[Amps] 2 x 4-250A Amp

[Peter Chadwick]

My experience with grounded cathode, passive grid, 4-250As - for what it's worth.
I found better linearity running in AB2, with some NFB from 10 ohm resistors, un-bypassed RF wise, in the filament circuit. Carbon preferred, but I suppose metal film would work. Incidentally, RF returns should be to the filament pin 1. To do this, you need a filament choke, but there's not much to it - you only about 10microhenries even for 160m operation. So the filaments are held above ground RF wise by the choke, and there's a 10 ohm or so resistor from each pin 1 to ground, with a series blocking capacitor. That needs to be a reasonable RF type, since the full peak RF plate current flows through it. You would also want a choke from the centre point to the meter. It might be easier to meter the HV negative and just run through a small (10 microhenry agian) choke to ground from the middle point of the filaments.
Because AB2 runs grid current, the bias supply needs to be shunt stabilised, and I built a simple thing with three high voltage transistors.. Needs well bypassing to keep the RF out.....I run a series regulator for the screen supply using a 6L6G my father bought pre WW2, a 6SJ7 and a VR150. You need to check with a scope that it isn't oscillating at some weird frequency, but that applies to all regulators.
I have parasitic suppressors in the grid feeds - 47 ohm, 5 watt carbon resistors with shunt coils of 2 turns, 1 inch diameter, spaced 1/4 inch, #14AWG. The plate suppressors are carbon rod, 20 watt, 47 ohm, with similar shunt coils.
I run at 300 volts on the screen, about -60 odd of bias, and 3800 volts on the plates. I can get about 1050 watts at -30dB IMD 2 tone, relative to each tone, although that's pushing the plate dissipation.
This amp was originally a commercial amp in the top of a tx rated at 900 watts PEP in independent or single sideband. The design started as a 350 watt for the UK army, and used a pair of 4-125s. When they wanted a higher power version, they upped the HV, fitted 4-250As and a bigger filament transformer. They didn't put in forced air cooling round the tube sockets, or uprate the various ceramic capacitors in the PA tuning......they also changed the colour scheme and sold it as a commercial tx for maritime use. So it had some problems. Mine is over 40 years old, and the capacitors are playing up, so I'm in the middle of major rebuild - have been for some months, but work has got in the way. I did the tank coil for 10/12/15/17 and 20 this week -  put a piece of 2 inch diameter aluminium mast in the lathe, clamped the 5mm diameter copper tube between bits of wood as a friction block, and set the lathe to 5 tpi and off we went. With the coil in place yesterday, I determined the taps: using the AADE inductance meter, they all worked out where I had calculated! I hard soldered (using silver solder that melts at 610 deg C) the tap leads on, and left it soaking overnight in Coca Cola to clean it up..........

The 4-250A has the same filament as a 3-500Z, and the rest of the internals are the same as a 4-400, just the 4-400 has a bigger anode. The 4X500 is the same internals with a different base and an external anode. So do make sure that there's air over the sockets, or you'll get problems in the long term - just as people report with 3-500Zs. Plenty of air round the tube, too, and a plate connection that has good heat transfer to the air. If you have chimneys round the tubes, blow well from underneath. I don't, but I have a Muffin fan under each socket blowing directly onto the socket, and big fan above blowing straight down onto the plate circuit.
Good luck. Feel free to contact me directly if I can help further.
73
Peter G3RZP