Puzzled by 6SN7 cathode connection

[jordan86]

Can anyone explain to me what is going on with this 6SN7's cathodes (V1)? Or maybe just tell me if this arrangement has a proper name, so I can read up on it?  I've seen resistors and capacitors across triode plates (cut control, snubbers, smoothing caps, etc) and I've seen shared cathode resistors, but I've never seen the cathodes connected ACROSS a resistor like this. Is it corrective, like allowing some sort of "crosstalk" or "feedback" to keep the triodes more closely matched/biased?

[jordan86]

No takers, huh?

[sluckey]

I don't know why. Hammond had some smart people working for them back then. Most of those old tone cabinets had that circuit in them. Some later models even replaced the crossover resistor with a pot and called it "Gain". You're gonna have to do your own research on this one. Captain Foldback is a good starting point. May even have to visit some hi-fi forums.

Remember, that tube is not the PI. Phase inversion/splitter occurred somewhere before that input plug.

[PRR]

Differential input with a little common-mode rejection. May reduce hum in long cross-church lines. Based on a long-tail pair but you need a delta-wye transformation to see it. As Slucky says, the cross resistor can change gain over a small range (such as to balance an odd lot of tone cabinets), and simpler than a classic long-tail.

[jordan86]

Thank you guys! I will certainly do some reading. I just recently learned about differential inputs, but was not familiar with common-mode rejection. My laymen's brain understands it to be in the realm of phase cancelling or balancing. Like in balanced audio. So my curiosity is satisfied. But I will dig some more just to more fully understand.

And yes, Sluckey, you've been very helpful in alerting me to the fact that the 6SN7 is NOT the PI. That's why I am doing a more low gain preamp to start with. I have no clue what an amp will sound like with a stage between the PI and power tubes. But boy, am I excited to hear!

[nandrewjackson]

Aside from the 6SN7 not being the phase inverter, the OT has me lost.


Apparently half of the OT secondary is center tapped as the ground for the output tubes, and the other half is the actual speaker output.


That's new to me.

[tubeswell]

Aside from the 6SN7 not being the phase inverter, the OT has me lost.


Apparently half of the OT secondary is center tapped as the ground for the output tubes, and the other half is the actual speaker output.


That's new to me.

Phase inversion via the 6V6 cathodes from the centre-tapped secondary it seems. (Don't know what the Pri:Sec turns ratio is for this winding, but knowing that would probably help to confirm this)

[HotBluePlates]

Aside from the 6SN7 not being the phase inverter, the OT has me lost.

Phase inversion via the 6V6 cathodes from the centre-tapped secondary it seems. ...

Sluckey said the "6SN7 is not the PI" and that the push-pull signals were created in "the box before this one."

The center-tapped secondary winding looks to me like a "tertiary feedback winding" and the feedback is coupled at the output tube cathodes.

[jordan86]

I guess it would be helpful to find a schematic for the “box before this one”. I will do some digging.

[tubeswell]

"the box before this one."

Those are labelled with two different signal inputs, so it seems there is no previous PI in the box before this one. Seeing has how the output stage is push-pull, then the 6SN7 must function as a differential amplifier when only one of the inputs is being supplied with a signal.

Phase inversion via the 6V6 cathodes from the centre-tapped secondary it seems. ...

The center-tapped secondary winding looks to me like a "tertiary feedback winding" and the feedback is coupled at the output tube cathodes.

Hmm, yes I also did wonder whether that secondary centre tapped winding could be cathode feedback,  I guess it depends on the relative phase of the tubes' plate connections w.r.t. the primary. If it was positive 'cathode current feedback', that would be weird because it would lead to oscillation. So not part of the PI then...

[jordan86]

So if I wanted to test the 6SN7 as a functional phase splitter, could I connect pin 1 on each of the sockets (which feed the grids of the 6SN7) with the same signal? Say after a gain stage and simple tone and volume controls? Maybe coming off a volume pot?

A friend (Hammond enthusiast) mentioned I would need to feed it a balanced signal to test it. I don’t think that would be balanced though.

[sluckey]

So if I wanted to test the 6SN7 as a functional phase splitter, could I connect pin 1 on each of the sockets (which feed the grids of the 6SN7) with the same signal? Say after a gain stage and simple tone and volume controls? Maybe coming off a volume pot?

That tube is a differential amp. It would be very easy to convert it to a phase inverter with a more friendly unbalanced input. Look at this...

http://sluckeyamps.com/rocky/rocky.pdf

[PRR]

... the OT has me lost.   Apparently half of the OT secondary is center tapped as the ground for the output tubes, and the other half is the actual speaker output.

As said: one is NFB (may be small wire, not a winding-space problem). The other is the speakers. Now the speaker lines may be totally un-grounded and also somewhat isolated against external crap getting into the amplifier guts. That may be better in the case of long lines. (It does open a safety issue: what if the OT insulation breaks-down while someone touches the speaker line? But Hammond's quality OT and plug system makes this less likely.)

[jordan86]

Just discovering more about this chassis and tone cabinet. The first stage 6sn7 was typically fed from a Hammond AO-28 or similar chassis. Which appears to have an OT driven by a 12BH7.

http://captain-foldback.com/Hammond_sub/schematics/ao28.jpg

This is way beyond my pay grade. Not sure at all now how I can build a preamp into the H1 poweramp.

[sluckey]

Remember a long time ago in another thread (and again in reply #2)... I told you there was no phase inverter on your chassis? Well, T3 on that AO-28 chassis ***IS*** the PI for your chassis. So, you must add a preamp and a PI into the H1 chassis if you want to use the original H1 circuitry.

[jordan86]

Remember a long time ago in another thread (and again in reply #2)... I told you there was no phase inverter on your chassis? Well, T3 on that AO-28 chassis ***IS*** the PI for your chassis. So, you must add a preamp and a PI into the H1 chassis if you want to use the original H1 circuitry.

I remember clearly. Thanks for that instruction, Sluckey! And I have done just that! So theoretically...the 6SN7 should be happy to see two inputs coming off a very standard LTPI?

[sluckey]

Should work just fine. Please post your as-built schematic in this thread.

[jordan86]

Should work just fine. Please post your as-built schematic in this thread.

Here it is. Only change from this revision is 1M grid leaks coming off the LTPI on the 6SN7 grids. And V1 is on its own node. All the preamp filtering and cathodes are grounded together at the input jack.