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previous: Chris Hurley Right now, I've got two normal 12ax... -- 7/26/2000 2:59 AM View Thread

Re: Self Split EL34 Output, and Getting more Power amp gain

7/27/2000 11:23 PM
Re: Self Split EL34 Output, and Getting more Power amp gain
Hi Chris,  
IIRC a cathode-coupled type splitter in a preamp can require twice the driving signal into its single grid input (usually the *upper* tube section on a schematic) to achieve the same output from the *upper* triode compared to what it would take if that tube section were wired as an ordinary gain stage, with other current values and voltages unchanged. (Discounting the possibility that twice some signals may clip the splitter input before its output could completely double.) Seem to recall the splitting pair have the same voltage differential between their out-of-phase signal outputs as a single tube section would have versus ground. This would mean if the single section was driven barely to the point of distortion by a certain input level, it would only be halfway there in each tube when the splitter configuration is used. The output of the single section would be the same RMS voltage as the entire splitter, but the splitter has twice the headroom, this makes logical sense, with twice the tubes you should be able to get twice as much of SOMETHING ;)  
Probably works the same way with power tubes, you may need to add that additional gain stage if you can't salvage some lost gain from the preamp in a few places. I like power tube distortion best, and do not fear driving them with more than an excess of clean signal when all I want is their particular distortion characteristics. You would find if you have a clean preamp driver circuit isolated from the power tube input, it can put out say 50VRMS which could be measured into *no* load. Once you were sure it was clean, when you connect it to drive the power tube you would no longer be able to see the full 50VRMS if you checked, since any excess over the effective grid bias would clip the former sine wave you saw on the scope. Or with some tubes the plate may reach saturation and clip the output before the input clips, but that will be reflected back to the grid and you will still see the clipped signal where you know it really is clean coming from the driver stage.  
Hope this helps,