Re: 811A any good?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> 
> Hi Jan, Matt, Ralph!
> 
> Date: 23 January 2001 19:59
> Subject: Re: 811A any good?
> 
> >Your right that the average power rating for the 811A is
> "45W CCS and 65W ICS
> >averaged over any audio-frequency cycle of sine-wave form".
> Maximum plate
> >input (pulse peak) is in the range of 230-260 Watts
> (radar-type
> >applications). It's usability depends critically on the
> type of circuit and
> >the type of signal.
> 
> I think there's slight confusion here over power output and
> plate dissipation.  The 45W CCS and 65W ICAS ratings refer
> to the amount of power which can be lost in the anode of the
> valve as heat.  RCA's "Transmitting Tubes" manual (I have
> the reprint) makes it clearer - under ICAS conditions and
> class C biasing, you can put 260W into the tube, the plate
> dissipates 60W as heat and 200W rf comes out, *continuous
> wave*, per tube.
> 
> I suspect our friend Terman would say you can squeeze a fair
> bit more under low duty cycle operation, e.g. ten
> microseconds on, ten milliseconds off.  Is that what John
> Freau does with his massive 833 coils?  I currently have a
> pair of 813s awaiting similar treatment as and when I get
> the time.  I anticipate 750W out (cw) from the pair with any
> luck. But no idea when I'll get the proverbial round tuit.
> It may be some time.
> 
> Dunckx

	The 65W ICAS rating is an AVERAGE power dissipation rating, which
should apply independent of the duty factor at which the tube is
operated.  In intermittent service the PEAK dissipation can surely
exceed 65W by the ratio of the OFF time to the ON time, provided they
are both smaller than the thermal time constant of the plate, which is
probably at least a second.  So, for the example given, the peak
dissipation could, in principle, be 6500 watts.  Of course, the tube
doesn't have enough emission to come anywhere near that.  Operation at
the highest plate voltage at which the tube won't experience gas or
insulation breakdown will be advantages to maximize the peak power.  

	As an example of pulsed operation, some of the early VHF radar
transmitters operated tubes rated for 100W dissipation at a plate
voltage of 15000 and a peak plate current of several amperes.  With the
right tubes this should also be possible in TC service, and wonder if
anyone has tried it (extremely suply voltage)?  Eimac tubes like the
304TL or TH will stand a lot of voltage, as will the VT-127/227/327
which were intended to be used in the service mentioned.  As a further
note, most of these transmitters were self-pulsed.  At the right
combination of grid bypass/coupling capacitor and "grid leak" the tube
will go into conduction, oscillate, and pulse itself off.  One of these
days, when "I have the time" I intend to experiment with this scheme,
using some VT-227's I already have, plus a 15000V transformer from such
a radar transmitter.  Would sure like to see somebody try it sooner.

Ed 

P.S. Haven't used an 811 or 812 for many years, but seem to remember
they "liked to go gassy".  Suspect operation at say 3000V might result
in problems, but sure would up the power output.