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Self Split EL34 Output, and Getting more Power amp gain

7/23/2000 4:52 PM
Chris Hurley
Self Split EL34 Output, and Getting more Power amp gain
Hi guys...  
I'm currently experimenting with a self-split push-pull EL34 output section. I originally started with a Pair of SV83's, which were not loud enough for band use in this configuration. The SV83's seemed to be contributing a bit of overdrive to the sound when everything was dimed, but the EL34's do not appear to do the same.  
For anyone who is unfamiliar with this arrangement, there is no seperate phase splitter. One of the power tubes is driven normally and has a cathode resistor which is connected to ground. The other tube's grid is grounded and its cathode is connected to the cathode of the first tube.  
I'm interested to know if anyone else has employed this topology in a guitar amp, but my main question is more straightforward:  
I'd like to be able to push the output section a bit harder. Since I can't hear much of any change in distortion by varying the pot right in front of my output stage, I'm assuming that there is more volume to be had in this circuit, as well.  
I'm afriad that simply adding another conventional gain stage will invoke more preamp distortion, which I simply don't require(got plenty already). I'd like to be able to maximize the volume from this configuration, as well as get a little bark from the poweramp.  
Any suggestions? (other than 'switch to a schmitt') :)
7/24/2000 11:17 AM

Hey Chris:  
With different countries and different generations/training represented on Ampage terminology gets fuzzy at times. I'm pretty sure that your "topology" is commonly referred to as "cathode coupled." The only reason I make the reference is due to the "self-splitting" design posted a few months ago that drove the second tube in a PP pair from a signal derived from the screen grid of the first tube of the pair - I still don't have a commonly accepted name for that circuit and I thought that this was what your heading alluded to.  
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7/24/2000 4:44 PM
Steve M.

If I may so bold, I believe Chris is experimenting with this circuit:  
Compact Hi-fi Power amp  
I have book that has another version of this circuit in it (using 6V6s) and refers to the topology as "grounded grid".  
7/24/2000 6:04 PM
Chris Hurley

The "Compact Hi-Fi" schem is actually what I am working from. Thanks for noting that.  
I now have a couple of new terms to search for... "grounded grid" and "coupled cathode". Maybe a usenet search will turn up further info.  
It occurs to me that perhaps another triode *will* be satisfactory. I'd imagine I have some loss in the tone stack that could be replenished. The problem with that idea is that I've now added another tube, which *could* also be used as a schmitt PI...
7/24/2000 12:23 PM
Mark Buckingham

Here's a sick thought:  
How about driving the el34's with one of your sv83's?
7/25/2000 1:32 AM
Don Symes

Is there even the slightest sense in separating the screens?  
I'm also thinking (without justification) that R6 could be a little smaller.  
Other than that, you should be able to get there by slapping it a little harder - maybe convert teh last stage to a CF?
7/26/2000 2:59 AM
Chris Hurley

Right now, I've got two normal 12ax7 gain stages, a third gain stage driving the cathode follower, which drives the tone stack, followed by a master level, then the output tubes.  
There are dual 470k voltage dividers after 1M volume pots in between stages 1+2 and 2+3.  
I'm trying not to get more preamp distortion (as I have plenty), but I'd sure like to juice the output section a little bit.
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