Could really use some help here!

[kendt]

Many yrs ago i was supplied with an amp schematic by a experienced amp designer. It was a 300b driven by 6de7,s. It sounded very nice but i no longer have it or the schematic. I do still have some 6de7's and would like to do some experimenting with other output tubes as a learning experience. I have been trying to find similar schematics to help me determine the operating points for the 6de7 but have found nothing. As i recall the two haves of the 6de7 were directly coupled to each other. I would like to keep the same set up if i could. Can somebody help me to determine a suitable set up for this tube as a driver.

 

[dsavitsk]

It seems likely that both halves were used as common cathode amplifiers -- neither has enough mu to fully drive a 300B so you'll probably need them both in conjunction. DC coupling them will require a fairly large B+. Doable, but not that efficient. If done this way, put a CCS on the plate of each and choose your favorite cathode bias mechanism. Cap coupling the stages will make things much much easier.

Alternatively, you can run the second side as a cathode follower. You lose a bunch of gain, but if you don't need it, it is easier to DC couple. In this case, put a CCS on the plate of the first stage and a CCS on the cathode of the second. You can basically set the current of each and forget it as they will adjust their own bias.

 

[kendt]

Thanks for the info. Thinking back more I recall the set up could be swapped with 6sn7 by changing only two resistors(and the sockets of course) The only cap directly in the signal path was between the second stage of the driver tube and the 300b,s. I wish I could find the schematic I had for it!