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Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
After doing a lot of online searching and reading through old post I think I've settled on doing cathode modulation for my 833C Vttc. I've never audio modulated anything so if you guys/girls would take a looking at this schematic and let me know if I'm missing something. I have a few options for the modulation tube (4-125, 811A, 813). Would a 120/12v power transformer be a bad choice for the modulation circuit? I was not planning on using another mot to power the modulation circuit because I have several 120/480v transformers laying around. Am I heading in the right direction?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The whole contraption looks a bity dodgy to me, old thing.
Let's start by changing the modulation valve into a beam power output tetrode. A triode will always try to feed some of its output back into the input, if not by rotten wiring layout with poor decoupling, then by the Miller Effect. Who knows what fo might be for your modulator, what motor-boating, squegging, and filament hum it may yet produce?
Then there's the non-linear transfer function of the transformer, which will certainly limit bandwidth and quality in cathode modulation.
You may need a means of independently setting the grid bias of the modulator valve, so you can park it in the most linear section of the valve's characteristic curve.
There's also the tendency of the ever unpopular cathode modulator - a beastly compromise - to 'down modulate' by which I mean the reduction in average power when compared with 100% anode modulation.
Cathode modulation of an LC RF oscillator will without doubt cause some frequency modulation, in addition to all the harmonics up into the VHF which it will broadcast over a wide area through the mains and through the air. I'd check the whole thing over with an absorption wavemeter to see just what it is radiating and whether there is any parasitic oscillation - a common scenario unless measures are taken to prevent it.
What would I do? Get rid of the modulator valve and the modulation transformer, and modulate the cathode current with an IGBT. This removes the disgrace of the transformer and its wretched transfer function all in one go. Less heat, less costly HT supply, smaller chassis ... signs and wonders!
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Thanks a ton for the reply. I have a beam power tetrode but I really wanted to get this VTTC going with the 833 because of its looks. I will give the IGBT modulation a try and thanks for the help.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
U4R1A wrote ...
Thanks a ton for the reply. I have a beam power tetrode but I really wanted to get this VTTC going with the 833 because of its looks. I will give the IGBT modulation a try and thanks for the help.
I didn't intend that the 833 oscillator valve should be replaced with a tetrode, but the modulator valve where you need more stability so it isn't pulled or otherwise interfered with by the fact of its coupling to a not very well controlled power oscillator.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Ohhh... Now I'm following you. I have a brand new 4-400 setting on my shelf. So if I decided to go all tubes how would I set this up without spending a ton? Thanks for your time.
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VTTC triode audio mod
