[Amps] Alpha 8410 using 4CX1500B - why?

Wed Oct 29 18:45:29 EDT 2014

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 at ludens.cl>
To: <amps at 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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