Non-Pultec ideas for S-217-D clone?
« on: March 18, 2021, 03:02:50 PM »
I came upon a S-217-D Clone (non orig, not cinemag) and I'm exploring whether it's worth putting it to use in ways other than a Pultec build...or selling it for someone else' pultec build and buying else.

Thoughts welcome.


Re: Non-Pultec ideas for S-217-D clone?
« Reply #1 on: March 19, 2021, 02:22:35 AM »
You could use it in a line O/P amp in a Mic amplifier..?


Re: Non-Pultec ideas for S-217-D clone?
« Reply #2 on: March 19, 2021, 06:19:53 PM »
The Peerless catalogs state that it can be used single-ended or in PP, but was designed for parafeed. (However, people on this forum have noted that many of the UTC transformers specified for either SE or PP don't sound that great when used one way or the other - SE, I think it was - so YMMV). You don't have to use the feedback winding if it's not needed for whatever you choose (ground it if you don't use it). But keep in mind that you can only DC couple it in PP (as in the EQP-1) where you can trim out any imbalance between the tubes (it takes 0ma unbalanced DC).

With a 12,500 ohm nominal primary, you could try it SE with any cap coupled 12AU7, 12AT7, 12BH7 etc output stage and see how it sounds. Of course, that primary should also work with SRPP or WCF stages as well (even if not ideal, impedance-wise). But I wouldn't think it would sound that great in PP without the feedback winding. Normally, you'd expect a 20k or higher nominal primary impedance with PP 12AU7s or similar tubes for low distortion. The feedback would fix that. But global feedback is tricky unless you're copying an existing design. (Although that shouldn't necessarily scare you off from trying it w/feedback, depending on your experience and willingness to work through any issues that might arise).

At a +20dbm max level, it's a little too low level to use for a classic varimu compressor build (e.g., using PP 6V6/6L6 etc, though there are a couple of designs that use 12A_7 type outputs). So your best bet is probably a mic/line amp, taking the above considerations into account.

If any of this puts you off, you might just consider selling it and buying something else.

Just my 2 cents, thinking out loud.
« Last Edit: March 19, 2021, 06:51:26 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.

Recording Engineer

Re: Non-Pultec ideas for S-217-D clone?
« Reply #3 on: March 20, 2021, 02:05:44 PM »
I’ve the Cinemag version. At some point, I’m thinking an RFT CM-7151 type of circuit.

Re: Non-Pultec ideas for S-217-D clone?
« Reply #4 on: March 20, 2021, 02:25:12 PM »
A tertiary winding - such as on the Peerless - with a suitable value variable resistive divider (pot or switch) across it can be used to control the feedback ratio and, therefore, gain.

Sometimes in a single-ended circuit (with the Peerless you'd want this to be parafeed so no DC current), we might also find that the feedback return is better served by being at a point in the circuit which requires the opposite polarity.  The tertiary allows you to choose which side of the winding to use as the feedback output point.

One simple case: We have two grounded cathode amplifier stages and, we'd normally return a portion of the signal from the 2nd anode to a point on the 1st valve's cathode for the negative feedback..  To allow adjustment of this, we'd need blocking capacitors in our feedback loop to stop any big pops, swishes, or the upsetting of the DC bias points of our stages.

By taking the tertiary winding as the feedback take off point, this signal is already blocked of DC and, if we use an inverted signal from the tertiary, we could bring the feedback back into the first valve's grid via the bottom of an input transformer.
This point is usually also at ground potential so, a pot and a couple of resistors gives us our variable gain.

Many more ideas for something like the Peerless of course, including ones where we use it push-pull.

D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Re: Non-Pultec ideas for S-217-D clone? New
« Reply #5 on: March 21, 2021, 02:45:27 PM »
I am liking the idea of a mic pre. I'm not skilled in the arts of circuit design so forgive any foolish thoughts I might have here.

Input transformer candidates include a Triad HS-1 and a Peerless 4665 (on hand), if it makes sense of course.

Re: feedback/ existing designs, the only ones I'm familiar with, at least in terms of tertiary feedback from the OT are the Pultec and the Altec 428b. I suspect the Pultec amplifier could be built with one of the above input transformers in place of the (HS56-EQ-HS-29) to feed the amplifier circuit, but would that yield sufficient gain for a mic pre?

The 428b's IT is basically a 4665 and the OT has mysterious specs, though Doug suggest the primary is in the ballpark of 25k~30K given the circuit design, so to make that work with the S217D, I assume v2 would need to be changed to something like a 12au7 as in the pultec, which I imagine would require some other adjustments to the circuit, and the big question of whether it would sound good.

If I'm way off please forgive/correct me. I'm open to other existing designs, or even a fresh design if anyone is interested in helping to work it out, though I'd want to keep things fairly simple if at all possible.

I appreciate the input already.

« Last Edit: March 21, 2021, 03:02:21 PM by thecr4ne »


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