dropping plate voltage

[Crusher]

Got a friend that is building a power supply for a single 8877. Problem is, it is a 2800V 2A CCS. Once rectified it will be near 4KV. My friend wants to drop the voltage about 400-500 volts. I told him to get a variac so he can adjust it. Too Rich for his tastes. Anyways, I was wondering using a fixed resistor. What is the best way to drop the voltage? Drop it down from the primary side or secondary/HV side before rectifiers as voltage is lower. If I do it Primary side, the transformer is 220V. What just one side need to be dropped or both legs? Anyone do this before?

 

[Captain Kilowatt]

Using resistors is an extremely poor way of lowering the voltage. The regulation will be very poor with the voltage dropping under load and soaring between voice peaks or in RX mode. In any event the best place is to put them in the primary but remember that the power they will need to handle is the maximum voltage dropped multiplied by the current drawn in amps. If you dropped the primary by 10% or about 20 volts and were drawing 5 amps from the line then you would be dissipating over 100 watts in the resistor. I still would'nt do it as I said it's a poor way to do it. If the tube won't handle that voltage then either get another tube or another transformer or pony up for the variac which is the proper way to do it. Another way is if you can locate an old high current filament transformer and wire it to buck the primary voltage.

 

[Shockwave]

If the amp is for HF use it will perform real nice with that 4KV on the plate. The tube will easily handle that in continuous duty service. Just bias it with a 25 volt, 50 watt zener diode on the cathode. That will just push the tube into class AB1 for clean linear operation.

 

[Kamikaze]

Yeah 4 KV on an 8877 will work just fine. Be damn sure there's a way to read grid current though.

Your friend could use a filament transformer wired in the primary of the plate transformer in the bucking configuration. Just get one with the right current ration. This assumes the amp in question isn't the cheesy kind with the plate AND filament windings on one core.

A much cheaper variac than you have probably been looking at will also work if you have a 250 volt primary plate transformer. Use a 125 volt variac in series with the winding. Use one endtap and the wiper. As you dial the variac away from the endtap the autotransformer will exhibit inductive reactance and will lower your primary voltage. Depending on the variac it should work from a littyle under half the power supply B+ all the way up to full. Just make sure the variac is designed to handle enough current. A 15 amp job would work great. This allows use of variacs most people wouoldn't take a second look at.