Interesting problem cascading EF86s

[mumford]

I am working on converting a Webster Electric PA. It has three separate EF86 inputs that join at a simple mixer into another EF86 followed by a James stack. Up to that point, it matches this schematic: http://2.bp.blogspot.com/-JySFGsgNk0k/T ... ematic.gif The power section is different.

The EF86 initial channels feature 220k plate resistors, 1M5 screen resistors, .1 uF screen bypass caps, and a shared 750 ohm/200uF cathode. All three are followed by a .02 uF coupling cap and a 1M volume, after which they join together into the second EF86 gain stage.

Although I don't have the voltages in front of me, I believe Va on V1 is 90v, and the screen voltage is 70v; V2 and V3 are more like 170v on Va and 120v on the screens. (Obviously some resistors have drifted.)

Each of these channels work fine on their own, if kind of blah. They have a lot of headroom, but get pretty distorted with the volume up most of the way.

I have been experimenting with just cascading one of the inputs into another, but I'm getting a weird problem. What I've done is connected the output of V1's coupling cap to the grid of V2. I also added a 1M grid leak after the cap. If I crank the volume, I get some very low volume guitar signal with no treble. Mostly what I get, though is kind of a gated "blat" if I hit the strings hard. Am I just dumping V2 into cutoff with the gain of V1?

Admittedly, my plan is to reduce the plate resistors, separate the cathodes, etc., to cascade some milder stages, but this was an easy experiment to try. When I tried some jumpers, both V1 and V2 work fine by themselves, just not cascaded. Any thoughts?

[JazzGuitarGimp]

I think getting the first stage on its own cathode R & C will work better. iIRC, you're only supposed to share the cathode R & C in preamp tubes when the signal through both triodes are in phase. In this case, they are out of phase.

HTH,
Lou

[mumford]

Thanks, I'll split them. I also want to reduce the huge cap.

[mumford]

Splitting the cathodes made it work, thanks.

[briane]

EF86's have hella gain - absolutely no need to cascade.

maybe just fix the channel?