S-217D transformer substitute

[Quayhog]

How important is the center tap terminal on the Peerless S-217D transformer?From my study of the Pultec circuit it looks like the CT with a 1000 ohm resistor is the kathode resistor for the 12AX7.  Could you use an artificial ct like with heater circuits?

I have found a 600 ohm line output push pull transformer that is currently connected to each plate of a 6SN7 tube.  It has tertiary winding providing feedback to the kathode of a 6SL7.  The schematic shows one side of the feedback winding going to ground and the other side through a 10k resistor feeding one side of a 6SL7's Kathode.

I was thinking of the practcalitity of using a 2k or other appropriate value resistor on each leg of the Pultec's 12aX7's 820 ohm resistor.  Will it work?  I don't have the transformer in hand, I just won an eBay auction this morning.  I'll see it and inspect and test after delivery.

 

[vari-mu]

Hi , Quayhog !
You can try this circuit FB.
It works, but there are nuances.
Excitation at high frequencies .

 

[Quayhog]

The Pultec EQP1A is a mysterious and complex circuit.  That's part of what makes it fascinating to me.  The unobtanium Peerless S-217D  with its tertiary feedback coil is part of its complexity.

I have found a line output transformer from a Dukane 2A40A that has a tertiary feedback winding, but, it lacks the centertap.  I'm thinking of using it in a DIY build.  I'm going to try to make an artificial CT in a similar way we commonly do to heater circuits.

HOW IMPORTANT IS THE INTEGRAL CENTER TAP ON THE TERTIARY WINDING?

I have two complete restored 2A40 preamps that sound wonderful.  The one that donated my sample was from a barn fresh unit with a lot of missing parts, so nothing was harmed by further robbing it for another project.

 

[shabtek]

Did not look at pultec-- but the tertiary ct is the return/ path to ground for the ax7 iirc so an alternate path will be needed. RDH has good info on various forms of feedback including tertiary windings.
Id try to establish bias with a resistor from each cathode to ground,  and fine tune response with RC and tertiary across cathodes.