Compact ~20WPC Tube Amp

Alot of solid recommendations and yet, not a mention of the speakers in question. The OP hasn’t mentioned what speakers he is using that are admittedly somewhere around 45-50 years old. He hasn’t mentiontioned impedence, brand, whether the crossovers have been updated to specification and whether the drivers are in proper working order. If it was my money, before I spent a dime on anything further, I would think it prudent to have the speakers tested by someone competent. It’s likely there is an amp/speaker mismatch but how do you know? And how can we make appropriate recommendations?

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.

@aricaudio 

I much appreciate your informed reply. I continue to learn from forum knowledgeable participants such as you and Ralph (atmasphere).

Charles

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).

As you know a circuit like this will result in a higher output impedance so wouldn't be the best application for driving a grid that has high capacitance and needs a lot of swing. The 300b has nearly the same mu as the power tubes we use in our OTLs. We also built an OTL using 300bs for a demonstration so I'm not speaking idly.

We don't run the tube with a lot of current either, despite that, direct coupled to the grid of a 300b will have no problem driving it to saturation. At that point the 6SN7 section will saturate too; when replaced by a much gutsier tube the difference was less than a 1% change so its very obvious that the 6SN7 can do this job quite well.

We use the 6SN7 driving the 6AS7G; if you compare specs to a 300b you see that the former has slightly higher grid capacitance and slightly lower mu; the 300b is easier to drive. Despite that we can get one 6SN7 section to drive as many a 14 6AS7G grids with no worries. One grid of a 300b would be like a walk in the park.

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