PPI Master w/cathode bias

[skyboltone]

Greetings Lads:
Will this work? I think. Help.

[img:800:593]http://tinypic.com/6b9fjol.jpg[/img]

edited out 2.2meg resistors.

[oldhousescott]

You don't need the 2M2 resistors. The grids of the output tubes are, as you note, tied down by the 220k resistors, and the outputs of the PI are always loaded by the 250k pots -- even if the wipers lose contact.

[stoo]

Why not just substitute 250K dual pot for the 220K resistors and take the wipers to the grids?
Stew

[skyboltone]

Why not just substitute 250K dual pot for the 220K resistors and take the wipers to the grids?
Stew

I don't think so Stoo. The 220s are there to keep the power tube grids 220ohms above ground providing correct bias. Your way you could ground the grids with the wipers at 0. Wired the way I show, the signal from the PI goes to ground with the wipers taking an adjusable PI signal to the grids.

What I need is somebody smarter than me to tell me if 250K is too small and that I"ll loose too much gain from the PI. Maybe those need to be 1 meg pots. I dunno.

[dartanion]

Go with a 1M ganged pot.

Here's the standard Post PI MV that the 18watt community uses:

[img:400:400]http://www.18watt.com/storage/18wattMV.gif[/img]

You may consider biasing the PI a little warmer as the one in your example will tend to be fizzy with that 1.2K Rk. An 820 is a good compromise

[stoo]

Why not just substitute 250K dual pot for the 220K resistors and take the wipers to the grids?
Stew

I don't think so Stoo. The 220s are there to keep the power tube grids 220ohms above ground providing correct bias. Your way you could ground the grids with the wipers at 0. Wired the way I show, the signal from the PI goes to ground with the wipers taking an adjusable PI signal to the grids.

What I need is somebody smarter than me to tell me if 250K is too small and that I"ll loose too much gain from the PI. Maybe those need to be 1 meg pots. I dunno.

Umm...have a look at the 18 watt schematic you posted! are not the grids grounded out when the mv os on zero?!! Well there's still the 8.2k grid stopper but otherwise the same as I suggested. Actually yours are too!
Stew

[skyboltone]

Go with a 1M ganged pot.

Here's the standard Post PI MV that the 18watt community uses:

[img:400:400]http://www.18watt.com/storage/18wattMV.gif[/img]

You may consider biasing the PI a little warmer as the one in your example will tend to be fizzy with that 1.2K Rk. An 820 is a good compromise

Thanks Dartinion, RWC eh? Weather best by government test.....

Anyway, I was hoping somebody from the 18watt group would jump in. The scheme you've posted is altogether different than the Matchless Spitfire I've shown. So the grids are 500K above ground. And a 150u cathode cap! Whoop! I can fool all around with this between what I've shown and yours I think and see what happens. I'll check out the 820 RK too.

http://forum.metroamp.com/viewtopic.php ... c&start=60

These guys on the above thread figure theirs will do everything including make your daughters marry rich. But they're playing with fixed bias so there you go.

Thanks
Dan

[skyboltone]

Why not just substitute 250K dual pot for the 220K resistors and take the wipers to the grids?
Stew

I don't think so Stoo. The 220s are there to keep the power tube grids 220ohms above ground providing correct bias. Your way you could ground the grids with the wipers at 0. Wired the way I show, the signal from the PI goes to ground with the wipers taking an adjusable PI signal to the grids.

What I need is somebody smarter than me to tell me if 250K is too small and that I"ll loose too much gain from the PI. Maybe those need to be 1 meg pots. I dunno.

Umm...have a look at the 18 watt schematic you posted! are not the grids grounded out when the mv os on zero?!! Well there's still the 8.2k grid stopper but otherwise the same as I suggested. Actually yours are too!
Stew

Yup. You be right on both counts Stew. Hmmmmmm.

[skyboltone]

Why not just substitute 250K dual pot for the 220K resistors and take the wipers to the grids?
Stew

Umm...have a look at the 18 watt schematic you posted! are not the grids grounded out when the mv os on zero?!! Well there's still the 8.2k grid stopper but otherwise the same as I suggested. Actually yours are too!
Stew

Yeah man. The more I look at it. Ok. I'll try it. It's down the road a bit but when the time comes. I'll keep you posted. Like this:
[IMG:800:593]http://i11.tinypic.com/4vs0fgw.jpg[/img]

Dan

[stoo]

Please do! Interesting stuff! This is basically what that 30 odd page post over on MetroAmp is about except they're talking fixed bias. So instead of grounding out the low end of the pots, the bias supply is there instead.
Stew

[David Root]

One meg pots will be more susceptible to parasitic oscillation/RFI. That's why the Rich Mod on the Metro forum uses 250K, and the original Ken Fischer version used 100K pots.

I've built the Rich Mod with 250K pots, it works as advertized and no RFI worries either.

[dartanion]

One meg pots will be more susceptible to parasitic oscillation/RFI. That's why the Rich Mod on the Metro forum uses 250K, and the original Ken Fischer version used 100K pots.

I've built the Rich Mod with 250K pots, it works as advertized and no RFI worries either.

The schematic I posted is a tried and true PPIMV for the 18watt platform. Although you do have larger grid stoppers on the power tubes that will help quell high frequency oscillations.

[skyboltone]

Ok I got my Hammond Organ amp from which this little guy will grow. It turns out to be an A0-35 reverberation amplifier. This one:
[img::]http://www.captain-foldback.com/Hammond ... s/AO35.JPG[/img]

I was going to go straight Matchless Spitfire with the "aforediscussed" master volume mod but I've been looking at some Carman Ghia schemes which also has a very simple front ends. I know, it all depends on what you want. The Matchless uses parallel plates on a single 12AX7 with a tone control and no recovery stage. The Ghia uses a more conventional front end. I hate to post both of these because though the Spitfire is out there on the net the Ghia owner gets upset when his scheme shows up on line.

What sorts of differences would you predict coming from these two schemes.

[tictac]

You need 4 caps after the PI for that kind of MV, 2 between the PI plates and the MV pot and 2 between the wiper and the power tube grids. Also you need to up the uf value since your putting the caps in series. Some have said x10 value but you can adjust to your own tastes.

[dartanion]

The AO-35 is VERY close to a Ghia. That's what he used to base his design from regardless of what he says.