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6av5 as triode? Redux

On 7-6 robertpeterson56 asked about using the 6AV5 strapped as a triode but was concerned because of the low g2 rating of 175V. In the discussion that followed, one suggestion was to tie G2 to the cathode instead of the plate. There was speculation that this wouldn't work but nothing definite. The curves in the 6AV5 info I have only go to 75V and plate current is really low at that point.

Since then I've played around with a 6AV5 strapped both ways and can report the following:

1. Strapping g2 to the cathode indeed doesn't work. With 0V on g1 and 400V on the plate, plate current was just 10mA. g1 would need to be driven + for any appreciable plate current and I doubt that it was designed for this service.

2. Strapping g2 to the plate (thru a 1K stopper resistor) seemed to work well as a triode. I ran this up to about 350V with appropriate g1 negative bias and nothing let go. As a result, I'm reasonably confident that the 175V g2 rating is very conservative and can be disregarded for conventional triode strapping.

3. The 6AV5 appears to be one tough little tube. I ran it as a triode @ 350V with combined plate/g2 current ~60mA for about 5 min. There were no red spots on the plate or glowing g2 wires that I could see. This is 21 watts for a tube with a combined plate/g2 rating of 13.5 watts...not bad!

4. I set up the triode strapped 6AV5 at a couple of operating points specified for the 6B4G (g1 V & plate V) and compared resulting plate current. Surprisingly, the current was reasonably close to what was specified for a 6B4 under these conditions. Obviously, a couple of points is not a set of curves but I believe there's more than coincidence at work here. Maybe Sylvania or whomever was justified in using a triode strapped 6AV5 as a modern day replacement for the 6B4G in govt apps. Wonder if discerning ears could tell the difference?

5. The 6AV5 absolutely requires G1 and G2 stopper resistors when triode strapped to prevent oscillation. I initially set up w/o them and ended up wiping out the lower end of the FM band as I powered the thing up. Position of my had around the tube changed the frequency.

Bottom line: The 6AV5 and possibly other neglected TV sweep tubes are probably worth pursuing as triode strapped audio output devices. g2 voltage ratings can probably be uprated to reasonably high plate voltage say on the order of 350-400V w/o failure (triode service only).

AS always, YMMV, so do your own homework!

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Topic - 6av5 as triode? Redux - Steve O 19:58:55 07/09/05 (6)

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