In Reply to: Re: Another attempt (no 3.) at 813 design posted by Bas Horneman on July 3, 2002 at 14:34:57:
Hello Bas.Grid resistance? I don't understand.
I use a 6BX7 to drive my 813s. As Fatbottle suggests, I have a 6SN7 DC coupled to the 6BX7 which, being a diff pair (p-p) sits on a pentode sink. The 6BX7 is cap-coupled (0.47 uF but should be a bit bigger) to the 813s. Each 813 has a 33k grid leak and there are 4.7k grid stoppers - whicxh are probably not necessary, but I ahve had enough of parasitic oscillations!
The main problem with driving large valves is the capacitance. 813 has a mu of about 8 so the gain will be somewhat less than this - perhaps 5 - I have not measured it. What I am getting at is that the consequent Miller capacitance is unlikely to be a serious problem especially if you use a valve such as 6BX7 that has a low anode resistance. My 6BX7 runs 15mA per side and uses 15k anode resistors.
best of luck
7N7
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Follow Ups
- Re: Another attempt (no 3.) at 813 design - 7N7 15:23:56 07/03/02 (1)
- Sounds good to me. - Bas Horneman 00:22:49 07/04/02 (0)