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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
ok hears an idia, clamp tube modulation for the screen. it is just an idea for a screen power supply but along the lines of what I have in mind. there may have to be differant operation moads one for spark length and modulation. Hear is a web sight that has another tube clamp circuit for any body that is interested with another tube clamp what do you think. any more reading for me is welcome
if anybody wants to know I am working on the chimny. when it is done I'll take a picture.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
That's a fairly standard screen grid modulation circuit. It's not nearly as efficient as anode modulation (you cannot get 100% modulation) but is obviously much the cheaper of the two options, since no additional big power valves, costly mod transformers and HV supplies are required.
In pre-SSB days, you would sometimes see it used in 'B' sets, where RT and MCW were secondary to the transmitter's main WT role.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
could I use a simular circuit to controle the screen for different modes of operation and use another tube to get modulation using the controle grid. do you think this is better way to go
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
No need for a control grid modulation - in that circuit you have both a shunt regulator and a modulator in one, though you will have to adjust the circuit values and tweek it up a bit for use with a big power valve.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Control grid modulation needs an anode supply voltage which is twice as high as would be needed for the same circuit without modulation. This means that you will have much higher anode dissipation at low modulation levels, and will be lucky to have an anode efficiency higher than about 40%. Unless you are trying to make something with a very wide bandwidth, where transformers would be troublesome, I'd give control grid modulation a miss.
As ever, others may think differently, so check with them too!
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
As for modulation, you can modulate the cathode, but on such high voltage tube, that cant be easily done with normal semiconductors. You COULD do a mosfet half bridge cascade, but that seems a bit much. You can perhaps use a small pentode or tetrode to connect and disconnect the cathode from ground, and modulate its grid, i think. Hehe. I know there is a way you could, but i do not know how difficult this would be or how well it would work. You could perhaps drive the pentode's grid with a PWM and modulate that?
With lower voltage tubes, like an 811A or so, you can use a mosfet or IGBT at the cathode, and have that modulate it.
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need help with a tetrode screen supply
