Wow! This thread got lively, and no name-calling yet, either.
Cool.
Uh Jay, I gotta differ with you about the Eimac 4CX350A tube. They rate the tube's control grid at zero Watts. A pretty good indication they want the tube held strictly to clas AB1, no grid current. If you have a way to enforce this, the tube takes next-to-NO drive to make it work, but can be easily damaged by excess grid dissipation. If you believe the rated "zero" Watt control-grid rating, an ALC setup that holds the drive level just below the point where it draws grid current will keep you out of trouble.
As a result I NEVER recommend this type to an AM operator. Never. Not a chance in Hades those folks will stick to anything that even
sounds like a, er, uh, "limit".
The '350A has almost 3 times the voltage gain of a '250. The higher sensitivity of the 350's control grid is what makes it so fragile, with NO rating for any grid current at all. The control-grid wires are VERY thin. Placing them closer together than the 2-Watt grid in a '250 is what boosts the tube's "mu". But to leave enough space between the grid wires for the electrons to squeeze through, those grid wires don't have enough metal in them to dissipate any heat at all. I have seen them last a few weeks or a month in an AM amplifier, doubtless running the grid positive, and drawing grid-leak current. The power was impressive, but the tube life was short.
But if you can implement an ALC connection to a SSB radio, and keep the drive level limited, that tube will rock and roll.
Using it in a Pride will be risky, but the stock bias control shouldn't turn down far enough to get a proper zero-signal idle current. The one time I got talked into trying this, the bias zener voltage got cut by about two-thirds. Calls for 20 or 25 Volts, instead of the 50 to 60 Volts for a '250B, or 70 to 80 for a '250R.
Speaking of '250 tubes, the 250R takes a higher negative bias voltage to obtain the same idle current. Dropping a '250R in a Pride may cause the tube to run hot, unless the range of the bias control is stretched to feed another 15 or 30 Volts of negative bias to the grid circuit. For customers who want to use either type, we add an additional 20-Volt bais zener with a SPST switch across it. Open for the 'R, closed for a 'B. Made more sense than altering the bias control to "turn farther".
According to Eimac's description, the 8930 is a '250R but with a larger anode ring. That anode ring should be good for around 350 Watts, more or less. Using it in a Pride requires the same attention that a '250R does. Same tube. If the bias control won't turn the idle current low enough, it will run hot. Otherwise, it's an upgrade for ruggedness, mostly.
The so-called Russky "4CX400" tube is also known as a GS-36B. The actual type number uses characters not in the latin (english) alphabet, but that's what the guy
http://stores.ebay.com/id=56152411&ssPageName=STRK:MEWA:MESST on Ebay selling them for around 100 bucks calls it. The base is wired the same, and the heater voltage/current should be close enough to serve as a sub in the Pride. It shows the same kind of low drive-voltage sensitivity as the Eimac '350B, but has a 2-Watt grid rating, same as the 4CX250 tubes. It looks as if this tube will drop into a Pride so long as the bias voltage is altered, but I've never tried it. Might have hidden drawbacks, might not. Keep promising myself I'll buy one of them to try, next time I have a hundred bucks do just blow on a toy.
Alas, my Pride is waiting for me to design the prototype board for the low-voltage power supply. Got it cheap since boards were missing. Figured it would goose the incentive to finish the design for that board. Then life got busy.
On the other hand, if you get tired of your H.V. boards burning up, I DO have a solution that that little defect.
Nomad Radio Heavy-Duty HV
Doesn't look like I'll be running out of that one any time soon.
73
