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Re: Back to the Drawing Board....

Well, okay, here we go :-). First, reapply the bypass cap to the 6N7 cathode since this seemed to stabilize it more. Next, remove the preamp from the circuit, short the input of the amp with a shorting plug, turn it on and see if it oscillates. If not, then there's either an excess of gain or an RF or ground loop thing with the preamp that's causing a problem. If you still get oscillation, try fixing the bias supply as Jim suggested--use a 6.3 volt tranny, hook the secondary to the 6.3 volt filaments, and use the 120V primary for the bias supply. Take one 120V primary and attach a single reversed diode, which goes to the filter cap, and ground the other primary. This makes a half-wave supply and it may add some noise but it should be stable.

Let me know what happens when you short the input.

Also, describe for me how you're powering the 845 filaments. 10VAC with a center tap? Is the supply grounded properly? How are you measuring the current? Your bias should be around -100 volts. If you're bias voltage is excessive (-170) the 845 may be going into cutoff.

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