Oddball Preamp Circuit?

[jbefumo]

I stumbled upon this schematic for the Allen Class Act amp, and am intrigued by the post-volume-control preamp stage, which appears to be combining common cathode and cathode-follower signals from a single tube (see attached.) First of all, is there a name for this? I'd like to learn more about it, so knowing the proper term would be a big help. I tried prototyping it and it seems to sound good. Examining the output in Multisim, it also looks as if the two signals are slightly out of phase, and I'm wondering if this is an intrinsic aspect of the design, or an incidental effect.

Thanks.

Joe

[M Fowler]

Looking for the layout of this amp.

[10thTx]

combining common cathode and cathode-follower signals from a single tube

I am not seeing a cathode follower there?

Maybe I am missing something. Looks like a common voltage divider that is grounded "above" the cathode resistor of the previous gain stage. And then negative feedback off a cathode in that last gain stage.

With respect, 10thtx

[surfsup]

I am not seeing a cathode follower there?

Maybe I am missing something. Looks like a common voltage divider that is grounded "above" the cathode resistor of the previous gain stage. And then negative feedback off a cathode in that last gain stage.

With respect, 10thtx

Either am I. Considering there are two versions of this schematic floating around, the circuit looks to be shunting bass frequencies with a knee at 35Hz (or 159Hz for the other version) back to the cathode of the stage (which is inverted from the plate) to reduce bass frequency output as local feedback, as well as a functioning voltage divider going into the grid of the next stage.

[jbefumo]

Sorry - wrong terminology - I was just referring to the signal being taken off of the cathode and mixed with the plate output through those two 470k resistors. Is there a proper name for that topology?



[quote="10thTx"][quote]combining common cathode and cathode-follower signals from a single tube [/quote]

I am not seeing a cathode follower there?

Maybe I am missing something. Looks like a common voltage divider that is grounded "above" the cathode resistor of the previous gain stage. And then negative feedback off a cathode in that last gain stage.

With respect, 10thtx[/quote]

[jbefumo]

Oh - wait -- is that a feedback TO the cathode? This is definitely skirting the fringes of my current theoretical limitations.

[jbefumo]

Perhaps a more succinct question would be: What is the purpose/effect of connecting the cathode to the output of the stage as shown?

[martin manning]

Yes, there is a bit of negative FB on V1a, but it's so small it will have virtually no effect on the gain or frequency response of that stage. The only significant thing I see happening is that V2b's grid is elevated to V1a's cathode potential (~3V), which is going to make V2b's bias pretty cold.

[jbefumo]

Hmm, so the connection of pin 3 of V2, via the 470k resistor, to the grid (pin 7) of V2 is solely to raise the bias of that stage?

When I put a scope on pins 1 and 3 of V2, I see a signal at the cathode, smaller than that at the plate, and almost, but not quite 180 degrees out of phase. My initial impression was that the two signals were being combined at pin 7 (grid) of the next stage. My sense was that this would reduce the overall level of the signal. Is this not what's happening?

[Andy Le Blanc]

its adapting a high gain pre to SE power side, picture the coupling cap going to the cathode, what happens?
Then add the voltage divider to the next stage.

[Firestorm]

To me, it looks like the output of V2a is mostly being voltage-divided in half (with a little feedback, too). Similar to V1b where at least half the output is attenuated (with the volume up full). That's a lot of gain stages in front of the tone stack. If you don't dump some of the signal, it would be an unmanageable monster.

[jbefumo]

Okay, I see now -- thanks! (Have to confess that I had to simulate what you described in Multisim to grok it.)

[quote="Andy Le Blanc"]its adapting a high gain pre to SE power side, picture the coupling cap going to the cathode, what happens?
Then add the voltage divider to the next stage.[/quote]

[VacuumVoodoo]

This amp schematic is nothing more then Sovtek MIG-50 preamp into a SE output. As for that 3rd and 4th stage duo. Clue: some people say those MIgs have cool overdrive and distortion.

[jbefumo]

I never checked those out - thanks.

I wired this up with a 3PDT switch to defeat that loop, and really like the result. Engaging it cleans things up, reduces gain, and makes the whole thing more articulate.

[jbefumo]

So ... is there a NAME for that circuit topology?

[quote="VacuumVoodoo"]This amp schematic is nothing more then Sovtek MIG-50 preamp into a SE output. As for that 3rd and 4th stage duo. Clue: some people say those MIgs have cool overdrive and distortion.[/quote]