How to design a direct coupled SE amplifier?

[keithpeter]

Hello all

http://www.the-planet.org/6EM7.html

I came across the above circuit that cleverly uses a dual triode.

How would I recalculate this to suit (say) a 6J5/6V6 (triode mode) or similar. I'd like to work out how to do this myself rather than follow a crib sheet.

I've sussed that the trick is to use a high voltage drop across the cathode of the output triode and take that as the working point for the first triode.

PS if the first triode gets pulled out or has a poor base connection, is the output valve toast as well like in the Loftin White?

Cheers

[Miles Prower]

Originally Posted by keithpeter

How would I recalculate this to suit (say) a 6J5/6V6 (triode mode) or similar. I'd like to work out how to do this myself rather than follow a crib sheet.

Figuring DC coupling is NBD. Just figure your loadline in the usual manner. Once you have the input stage calculated, you can figure out what your DC voltages to ground will be. All that's required is elevating the Vpp so that the Vgk and Vpk will be the same as you figured from the loadline.

PS if the first triode gets pulled out or has a poor base connection, is the output valve toast as well like in the Loftin White?

That'll poof it for sure. Neither the 6J5 nor 6V6 were designed to withstand the grid current that will flow if it gets hit with near the whole Vpp. In this case, your best bet would be to sample the final's cathode voltage to a window detector or "crow bar" that will disconnect the HV DC in cases where excessive voltage is detected. That just might help protect your final in case something like that goes wrong.

Also, pay careful attention to your Vhk ratings, and add heater lift voltage to make certain that this rating is not exceeded.

[keithpeter]

Originally Posted by Miles Prower

Figuring DC coupling is NBD. Just figure your loadline in the usual manner. Once you have the input stage calculated, you can figure out what your DC voltages to ground will be. All that's required is elevating the Vpp so that the Vgk and Vpk will be the same as you figured from the loadline.

Thanks Miles, I'll play with some load lines. And look up protection circuits as 6V6s come expensive these days (used to take carrier bags full of Octal valves home as a teenager working in a music shop).