what is the typical gain of a PI stage for guitar power amp? I am going to use 6v6 tubes in push pull. Right now, i have a pair of 6CG7 tubes that i wanted to use for the PI. I am debating if i should use LTP or cathodyne (plus an voltage amp stage before it). I would like the power amp to be stand alone, which means i dont have an specific preamp circuit in mind to determine the gain, so i would like something more general....but i need to make sure that 6CG7 will have enough gain,....
It depends: gNFB or no gNFB? The 6V6 requires right around 20Vp in Class AB1 PP. If you design for the usual input sensitivity of 1.0Vrms (1.4Vp) you need:
Av= 20 / 1.4= 14.28
A 6CG7 as an LTP won't have that much gain (more like ~8). One section of the 6CG7 as a voltage amp driving the other half as a cathodyne just might come close if you have the voltage reserve for a big enough passive plate load, or you use a lower voltage with an active plate load.
If you include gNFB, then one 6CG7 isn't going to have enough gain.
Since this is a guitar amp, input sensitivity should be much higher, so aim for a voltage gain of something like 50 ~ 150. Global NFB is not particularly common in guitar amps, so you don't need to take that into account.
Dont want to throw any wood in the fire here (even because i know i will be not using feedback) but all the schematics ive seen for the JCM800 have feedback... Also one of my favorite amps, mesa dual rect, also has feedback. Is there a difference between global NFB and the NFB used on those amps???
Yes they have global NFB, but not much. Just a 'taste' of it. It's not an absolute requirement or anything.
Generally you want as much gain as possible in a guitar amp, especially since it sounds like you like distortion sounds. You'll probably want to stick to an ECC83 or ECC81 in the PI (assuming its a long tailed pair). The two are pretty much interchangable in a guitar amp, so you can try both.
For a PP 6v6 amp you could need over 40v peak swing to hit zero volts on the grid. Since it's a guitar amp, you probably want the capability of overdriving the outputs. Even if you don't intend to have overdrive it's ok to have it lurking in case you want it by turning the volume up more. That's how the classic guitar amps work anyway. My '71 Fender has 440v on the plates and it takes 35-43volts of bias to result in about 12.5watts at idle. The outputs clip when the grid hits Ov. So I need measurably over 45vpeak. Bottom line is you probably want a LTPI 12at7 if you want the common approach.
Or use the 6CG7 in a manner that results in adequate output, assuming they can achieve that.
Or use the 6CG7 in a manner that results in adequate output, assuming they can achieve that.
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