OTL SET 300B drives F4?????

[ro9397]

Finally finished reading all 200 pgs (kidding) of the 6moons F4 review. Read it yourself if inclined. Bring water. It's not a stretch to say it was overall very favorable, whether the F4 was used by itself or being driven by a lovely 300B SET inserted between the preamp & F4. FYI I have been trying to nail down a short list of premium affordable power amps, w/o any bias for tube/ss or architecture, & sight unseen/sound unheard, based on literally more than one hundred hours of research, I have to say I've narrowed it down to the F4.

W/ the 300B SET inserted between pre & F4, apparently an 8 to 25-Ohm resistor load must be paralleled across the SET ouput wires to provide a more reasonable load for the SET. Remember, tubes want a load or they can get rather unhappy.

My question is, if, as the reviewer suggested, someone ("...anyone...anyone?") was building a one-box SET 300B/F4 (way too complex for me) OR just sourcing an SET 300B for this purpose...

What good is the transformer? Why not OTL? Would not a 300B SET function best sans tranny, directly driving Nelson's wonderful little F4 invention? I'm guessing the F4 has a 40k input impedance. Is not the tranny's sole purpose to convert the tube's high voltage to current for the speaker's low-impedance electro-mechanical load? W/ the F4 supplying the current the tranny (the single most expensive part of this whole chain) looks like nothing but a distortion-laden boat anchor. Of course this eliminates the need for the load resistor too.

Please fill me in..........Inquiring minds want to know!

[dsavitsk]

Output impedance from the plate of a 300B w/o a transformer (loaded with a CCS or choke) is about 700R (with a resistor it drops slightly) and gain from the single tube stage would be 3 or 4, or something like that. So, in theory, not a bad preamp. A little noisy maybe, though. Other DHT's might work a little better here.

But, without the transformer, you probably want to drop the driver tube too since the transformer is trading voltage for current, and if you just tap the plate of a typical 300B amp, the voltage will be very high. Plus, without the transformer, you are going to have to cap couple, and I'd take a transformer over a cap to couple any day -- really. The other problem is that the 300B is hard to drive, so you are going to need a pretty hefty stage before it to drive it, which brings back a driver tube, but perhaps a low mu tube like the 12b4 would work. Ideal might be a low mu driver, and a OPT with a 600R tap instead of the typical 8.

[ro9397]

Please Mr. Tube Brainiac sir, don't leave!

Tell us more....

I couldn't agree more about prefering a tranny over a cap, as you say, any day.....really.....Steve McCormack didn't choose an input coupling tranny for his $7k DNA-500 powerhouse because of a lack of listening experience! I'm pretty sure there are great sounding tranny-coupled ribbon mics...etc....

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?

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)?

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.........

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

The idea of making my own SET doesn't sound so sexy, but OTOH if the cost/reward ratio was good enough....hmmmmmm.....

Is your avatar a picture of the wrestler Pepper Gomez from the '60s (19, not 18)?

[dsavitsk]

Originally posted by ro9397
Please Mr. Tube Brainiac sir, don't leave!

Tell us more....

I saw your post in the tube forum, so I'll respond there too. But, there are lots of people there who know a ton more than me, so hopefully they'll chime in as well.

Originally posted by ro9397
Is your avatar a picture of the wrestler Pepper Gomez from the '60s (19, not 18)?

Close. It's Reggie "The Crusher" Lisowski