I have a perverse yen to do an amp project with a pentode first stage, and was thinking of one of these two (it helps that I also have both). I also thought of the 12HG7, but the transconductance is too much for what I have in mind. Any comments regarding linearity, etc., or alternate tubes?.
wrenchone said:
I believe the 12BY7A has a controlled warm-up filament, and is reputed to be slightly quieter than the 6CL6. The 6CL6 however seems more readily available, and most tests show that these tubes are nearly equivalent in use, running at similar operating points. Both are capable of swinging gigantic voltage at low distortion. User Boris_the_Blade has done some extensive 6CL6 differential amp tests, and uses them as a driver for a PPP EL34 amp. You can also find some pentode comparisons on Pete Millett's site (Pentodes).
Personally, I'd use whichever I had more of at the time 😀
I had a Citation II which used 12BY7A pentodes in the input and driver stages, not sure that the 6CL6 differs significantly, but I had a lot of problems with microphonic 12BY7A particularly in the first stage. Either way you should probably plan on getting few extras so you can select the least microphonic ones for use.
The "best" 12BY7s in a "Duece" are RCA in the voltage gain positions and GE JAN in the splitter positions. Set up a "12" V. heater supply and RCA 7054s enter the picture.
Remember, the Cit. 2 has well over 30 dB. of NFB in 3 nested loops. The outer, global, loop applies only a few dB. Stu Hegeman leveraged bandwidth throughout the design to make the "gobs" of NFB work well.
Remember, the Cit. 2 has well over 30 dB. of NFB in 3 nested loops. The outer, global, loop applies only a few dB. Stu Hegeman leveraged bandwidth throughout the design to make the "gobs" of NFB work well.
Eli Duttman said:
Have you determined this by measurement, or by listening? The reason I ask is, after much experimentation, I agree 100% on a "sonic" basis. Another good trick is a 12BV7 for the first stage, which has slightly higher transconductance and gives you a touch of extra global-NFB -- I use some early ones with the smoked glass that look sweet in that position. 😀
wrenchone said:
You can check it out at Frank's: 12BY7A. A good loadline for the 12BY7A is going to require a rather stiff plate current, and lotsavolts. Of course, this being designed as a vid amp for colour TVs, it needed enough current sourcing to avoid slewing at a 4.0MHz top end, and high voltage is easy to come by in a colour TV set. I've got a few of 'em, but so far, I haven't needed that much gain. It could be one of those types that're gonna be hard to come by since lots of ham rigs used these as RF drivers.
HollowState said:
That's exactly what I run. You can get matched 6CL6s in the same form, too. Nice stuff!
What I'm probably going to use first off after some thought is a 6AH6 in full pentode mode driving a source follower, which in turn will drive an enhanced triode (screen drive) mode sweep tube. The 6AH6 has almost as much transconductance as the 6CL6 and 12BY7 and doesn't require as much plate current to get the benefit of its goodness. Cheap and plentiful, too. Comments?
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