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In Reply to: RE: 6AF4A Useful? posted by PakProtector on May 17, 2017 at 16:24:45
The 5842 and 6C45П (6s45p) come annoying close to oscillating, while sitting in the cardboard box. FWIW, I say take no prisoners. A 10 Kohm CC stopper on each grid connection, a 100 Ω CF stopper on the plate connection, and ferrite beads on the heater wires is (IMO) not overkill.
The "in box oscillators" were designed for grounded grid RF duty. Setting them up common cathode requires stringent precautions. I've seen documentation with RF chokes in the heater wiring, when set up grounded grid.
I just increased the 1500 ohms CC grid stopper on my 6gk5 to a 10k ohm CC.
The 6gk5 has a in circuit Miller of about 45pf.
1500 ohms gave a HF -3db point of over 2 meg Hz.
10k ohms gives a HF -3db point of 350kHz.
Should I do more?
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"Still Working the Problem"
I don't know. As gm increases, I increase the stopper value. My seat of the pants value for the low gm 'X7 triode is 100 ohms. OTOH, the high gm 5842 has 4 grid connections and I advocate 4X 10 Kohm parts arranged in a "cone" on that tube.
A tiny ferrite bead cemented around, but not touching, the grid's socket lug(s) is another anti-parasitic oscillation method. I like that idea on the "upper" triode of a cascode, as that grid should be at AC ground.
Eli, I looked into these more after getting home from work. I missed the dual grid connections earlier, and like you say, that means they're designed for UHF grounded grid service. They want to oscillate, and they need the shortest possible connections from the grid pins to RF ground. And the 5842 you mentioned - four grid connections and transconductance of 25,000-30,000 umhos! Holy Moly! Sorry, this is just too much to deal with. Any piece of audio gear built with these tubes needs to be vetted with a microwave spectrum analyzer. Even then, the circuit might age into instability in a year. That's exactly the kind of thing I don't want to deal with in hi-fi equipment that should be routinely and unconditionally stable. Now I know why the 6AF4 is so cheap.
Anyway, I've gone back to published tube curves in search of other types for use as choke-loaded CFs. I'll post more in the thread below. Thanks for your comments on these little triodes.
Buy Chinese. Bury freedom.
it did not help me confusing 6AH4 with the 6AF4 either.
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