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Old 10th December 2016, 07:37 PM   #1
AC439 is offline AC439  United States
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Question Is it common to have different gain in same envelope?

Hi all,

I have just converted my 12au7 preamp from SRPP to Mu-follower topology and love it. Besides the new sound I like, also higher gain as well.

Now I have noticed one slight problem, L/R gain imbalance. Although I used two 12au7, they are not same brand. Therefore, in order to achieve best balance, I used one tube as the top part and another tube as the bottom part. The two tubes also have different gains as well.

I swapped the top and bottom tubes and I got a lower overall gain but a balanced L/R output. I know the bottom tube determines the gain and the top is basically a cathode follower. Therefore, I concluded one tube is imbalanced inside the same envelope - one triode gives higher gain then the other inside the same envelope!

Is this common ? I am relatively new to tubes but these 12au7s are old and the prints are coming off and hard to read. But they do sound good to me (ok, I know you guys would say get 6sn7 but that's all I have to play with for now).

Thanks.
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Old 10th December 2016, 08:01 PM   #2
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Tubes are mechanical creatures, assembled by human hands, even at the best of days they will easily have +-30% differences from tube to tube ( or from
triode to triode in the same envelope as an ecc82)

Designers of the past know this and made circuits that did not depend ( as much) and usually tolerated randomly selected tubes.
( hint : feedback)

When proper design did not help tubes had to be selected, some did this
in-house and usually marked the selected tubes in some way ( "red tip tube"),
The rest of the tubes were sold as new, which they were. Some of these
are still on the market today and sold as NOS.

You have two roads here : alter your circuit to accept tubes with wide
tolerances or select tubes ( or pay someone to select, and you have to be clear
what grounds the selections should be done).
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Last edited by jazbo8; 11th December 2016 at 06:12 AM. Reason: Rule 18
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Old 10th December 2016, 08:14 PM   #3
AC439 is offline AC439  United States
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Thanks for insight. I knew there are certainly differences triode to triode in the same envelope but did not expect such a big difference in the tube I'm using.

I usually measure and match the resistors I use between L/R channels so I will get the best possible match. I don't think there's anything in the schematic I want to alter, especially its a very standard Mu-follower circuit.

I think I'm down to these two - 1) Find more 12au7 and swap them till I find the one I like in terms of sound and balance, or 2) add a balance pot (not what I prefer but can be a quick fix till I find the right tube).
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Old 10th December 2016, 08:26 PM   #4
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Yes, these are your options in a non feedback amp.
Some tube vendors offers tubes that are "matched" ( both triodes alike)
and possible a pair of tubes with similar values, i guess you want tubes with
similar gm as u-followers will auto-correct their bias.
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Last edited by jazbo8; 11th December 2016 at 06:13 AM. Reason: Rule 18, spelling
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Old 10th December 2016, 11:12 PM   #5
DF96 is offline DF96  England
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The gain in an SRPP (and even more so in a mu follower) is set largely by mu, not gm. Mu tends to vary less among samples of a particular valve than gm. However, two different designs of, say, 12AU7 could have somewhat different mu.

I have seen rumours that RFT twin triodes are often particularly well matched. You can find these labelled as RFT, but also under other NOS brands such as 'Mazda foreign' (often with blue print on the glass). Note sure how many of these made it across the Atlantic, though.
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Old 11th December 2016, 10:07 AM   #6
euro21 is offline euro21  Hungary
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Some vendor select -Gm- "matched" tube pairs near/within 1%.

For example:
2 NOS RCA Conn 12AU7A Clear Top Audio Tubes U.S.A. 1960's | eBay

If Gm is almost equal between two halves, the inner structure also very similar, so tube halves output impedance (Rp) is similar.

Mu=Gm*Rp

Almost equal Gm probably means almost equal Mu values.

/ BTW RCA clear top is one of the most "musical" 12AU7 tube. /
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Old 11th December 2016, 11:45 AM   #7
AC439 is offline AC439  United States
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Default Pics of my tubes....

Thanks for all the info. Here's a pic of all my 12AU7.

The left two are being used in the mu-follower. The one on the far left is the top tube, second from the left is the lower tube (the suspect that gives imbalance but higher gain). I could hardly read what's print on it. I am not even able to tell what brand it is.

The two on the right side are used on another preamp I built - two stage direct coupling with second stage as a cathode follower. My idea was to minimize usage of coupling capacitor. The result was so-so, clean sound as a preamp but not too much like a tube amp. I suspect this may be the Sylvania tubes ? (not too good).

Is current production also a good option such as JJ tubes ?
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Old 11th December 2016, 12:16 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by euro21
If Gm is almost equal between two halves, the inner structure also very similar, so tube halves output impedance (Rp) is similar.

Mu=Gm*Rp

Almost equal Gm probably means almost equal Mu values.
I would not assume that. Different things set mu and gm, so you can certainly have similar mu with very different gm. I would not be too surprised to see similar gm and different mu. Mu is set largely by geometry, while gm comes from geometry and other things such as cathode emissivity.
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Old 11th December 2016, 02:33 PM   #9
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Quote:
Originally Posted by AC439 View Post
Thanks for all the info. Here's a pic of all my 12AU7.

The left two are being used in the mu-follower. The one on the far left is the top tube, second from the left is the lower tube (the suspect that gives imbalance but higher gain). I could hardly read what's print on it. I am not even able to tell what brand it is.

The two on the right side are used on another preamp I built - two stage direct coupling with second stage as a cathode follower. My idea was to minimize usage of coupling capacitor. The result was so-so, clean sound as a preamp but not too much like a tube amp. I suspect this may be the Sylvania tubes ? (not too good).

Is current production also a good option such as JJ tubes ?
clean is what you aim for in that design. but if you want less of that, use resistors other than metal films. And no metalized polypropylene caps of any brand. tube changes are typically a 5% change. But if you aim for the circuit to have a certain character, it doesn't matter what tube gets rolled in.

That is why nos tubes are not a big deal to me.

JJ tubes are ok, never had issues with them.

Last edited by DavesNotHere; 11th December 2016 at 02:41 PM. Reason: jj tubes are ok
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Old 11th December 2016, 02:37 PM   #10
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most of Sylvania's tubes were made by Amperex.

Any deviation with gain is due to age and wear. If it is just gain loss with age (just sitting there), usually running the tube for a few days without plate voltage clears it up.
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