4-400 Zener Bias voltage

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Gavin Williams:
I have an am well in fact two of them, that is a copy of the Economy 2kW amp 81 ARRL handbook using a pair of 4-400 valves.

It specifies a 8.2volt 50w zener to develop the bias.

IF I was to up the voltage of the zener to say either 16 or 51 volts  when running CW would this have any benefit in term of efficiency and power output, or what voltage should I put it up to for CW operation?.


Lou Parascondola:
Are you running this amp in grounded grid?  What is your plate voltage?  If you have a plate voltage of 3KV and running GG you probably don't want any operating bias at all.  Usually a pair of 4-400's at 3KV self bias themselves to a decent idle current, only when you approach maybe 4KV on the plate would you begin to need about 8 volts of operating bias.  If you up the bias voltage to about 15v the tubes will be lax and you'll need a lot of drive to get them going.  You should shoot for about 150-180ma I'd say.  Case in point, if you want to run 4-400's in an SB-220 amp, you should short out the zener diode or else the tubes will be biased way down to low in idle current.  These tubes are NOT like a GS-35B or some other type of Russian tubes that need goobs bias voltage.

Gavin Williams:
Yep, GG, and around 3Kv in plate voltage.  All I was thinking about was increasing the efficiency for CW only.  The original ARRL circuit shows 8.2 volt zener and talks about 3400 on the anodes.

The issue about how much drive is another consideration, I have about 150 watts of drive available, but would prefer to run the exciter throttled back a bit.

Other thought is, upping the plate voltage, that can be done quite easily, I have a good few HV transformers around.

My main and only aim is to be able to run 1000 watts output in CW, so want to upp the efficiency a bit.


Tom Rauch:
First, amplifying CW requires a linear amplifier. It isn't as critical as SSB with strong multiple tones, but CW still is amplitude modulated.

If you chop the rising or falling edge by biasing into cutoff, your signal would have a very wide bandwidth at that moment of time. In other words, it would have very wide keyclicks. Since CW is amplitude modulation, you cannot have grossly non-linear transfer function at any part of the transmitted rise or fall without incurring bandwidth issues.

This means you can safely push the amplifier near cutoff, so long as it does not go into cutoff, between dots and dashes. If you set bias at just a few mA it would be safe. The result of this is the "between key" dissipation would be less, reducing heat. Peak efficiency would not change much, which means key-down efficiency would not change much, but anode heat between dot and dashes would be nearly eliminated. Since normal SSB dissipation is often set at ~50% of rated dissipation in AB2 service,  you could shed about 400 watts of unnecessary heat between dot and dash periods.

Whatever you do, don't make the mistake of thinking an external CW amplifier can be class C.

73 Tom

Phil Chambley, Sr.:
I think you're trying to go the wrong way with the bias.

Make sure you have a good tube and your input circuit is correctly matching the exciter to the cathode load.

Then, drop the bias zener voltage, or eliminate it altogether and try driving the amp with a hundred watts or so.

Phil C. Sr.


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