300b lovers

To recap, there are different challenges associated with low and mid-frequency distortion vs high frequency distortion. HF distortion is very often caused by nonlinear current delivery into a capacitance, and stray capacitance is everywhere in audio design. Sometimes you can reduce the capacitance using various methods such as cascode circuits, or pentodes (which are electrically similar), or take the alternate approach of increasing the drive current severalfold.

The 300B is a bottleneck in many SET amplifiers. The 80 pF load isn’t so bad, but the 300B requires 70 to 80 volts to clip it, and if the driver circuit is A2 capable, 100 volts. And ... the 300B has lower distortion than many, if not most, driver circuits, which defeats the entire purpose of using an expensive DHT like the 300B.

What looked like a simple problem is not simple at all, if you want to hear what the 300B actually sounds like, instead of a distorting driver stage. You have to deliver extremely low distortion into a capacitive load, over a range of hundreds of volts (if using PP output devices). This is no longer trivial. The common RC coupling seen in many amplifiers may not be up to the task.

We found transformer coupling with dedicated power tubes, themselves operating balanced Class A mode, gave the lowest distortion. Transformer coupling also allows A2 drive, with the 300B smoothly transitioning into the positive-grid region with no glitching. Although the 300B is not rated for A2 operation, we’ve found no indications of harm, although steady-state operation into A2 might overheat the grid, so not suitable as a guitar amp.

Now if feedback enters the picture, the design criteria all change. Forward gain goes up by as much as 10~20 dB, different parts of the circuit get optimized, and stability at high frequencies, particularly transient overload, become important. Nested loop feedback (2nd-order or higher) gives even lower distortion, but long settling times (after transient overload) can be problematic (because the different loops have different recovery times). You can have even more fun with modern feedforward techniques, but now we need serious computer modeling to pull that off and still have a stable amplifier.

The Raven and Blackbird are non-feedback amplifiers, with the cathodes bypassed so local feedback does not apply, either. So there are no stability criteria or load stability issues. The distortion is simply the distortion of the matched pairs used in the preamp or amplifier. It has similarities to a SET amplifier in terms of a simple harmonic structure, but the pair-matching and balanced operation reduce distortion by about 30~35 dB ... without feedback or any associated stability or settling-time issues.

@lynn_olson Our OTLs are zero feedback too. We get a similar reduction in distortion over SETs.

Slewing, by contrast, is part of the amplifier running out of current, not voltage. Specifically, current available to charge a capacitance. Now, 80 pF isn’t much capacitance, but tube circuits are inherently high impedance (compared to solid-state) and operate at fairly low currents (again, compared to solid-state).

The load the driver sees in our OTLs is considerably higher than just 80pf; more like 200pf. We tie the cathode resistors of the driver tube to B- which in our smaller amps is about -300V. We put some current though there too, so the 6SN7 in question has to have a -GTA or -GTB suffix to handle both the current and Voltage to which is subjected. But they hold together in that circuit for years and even decades.

Driving a 300b in an SET has a lot in common with sitting on a park bench by comparison. We built an OTL using four 300bs about 25 years ago using the same kind of driver circuit. It worked fine. There's no problem at all using this technique as seen in the schematic that was posted above.

Ralph, you’re the acknowledged OTL expert. How does an OTL amp work without feedback? I’d like to know. The last I checked, tubes designed for series regulator use like the 6080, 6AS7, or the Russian 6C33C have a Zout on the cathode side somewhere around 100 ohms. I’m not an expert on this class of tubes, but I wouldn’t expect any tube to have a Zout in the 8 ohm or less range.

(For the reader following along: a Zout of 8 ohms gives a damping factor of 1, a Zout of 2 ohms a damping factor of 4, and so on. Zout in the 0.1 ohm range, typical of transistor amps, gives a damping factor of 80. It should be mentioned that damping factors much greater than this are kind of pointless, since the speaker cable, which is in series with the DCR of the lowpass inductor in the crossover, will typically have a DC resistance of 0.1 ohm or so. I don’t expect speaker cables to use superconductors any time soon, so we’re stuck with copper or silver at room temperatures.)

I’ll admit this is kind of an idle curiosity since I will never design an OTL amplifier. The amps I’m interested in use transformers to solve various circuit-design problems. But I’m always curious how things work, whether solid-state, vacuum tube, or Class D, or hybrids of all three (like ZOTL’s).

@lynn_olson You might want to study the Wiggins Circlotron patent for the answer, or look at the setup/spec sheet of one of ElectroVoice's amps that used the Wiggins patent. Here is an example. 

I recently restored one of these (A20C model) that I had inherited about 20 years ago. Like you, I was very curious about it since EV and the patent claimed no zero crossing artifact if the amp is biased class B. I wanted to see if that was true. It is! With a sine wave input even at 1 milliWatt the output is excellent. Its first Watt performance (and sound) is really quite good.  

In the Features paragraph at the link you'll see that the primary impedance of the output transformer is 1/4 that of conventional PP amplifier circuits. That applies to an OTL in the same manner. Since the Circlotron allows for a fully differential/balanced Voltage amplifier and drive circuit, its symmetry means that distortion will be lower. So you can run it without feedback (you can do that with a totem pole circuit as well but its not nearly as easy).

We were the first to manufacture a Circlotron OTL and as such they were also the first really reliable OTLs as well because they were unconditionally stable with any output or load condition. I think that is due to the symmetry of the design and the lack of feedback, which can be a destabilizing factor especially if the amp operates near its phase margin limits (which can change with the load).

The Futterman OTL circuit employed positive feedback to equalize the drive to the top and bottom tubes of the totem pole output circuit, so that design seems to need negative feedback to really function right.

We've run feedback as well but do it with balanced feedback loops mixed with the audio in a manner similar to an opamp. This is because the cathode circuit is unavailable (there's a CCS circuit there after all); after doing that it took a while but I finally realized there's an advantage doing it that way because the feedback signal is not distorted by the tube as it would if the feedback were received in the cathode. In this manner there is less higher ordered harmonic and intermodulation generation caused by the feedback (meaning it sounds more relaxed).

I am convinced feedback to the cathode of the input tube is a reason why feedback has gained a bad rap in high end audio. Funny how traditions like that hang on for so long.

That’s a subtle aspect of feedback theory that is often overlooked. The summing node must be distortionless, and also free of overshoot or slewing artifacts. Applying input signal to a grid, and feedback to the cathode, impresses tube distortion (of the input tube) on the entire feedback loop.

Of course, we can get in trouble with a differential circuit as well, since the summing node is spread across two grid/cathode circuits, not one. Assuming perfect match, the nonlinearities of the pair should cancel. In practice, we should expect 2~3% gain mismatch in the differential pair, so there will be a residue of mismatch and associated nonlinearity, but not much.

If I understand your previous post correctly, the Zout of a Circlotron amplifier will be high, maybe in the 10 ohm or higher range. Or there is local feedback somewhere in there, reducing it to "normal" levels of an ohm or less.

As you can see, Don and I are taking a brute-force approach to distortion reduction.  Drivers that run at 40 mA per tube, and balanced deep Class A operation. The even-order distortion of the driver section is cancelled in the primary of the interstage transformer, reducing driver distortion even further. This presents a highly symmetric drive to the paired 300B grids.