triode – wauwatosa tube factory

I’ve had a few 6CB5As kicking around for a while waiting for a project. The 6CB5A has been documented by Thomas Mayer and Ale Moglia, among others, as a great option for triode strapping. Thing is, I like trying new things when I build and repeating a cap coupled formula for a two stage single-ended amp just wasn’t making it to my short list of projects (which I don’t quite have time for anyways).

A recent discussion reminded me of an idea to use the shunt cascode topology to direct couple to a single-ended output. It required some extra power supply rails, including a fairly large negative rail. These requirements aren’t anything too unusual; you see them with Morgan Jone’s Crystal Palace amplifier or any kind of fixed bias scheme (in a way).

Anyways, the more recent discussion reminded me of a thread discussing a novel way of direct coupling two stages by stacking the power supplies. This is kind of similar to the Free Lunch AKA Monkey on a Stick arrangement. Applying this idea to my original shunt cascode brainstorm lead me to this:

We have a shunt cascode input stage. The output resistor (R2) idles at about 75V across it. This feeds a MOSFET source follower, which will have just a couple volts less on its output. So we have a very low impedance output at around 75V above ground and we want to direct couple that to the next stage. This is where I think it gets exciting.

We raise the cathode of the output stage so that it is positive relative to the grid (at 75V) by floating the output tube power supply (V2). The voltage we float it at is roughly equal to the target bias voltage plus half the target output swing. In other words, we raise it by twice the bias voltage for A1 or twice plus a bit for A2.

The output tube anode is connected normally and the cathode returns to the point where the output supply (V2) floats on the bias supply (V1). Our input stage is powered by another supply floating on the bias supply (V3). Our input and grid driving circuits are all referenced to ground and direct coupled. We can set the output bias by adjusting the current through R2.

Here’s a more fleshed out version:

It looks like a lot in the schematic, but I’ve already got shunt cascode and grid driver circuits on small PCBs. The power supplies don’t need to be anything exotic in this case as the input has decent PSRR already. The higher current output could use simple CLC filters as well.

Will I build it? I hope eventually. By summer I hope to have the workshop basically finished. I’m already enjoying having all the tools and parts in one (heated) space!

One of my first tube projects was a TubeLab SSE. In fact, I still use this amp as a reference whenever I build something new.

George, of TubeLab, is notorious for torturing tubes and generally just blowing crap up in his experiments. He is also a fervent supporter of DIY and frequent poster on diyaudio.com.

A couple of my recent posts have looked at ways of applying local feedback to pentodes in order to force them into more triode-like behavior. It’s funny how experiments and research in the DIY hobby community converge. Here’s yet one more example.

Yesterday George posted some exciting but cryptic experiences with a new design here. The challenge, in his own words:

[With triode strapping] the pentode takes on triode like qualities with the associated triode disadvantages, most notably the inability to pull its plate down near the cathode voltage thus limiting the available power output. Another issue that needs to be overcome is the screen grid voltage limitations of most TV sweep tubes. Wire them as a triode, and most will eventually blow up when left alone idling which is worse case for a class A amp (maximum dissipation).

This is a great summary of the limits of single-ended triode amplification. Power is limited to single-digits by tube perveance, voltage maximums, and the ability to dissipate heat. Pentodes, able to swing outputs much closer to zero and operated with fixed screen voltages, go a long way towards solving the conundrum. But the trade-off is linearity and output impedance (which is why SETs are popular in the first place).

George goes on to tease his new design:

I arrived at a new topology that I can’t find anywhere in recorded vacuum tube history…..yes, there are several close similarities, but this is truly unique…I called this topology the Composite Electron Device for lack of a better name, since it is a composite of a vacuum tube pentode, a mosfet, and a hand full of discrete parts to create triode like curves.

We don’t get any schematics (yet), but he finally gets to the measurements pudding:

THD was 0.197% at 100 mW, rising to 0.235% at 1 watt. 5 watts brings 0.662%, 10 watts 1.61%, with 2.48% at 15 watts and 4.04% at 20 watts clipping sets in at 20.8 watts where the THD hits 5%.

So what is it, this single-ended not a tridoe? We know there’s some local feedback going on, but George says this is actually something new under the sun. That doesn’t happen everyday in tube land, so I’m following this one with a lot of interest.

By the way, if you want to build a traditional SET and prefer a PCB, take a look at George’s TSEII or original SSE designs!