What kind of PI is this? Marshall 20W

[Matt T.]

What kind of PI is the one in the attached schematic...ie. the Marshall 20W Bass. What are the pros/cons vs. a long tail pair?

[DirtyGeorge]

I don't know what this PI is called, but as for pros and cons I would expect it to be less balanced than a LTP. I suppose the difference in plate loads would help in that respect. It should handle a larger signal swing since there is no tail resistor to drop voltage. And you don't need a coupling cap between the volume control and the input since the grids are referenced to ground.

DG

[hasserl]

bump

I'm interested in learning more about this PI too. Can anyone compare these 20 watt Bass amps to the more popular 18 watt?

[Ptron]

I think D.G. pretty much said it, but here's some info paraphrased from Kevin O'Conner's TUT1:

It's a predecessor to the long tail pair called a Schmitt splitter. The second grid is AC grounded through the .1 uFcap and DC referenced to ground through the 1M resistor. It's big disadvantage is balance accuracy. The cure for this is to direct couple the input to the last stage(not done here), raising the grid and cathode voltage up to say the 100V range. The changes in cathode current due to the signal are now much smaller compared to the overall current and the cathode resistor functions more accurately as a constant current source.

Maybe Marshall used it for it's simplicity? It does use a few less components than a standard LTP.

[Matt T.]

Thanks for the responses.

Ptron,
I'm considering putting this circuit into my Gibson GA15 (Trace Elliot Velocette in different clothing). The voltages are almost exactly the same and it's SS rectified. Since I have only one input jack I was thinking of just making it single-channel...which means I'll have an extra triode (my amp has only two 12AX7s). So I thought about shoe-horning a direct-coupled cathode follower after the first stage. The cathode voltage on that cathode follower should be in the 100vdc ballpark. But I expected to put the volume/tone controls after the CF...can anybody think of a way to direct-couple the CF to the PI input and still have the volume/tone controls?

Anybody got any better ideas/circuits to put into this SS rectified, 2x 12AX7, 2x EL84, cathode biased amp?

I thought I had read that many people thought the new 20W HW sounded better than the 18W HW...is that correct? I also read that somebody brought an all original 20W to the most recent Tonefest in Florida and it got rave reviews...seems everybody was gushing about it. So that's what got me interested in this circuit.

[Ptron]

Well, I'm not really an expert here, but I would think the imbalance would be part of the character of this amp that people like so much and you might not want to "correct" it. If you did, it would probably just be easier to use a long tail pair.

[DirtyGeorge]

I'm with Ptron on this one. Keep the PI as is. Use the direct coupled CF off the first stage to buffer the tone control.

DG

[hasserl]

Or, use both triodes in a cascode stage as in the Weber Smokin Joe II : https://taweber.powweb.com/store/smokeII_schem.jpg

Or, use the second triode as a second gain stage after the Vol/Tone controls.

Or, ditch the 1st 12AX7 and wire the socket up for an EF86.

Or....

[Matt T.]

Thanks hasserl, there are a bunch of different ways to skin this cat.

I had a long post written up but as usual this site makes my browser crash so I lost it. But I at least wanted to post that Marshall revised this PI and apparently used the revised PI in most amps...including the 2061X reissue. Schematic here.

Lots of good info here.