The AM Forum

Does anyone know how much peak to peak grid voltage is required to drive my project 4-125A amp to full output in AB1?

Current design is as follows: 50 ohm untuned input, through a toroid transmission-line transformer (bifilar wound, 1:4 impedance ratio) loaded with a 200 ohm noninductive resistor, to the grids of four paralleled 4-125A's. Plate voltage will be either 2600 or 3200v under load (transformer taps). Screen voltage will be well shunt-regulated at 600v. According to Eimac's Care and Feeding of Power Tubes, higher screen voltages reduce the amount of grid voltage required for a given output.

There is a lot of Cin (43 pf for the four tubes alone) but the Command Set transmitter seems to be able to tune out the reactance just fine. My HW-22 isn't happy with it though, since it has nonadjustable loading.

I measure about 140 volts p-p at the grids with the ARC-5 putting out its full 12W of unmodulated carrier. At 100% modulation there should then be 280 v p-p. Is this enough grid swing or will I need to go to a 9:1 (i.e. trifilar winding) and a 450 ohm load resistor? Or is it already too much and I should use a 1:1 unun and a 50 ohm resistor?

(I'd just try it and see for myself, but I don't have the HV and screen supplies completed yet. I'm still working on the main chassis you can see in the avatar).
thanks for any hints.
Charles

Hi,

  The info I have from my Eimac data sheet:

At 1500 volts B+  90 volts
At  2000              94 volts
At  2500              96 volts

All of the above are with a screen voltage of  600 volts.
The gird voltage is negative the same voltage as drive voltage.

  Get out your Ohms law to figure your load resistor and turns ratio.

                                                  Wayne, kd7rur

Thanks for the info. I emailed Eimac and they faxed me their data from their archives. The curves included were at 350v screen though. It also shows complete cutoff around -95v.

It sounds like 100v p-p will be about right for grid swing. That implies I should use a 1:1 instead of a 1:2 unun and a 50 ohm swamping resistor. No point in making too much power gain and requiring 1 watt to drive it! 12.5 watts across 50 ohms is 25 v rms or 70 v p-p, and at PEP (50 watts) 140 v p-p. So the Command Set won't even be running at max output, and hopefully it will be cleaner with lighter loading.

I wouldn't need the input transformer at all for a 50 ohm input except that I'm using a bridge neutralizing circuit so the "cold" end of the grid drive source has to be slightly off RF ground. The Care and Feeding bible suggests using the screen as a neutralizing element. Could be interesting but I've never seen this in a "ham" design. Wonder why?

thanks
Charles