Fake PI

[LeftyStrat]

While I am still gathering parts for my planned amp builds (Five!?!), I realized I have parts on hand for an old idea of mine.

I want to put the classic blackface preamp (ala Alembic FB2) and the classic plexi preamp into a single rack space, with the send part of the Dumbleator for each preamp output.

Given that part of the magic of the Marshall preamp is PI distortion, I had planned on having a "Fake PI," however I realize I have never actually seen a schematic for such an implementation.

Would a normal gain stage be sufficient, or is there more to emulating the PI distortion?

[diagrammatiks]

you can't do it with a single ended amp.

even if you could the output stage would still be single ended and it won't sound right.

[Reeltarded]

Umm couldn't you make both pres to the tails, and work it out from there?

I was thinking about something similar.

[Cliff Schecht]

While I am still gathering parts for my planned amp builds (Five!?!), I realized I have parts on hand for an old idea of mine.

I want to put the classic blackface preamp (ala Alembic FB2) and the classic plexi preamp into a single rack space, with the send part of the Dumbleator for each preamp output.

Given that part of the magic of the Marshall preamp is PI distortion, I had planned on having a "Fake PI," however I realize I have never actually seen a schematic for such an implementation.

Would a normal gain stage be sufficient, or is there more to emulating the PI distortion?

Well think about the phenomenon of your power tubes starting to draw grid current as you push them harder and harder. This causes the phase inverter to clip as the grids start drawing more current than the PI can supply. You will have a helluva time emulating power tube grid conduction without having a power tube to cause stuff to clip earlier.

Marshalls do clip in the PI but with the introduction of the master volume the emphasis eventually moved to the preamp before the TS/MV/PI setup. IME you can get a nice Marshall sound with just the preamp up through the tonestack (meaning include the tonestack!). This and the coupling caps are honestly a big part of that sound.

[Reeltarded]

What about simple circuts up to the PI out, and then some simple resistance control, and all this is per channel?

Hold on... PPIMV.. ok, I am not smart enough to imagine something that I know won't catch fire after that.

[9pins]

treat a PI as differential stage, but grab only one side of the output.
might try putting a 1m pot across the outputs of one you have up and running and see what you get as a summed out, swing it back and forth between the sides.

Look at a Ampeg see saw with the fixed resistors in the plate circuit

[oldhousescott]

You will have a helluva time emulating power tube grid conduction without having a power tube to cause stuff to clip earlier.

Couldn't you do something with zeners and regular diodes to simulate the grid conduction? Maybe something like this:

[Reeltarded]

That looks like a 4 bedroom house with a hot tub.

[John_P_WI]

Lefty,

I don't have time to search, but look for posts from or pm "dartanion" .

He discusses this and does this IIRC in some of his posts.

Good luck

[9pins]

If you start throwing sand at the circuit, you'd be better off going to a IC based emulator with a tube SE power side, 5w pod.

do ya need a center tap?

[Jana]

I think this is getting more complicated than it should be.

Just build the PI like it was going into a P-P amp.

Leave off the coupling cap from the bottom side (since you are only driving one half of the P-P power tubes).

Replace the bias resistor with an equivalent pot if you want a master volume there. If you like the 220K bias feed resistors in a Marshall, use a 250K pot. If you like 100K bias feeds, use a 100K pot. Since this is now cathode biased SE, you don't need the bias circuit. The pot, or resistor if not using a pot, goes to ground and not the bias supply.

As for trying to simulate that condition of drawing grid current--don't bother. The best sounding marshalls are the ones that clip in the PI before the power tubes start drawing grid current.

[LeftyStrat]

you can't do it with a single ended amp.

even if you could the output stage would still be single ended and it won't sound right.

I'm not planning on doing single ended. Just the preamp with a line out. This would go into a multi effects unit and then into a stereo tube amp.

And I'm not really looking for high gain tones, just a little bit of the Marshall crunch.

Perhaps another way would be to do the Marshall preamp into a pentode like an EF86.

[Reeltarded]

I think this is getting more complicated than it should be.

As for trying to simulate that condition of drawing grid current--don't bother. The best sounding marshalls are the ones that clip in the PI before the power tubes start drawing grid current.

Me too, maybe.

Also, that point about the PI driving, it sounds like splatty shit when you use. PPIMV and don't limit the feed onto the PI with something. I use a second master or diodes to do just that. I think without the loading, it probably works pretty nice though. A hunch.

[Jana]

If you are just going for line out, then just do what I described and the wiper from the pot I described is your line out.

Might I suggest though that you use a 12AU7 or other small triode as the "power tube" and use a reverb driver transformer for your OT. Then you could include the NFB network just like a full amp. Drive a dummy load with the OT--a 10 watt wirewound resistor should do fine. Maybe include a speaker jack for driving a little speaker for a bedroom amp.

Just some thoughts.

[LeftyStrat]

If you are just going for line out, then just do what I described and the wiper from the pot I described is your line out.

Might I suggest though that you use a 12AU7 or other small triode as the "power tube" and use a reverb driver transformer for your OT. Then you could include the NFB network just like a full amp. Drive a dummy load with the OT--a 10 watt wirewound resistor should do fine. Maybe include a speaker jack for driving a little speaker for a bedroom amp.

Just some thoughts.

Interesting. I have a boatload of 12sn7's lying around, I could go the AX84 October Studio route I suppose.