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Old 3rd January 2012, 05:28 PM   #1
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Default SE board + interstage trans. + 211

Since I'm not a tube expert, I'd like to have some suggestions if I want to put an interstage transformer between the SE board and a 211 for a SE amp.
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Old 14th January 2012, 08:19 PM   #2
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This is what Georges answered in another thread but I think it should be here if someone else wants to do a similar project:



I have never been a big fan of driver transformers since I believe that a mosfet follower is a better solution, but there are some nice sounding amps out there that use them. Both the 845 and the 211 respond well to A2 operation. The 211 is the easier of the two to drive and a 45 can develop the voltage to drive it easilly. Whether it can push enough current into the grid of a 211 to squeeze 40+ watts out of it without a little help from some sand, I don't know. Even if you don't expect to fully saturate the 211 you should be able to get 30 to 35 watts using a 45 for a driver and a 1:1 driver transformer.

I would look for a transformer that goes from 5K ohms to 5K ohms that can eat 25 to 30 mA of primary current (the sweet spot for a 45). You will want a good resistive load on the secondary in the 5K to 10 K range. Measure the frequency response at the driver transformer secondary with no 211 in the socket and no high voltge turned on. Tweak the resistor value to get a flat response with minimum distortion. A 211 will need about 150 volts P-P of clean drive, a bit more if you want to do A2. Your negative bias should be able of going to -80 volts or so.

If your negative bias can go to -200 volts and you can get 300 to 350 volts P-P of drive, you can also run 845's in the same amp. I can swap between the two tubes by readjusting the bias in my amp.
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