Steel String Singer PI

[txbluesboy]

In another thread, Funk said you could take aFender AB763 circuit, use a low plate skyline preamp section, ditch the tremolo and use that tube for the PI then use the PI tube location for an Ampeg style driver section. I assume he means the SVT driver, however it has two more tubes, a driver type section and a cathode follower. Do you just use the driver section without the cathode follower? Also can anyone explain this PI to a simple Texas boy? Thanks

[Fischerman]

The Ampeg has a lot of stuff going on there. I think it goes Gain Stage-Cathodyne and the cathodyne is the 'splitter'...then each side goes through another gain stage and then into the two cathode followers (the CFs being the 'drivers'). It's like 5 triodes...or 6 if you count that first gain stage. There's also all sorts of oscillation prevention going on in there.

There is a chapter on the SVT in TUT3 and KOC gives a simplified version of the SVT splitter/driver kludge. Going from memory: Basically it's just a LTPI driving CFs which then drive the power tubes. The CFs are a 12AT7. I'm hesitant to say more because I can't remember the details. But it's really simple the way he did it. Also, I've read that KOC devotes a chapter in a later TUT (maybe TUT5 or 6) to Dumble amps. I'd be interested to read that.

I think when using CFs as drivers you can lower the bias splitters because the CFs have lower output Z. So maybe instead of 220k splitters maybe try 100k.

[txbluesboy]

Thanks, I'll get TUT 3 and ask KOC about the volume with a Dumble section.

[Fischerman]

Hey tx,
Somebody PM'd me about this and I thought I'd post my response. It details the circuit but you'll have to draw it out yourself.

It's in the book so I don't have any electronic form of it. It's just .1uF couplers out of a regular LTPI into the grids of the cathode followers. The cathode follower cathodes each have a 1K resistor and a 22uF cap in parallel and each one of those 1K//22uF pairs connect to seperate 33K resistors to ground. The signal is taken from the junction of the 1K//22uF and 33k resistors...then to the output couplers (.1uF). The output couplers/bias splitters/bias circuit/power tube grid stoppers/screen grid resistors/NFB/etc. are all normal except the bias splitters are reduced to 100K. There are also 470K resistors from each cathode follower grid to the junction of the 1K//22uF and 33K (these bias the cathode followers). That's it. Pretty simple when you draw it out.

The 22uF caps are just 25v.

Note: the CF plates just connect straight to B+...same spot as the PI plate resistors. HTH.

Question: WTF is the word 'resistor' so friggin hard to type correctly the first time!

[ODwan]

Also, I've read that KOC devotes a chapter in a later TUT (maybe TUT5 or 6) to Dumble amps. I'd be interested to read that.

Hi,
KoC's Dumble chapter is in TUT6. I've read it and must say I'm a little dissapointed. It's just an analytic rundown of the basic ODS circuit with some suggestions for mods. Most of which are already discussed on this forum. I've learned a lot more by reading and lurking at this great place!
The rest of the book is great, however!
Timo

[txbluesboy]

Thanks Fischerman, When he said " Its in the book" did he mean some Dumble book, as this is what is in the SSS, or is this KOC's simplified Ampeg circuit. Or are they one and the same? Anyway this looks like what I kind of suspected since the SSS does'nt have but one extra PI tube as far as I can tell. Thanks again

[txbluesboy]

Also the most important thing---how does this cathode follower in the PI affect the sound of the amp?

[Fischerman]

When 'I' said "it's in the book" I meant TUT3.

As far as how it sounds I've never tried it so I dunno.

[chris_sanford]

Guys,

Not saying that Dumble did it this way, but you can check out the Fender 'Super-Twin' for one example of how to do a dual-tube PI. You could lose the coupling caps on the CF portion if you applied the the negative bias voltage to the tail(s) of the CFs.

chris