Which amp design for DHT tube input?

[andyjevans]

I presently have an all-DHT setup - 26 (mu 8.5, Rp 7.5k) into 46 (mu 5.5, Rp 2.5K ) with a Lundahl transformer output which I could wire as SE or PP out. I could also step down if necessary from the 2.5K O/P impedance.

This currently has a 300b output which isn't enough to drive my Mark Audio Alpair 10 full range speakers. They're about 88db or 89db, but my room is small and I listen to classical and jazz, so I don't need huge power - maybe a good 10 watts, max 20w.

I have almost no experience of building a solid state output stage, but I build all my tube equipment. Know very little about Mosfet circuit theory, though enough to know I want class A and no caps in the output. I have most of the hardware - 25-0-25v (approx - could be up to 35v) power supply in different box, and some big heat sinks.

Can you guys recommend a really good DIY output circuit, which I could drive with DHT input stages (SE or PP)? Thinking of putting the output stage in a separate box from the DHT inputs. This may sound interesting or even great, so it may be a worthwhile experiment. I can build on tag strips for instance, or very open to a PCB. I've looked at Nelson's designs with awe, but I don't know enough theory to evaluate the many options. If I had a design I could go ahead and build it and report back!

Andy

[andyjevans]

Here's the heatsinks - not sure what width to make the chassis.

[Nelson Pass]

Maybe you want to take a look at the F4 circuit or something like it.

FIRST WATT HOME

[andyjevans]

Hello Nelson - good to hear from you. Yes - I did think F4. I'd need a PCB maybe?

What other choices do I have? I've just been looking at the Arch Nemesis, for example. An output transformer isn't impossible! I'm in UK as you see, and I'd have to find a source of parts like Farnell. I can get Digikey parts, though.

andy

[d to the g]

The arch nemesis is already running more than an amp through the OPT, and only does 5 or 6 watts. I'd think you'd run into bandwidth issues with the size of the wiring/opt before you could effectively scale this up.

A DHT transformer coupled to the F4 would certainly be possible (and interesting).

[Nelson Pass]

Originally Posted by d to the g

The arch nemesis is already running more than an amp through the OPT, and only does 5 or 6 watts. I'd think you'd run into bandwidth issues with the size of the wiring/opt before you could effectively scale this up.

Jan Didden has some larger transformers which appear to be able to take
a higher bias current, and from his figures he seems to get good bandwidth,
so I think more power is certainly possible, although a bit expensive.

[andyjevans]

The F4 appeals because as Nelson says, that's what it was designed for. Seems I will run into those tedious problems of finding 2SJ108s - I've been searching through many threads. Is there any recent discussion of how to get around this part not being available - substitutes etc? The other devices are stocked by Farnell in UK.

Andy

[Nelson Pass]

The 2SJ108 is really the 2SJ109 in a slightly smaller package. I think you
can probably find some of those.

[Zen Mod]

JB Mode ON :

"gotch ya !!"

JB Mode OFF .......



(it is just 1/2 of 2SJ109 , so it's 2SJ74 lookalike )

[andyjevans]

I just had a scary thought that Semisouth would in the near future come up with a plug-in solid state version of a 300b.......

Or at least something rather like it. I mean, how many tube amps are there lying about with OPTs just ready for such things. You might need something that would work into a 3K to 6k primary. Push pull should be fine (then there's parafeed....).

I wonder how far we are away from such a device...... tube builders are more used to seeing curves as below - wonder what they look like for some of the SIC devices....