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previous: Ray Ivers Chris,I just finish... -- 6/4/2000 5:54 PM View Thread

Re: Building a 'clean' PI

6/4/2000 10:44 PM
Chris Harden
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Re: Building a 'clean' PI
I've only just found this post, because the nice people at AMPAGE have had a reorganisation and spit the guitar amp section up. Nevermind, I'm here now.  
 
Firstly, the circuit I'm using. I don't know the name for it so I'll explain as best I can.  
 
Take both sections of a 12AT7 and connect the cathodes to a common cathode resistor. The outputs to each O/P tube come from the two anodes via caps. One gate is connected to the output of the previous stage (via master volume), the other is connected (via cap) to a potential divider across the outputs of the two sides (the inputs to the O/P tubes). It's similar to a long-tailed pair except for the spare grid being connected across the outputs rather than to ground, which should give better symmetry, because as soon as the sides are un-equal, a signal appears at the spare grid to correct it.  
 
I have a blank sheet to work with, you mention the B+ being a little low, what would be better? I'm going over to EL34's soon, which will be run at 550V, but as I think I mentioned, I'm waiting for parts.  
 
The rest of the preamp at the moment is as follows, 1/2 12AT7 I/P stage, tone stack, 1/2 12AX7 common cathode, master volume, PI. I have more stages to add, but I'm working on the clean tone first.  
 
I've also been playing with a current source instead of the cathode resistor in the PI, I may have mentioned earlier. I changed it for a resistor when the circuit wasn't working right, but it didn't seem to make much difference.  
 
Thanks for your thoughts  
 
Chris

 
Replies:
Ray Ivers Chris,I would call ... -- 6/5/2000 2:59 AM