[Amps] Alpha 8410 using 4CX1500B - why?

[Manfred Mornhinweg]

To all tube specialists out there,

I wonder whether the 4CX1500B is _intrinsically_ a low distortion tube, by any 
special construction techniques, or if the good IMD rating comes simply from the 
operating conditions chosen for the spec sheet!

In the EIMAC datasheet, this tube is specified for -43dB 3rd order IMD, which is 
excellent, when operating at 2900V, 300mA idling current, producing 1100W 
output. That very high idling current means 870W of plate dissipation at zero 
drive!!! That kind of biasing is suspiciously close to class A. At full power, 
single tone, the plate efficiency is only 53%. With a two-tone signal, 
efficiency is 35%.

I wonder how this IMD spec compares to that of other tetrodes when run at such 
high idle current, or how high the IMD of the 4CX1500B gets with a more 
reasonable idling current, such as 50mA.

Not many datasheets give useful information about IMD at different bias levels, 
so its hard to compare on paper.

Call me a cheapskate, if you will, but burning up 870W of power at the plate 
when idling, plus 60W all the time in the filament, to get 1100W output at 53% 
plate efficiency, looks dismally poor to me.

This last contest weekend I was at the CE6TC contest site. It was a two station 
setup, one equipped with an Alpha 89, the other with a Ten Tec. Both amps worked 
great, but the streams of hot air blown out by the amps, overheating the room, 
are a testimony to how antiquated that technology is. Overall there was more 
heat than RF power. I really think we need something better.

Has anybody here tried to run a large tube, such as the 4CX1500B, in linearized 
class C? I mean, build a pretty normal amplifier, with a somewhat lowish plate 
voltage, and add a linearizer circuit that compares output amplitude to drive 
amplitude, and controls the grid bias to keep the amplitude ratio constant. The 
tube's operating point would move from almost pure class B at low signal levels, 
to narrow conduction angle class C during higher signal times, and back to wider 
conduction angle class C at peak power, with conduction angle controlled by the 
bias, to achieve good linearity. The grid bias voltage range would be restricted 
to the range from what causes a small idle current, down to deep plate cutoff.

To accomodate less than perfect SWR, the gain (ratio of output voltage amplitude 
to drive voltage amplitude) needs to be variable depending on loading 
conditions. This can be accomodate automatically, by the circuit increasing this 
ratio up to just below the point where peak clipping starts. Said more simply, 
if the grid bias  set by the circuit reaches the limit of the allowed range 
during peaks, the gain is automatically reduced, with a slow decay, or the 
exciter's output is reduced through ALC action.

  The cost of this grid bias modulation circuit is almost nothing, its 
complexity is modest, the rest of the amplifier is basically just as you 
oldtimers know it, but there would be almost no idle current, the full power 
efficiency would be like 80%, and the two-tone efficiency would be like 60%. 
That allows far lower heat production, lower consumption, smaller, lighter, less 
expensive power supply components, a smaller and less noisy blower... lots of 
good things.

Has anybody tested this? Does anybody expect any big trouble with it? Is there 
anything I'm not noticing, causing phase distortion or so? Or has nobody done 
it, simply because nobody cares enough about power consumption, heating, and 
blower noise?

And about Alpha using the 1500b instead of the 1000 now, somewhere (could have 
been on this very forum) I read that it was mainly because of availability 
problems of the 1000. They are still around, but the future seems to look a 
little more stable for the 1500b than the 1000.


Manfred


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