Some more things I forgot to mention about the YC-156. Where directly heated thorium cathodes can be run at slightly reduced filament voltage for increased life, don't try this with an oxide coated cathode. It has the exact opposite effect on these tubes and shortens the life.
The YC-156 was also made for one specific purpose that is now becoming obsolete. It was custom designed for use in many MRI machines and may even be out of production as we speak. Apparently the RF generators in these devices have progressed to the point where this tube is no longer used.
That being said, the tube is fragile and may become difficult to replace in the future. Buy a spare now and I strongly recommend using some type of grid over load protection. I believe the tube has a 5KW anode dissipation but the grid is no stronger then the 8877 at only 25 watts. This makes for some very low IMD when used as a linear but easy to melt the grid.
I personally know more than a couple MRI technicians. NO, I don't have a good line on surplus tubes, lemme get that out of the way now. They already sell to Alan.
BUT, the YC-156 IS going away, and as you stated, may be out of production already. HOWEVER, the 179 is still being produced. It won't 'bolt in', but an adapter plate can be made to make it work. It's basically the same tube, SLIGHT updates inside (I've heard it reported that the 179 does NOT need to be 'debarnacled' to run it in SSB service, but I would still perform the practice, JUST IN CASE!!!!).
I've also been involved in more than one argument about the grid structure. In an MRI machine, they pulse drive the grid. Because of this, the grid structure is 'isolated' mechanically..... Meaning, the 25 watts is 'free air' dissipation, like mounting a TO220 transistor sans heat sink. Since we (radio hobbyists) use the grid strapped straight to ground, usually on a nice thick chassis plate, there IS a heat sinking effect.... HOWEVER, I don't believe you NEED to drive the hell out of this tube, as the gain is pretty high.
I spoke with Tony King via email for quite some time on the YC-156 tube, and he agreed that it DOES get at least some benefit from the heat sink effect, as has VE7RF and more than a couple other 'large' amp builders. HOWEVER, I also have been argued on the other side of the coin that there is NO heat sinking effect, and the opponents of the idea pointed to pictures ON Tony Kings website to proof.
HOWEVER, I've put MORE than 25 watts through the grid, and not had a problem **wink** There is also other users that report the same.
The statement "gain of a tetrode" comes from a statement made by Reid Brandon. NO, it doesn't have the gain of a modern day tet, but at the same time, this isn't your Daddies 3-500Z, either!
As you pointed out, you do NOT ramp the voltage to extend tube life. Tube life in these types is maintained and extended by KEEPING THE FILAMENT AT IT'S RATED TEMPERATURE!!!!!
If you'd like to make it even BETTER, there is another cooling tower design I've used (given to me by someone else, in the interest of clarity) that bumps the Pdiss up about another 1 Kw, in typical amateur / CBer service. Instead of the standard chimney, you can use a box assembly and then drill HUGE holes between the anode and filament areas. This will allow you to force more air through the anode, and airflow = dissipation. Again, the thing you'd really have to watch here is the Grid diss, since it would be pretty hard to run that thing to 6 Kw and only hit the grid at 25 watts... You'd be pretty damn good with the loading! HOWEVER, for the cost incurred, I'd go with this method.... Just make sure you size the blower larger, or you run into backflow issues since the holes are now bigger (I've found that if you follow Eimac's recommended drilling patters, you end up with more backflow from the chassis than you do from the anode! Collins and a couple others figured this out years ago and designed 'top down' cooling, where you use a fan on the lower assembly and a blower on the anode cavity. THEN, the anode is always breathing the coolest air, the tank circuit also gets cooled, etc.... BUT, you have to have a pretty good airflow engineer to get the thing to work correctly from what I've read).
The YC is a 3-500 dollar tube. The 8171 is > 1000. I've successfully seen 4 YC156s in operation in a 30Kw amp. On 10. Buy a pair, you'll probably have your kids selling BOTH tubes after we're all done and gone.
--Toll_Free
