What PI in this Ampex 2010?

[tonewood]

What PI is this? Trying to figure out how it works & if I would get more gain if I put a 12ax7 in. I'd rather not pop in another tube until I understand what is going on.

Also, any ideas on why R23 is on the cathode? Why not just adjust R2 for the correct bias?

THANKS!

[tonewood]

DC Long Tailed Pair?

[rdjones]

What PI is this? Trying to figure out how it works & if I would get more gain if I put a 12ax7 in. I'd rather not pop in another tube until I understand what is going on.

Merlin calls it the DC coupled long tail pair, I've also heard it referred to as "short tail pair" because of the single tail resistor.
It works the same way a long tail does with the drive for the second phase coming from the cathode, the second grid is AC grounded through a cap.
You won't hurt anything by trying a 12AX7 there but the additional gain may increase the inherent imbalance.
This is sometimes good for guitar amps, not so good for HiFi.
I would not hesitate to play a guitar through that circuit !

Also, any ideas on why R23 is on the cathode? Why not just adjust R2 for the correct bias?

Not sure but I think with C12 tied to the cathode instead of ground that C12 and R23 form a small bit of local feedback for the pentode.
In any case R2 does set the bias.

reddog Steve

[LeftyStrat]

What I find even more interesting is the "Bass and Treble Compensation" on the negative feedback.

I would certainly be interesting to hear this circuit.

[tonewood]

What I find even more interesting is the "Bass and Treble Compensation" on the negative feedback.

I would certainly be interesting to hear this circuit.

That's to compensate for the very small enclosure that the speaker is in.
http://www.audiokarma.org/forums/showth ... p?t=186703

[zjokka]

Hi guys, this thread really helped me out while working on this Ampex 2010... I don't know how rare they are in the US, but they seem wanted by hifi freaks.

What I find even more interesting is the "Bass and Treble Compensation" on the negative feedback.
I would certainly be interesting to hear this circuit.

It's probably a genius invention, but not of much use for use as a guitar amp... there are other issues too, but disconnecting the NFB network is a first step towards unleashing a lot of gain.

What PI is this? Trying to figure out how it works & if I would get more gain if I put a 12ax7 in. I'd rather not pop in another tube until I understand what is going on.

Merlin calls it the DC coupled long tail pair, I've also heard it referred to as "short tail pair" because of the single tail resistor.
It works the same way a long tail does with the drive for the second phase coming from the cathode, the second grid is AC grounded through a cap.
You won't hurt anything by trying a 12AX7 there but the additional gain may increase the inherent imbalance.
This is sometimes good for guitar amps, not so good for HiFi.
I would not hesitate to play a guitar through that circuit !

I didn't, and took some tinkering, but it really crunches like a Tweed Deluxe when cranked. However, the DC coupling to the PI makes it impossible to insert a simple tone and gain control. So the next step is to convert to 12AX7 LTP and insert tone and gain control. Then I should have a look at the biasing of the EF86, and maybe increase the gain a bit - but still following Merlin's pointers

The mains transformer is a 240V replacement (I'm in EU) which remarkably exactly fits the spot of the original one. One thing confuses me: it has a separate winding (yellow) feeding the filaments of the 6AX7GT with 6.6V. All while the preamp filaments feed off a separate winding -- is this common? Also the rectifier seems to be red plating... supply voltage cannot be the problem, looking at the spec sheet. Do I need to hunt down a replacement? Was hoping the yellow wires wire a 5V winding wrongly connected to the 6AX5, so I could rewire the socket for a 5Y3.

[Stevem]

Go to the Aiken amps site to his white papers / tech section.

He has the best info on all types of PI sections!

[zjokka]

Go to the Aiken amps site to his white papers / tech section.
He has the best info on all types of PI sections!

I don't need to - I have Merlin Blencowe's book dispelling the myth of the unequal plate resistors on p.158 while Aiken still maintains:

When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal.