Someone recently asked me about the feasibility of running electrostatic speaker panels directly from Directly-Heated Triodes (DHTs). He had been running resistor loaded pentodes as output tubes, but was concerned about the high voltage and power needed if he was going to use push-pull 211s. He wondered whether choke loads would be better or even if the primary of an output transformer could be used. Here are my thoughts:

Running electrostatic panels directly from resistive-loaded tubes is a really inefficient proposition. Nearly half the power is dissipated in the load resistors and since each tube in the push-pull pair is separate, there is no ability to “slide” into class B. Even if you want to run only class A, having the ability to get close to class B on signal peaks (what the Radiotron Designer’s Handbook calls “Limiting class-A”) can give you a nice boost in power. If you want to increase the standing current to allow better drive of the capacitive load, even more power gets dissipated, and the plate load resistors have to be lowered, increasing distortion.

Using chokes are great in this application because they are energy-storage devices. Thus they allow the plate to swing above the supply voltage, thereby doubling the voltage swing for a given supply voltage. The power lost in the load resistors is gone. Chokes present a nearly infinite load resistance to the triode, which makes it run very linearly and at maximum gain.

However, you really don’t want to use two separate chokes in a push-pull amplifier, for the reasons mentioned above about the inability to go into class-B mode. Also, each choke requires a substantial air gap to handle the DC current and this makes the choke larger and less optimum than if there were no air gap. The best solution is a center-tapped choke - essentially a push-pull output transformer without the secondary. Since the DC currents cancel, then no air gap is needed, resulting in a smaller core and fewer turns of wire - things that help both the low and high-frequency response. Also, if the leakage inductance between the two ends of the choke is kept low, then the push-pull amp can then run in class-AB, which can give much higher power output.

The problem with running tubes like the 211 in this mode is the high voltage involved. Conventionally-built chokes like the One Electron PRC series are not recommended for supply voltages above 600 volts. Don’t forget that the peak voltage will be at least twice as high as the supply voltage. (It can go higher than twice the supply voltage if the tube abruptly cuts off and there is significant leakage inductance in the transformer or choke.) If you want to run the tubes at 1200 to 1500 volts, then the choke would have to be insulated for 3KV or more. This can be done, but is not standard on off-the shelf units.

Another issue is the required inductance for good low-frequency response without distortion. You want the inductive reactance to be significantly higher than the tube’s plate resistance within the audio band. High-voltage, low current tubes make this minimum inductance quite high, which means lots of turns of fine wire.

Just as a feasibility test, take a high-impedance output transformer that you don’t care about (6K or higher plate-to-plate impedance) and use it as a center-tapped choke in the push-pull DHT amp. Float and insulate the core to reduce voltage stress (Warning - this means the case could be at high-voltage - take precautions!). Do not connect anything to the secondary - you don’t want to burn power in a load resistor. Then hook this set-up to the electrostats and slowly increase the supply voltage until: a.) you get good sound or b.) the transformer blows out. If you get scenario a, then you will be OK. If b occurs, then you will need a higher-voltage transformer.