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Old 25th May 2011, 04:23 PM   #1
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Default Gain of Phase Inverter

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,....
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Old 25th May 2011, 04:34 PM   #2
SY is offline SY  United States
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LTP will typically be 30-40% of the mu. Cathodyne is about 1, but with a voltage amp stage in front, it can be close to the mu.
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Old 25th May 2011, 04:36 PM   #3
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but what should be my goal in order to choose the tube? A gain of 10, 20, 50, 100???

thanks
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Old 25th May 2011, 04:39 PM   #4
SY is offline SY  United States
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That depends on what you're doing in the overall design. I know you're looking for a definite answer, but there *is* no definite answer. That's why there's all different kinds of tubes used in that application.
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Old 25th May 2011, 04:52 PM   #5
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hum...i was afraid i may get that type of answer...thanks anyways....
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Old 25th May 2011, 05:10 PM   #6
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Quote:
Originally Posted by hpupo View Post
but what should be my goal in order to choose the tube? A gain of 10, 20, 50, 100???
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.
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Old 25th May 2011, 05:21 PM   #7
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Quote:
Originally Posted by Miles Prower View Post
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:
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.
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Old 25th May 2011, 05:27 PM   #8
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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???
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Old 25th May 2011, 05:45 PM   #9
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I recently applied this circuit with a 6AN8A.
It has plenty of gain and sounds very good imho.

An big improvement over the conventional split-load driver in all ways.

rgds,

/tri-comp
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Old 27th May 2011, 09:02 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by hpupo View Post
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.
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