Short piece of wire attached to pin 9 of a 12Ax7. What is your role?

[Chuck H]

There have been other discussions of similar thought too. I remember reading here that one of Ken Fischer's "secrets" was intentionally induced borderline instability in some circuit or circuits to get the effect he wanted. Say what you will about his actual tech knowledge but his amps do sound better than the many clones. Anyway, I can second this. I had one rebuild homebrew long ago that had some minor instability but an excellent OD character otherwise. I moved the OT and the instability was fixed but the tone lost all it's bristley, hairy mojo. Admittedly that amp is still shelved, but the point is that you can intentionally locate, or create magnetic fields to effect the tone (and obviously the noise too) of an amp. I would think this is especially true of higher gain and early preamp circuits.

Chuck

[mooreamps]

I have no trouble imagining a little 3VAC antenna introducing enough reverse phase AC to cancel some other hum source.

I have a great deal of trouble imaging it. Pin 9 is ground. :|

-g

[R.G.]

I have a great deal of trouble imaging it. Pin 9 is ground.

Hmmm. I guess we'd have to know exactly which tube position it was in the pictures to know that for sure, going just from the comment that some tubes in the referenced amps get DC heaters and some don't.

Do all MB amps have pin 9 ground for all 9 pin tubes?

[J M Fahey]

Not taking any sides here, but:
*If* 6.3V filaments are wired in parallel, and *if* a center tap is used, (2 x 100 ohm ), then neither 4+5 o 9 is ground.
Pin 9 becomes "a little 3 VAC source" and the small antenna may well be capable to induce hum at will, to cancel some previously present there.
And we all know filament voltage induces hum in high gain, high impedance circuits., don't we?
That's what filament wires are twisted for, and why their voltage to ground is balanced.

[mooreamps]

I had to look at the print again. It is floating at 3vac ; execpt for V1 which is DC.
But, no current flow, so no magnetic field ; at least not that I can see.

RG. I looked at several MB prints. All of them I looked at float pin 9.

-g

[J M Fahey]

Yeah, fully agree, no current flow.
What I think possible is only electrostatic, capacitive coupling.
Induced/coupled voltages are very low, as previously calculated, but very high gain brings up the ugly side.
Remember the hum that appears when you put your hand a few inches from a sensitive point.
Not current flow there either.
High gain amps are difficult beasts to tame, if at all.

[R.G.]

I had to look at the print again. It is floating at 3vac ; execpt for V1 which is DC.
But, no current flow, so no magnetic field ; at least not that I can see.

RG. I looked at several MB prints. All of them I looked at float pin 9.

OK then, pin 9 waves around at 3V (roughly) of AC. As I said in my post:

A wire all by itself is kind of a monode - no possible current flow, and hence electronically a "don't care" until one remembers that the wire on pin 9 carries the voltage of pin 9 and that all conductors are capacitively connected to every other conductor in the universe...

Attaching a wire to pin 9 and laying it near a high impedance point maximizes the capacitance from pin 9 to the high impedance point. The capacitance is small, but for something like a grid with a grid leak of 1M or so, even small capacitances let some signal get transferred.

It's electrostatic field and capacitive voltage transfer. So whether current flows or not, signal gets transferred. Current flow is immaterial in this case.

[Pedro Vecino]

Hello everyone. As I mentioned, I received another Dual but did not bring the wire. I connected one identical to pin 9 in V2 and in the same way, and after putting the preamp into oscillation (simply replacing two cathode bypass capacitors with long pins and using another model as first 12AX7) no results here. No relation with prevention of oscillations.
I also make noise (hum) tests in different ways without results. Does not affect nothing. At least I have not seen it

The only explanation I can find might be a factory resistance test to check the intensity which supports the track. Previosly in roadking was a problem with a track (+B) that could not support the traffic of electrons. From the company called for cutting the track and replace it with a wire.

Mr. Fahey: Gracias por las felicitaciones. La copa se la guardaremos para la próxima vez. ¡Suerte! .

[R.G.]

...no results here. No relation with prevention of oscillations...I also make noise (hum) tests in different ways without results. Does not affect nothing. At least I have not seen it

The only explanation I can find might be a factory resistance test to check the intensity which supports the track. Previosly in roadking was a problem with a track (+B) that could not support the traffic of electrons. From the company called for cutting the track and replace it with a wire.

Thanks for adding some real data. I guess for the time being it's a bit of a mystery.

[mooreamps]

Why doesn't someone send an e-mail to Mesa Boogie ; and ask one of them ?

[R.G.]

Why doesn't someone send an e-mail to Mesa Boogie ; and ask one of them ?

Good idea Gary. Why don't you?

[mooreamps]

Good idea Gary. Why don't you?

I don't think they would reply to me. I did not purchase their amplifier, and I'm sure the good folks at Mesa know me well enough I never would. It really should be requested by an actual Mesa customer.

-g

[Enzo]

ANy time I contact a manufacturer, I always start out, "I have a customer's model..." That makes ME the surrogate owner.

[mooreamps]

ANy time I contact a manufacturer, I always start out, "I have a customer's model..." That makes ME the surrogate owner.

Good. You write them.

-g

[R.G.]

Good. You write them.
-g

“No – no – I reckon it wouldn’t hardly do, Ben. You see, Aunt Polly’s awful particular about this fence – right here on the street, you know – but if it was the back fence I wouldn’t mind and she wouldn’t. Yes, she’s awful particular about this fence; it’s got to be done very careful; I reckon there ain’t one boy in a thousand, maybe two thousand, that can do it the way it’s got to be done.”
Samuel Clemens, writing as Mark Twain