SET "interstage coupler" for First Watt F4

[ro9397]

This started in the Pass forum, but seems more appropriate here.

My system, maybe the one I'll die with (hope that's a loooong way's off) is about done except for the amp. I've researched, embarrassed to say, almost certainly over 100 hours & narrowed it down to the SS First Watt F4 by "Papa" Nelson Pass, a unique (really) unity-voltage-gain current-source only, 25W stereo/100W balanced mono http://www.firstwatt.com/products/f4.htm more links if interested. My system appears to meet minimum voltage gain requirements.

A respected reviewer & lover of SET 300B loved the F4 alone, but when his SET 300B was inserted between the preamp & the F4 it became possibly the best amplifier ever. (A 20-Ohm resistor is required across the SET outputs for a proper load impedance; the F4 input Z is 48k-Ohm SE/96k-Ohm balanced.) In esence, this strange system may be the perfect "hybrid" amp: an SET driving a unity-gain SS current-source; an SET w/ no current requirements...

Questions for a Stereo SET "interstage coupler" to be inserted between a SS preamp (20 dB gain & 10V RMS maximum output) & the First Watt F4 unity-gain current-source:

What is ideal impedance value for output transformer taps? I assume 600-Ohm beats the normal 8-16-Ohm, thus eliminating the need for the 20-Ohm output resistor.

Your vote for the best output trannies available new? (No used except for compelling reasons.) If your "best" is in the mega-thousand USD range I'll be interested in your other podium choices...

Estimated tranny cost, please specify each or pair; I'm easily confused.

What is the ideal SET for this use?

If needed, what driver tubes?

I'm of course interested in a DIY schematic diagram.

How might one quantify any potential performance advantage of the above custom SET "interstage coupler" vs. a normal SET power amp used in the same way?

Best regards to all.

[dsavitsk]

Before going too far, it might be worth noting that this sort of design (a tube gain stage followed by a solid state buffer) is pretty common in the headphone world. So, there's really no reason not to do it with speakers, too.

Originally posted by ro9397
First, which other DHT's come to mind, that might better replace the 300B for this admittedly very very strange but possibly just as wonderful purpose?

There are quite a few. Emission Labs makes a few that have a ton of gain and might be worth looking into as they won't need drivers.

If you want to stay with a power amp, the 45 is the gold standard -- 2A3's are nice too. Or, there are lots of preamp tubes: 01A, 26, 1G4, 1H4, 205-D, 50, etc. There are tons, so it is worth a search for DHT preamps, or looking through an old tube manual.

Originally posted by ro9397
How much money could/might/should be saved in labor & materials building an imaginary/theoretical SET "interstage-coupler" to be inserted between a SS preamp & the F4 (vs. a regular SET driving a highly reactive speaker load)?

Probably none. Driving speakers is probably easier as the noise floor can be a little higher than with a preamp.

Originally posted by ro9397
How much might be saved on a 600-Ohm output tranny vs. a normal 8-16-Ohm tranny? Would you recommend spending the savings on a more exotic tranny, such as silver or one of Elliano's partial silver types.....hmmmmmm.........

Probably none here, either. But, I do think electra-print should be on your short list of transformers in part because they are very nice, and in part because Jack will wind you anything you want. Other winders I'd consider would be Magnequest, Audio Note, Lundahl, Tango, Tamura, and I'm sure a bunch that I've forgotten. I always buy from Jack anymore.

Originally posted by ro9397
BTW, my state of the art SS preamp has 20 dB gain & 10V maximum output SE or balanced per phase.

My sense would be that more components is worse than less, unless they do something specific. I'd drop the solid state pre as it doesn't do anything for you.

Originally posted by ro9397
Your vote for the best output trannies available new? (No used except for compelling reasons.) If your "best" is in the mega-thousand USD range I'll be interested in your other podium choices...

see above

Originally posted by ro9397
Estimated tranny cost, please specify each or pair; I'm easily confused.

can really vary by manufacturer and materials, and the tube you use. A 300B transformer needs to be much larger, and thus costs much more, than an 01A tranny.

Originally posted by ro9397
What is the ideal SET for this use?

See above

Originally posted by ro9397
If needed, what driver tubes?

If you can keep gain needs low, then a highish gain DHT driving a power tube is probably your best bet. Otherwise, something with low distortion -- 5842, E180F, E810F, 5687, 7119, 6H30, etc. There are lots of options, and it is never an easy decision.

Originally posted by ro9397
How might one quantify any potential performance advantage of the above custom SET "interstage coupler" vs. a normal SET power amp used in the same way?

Tubes sound, on average, 2 units better than comparable solid state devices ...

[Gordy]

Originally posted by ro9397
I've researched, embarrassed to say, almost certainly over 100 hours...

100 hours? Novice!

Originally posted by ro9397

20 dB gain & 10V RMS maximum output

10 V RMS is about 28 V peak to peak which may be sufficient depending on your speakers / room / desire for volume. Try it first, before worrying about another amp in between.

If there is not enough volume I would tend towards ditching the SS preamplifier and use a single-ended valve preamplifier with decent gain.


Good luck

[ro9397]

Gordy
Good point! I haven't been punching into a time clock, but I probably underestimated the time out of embarassment... This search has probably been going on for decades....