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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Dr. Kilovolt wrote ...
Thanks Steve, that cleared some things up a bit for me. But I'm still not sure about the grid feedback network - the capacitor just blocks DC, or does something else too?
To my mind, it passes the required AC current to drive the grid, and stores the self-bias voltage between cycles. If the RC time constant is wrong, you get the dreaded "squegging".
Tubes beginning in "P" are for 300mA constant-current operation. They all draw 300mA, but the bigger tubes have a higher heater voltage. The heaters were supposed to be connected in series across the mains, like Christmas lights.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
okay, I'm now drawing 10cm white hot arks from the flyback, and the plate not even slightly red.
But what worries me, is that the cathode(??) seems to get hotter when the tube runs, it looks almost like as if someone upped the heater voltage during the run, but that's not the case.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
A RF current component traveling along the cathode? (as for any metal piece in a RF field). It will rise more the temperature of the cathode. That's why, I think, the fillament circuits in TVs have some varistors in serie, to get back the current at the right value.
Just decrease a bit the fillament voltage, or run as it is, because PL500-504 are cheap.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
vasil wrote ...
A RF current component traveling along the cathode? (as for any metal piece in a RF field). It will rise more the temperature of the cathode. That's why, I think, the fillament circuits in TVs have some varistors in serie, to get back the current at the right value.
Just decrease a bit the fillament voltage, or run as it is, because PL500-504 are cheap.
I think the varistor was to limit the inrush current (I need one too, one of the filament connections gets white-hot for a moment after turning the heater supply on...) So you think this is ok? The anode is not red at all.
I know PL504's are cheap, but mine is 509 which is twice as powerful.
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Tuning the vacuum tube oscillator
