Preamp Tube Channel Balance

I recently acquired a rebuilt and fully calibrated Hickok 6000 tube tester. Like most of you, I have acquired a very large number of various preamp and power tubes. I also never seem to throw a tube out, even though they are bad! I decided to go through my entire collection on the Hickok, and was a little shocked at the results. Out of 25-30 12au7s and 12ax7's, I have only three Mullard CV4003's that are perfectly balanced. The rest are off by as much as 20%, with a few complexly dead channels as well. The Hickok runs a B+ of 190v, which is less than these tubes see in a VTA-70. Is there a way to test the L/R balance of a 12AU7 preamp tube in a VTA low gain board, while in-circuit? Does a 20% channel difference in a 12au7, result in a 20% difference at the outputs, or does the amp somehow correct for this? I also tested 20 EL34s, and saw a 10-20% variation on those as well. My brand new Shuguang's consistently tested 15% higher than the same tube after 1 years use. Thanks,

Mike

Not familiar with that tube tester but I'm wondering when you say 20% what exactly are you measuring?  Transconductance?  The measurement you really need at least for the the VA (center) position is gain.  I think that gets measured differently.  I don't know.  NOS tubes can be a crap shoot.

I am measuring "Mutual Conductance" on the Hickok 6000. As an example, I have a RCA 12au7 that measures 2200 micromhos for Triode #1, and 2000 for Triode #2.

Mike

mazeeff wrote:I recently acquired a rebuilt and fully calibrated Hickok 6000 tube tester. Like most of you, I have acquired a very large number of various preamp and power tubes. I also never seem to throw a tube out, even though they are bad! I decided to go through my entire collection on the Hickok, and was a little shocked at the results. Out of 25-30 12au7s and 12ax7's, I have only three Mullard CV4003's that are perfectly balanced. The rest are off by as much as 20%, with a few complexly dead channels as well. The Hickok runs a B+ of 190v, which is less than these tubes see in a VTA-70. Is there a way to test the L/R balance of a 12AU7 preamp tube in a VTA low gain board, while in-circuit? Does a 20% channel difference in a 12au7, result in a 20% difference at the outputs, or does the amp somehow correct for this? I also tested 20 EL34s, and saw a 10-20% variation on those as well. My brand new Shuguang's consistently tested 15% higher than the same tube after 1 years use. Thanks,

Mike

Tubes, as mechanical devices, made by hand, is not perfect devices. In fact even at it's hayday
the spread between devices was significant. That's why amplifiers uses feedback that cancels the
effect of non-perfect devices.
You ask if a 20% difference in parameters will result in 20% of sound volume ? No, the answer is that
the circuit used is forgiving, and will be close to immune of these differences. Other designs
might not be that immune.
About powertubes, here it is a little more important, thus the need for matching powertubes
( close to the conditions they will be used ) to avoid saturating OT with unbalanced tubes
and overloading the feedback chain when two tubes have large difference in Gm.

It depends on the design of the amp, yesterdays designers was aware of this.

Thanks. If I focus on the three 12au7s used on the low gain board, where is it more important to have balanced triodes within a single tube. Preamp or phase splitters?

Mike

I guess Bob will fill in here, but as both positions seems fairly well designed +-10% should be
more then enough.

The preamp has local nfb, and the splitters are balanced with a current source. On top of
that nfb will furher reduce inbalance.

I use the most sensitive test equipment ever devised ... my ears.

Granted, a center driver with perfectly matched triodes, as well as matched tubes in the sides, should get you perfect balance, but even then, expect some inconsistencies in the room (and source) to throw things off. That's why I went with individual volume pots for each channel on my custom build ...



You'd be surprised how small a tweak it can take to make your music pOp ...

In my case, I do not have control over Left/Right balance. I am also a little confused as to how the nfb circuit can help restore balance to a slightly (+/- 10%) unbalanced preamp tube.

Mike

mazeeff wrote:In my case, I do not have control over Left/Right balance. I am also a little confused as to how the nfb circuit can help restore balance to a slightly (+/- 10%) unbalanced preamp tube.

Mike

The short version is that :
- an open-loop amp ( has no NFBB) is very dependent on it's component, both as "raw" amplification and
frequency behaviour.
- NFB will make the amp more or less independent of the tube as the amplification is (most) a
product of Rin / Rout  ( see https://en.wikipedia.org/wiki/Negative_feedback_amplifier )

A good verbal description is at http://www.learnabout-electronics.org/Amplifiers/amplifiers31.php

The VTA has NFB is several ways, both "local" and "global". Thus the effect of +-10% of a tube
does not really matter.

Got it. The links helped a lot. Thanks,

Mike