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Two years ago, I obtained some 810 transmitting tubes that I thought I’d like to use as output tubes. Jack Elliano of Electra-Print and I discussed driving power tubes in A2 mode, with Jack suggesting the need to supply power to the tube’s grid to get a lot of power out. So, he came up with the idea of using the TDA2030 loaded with a step-up transformer. He sent me the first one to try. My first experiment was driving an 810 tube at 1050V, 150mA bias. I got 55W RMS out of this tube with the Electra-Print 5.5K 300mA output transformer.
Jack continued to improve his step- up transformer (the current model is A2LD7). He was kind enough to send me his updated ones. Last year, I built another pair of monoblock power amps using the Svetlana SV811-10. Because the SV811-10 is a very tough tube and I wanted to get maximum power out of it, I set the bias point at 770V 116mA. That was about 90W of plate dissipation and I got 25-27W RMS output power per channel. I lent these monoblocks to my friend, who owns a coffee shop, for about a year. They were turned on 12 hours a day, 7 days a week, playing all kinds of music for the customers throughout the year without any failure. The SV811-10 is a truly great tube!
above: The SV81 1-10 monoblock power amp.
I also recommend the 211 and GM70 tubes, which are easy to drive due to higher gain than other low mu triodes. You can expect to get output of 40W RMS or more from these. However, you won’t be able to drive the 845 because this tube needs much more voltage drive than the 211 or GM70.
You can find an A2 article written by Jack Elliano at electraprint, as well as the sample schematic (Fig. 1), which is shown here with LM1875 instead ofTDA2030. I chose LM1875, which I preferred over TDA2030, be cause it needs a minimum gain of 15, while TDA2030 needs 20. You may use SV572-10 if conservative rating is a concern.
FIGURE 1: Amp circuit.