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Ceramic Disk Capacitors are used as RF bypass capacitors in RF circuitry--that is to short RF to ground.? And Capacitors can be also used to block DC and low frequency current form passing through them. How to use the in your situation is described below.

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Use a set of High Voltage Ceramic disk capacitors, wired from each of the two filament leads to ground—this will allow the high frequency RF to pass through and block the low frequency AC filament voltage..?

-Disk ceramic Capacitors are also used in High Power Tube Amplifiers as a DC blocking capacitor to Pass through only the RF from the plate to the PA plate circuit and then the antennas system.?

-Since this works in the plate circuit and it will work in the ground/common circuit too.?

-I am not certain about the voltage requirement for this filament grounding circuit.? Look at the Filament transformer’s insulation value.? You aren’t going to need much higher voltage for the capacitors than that.

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Turns out, high voltage disk capacitors are becoming hard to find:? I found this at RF Parts:

? 0.01 micro farad @ 2KV

-Assuming you will use a 0.05 microfarad capacitance value you will need 10 of these to make your filament grounding system.? Double than quantity if you need a high voltage of 4KV.

-Alternate choice but more expensive are doorknob capacitors.? While these might be awkward to mount a capacitance above 2000pf may work for you, these will certainly work and look impressive.? You are going to need one of these door-knob capacitors for your amplifier plate circuit too.---just buy 3 and you’ll be set :>).

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A capacitance of? 0.05 microfarad will present only 3.8 Ohms (2 x 1.8) resistance to the RF signal at 1.8 MHz even lower resistance as the frequency goes up.? But will present 106K Ohms (2 x 53K) ohms to the 6.3 volts 60 Hz AC voltage.? Use this calculator to determine alternate capacitance values if you desire:

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73,

Dave, w6de

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Hi, I've been trying to gather parts for a 3CX1200D7 build. I had trouble locating a sufficient filament transformer. Tube requires 6.3V @ 25A. Ended up finding one for 6.3V @ 28A - Would like to have more overhead but thought it would be the best I'd find, and only $50 brand new! Perfect!

BUT In my haste to complete an order of needed parts, I wasn't paying close enough attention. It turns out that it doesn't have a CT. (it's dual secondary - parallel is 6.3V@28A and series is 12.6V@14A) - D'oh!

Now what... when I used to play with guitar/audio amps, a semi-common thing to do would be to take a pair of low value resistors, put one end of each to each end of the filament winding, and ground the joint in the middle for a fake CT. Unsure how this will affect a cathode bias.

I've also heard of using the secondary winding of a totally separate transformer that does have a CT, and using it in parallel across my filament winding, then using that transformers centre tap. Further, I've heard of using a pair of diodes, each anode to each end of my filament winding and tie the cathodes together in the middle... or maybe that was t'other way around... either way I dunno about this either.

Anyways, obviously the best way forward would be to actually use a proper transformer, but since I can't find one and I already made this oopsie... I'm wondering what the next-best-way forward is? :)

Thanks :)