Chuck said?Your guess is correct, it¡¯s a 3CPX-15000B7.? I think it would be a decent choice for a quiet amplifier at 2 ¨C 3 KW out with a very low air flow requirement, especially compared to a 8877.? No socket required and very solid grid to ground connectivity.? High gain also so only 50W of drive needed for the 2 ¨C 3 KW out.?
Oh my, this brings back memories!?
I was responsible for introducing that tube in the 80's. There may have been 2 customers for it but I remember only Harris RFC in Rochester NY.
I never wrote up a story on my experiences related to the 3CX15,000B7 but I'll put some of it down here, as I am surprised to hear there are two tubes that are still in existence.?
Harris designed a 10kW HF amplifier to compete with Rockwell Collins in the military programs that were coming out, namely the Falcon program. Rockwell got that contract which resulted in the HF8022 with a 4CX15,000A in the PA, driven by 4CX350A's. An old design they dusted off and quoted with little or no R&D required.
Harris RFC chose the "new" 3CX15,000B7 driven by a pair of 3CX800A7's. John Lundberg, W2TX, and others developed the prototype and for whatever reason took the driver plate voltage off the center tap of the 3-ph plate transformer, obtaining one half the PA plate voltage with no other iron required. It saved space surely but that became the Achilles heel of the amplifier.
There were a number of microprocessors doing auto tuning and loading and I believe that added to the issue of cases where extreme plate current and arcing in 3CX800A7's occurred.
Eimac's engineering manager was an ex-RCA engineer who came to San Carlos via a stint at the Eimac Salt Lake City plant. With pulse experience, no rf background at all, his John Wayne style of managing personnel was atrocious. His telling production to remove a platinum ribbon from the 3CX400 had disastrous results when Motorola PA's mostly used in UHF communications systems started failing. That just accelerated big M's switch to SS.?
His recommendation to Harris to solve the arcing problem was adding a vacuum relay to the B+ going to the drivers to interrupt B+ in the event of an arc. Fuses were out of the question.
Harris did some work-around and my boss W6TC and I flew up to Rochester to discuss the problems with the buyer, Frank K2OS. I didn't set up the trip, going along on George's coat tails as he wanted to make a sale. We arrived that morning un-announced and the receptionist called Frank and said we were there to see him, he came out and quickly ushered us behind a coat rack to hide us from view! He said management had proclaimed no communications with Eimac, we were shocked to say the least and left a few minutes later. I visited Frank at his home and he was a genuinely nice person but the Harris RFC CEO or someone in the food chain there was an A-hole.
Harris sold a couple of the 10 kW amps to AT&T Longlines for HF stations and the tubes worked as far as I know, perhaps they solved the arcing problems.?The YC156 / YC179 used the exact same grids and cathodes as the 15,000B7 and they were 99.9% reliable.? I remember one YC156 was returned during the time Erbtec developed the MRI amplifier, a Brit whose call I don't remember was the rf designer, and GE Medical went on to sell hundreds of MRI systems with that tube in them. Now all SS of course.? ??
So who is going to be the lucky person to put the 15000B7's to use? I suspect those are the last of unused tubes of this kind on the planet. Rare as hen's teeth!
Maybe I'll post my Falcon Program story (developing the 4CX350AC driver tube).? That's another rare tube and BWT I'd buy unused Eimac 4CX350AC's in case someone finds any contact me.?
73, Reid W6MTF? ? ? ??
Brandon DX