• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

unusual SET amp design, your thoughts please

As I was looking for a starting point to try an SET amp in my system I came across this unusual 300b amplifier:

Softone Model8 technical description

notable features:
1.Three stage direct coupled and fixed bias circuit
2.Second stage is composed of Pch-MOSFET
3.Automatic plate current adjustment function
4.Operation of filament with DC/DC converter
5.solid state rectifier
6.R-core transformers for power and output

your thought on this vs a traditional SET 300b would be appreciated. I am trying to select an affordable SET amp.
thank you.
 

ballpencil

Member
2013-11-15 4:44 am
Interesting.. thread subscribed. Tube purist will quickly dismiss the use of MOSFET. Even worse, it's not working as follower (as some would still tolerate) but as a gain stage. Not me though, if i had the money for pair of 300B, i might try it. Not an expert either.. just here to see what others would say.
 

Ludwig Haus

Disabled Account
2015-06-14 5:44 am
It varies
I'm looking at both options. The above amp being $880 caught my attention, but building for me would mean using an existing tested design. No amp designer I am. Also considering kits, such as Bottlehead.
Gera

Both? = if not DIY, go with Bottlehead, hands down.

12Au7 is mediocre at best, and the input stage is cathode biased, then the follower, blah.. and 300B ok.

If you were to borrow something from this design and DIY, the split rail supply is 100% the best (IME), go with two stage high mu, low Rp input stage and DC couple to the 300B. NO cathode bypass, but you'd need an input transformer (small nickel cored units the size of match box are excellent and dont cost more than half an arm).

But maybe its all about beer and catching fish?.

Good fishing.


LH/S
 
Last edited:

jazbo8

diyAudio Moderator Emeritus
2011-01-05 8:34 am
In Transient
12Au7 is mediocre at best, and the input stage is cathode biased, then the follower, blah..
Agreed, there are better tubes than the 12AU7. But the P-FET is not just a follower, it has gain.

go with two stage high mu, low Rp input stage and DC couple to the 300B.
What tubes would you suggest for the job? How do you get enough gain for A2 operation?
 
I'm a habitual mongrelizer of analog technologies, so it looks good to me! The 12AU7 is operated at low gain -- it shouldn't dominate the distortion profile. No caps in the signal path, but electrolytic decoupling caps might have some impact on sonics. The autobias circuit looks unique (to me, anyway ;)) and could be troublesome if the designer didn't wring it out thoroughly. The quality of documentation suggests that he knows what he's doing. Distortion waveform and spectra are absent, however, so the sonic footprint is difficult to guess.