[Amps] EIMAC vs Amperex 3-500

[gudguyham at aol.com]

When you talk 200ma is that for one tube or two?  What mode are you figuring on?  Using SSB (ICAS) peak grid current of 200ma per tube and 400ma for two is no big deal.  The CCS rating on a 3-500 is about 125-150ma per tube.  That is key down 24/7.  Therefore with an ICAS rating a peak of 200ma per tube is nothing.  Unlike indirectly heated cathodes peak grid current over time isn’t harmful.  Henry ran 3-500 tubes at high output by raising the plate voltage up to 3800-4000 in their amps.  Raising the plate voltage lowers grid current.  The only thing that usually  destroys tubes like 811a/572b/3-500 which many amateur amps have is over dissipation over time, not necessarily high peak grid current.  A set of good 811a tubes used in SSB can tolerate around 1100 pep output without compression.  That kind of power would be relegated to DX type operation only!  
Four 572b tubes can easily take 1 amp of plate current and 200mills of grid current.  In a mono band high efficiency tank circuit that will give you 1600plus pep output.  That of course will be an ICAS rating.  Used in DXing type SSB they don’t even show color at beyond 1600w pep.  
Dissipation over time kills these tubes not grid current when the amp is properly operated.  Too many people agonize over grid current. If you are running RTTY or FT8 in a contest type mode then you’ll probably want to watch the grid current more closely, but most likely not overheating the tube would be more of a problem.

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On Friday, October 21, 2022, 9:30 PM, john.keating at outlook.com <john.keating at outlook.com> wrote:

Spec sheets for these tubes appear identical. Yet they behave 
differently in my dual 3-500 amp. I can drive the Amperex at 90w for 
near-legal limit out (power gain of about 16x) without exceeding 200mA 
grid current; Plate current is just about 800mA for the pair. With the 
Eimacs, I can't exceed 75W drive without exceeding 200mA grid current 
(running about 720mA plate), and they do give the same 16x power 
amplification at that drive. At 90W drive, the Eimacs exceed the 
recommended grid current limit. Wondering if the amp brain trust can 
enlighten me on the difference and cause.

73, John / AI6LY
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