Shared 12AX7

[DavebF]

I've noticed something strange on an amp I'm building and I would like some opinions on it. In the amp the second stage and the third stage share the same 12AX7. With an audio generator fed into the first stage if I turn up the preamp gain control all the way (it's located between the 1st and 2nd stage) even if I ground the grid of the third stage have of the 12AX7 I can still get the output from the speaker! Doing some investigation with a scope I noticed as the output signal strength from the 2nd stage gets progressively louder (as measured at the plate of the first half on the tube) the signal starts to show up on the plate of the 3rd stage (second half of the 12AX7) at a very small level but, enough to eventually be amplified by the rest of the amp. Upon further investigation of this signal on the plate, I noticed that as I bring the scope probe near the center of the tube (from the bottom of the chassis) I can actually get the same signal induced into the probe/scope without even making physical contact to anything! Is this signal somehow getting into the third stage half on the 12AX7 from the second half internally to the tube? One further note that thought might be a culprit is that both half's of the 12AX7 use the same resistor and bypass capacitor network on the cathode for bias.

 

[rayma]

Is this signal somehow getting into the third stage half on the 12AX7 from the
second half internally to the tube?

Yes, there is coupling between the triodes of a 12AX7, mostly from one plate to the other grid, since that is normally at a higher impedance level. A bypassed cathode shouldn't transfer much signal between the sections.

 

[DavebF]

But, I have the grid of the second triode jumped directly to ground and on the scope no signal is appears on the grid but, it's on the plate? Shouldn't the grid jumped to eliminate the grid as the source?

 

[rayma]

Then it could be coupling to the plate circuit, since it's not possible to be to the cathode or grid. If you disconnect the plate voltage from the "receiver" plate, and leave the plate unconnected, you can see if the coupling still happens passively to the plate in the same magnitude.

 

[DavebF]

Good idea, I'll try that.

 

[nigelwright7557]

If you look up the 12ax7 datasheet you will see there is capacitance between all the internal parts of the tube.

I have to admit to having problems with cascade stages in 12ax7's before and gave up and used a 12au7. This could be why a lot of guitar amps parallel the 12ax7's electrodes. The gain is much less in a 12au7 (about 18 as opposed to 100) and doesn't suffer from inter-electrode capacitance so much.

 

[rayma]

Good idea, I'll try that.

Keep the second tube's grid grounded, and its cathode bypassed to ground (or grounded).

 

[DF96]

Some datasheets may give the anode-anode capacitance. Circuit strays will increase this.

 

[PRR]

...second stage and the third stage share the same 12AX7.
...both half's of the 12AX7 use the same resistor and bypass capacitor network on the cathode for bias.

Generally a bad idea. At some extreme it is a multivibrator. The cap has to be quite large (actually much larger than the forward path bass response) to not have at least some sneakback.

Fender(?) did it that way to save a penny. You do not have to be so cheap.

 

[DavebF]

I checked this morning and saw a small amount of signal on the common cathode bias resistor/capacitor network. It reduced when I jumped-in an additional capacitor across the cathode resistor. Thinking that could be the issue I ordered the parts to create separate bias circuits for the triodes (actually a third tube shares the same bias voltage). This portion of the amp is a carry-over from the original amp a Hammond AO-29.