Alternate transformers for Pultec EQP-1

[Quayhog]

In an old thread titled "Transformer List For Projects" CJ listed a whole bunch of different trannies that would be useful for projects. Surely with different sonic results.

One transformer he notes as an output is a UTC HA-133.  I happen to have one and I'm collecting the important bits for a Pultec DIY build. The HA-133 doesn't have the tertiary winding the original S-217D has.  Is that winding and it being the kathode resistor part of the circuits  magic? 

I also have a plate to 600 ohm output resistor that does have a non CT tertiary winding.  It was removed from a Dukane 2A40 mixer.  The mixers sound nice.

Which should I use since the original output transformers are impossible to find and when found are prohibitively expensive?  Seek out an old or go with modern reproductions.?

I have an original HS-56 and a vintage windings filter unit.  What would a good alternate be for the Triad HS-29 and S-217D?

 

[CJ]

that Pultec  MEQ-5  line amp sounds very good also, it is single ended so has different distortion character, and the OPT does not need the FB wind, that 6AQ5 is probably one of the best sounding and least used tubes out there, so you can get vintage NOS types on the cheap,


look at this, 5 premium GE NOS tubes for 2 bucks a pop,
which is a better line driver?  no brainer, 6AQ5 blows the 12AU7 out of the water and into the beach,
we have both the EQP and MEQ in house, and that MEQ sure sounds better than the EQP, a bit more noise because it is not Push Pull, but not that big a deal, plus, you do not need he expensive and transparent HS 29 or Sowter equivalent,

 

[Quayhog]

I have a partial sleeve of Telefinken EL90 just waiting for such a project.  I'll need to look into the MEQ-5 circuit.  The research is one of the joys of DIY.

I'm still curious if the Dukane 2A40 transformer could be made to work. While it does have the tertiary winding it isn't center tapped.  I'm trying to get my head around using it as a very poor mans S217D by using a false CT like one does with heater circuits.  I would probably need to clip the internal ground wire and reconnect to a separate lead.  I have a remote restored unit that a previous owner wired in HS-1 input transformers.  It sounds better than my Altec 1567a.

While I know weight isn't an empirical indication of quality the Dukane transformer weighs 22.8 oz while the HA-133 weighs 27.4 oz.

I'll try to post a schematic but in getting "file size" error

 

[Quayhog]

Attached is the Dukanes output transformer schematic.  It's a potted transformer so I'm not sure how I could disconnect the transformer ground lead.

 

[lassoharp]

Which should I use since the original output transformers are impossible to find and when found are prohibitively expensive?  Seek out an old or go with modern reproductions.?

If you're going to build the original EQP1A amp your only choice for 217D drop in sub is the Sowter reproduction the last time I checked.

The HA-133 is for single ended, plate coupled use, not pushpull.  Save it for a BA2 clone. 

If you want to experiment with a different makeup amp you could try the Dukane amp as it is drawn in the schematic.  Check the gain by injecting a small AC signal (~ 0.025V or so) into the V2 grid - at the top of R3.  Then measure voltage at the output secondary to compute the gain.  The Pultec needs around 20-25db (I think, double check it) makeup gain.  Going in at the PI may not give you enough so you could add another 6SN7 stage before that.  Might be easier than trying to slug through the redesign details of retrofitting that Dukane OT to the original EQP1 circuit. 

 

[CJ]

we have the turns and wire size and coil structure for the 217 OPT, we can transfer that to the more common EI core and see how it works,

this would save the hassle of obtaining L lams,

probably take a few days...