I’ve just completed (almost) a 36watt and would appreciate some feedback on the design and some answers to a few questions. Any advice or suggestions would be greatly appreciated.
The amp is basically the standard EF86 channel with tone and 6-way contour switch and the Lite IIb channel run into either side of the PI (no cathode follower). The Lite IIb channel is switchable between single triode, parallel triodes with shared cathode, and parallel triodes with split cathodes. From there it’s Richie’s 36watt Rev.5 with a hi-cut control and master volume. There is also a power scaling circuit built into the amp, which I haven’t hooked up yet. I would prefer to get everything else tied down before I start experimenting with this.
My questions are:
Is there an optimum voltage for the EF86? I changed the plate resistor from 220k to 180k to balance the ratio between the plate and screen but I feel like my overall voltages for this tube are low (118 on plate, 98 on screen).
Since I’m running each channel into a separate side of the PI, is adding a NFB control like the one on Ceriatones EF86/TMB a useful option or would it be better just to tie the PI tail resistor to ground?
One more. It would be nice use both channels but would require mixing them into one side of the PI. Does anyone have any experience with how this might affect the volume/gain/character of the amp?
The amp sounds wonderful although I haven’t had a chance to really crank it up yet, except through an attenuator. The EF86 channel is a gain monster but balances pretty well with the 12AX7 channel. Channel switching will be added once the basic amp is tweaked. I’ve posted a schematic, layout, and some crappy pictures here:
http://webpages.charter.net/rscogin/amps/
Thanks,
Richard
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