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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 6th December 2017, 08:59 PM   #1
GeorgK is offline GeorgK  Austria
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Default Strange volume control and tube behavior in 12AX7 twin-triode input stage

Hi all,

From time to time I stumble over guitar amps that show some strange volume control behavior: With volume fully turned down the amp plays at moderate volume, turning up the control just a little bit decreases volume to - almost! - zero, when turned up further, output volume increases again as you would expect.

I found out that this does not come from a faulty potentiometer (or any faulty part, I suppose) but from some unexpacted behavior of the tube used - always 12AX7.

I recently encountered this again on a Selmer Treble'n'Bass MkIII. The input stage looks like this (sorry for the quality):
tnb_input.jpg

This is fairly straightforward, with the first half of the 12AX7 feeding the tone stack, then the signal goes to the volume pot before reaching the grid if the second system.

Now what happens, with volume turned fully down (and the grid of the second triode therefore grounded!), is that the input signal appears at the plate of the second triode system (marked "1"), with identical level as at the input, but, surprisingly, out of phase.

Now turn volume a little bit up: The reversed signal mixes with the desired signal until there is a point of almost full cancellation. Turning up further simply drowns the reversed signal until it is not noticeable any more.

Now this may sound like just some unwanted crosstalk, but there is more to it. First, it only seems to occur with 12AX7s or ECC83s (no matter what manufacturer).
It also is crucial that the first system has a bypassed cathode resistor while the second has not.
Shortly, the tube is obviously running under some abnormal operational state, with the second triode system behaving rather like a grounded grid circuit.
Remarkably, it is even possible to stop this phenomenon and restore normal working condition, namely during operation. Everything works like expected then - until the amp is being turned off. Let it cool down then for some minutes and it starts all over again.

I did some reserarch on what's happening and would be happy to share my experiences if someone is interested or has been dealing with this phenomenon too. I can come up with some more observations, values and scope screenshots. I suppose there is a name for it, but I have no idea where to look for...
Maybe someone even knows the correct term?

Cheers
Georg
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Old 6th December 2017, 10:14 PM   #2
Malcolm Irving is offline Malcolm Irving  United Kingdom
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There is a small stray capacitance between the two plates in a 12ax7. (I think it’s about 40 or 50pF.)
Other than that, an inadequate HT filter cap, or poor grounding could couple signal from stage 1 to stage 2.
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Old 6th December 2017, 10:51 PM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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The 12ax7 has a gain of 100 so great care has to be taken with decoupling and layout.
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Old 6th December 2017, 11:50 PM   #4
PRR is offline PRR  United States
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GeorgK > possible to stop this phenomenon and restore normal working condition, namely during operation.

How? Playing? Knob?

When "nulled", is it a full-range null, boom, shrill, or variable?

The other possibility is excess ground resistance, but it would have to be >Kohm to be significant.

The third is stray capacitance. When I did a similar circuit, many high settings of Vol and Treb would squeal, until I poked pot-leads with a "Lead" pencil to find what made it squeal worse, and put more separation in there.

I would not think 50pFd a'-a''. Brimar sheet shows <1pFd. Page 2:
http://www.mif.pg.gda.pl/homepages/f...84/1/12AX7.pdf
I don't have a 12AX7 and a C-meter at hand to test that.
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Old 7th December 2017, 08:23 AM   #5
GeorgK is offline GeorgK  Austria
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Hello guys,

thanks for your interest and replies.
The point is - I do not think it's merely stray capacitance causing some kind of crosstalk between the two systems. Let me come up with some more observations. Some of these make more sense, some less. The reason why I do not post them all together is that I think may be difficult to follow.

And yes, my first thought was some weak HT filtering. No success.
Grounding looks ok.

Now for the first measurements:

Here is the signal at the input and output, volume pot fully down. Frequency in all shots at about 500Hz, but this is not important.
Upper trace is the input, lower one the output at plate of V2.
Scaling: 100mV/div
100mVOOP.jpg

I find it remarkable that they are practically identical in level. Coincidence?

Anyway, this signal does not have lots of headroom. If you apply roughly 500mVRMS on the input, the output already looks severely distorted:

500mVclip.jpg
500mV/div

Now for the cancellation:
By turning up volume control a bit you can almost cancel the output signal.
As, other than the disturb signal, this desired signal has to pass the tone stack it is slightly phase shifted, so for best canellation you may have to adjust the tone controls.
The result looks like this:
100mV_canc.jpg
100mV/div

The cancellation works over a wide frequency range, but may be kind of distorted, as an undistorted but phase-shifted signal (the actual audio signal that goes through the tone stack!)is being mixed with a maybe overdriven but exactly 180 degrees-shifted disturb signal.

Now, for the weirder things:

To stop this you simply have to lower the plate voltage of V1 for a moment.
This can be done by either momentarily shorting V1's cathode resistor or removing the HT. You can also turn off the amp, wait for about 5 seconds and turn it on before the heaters have cooled down. Amp works as expected then.

You have to do this on V1.
Temporarily removing HT from V2 has no effect.

The point is, the selmer mk3 has a silicone rectifier but no standby switch.
You can fake the switch by removing the HT fuse when powering up and put in the fuse after some seconds.
Obviously the phenomenon only occurs when HT is applied to "cold" tubes.

I have some more, but I have to leave this for this evening.

Cheers,
Georg

Last edited by GeorgK; 7th December 2017 at 08:43 AM.
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Old 7th December 2017, 09:55 AM   #6
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Quote:
Originally Posted by PRR View Post
...
I would not think 50pFd a'-a''. Brimar sheet shows <1pFd. Page 2:
http://www.mif.pg.gda.pl/homepages/f...84/1/12AX7.pdf
I don't have a 12AX7 and a C-meter at hand to test that.
Yes, sorry, my memory playing silly tricks! There is a long and interesting thread about it here:

Crosstalk between Triodes in Dual Triode Tubes

I think the '50' that I remembered was actually -50dB of cross-talk - Dohhh!
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Old 7th December 2017, 10:03 AM   #7
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Very mysterious! How about looking at the voltages, relative to ground, of the cathode and grid of V2?
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Old 7th December 2017, 10:17 AM   #8
GeorgK is offline GeorgK  Austria
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Nothing special here...
Ug=0V (of course, it is almost perfectly grounded by the volume pot)
Uc=1,8V

Maybe one point that makes the Selmer special is that it runs with rather high voltages, HT at the plate resistor is about 410V, and of course even higher when the heater is cold.
The resulting plate voltage during operation is about 215V.
This is the case for both systems.
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Old 7th December 2017, 10:27 AM   #9
Malcolm Irving is offline Malcolm Irving  United Kingdom
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OK. But how about some scope pictures of ac signal conditions at various points around V2? If V2 is conducting an ac signal plate current, there must be an ac signal voltage difference between cathode and grid (assuming the HT is constant).

Last edited by Malcolm Irving; 7th December 2017 at 10:33 AM.
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Old 7th December 2017, 11:16 AM   #10
GeorgK is offline GeorgK  Austria
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Yes, and it actually is there. You can find an "input" signal on the cathode.
In fact, V2 seems to operate like a grounded grid circuit. Output on the plate is in-phase with the signal on the cathode (and out of phase with the actual input). Remember, V2 has a non-bypassed cathode resistor.
I did not take a photo because the input signal trace looks quite noisy on my scope and is hard to capture on a photo, but it is obviously being fully amplified.

What happens when turning up volume slightly is that you apply a signal of roughly (tone stack!) the same phase to the grid of V2. Eventually, as you would expect, the output signal is being "cancelled" at the plate as soon as they are of equal level.

Questions are:
Where is that signal at the cathode coming from while the grid ist grounded by the pot? I think I can rule out the HV supply. Adding an additional filter cap changed nothing, and there is no signal visible on the supply.
And why does this stop when HV supply to v1(!) is being interrupted for a moment (or to be more exact, plate voltage is shortly lowered?
I tested this with different makes of 12AX7 and they all show the same effect.
Install let's say a 12AT7 and everything is fine.

btw, the ubiquitous Shuguang-made 12AX7 looks exactly like their 12AT7, so I would expect roughly the same plate-to-plate capacitance. Probably they just differ in their grid winding. I tested both. 12AT7 does not show the effect.

Last edited by GeorgK; 7th December 2017 at 11:29 AM.
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