Compact ~20WPC Tube Amp

Definitely planning to contact Aric!

Good idea! 😊

Charles

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

@charles1dad The 6SN7 has no worries driving a single DHT! That's not the problem- the coupling cap and the timing constants around it is. We ran into the same problem in our OTLs, but solved it by direct-coupling. We have a single 6SN7 thus driving 8 power triode grids with no distortion problems even when drawing substantial grid current. But to do this properly, you need a negative power supply and most SET designers aren't interested in the additional cost.

@atmasphere 

I am not an electrical engineer. I do listen intently when people who are successful at what they do offer their perspective and experiences. Israel  Blume is very accomplished with what he does and so is Aric. They both share the same opinion with regard to appropriate driver tubes for DHT tubes (Specifically 300b).

Either of them (And other amplifier builders who have similar view) are far more qualified than me to debate this point with you. I can say that their comments make plenty of sense and are backed up by their SET amplifier performance results. I just listen and read in an attempt to learn.

Charles 

Either of them (And other amplifier builders who have similar view) are far more qualified than me to debate this point with you. I can say that their comments make plenty of sense and are backed up by their SET amplifier performance results. I just listen and read in an attempt to learn.

@charles1dad 

What they are doing is a brute force thing by running lots of current through the tube so they can have lots of current drive on the grid.

Of course by simply paralleling elements in the 6SN7 they could do the same thing... but at any rate, there is a series impedance (the coupling cap) which limits how much current can be delivered to the power tube grid. So they are going for overkill.

But if you direct couple you solve this problem, although to do it effectively you have to run the driver tube with no voltage gain. That’s exactly how we’ve done it in our OTLs. The advantage of this approach is there is no overload recovery time- its instantaneous as there are no timing constants present between the driver and power tube grid. But it does make for what appears to be a more complex circuit, something that seems anathema to SET designers.

However, there is another advantage: the coupling cap that you do have to have to couple the signal to the driver tube can be a considerably lower value, usually by at least an order of magnitude, possibly more. This means that no matter what the coupling cap, the smaller cap will always sound better. So you have a double whammy.

The downside is the driver tube isn’t available for gain. But that’s OK- being a lower powered amp, an SET doesn’t need much gain (certainly less than we have in our OTLs).

The funny thing is you could execute this with the same number of tube sections as used in a traditional SET right now (three total, including the power tube). The reason you don’t see this is the fact that you have to build a negative power supply. But if you did, you could build an SET that was inherently lower distortion which means it would be more transparent.

@atmasphere 

The funny thing is you could execute this with the same number of tube sections as used in a traditional SET right now (three total, including the power tube). The reason you don’t see this is the fact that you have to build a negative power supply. But if you did, you could build an SET that was inherently lower distortion which means it would be more transparent.

Interesting you say “more transparent “. ironic then, one of the first things I noticed when I heard my 300b SET mono blocks, superb transparency and startling nuance/inner detail. As if the speakers weren’t even in the room!

I do not possess the technical background or expertise  to explain why this is exactly  so. I can only rely on what I hear. That initial  listening encounter was striking!

Charles