For these tubes we use a very fine gold plates grid wire, with a gain of 21x for the 20A and 20B, and 32x for the 30B. It is recommended to use these tubes with a SE transformer, or choke loaded. When you choose a plate resistor design, it can be done, but due to the high voltage over the plate resistor, you'll end up with very high supply voltage in that case. For this reason a SE transformer or plate Choke is more practical way, giving better performance.
The 20A, 20B and 30A tubes are based on older principles, where high gain tubes have much wider plate distance. This is still the best way to make a very linear DHT high gain tube. It will outdo any small size tube. We encourage you to check the tube curves of any miniature tube, like 6SN7, ECC88 or ECC82, and you will see this series presented here has much better curves.
However this results in higher working voltage, and larger size. This was not done any more after 1935, when miniaturization of tubes started to become important. With the new production tubes we make today, miniaturization is no requirement. So for high gain tubes, we can now focus on lowest possible distortion. We build these tubes again the original way, with wide plates. Check plate pictures here.
Distortion for small signal has been measured and is below 0,08% at 10 Volt output signal, at 400Volts plate voltage. For larger signal, distortion depends on the way the tube is used. Please note that when one DHT tube is driving another, some part of the uneven distortion of the second tube will be eliminated by the first tube. This harmonics cancellation is only with triodes, and they must be similar tubes, like drive a 300B with one of the tubes of this series.
For lowest possible mechanical noise with (any) high gain DHT pre-amp tube, use high quality tube sockets that do not pass the chassis vibrations to the tube. Chassis vibrations can come directly from the mains transformer, or by air from the loudspeaker. The best sockets for this are probably the Yamamoto Teflon, because Teflon is a flexible material. Use tight fitting Teflon tube dampers such as the Duende Criatura. They make the best damper for the EML tubes. They can be fit very nicely on the tube top, or one on the top and one on the bottom.
Gold plated, Wolfram Grid
