I think the suggestion that a power tubes' plate is supposed to glow a
"dull red" during normal operation is ignorant. If the plate is glowing,
the plate's maximum dissipation is obviously being exceeded and the life of
the tube(s) will suffer due to, in part, the following factors:
Plate voltage too high.
Grid voltage too low, (too small value grid leak resistor in tesla coils),
causing excessive plate current to flow.
Plate circuit out of tune.
Of course, it is tempting to observe and enjoy the longer spark that occurs
when we "max out" the plate dissipation of our transmitting tubes; but
remember, you are exceeding the maximum design parameters of the tube(s).
If you want your tubes to last for years, I would never advocate running
them with any kind of 'glow' other than the filaments. This is good just
plain good design etiquet. But thain again, they're you tubes, so if you
want to run them in 'supernova', be my guest. Just remember what others may
be thinking when they watch your TC's plates overheating.
Dr. Hankenstein
> [Original Message]
> From: Tesla list <tesla@xxxxxxxxxx>
> To: <tesla@xxxxxxxxxx>
> Date: 3/22/2006 6:16:26 PM
> Subject: Re: 833A's plate color
>
> Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
>
>
> Dull red is the normal color for 833s. Bright
> orange means its running too hot. Fab a chimney
> with forced air cooling (ref: older ARRL Radio
> Amateur's Handbooks for articles and
> photos). PVC is acceptable with a good blower, ie, 200-300 CFM.
>
> Dr. Resonance
>
>
>
>
> Hi Frank, and anyone contemplating a VTTC:
>
> If you possibly can, avail yourself of a
> reprint of RCA manual TT-5. I believe that
> "Antique Radio Supply of Tempe AZ still sells the
> reprints. There is a wealth of theory and
> measurement, application and circuit diagrams in this book. (<$15)
>
> Matt D.
>
>
> In a message dated 3/22/06 1:41:36 P.M. Eastern
> Standard Time, tesla@xxxxxxxxxx writes:
> Original poster: "Philip Chalk" <philip@xxxxxxxxxxxxxxxxx>
>
>
>
> Subject: 833A's plate color
>
> Original poster: "Angelini, Frank J" <FrankJAngelini@xxxxxxxxx>
>
> > I am running an 833A based coil. The tube is NOS RCA from a now
> >defunct AM radio station. I have approx. 3000 vac on the
> >plates. I'm getting a nice 18 - inch brush discharge, but the plate
> >color is bright orange. I have been told that I am pushing this
> >tube. Is this correct? What should my plate /current/current
> >be? How can this be measured?
> >
> >Thanks
>
>
> Hi Frank,
>
> Sorry, this turned out very long - hope it helps :-)
>
> I've long wanted to play with 833a's, & never met one up close, but am
> reasonably familiar with it - "bright orange" does sound like pushing it
> a bit, depends on your subjective perception, description etc of the
> term.
>
> According to RCA's TT-5 Transmitting Tubes manual :-
>
> "Plate shows an orange-red colour when tube is operated at its maximum
> CCS or ICAS ratings."
>
> 3000v rms is about 4,250v peaks, so it's a little above the ICAS max
> rating of 4000v **with forced air cooling** With natural cooling, it's
> 3,300v max (ICAS. 3000v max CCS)
>
> As a class C RF Osc/Amp the tube's max ICAS ratings include also :-
> Plate current = 500ma, Plate input = 2000W, Plate dissipation 450W. Here
> the max values co-incide, which isn't always so. At these values, you're
> getting 1,550 Watts output from the tube.
>
> Your input may well be above 2000W rms, and your plate dissipation sure
> sounds like its greater than its rated. As plate dissipation is simply
> the difference between the power input to the tube & the power output or
> extracted from it, to reduce plate dissipation you must either reduce
> input or increase output power. And power of course is the product of
> voltage & current. So reduce input power by reducing plate voltage,
> current or both & increase output by better impedance-matching from the
> tube to the load (coil system).
>
> Btw, CCS = 'Continuous Commercial Service' & ICAS = 'Intermittent
> Commercial & Amateur service'. Given that RCA say, even for CCS, which
> is like an A.M. broadcast station transmitting 24/7, that the plate
> 'should be' running orange-red, you probably don't have to worry too
> much if you're only doing short runs, with intervening cool-down periods
> - depends on just how bright "bright orange" is :-)
>
> If you haven't got any forced-air cooling on the tube, you should
> definitely add some. A heat-dissipating connector on the plate terminal
> could also help a little. RCA TT-5 again:-
>
> "With forced-air cooling, an air flow of 40 cubic feet per minute from a
> 2-inch-diameter nozzle directed vertically on the bulb between grid and
> plate seals is required to limit the temperature between these seals to
> 145°C." Do you have equipment that you can measure this temperature
> with ? (BE CAREFUL of the HV)
>
> Definitely measure the anode current if you can. Especially as you're
> feeding it with ac, this is not a completely trivial exercise. You
> really need a true RMS meter to get useful measurements, due to the
> 'spikey' nature of the anode current waveform. If you have virtually any
> kind of true RMS meter with about a 1, 2 or even 5 amp FSD scale you
> could use that. ( AC Current meter goes IN SERIES with the HV supply
> lead to the anode )
>
> BE EXTREMELY CAREFUL to not touch the meter or any associated wiring
> while the coil is running. Set up the meter, making sure it & it's
> leads are well-insulated & kept well away from anything else, then hands
> off, coil on & note reading. A 1/2 or 1A fuse in series with the HV
> supply lead if paranoid.
>
> Do you have an oscilloscope ? This certainly gives you more options.
>
> Sorry to preach again something that is constantly reiterated here, but
> at least for my peace of mind:- With ANY kind of High Voltage
> measurement you need to be very sure you know exactly what you're doing
> & TO BE EXTREMELY CAREFUL. Any such measurements around this sort of HV
> carry very real risk of serious injury or death at your very first
> mishap or careless slip-up.
>
> Some kind of protection ACROSS the current meter is worthwhile if you
> can arrange it - several 1kV diode strings back-to-back, or a tiny
> spark-gap of about 1/32 inch or less, etc. If the meter goes
> open-circuit for any reason, it'll have the full 3kV ac across its
> terminals & is very likely to blow-up, possibly literally, in protest.
>
> Reducing the plate voltage some will certainly give the tube a happier &
> longer life, as will reducing the filament supply a little - down to
> about 9.5v, or current-limiting it.
>
> Depending on the circuit, increasing -ve bias on the grid might also
> help, possibly simply by using a cathode resistor, bypassed; or reducing
> the grid current somehow - reducing coupling of feedback coil, or
> whatever - all depends on the circuit. Possibly you need to load the
> tube more heavily by increasing overall coupling from the tube to the
> secondary. Again - what's the circuit?
>
> Certainly taking any measurements you can will help getting a feel for
> the conditions, safety etc of the tube.
>
> If you'd like to email me more details, type of transformer & the
> circuit, or a link, or something, I might be able to offer you more
> specific suggestions. Haven't built a tube coil yet, but keen & getting
> close. Got a 4-250A lying around just asking for it. vk2poc@xxxxxxxxxxx
>
>
>
>
>