[Amps] Requesting opinions on a Drake L-4b's tubes

[John Lyles]

The author originating this discussion said that they had one 3-500Z 
that was the vertical thin fin plate, Eimac original, and one 3-500G 
that has the graphite anode presumably with the horizontal heat 
radiating structure. This would look like the Amperex/Covimag tubes that 
are no longer made, but are cloned in China. For that matter, the 
original 3-500Z from Eimac is long out of production as well.

I think it would be more risky to pair these two tubes together as they 
come from different manufacture with different plate physical design. 
There is no guarantee that the cutoff bias voltage is identical and that 
the mu is the same or within a tolerance. Operating this way might cause 
one tube to hog more current, have higher dissipation due to the idling 
current (Zsac), if they both share the same cathode bias setting.

It would be optimal to put the older style NOS tube into the amplifier 
with one of the original (weak?) tubes to see if it works better. Then 
try it with the other original tube. If the two tests show large 
difference in power capability, then I would suspect just one of the old 
tubes. At least start with this plan, and see how it works, report back 
here.

If anyone is interested in Amperex/Covimag 3-400Z or chimneys for either 
triode, get in touch.

73 and Happy Holidays

John

K5PRO