Welcome to tetrodes Manfred. Very few are designed for very low IMD and
excessive, IMO, idle current is a way to appease the masses in the ham
world.
The 600W dissipation Russian GU-74B was fraudulently changed to 800W as the
4CX800A by the Svetlana management as a way to market it to US hams and amp
manufacturers.
Original Russian IMD tests from Jan 1993 came up with -15/-24 dB 3rd and 5th
order respectively in a standard grounded cathode AB1 circuit with the
control grid swamped with 50 Ohms. Ep was 2200V, idle Ip 150 ma.
After all said and done they settled on a 24 Ohm cathode resistance as NFB,
a 250 ma idle current, -33.5/-36 dB IMD 3rd/5th and an efficiency of 56%.
That is 550W of idle power heat which helps explain the revised plate
dissipation spec.
Various ham manufacturers pushed it even harder with 2500-2600V Ep under
load and 625W idle power...per tube. No NFB. No wonder they died at an early
age.
Carl
KM1H
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: <amps@contesting.com>
Sent: Wednesday, October 29, 2014 5:03 PM
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?
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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