On Sun, Dec 3, 2023 at 01:58 AM, Dave w6de wrote:
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¡ªthis 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
I bought? .01uf @ 1.4 kv disc caps from RF parts.? Huge things at 1" diam x 3/16" thick.? ?
Also bought a bunch of Ceramite brand? disc ceramic 4700pf caps (.0047uf) @ 15 kv? from Henry radio.? They too will handle a lot of current....and are big things.
Then bought dozens of the chinese .01uf @ 30 kv disc caps off their? ebay store.? Massive things? at 1 3/8"? diam x 7/16" thick.? These hi pot tests to? 15 kv? ( limit of my hi-pot). Max current of all of em.? ?11m ops use em for coupling caps into the cathodes of GG amps...and stuff several kw through em. The chinese disc caps also come in 10 kv..and also 20 kv.? ?They are so cheap, and shipping is dirt cheap,? I decided to just stock pile 3 x doz of the 30 kv types.? ?I hi-pot tested all of em....and also the C... which was typ 9200-9600 pf.?
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I put 2 of em in series.... and wire that mess across the cathode of the 3x3, 3x6, YC-243, and bigger tubes.? Drive applied to the junction of the caps.? Done that way, the drive current splits in half...and makes for better use of caps.? ?Z at 60 hz is sky high.? Z at 1.8 mhz is low.?
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For the OP's situation, a small fil xfmr with the CT on the sec? will easily work.? Then the? CT becomes your fake CT....that can be used for the cathode return current.? ? Sec of small xfmr wire in parallel with his existing fil xfmr( that has no CT).
And like Reid sez.... the 120 vac primary of the? small fil xfmr has it's 120 vac primary leads taped off...... not used.?
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Jim? VE7RF
