.

Note to other owners, it was balanced way off by either method.