300b lovers

There's no feedback, and there's gobs of 2nd-harmonic distortion. Parts coloration works for or against that 2nd-harmonic distortion. Also, because power-supply rejection is zero, power supply coloration is right in your face.

Driving DHTs takes two to three times the swing of pentodes or beam tetrodes. This exposes driver nonlinearity as well. Even worse, in some designs, driver and output distortion partially cancels ... but only at certain power levels. So the distortion signature is strongly level-dependent, which is very undesirable.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

I suggested this earlier in this thread; and added to it that with the input circuit also being differential there is additional benefit. A fully differential circuit has harmonic cancellation at every stage of gain; not just at the output. This results in the 3rd harmonic being dominant; it is treated much like the 2nd by the human ear in that its innocuous. But compared to an amp that does not use this topology, the 3rd is at a lower amplitude, and succeeding harmonics fall off at a faster rate (than seen in an SET) on an exponential curve. So the 3rd is thus more effective at masking higher orders and since distortion is lower, the circuit can be smoother and more transparent.

The problem with two successive stages that are balanced and DC-coupled to each other is that DC drift is a big deal. A 1 volt shift on a 150 volt plate is normally inconsequential, but becomes a serious concern when the grids of the following stage have a 1 volt offset between them ... which is what DC coupling does.

A Mullard sidesteps this by direct-connecting the plate of the SE input stage to ONE driver grid. The other grid (of the driver) is AC-connected to ground through a 0.1uF cap and DC-connected to the other grid via a 100K ~ 220K resistor. As a result, the two driver grids always DC-track each other.

By contrast, if the Mullard input section is replaced with a DC-connected balanced or diff stage, then DC balance and drifting of the first stage becomes critical, requiring a servo circuit to always keep the plates of the input tube exactly matched. No thanks.

The Blackbird is fully balanced, input, driver, and output, with DC balance issues resolved by using transformer coupling. Transformers are incapable of passing DC from primary to secondary, since the coupling is magnetic. Charge/discharge issues associated with capacitors, as well as potential coloration, are also avoided since cap coupling is not used anywhere in the forward path.

The hard part is getting transformers of high enough quality ... this is where working directly with the transformer designer, making them a part of the design team, is essential. These are not off-the-shelf parts.

A minor side benefit is avoiding turn-on pops and clicks, since the circuit remains balanced in all modes of operation, without relying on servos to maintain balance.

As mentioned above, a part-Mullard is great way to build a PP DHT amplifier. Not too complex, a well-known circuit that behaves predictably, and capable of scaling up the driver so it has enough power to motivate DHT grids.

A quick note: Don and will not be making the part-Mullard or the suggestions made by others in this thread. The Raven and Blackbird are where our attention is, and that’s where it will stay. Our focus this year, and the next, is getting production moving smoothly, making sure the Raven and Blackbird are reliable as possible, and growing the customer base.

On my part, I’ll be completing the long-awaited "Beyond the Ariel" speaker project over on DIYaudio, with the assistance of Troy Crowe in Canada. I do my best work collaborating with others, and Gary Dahl, Bjorn Kolbrek, and Thom Mackris have made a real difference on that project. I’ve been meditating on an appropriate name for the speaker, and "Phoenix" feels right, considering how many times it has been re-born.

Scope pr0n. Same pix as earlier, just more zoomed in.

Don Sachs scope photo of Blackbird at 30% power (8 watts) at 1 kHz. Zero feedback, with no grid resistors to "trim" the response.