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    Building, troubleshooting and testing of these amplifiers should only be
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Differences between 6SN7GTA and 6SN7GTB?

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Hi pftrvlr,
I don't know off hand. The differences may be very minor, such as controlled heater warm up time.

Try them out. The worst that can happen is that they sound vastly different and you need to buy another pair. I doubt this will be the case.

-Chris
 
The only difference between the 'A' version and the 'B' version is that the 'B' version includes what's called a "controlled heater warm-up characteristic". This helps to reduce the voltage imbalance on power up between cold filaments when stringing all the filaments in series, as was frequently done as a cost reduction measure for TV set design.

The main difference between the original 6SN7GT is that the peak positive plate pulse rating was increased from 1200Vdc to 1500Vdc in vertical deflection use, and that the control grid was beefed up for greater reliability in Class C, postive control grid, operation. The plate dissipation was also increased to 5000mW from 3500mW.

For use as a small signal audio amp, it shouldn't make any difference, and except for extending the plate characteristic into positive grid territory, the characteristics look pretty much the same.
 
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Hi pftrvlr,
Is it OK to run 6SN7 in positive bias?
Not unless you can deal with very high distortion. This tube would require a driver tube. One would then ask, why do you need to do this?

Always consider the effect on your input signal as in positive bias, grid current flows and your nice high impedance drops way, way down. BTW, grid current can begin anywhere from a volt or less below ground, so try to keep your grid swing out of this area also.

-Chris
 
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Hi Miles,
I was thinking along those lines myself. For RF or some kind of HF driver yes, but not for the average audio circuit.

I wouldn't be surprised if there was an audio circuit, somewhere in this world where a circuit using the 6SN7 was designed to draw grid current.

-Chris
 
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