Compact ~20WPC Tube Amp

The Sun Audio 300B amp, if I’m not mistaken, uses two 6SN7 stages to drive the 300Bs and is the main issue with the amplifier distorting prematurely. The 300B and many other DHTs will draw grid current long before their power rating is reached and will "slew" causing distortion (the coupling cap is discharged faster than the driver can charge it leading to rolled off highs and sloppy bass). This is really common with all of the 2 stage 6SN7/300B amps out there. The 300B really needs a stout drive capable of kicking it in the @$$ to really allow it to open up. While a push pull tube amplifier will certainly give you the merits of more power, if you’re looking to stick with the overall sonic character of a single-ended amplifier, there are other options out there. My single-ended KT120 for example, with a solid 6SN7 SRPP driver will sound more like a 35 WPC amplifier, but can still produce 18 watts per channel with excellent cone control/damping. However it does use NFB as non DHT tubes really do not sound good without it unless they are "triode-strapped", which is an inherent form of local feedback. With most SETs the driver is the amplifier’s downfall- not the power rating of the amp, or the 300B. 

Best regards, Aric

@aricaudio 

With most SETs the driver is the amplifier’s downfall- not the power rating of the amp, or the 300B.

Aric this is precisely what the builder of my Coincident SET (And speakers) explained years ago. The main limitation with some SET amplifiers is the choice of weak/inadequate driver tube. Just as you say, this driver needs to be robust. The DHT output tube demands this. He chose the 6EM7 rather than the 6SN7 for that reason.

@atmasphere 

Good advice you gave to Israel Blume (Coincident). I’m happy that he took note of that wise counsel,😊

As you’ve pointed out before, higher speaker impedance means less amplifier stress/work and less distortion.

Charles

@charles1dad Exactly! The 6EM7 has two different triodes inside, one has high gain and low Gm so is used aptly as a voltage amp where its only job is to make the input signal larger, which is then cap-coupled to the higher Gm, lower gain and lower output impedance section of the tube. Then that is coupled to the 300Bs grid by a 2:1 interstage transformer so it makes a dog's lunch out of the 300Bs grid current and allows the amplifier to realize the full power of the 300B. My 300B is similar, but takes a slightly different road to get there (same principle however). 

Regarding speakers, another trick is to use an output transformer with a higher primary impedance to drive a lower impedance speaker. I've built amplifiers for several customers where their speaker's impedance profile dipped down heavily into 2 ohms, and the SET was able to drive them just fine, or more so surprisingly fine!

All the best, Aric

compact ________?

We really know what we own or have known,

My Cayin AT88 is not compact, a ’regular’ sized tube amp, 45 wpc. It had to sound as good as my Fisher 80az mono blocks made in 1958, it does, and gave me remote volume, which allowed me to use a vintage tube preamp with no remote anything.

If interested, get a later model with external bias, I needed 16 ohm taps, only m1 has those. later have 4ohm and 8 ohm taps

https://www.google.com/search?q=cayin+at88&rlz=1C1CHBF_enUS949US949&oq=cayin+at88&aqs=chrome..69i57j0i22i30l2.14365j0j7&sourceid=chrome&ie=UTF-8

6 moons review convinced me to try it, glad I did

https://6moons.com/audioreviews/cayin/a88t.html