Compact ~20WPC Tube Amp

@atmasphere

"If you like the 6SN7 that much, if I were you I’d be looking a better topologies to make it happen."

I have no need or desire to change the topology in my amp. This appears to be going in a much different direction than the OP’s question. Cheers, Aric

@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

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

Horses for courses.

I just want to clarify for the OP, that sure- the 6SN7 can be used as a driver with good results to a DHT like the 300B, such as SRPP, DC coupled cathode follower with a negative supply at the cathode, or grounded cathode with local feedback (anode follower). However my point was only that in the Sun Amp (which is very similar, if not identical to the JE Labs 300B SET and also the design posted by Morrison on DIY Audio projects), it is simply using the first section of the 6SN7 as a grounded cathode volt-amp DC coupled to the second triode and then capacitor coupled to the 300B grid. This amp does not do the 300B justice (I’ve built it before). There are many other ways to drive a 300B as well- a C3M pentode, a triode strapped 6V6, a 6EM7, etc. That is not to say that I do not LOVE the 6SN7 (I use it in many of my products) but used properly and not typically being asked to provide large amounts of current sounds best to me. Again, with all engineering there are trade offs and there is no one way to implement an amplifier circuit. I don’t believe economics are the key driven point with a negative supply either, if so, then many designers wouldn’t bother using an inter-stage transformer. As they say, there is more than one way to skin a cat.