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Strange volume control and tube behavior in 12AX7 twin-triode input stage
Strange volume control and tube behavior in 12AX7 twin-triode input stage
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Old 7th December 2017, 03:05 PM   #11
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Originally Posted by GeorgK View Post
Where is that signal at the cathode coming from ... ?
Any chance that the tag-board, or whatever the circuit is constructed on, has become conductive due to age and moisture absorption?
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Old 7th December 2017, 07:12 PM   #12
GeorgK is offline GeorgK  Austria
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Well, I had thought of that...
Anyway, nothing I could measure with an Ohmmeter.
And how could this explain this weird behavior regarding powering up hot or cold filaments? Why can I make it go away as long as power is not turned off and on again? And why only with 12AX7?
Besides the input inpedance of the cathode is pretty low, so I rather do not think that a significant amount of signal could creep in there.

TNB3 IP board.jpg
Here is a blurry photo of the input stage, the cap far right is V1 cathode resistor bypass, then Rc, Rc of V2, then plate resistors and decoupling caps of both systems. About in the middle of the pic you can see the components of the summing amp that comes after both input stages. Yes, the output of V1 is not too far away from the cathode of V2, but still... Bending the socket wires to different positions made no difference either.

I have to mention, I don't have the Selmer at hand any more. Everything I write down here comes from my notes.
But what I did try was to install a bypass cap over the cathode R of V2. Strange things happen:
If I install it with the amp powered off, the whole stage is ok. Overall Gain is higher of course.
But if I install it during operation (don't try this at home, folks! ), the signal on the cathode of course disappears but that signal on the output only gets lower by maybe 30 percent. Which makes me speculate that there is some crosstalk is happening (within the glass?) that still is modulating cathode current. Something that is only being triggered when power is switched on with cold tubes...

Also, everything is fine here when V1 is running without bypass cathode cap. I suppose it has something to do with gain of v1 being lower then. Unfortunately I have failed to check what happens with the cap being removed under operation.

It is not the first tiime I see such a phenomenon. I remember a couple of years ago, I had a Fender Pro Junior on my bench that showed the same thing. I did not have the time to do more measurements as on the Selmer, but finally installing a 12AT7 helped.
What the Fender Pro jr. and the Selmer TNB mk3 have in common is the lack of a standby switch and a similar input stage, although the Fender has no cathode bypass cap on the first stage.
And I am aware of the fact that also Pro Junior players are commonly not plagued by such issues.


Last edited by GeorgK; 7th December 2017 at 07:33 PM.
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Old 8th December 2017, 11:17 PM   #13
PRR is online now PRR  United States
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Originally Posted by GeorgK View Post
... input and output, volume pot fully down. ...I find it remarkable that they are practically identical in level. ....
I think it remarkable it plays, audibly, with "volume pot fully down". Excusing the typical >-50dB pot leakage, I would expect "no" sound. Most buyers/users would. So there is A Fault.

Is this only with the one Selmer? All of this model? Other models? brands?

I would really be thinking a bad connection. Looks soldered. Isn't. I have seen that happen at the factory and the amp worked well and hard for years (before catching fire). I have seen many oxide "contacts" re-make after disruption, including supply cycling. Tube pins/sockets are prolific bad-contact points. Bad contacts *inside* resistors and caps are rare but possible.
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Old 9th December 2017, 04:22 PM   #14
GeorgK is offline GeorgK  Austria
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thanks for your thoughts.
As I mentioned, I remember seeing a similar problem in a Fender Pro Junior.
Can't be a design problem regarding they must be produced in ten thousands a year.

It's not simply a matter of power-cycling, the difference obviously is whether the cathode is cold or pre-heated at the moment HT is applied.

I watched the "leak" signal at the moment when power is being turned off. For one or two seconds, as the voltages get lower, the signal decreases until a certain point is reached where it abruptly goes to zero. When the amp is being turned on again before this moment the leak signal stays. When turning it on afterwards (but before the heaters are fully cooled down) you get rid of it.

And as I mentioned, it has nothing to do with a leaking pot. This signal has not passed the tone stack (verified with square wave input) and appears at the plate of v2 with 180 phase shift.

Can be seen on either of both input channels, btw. They are identical except for the tone shaping.

Also not a matter of power supply load. Pulling the mixer tube and therefore kepping the signal away from the power amp does not change anything.

Is it a coincidence that, at low levels, it is exactly of the same level as the input? I don't think so.
In case of the Selmer, this is enough to let it play on, let's say, moderate bedroom volume.

The amp came to me for a different reason and the owner never bothered about it because he plays it loud only and never at home.

I do not think all Selmers suffer from this. The mk1 (and mk2 I think)has a tube rectifier and therefore is not prone of this anyway.

I even remember encountering something similar on my own 1968 Deluxe Reverb when trying out different (mostly old) input tubes. Did not investigate this any further because I thought it was a matter of the tubes used. As the Selmer is gone now I may have a chance to reproduce the effect on my amp.


Last edited by GeorgK; 9th December 2017 at 04:30 PM.
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