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Re: [Amps] Why does power output increase?

Subject: Re: [Amps] Why does power output increase?
From: Victor Rosenthal 4X6GP <>
Date: Sun, 5 Jun 2022 13:55:56 +0300
List-post: <>
This morning, for some reason, the phenomenon did not occur. I was getting close to 1400 watts the whole time. I always operate CW in the SSB position -- the warning in the manual against this is (I think) only relevant if the original slow t/r relay is in use, which hot-switches with semi-QSK. My amp has a reed/vacuum relay QSK circuit.

Because I do not use SSB, I have adjusted the idle current lower than normal. It is 140ma for two tubes. This requires a bit more drive but produces more output without generating clicks. It didn't change more than 5 ma (a slight drop) during my test. The Zener that is in there is a 50-watt one.

I agree that it is unlikely to have anything to do with the filament voltage.

I have noticed the phenomenon with two wattmeters, an Alpha 4510 and an Elecraft W2.

I've heard about using diodes instead of zeners, but the curve of current vs. forward drop of the diodes is sharper than that of zeners. The temperature characteristic also compares unfavorably. So I've stuck with zeners. I put in a 50 watt one because that is the one I had with the appropriate voltage.

I still wonder if the filament voltage should be measured while transmitting or on standby, given that even during a transmission the plate current is only drawn at a maximum of 50% of the time, and the amp is probably in standby with zero plate current most of the time (my biasing circuit cuts off the tubes in standby).

Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
On 04/06/2022 18:35, jim.thom wrote:
Date: Sat, 4 Jun 2022 13:26:32 +0000 (UTC)
From: "" <>
To: Shane Youhouse <>,  Victor Rosenthal 4X6GP
Cc: Amps reflector <>
Subject: Re: [Amps] Why does power output increase?

##  what is the idle current ???  Try it on SSB  (hi voltage)...and also on
CW  (low voltage).   IE: put xcvr on  SSB, mic gain on zero, key the PTT.

##  IF the resistor in the fil primary heated up, it's resistance would
INcrease, not decrease.   Tube emission would decrease.  PO would drop, not

##  Your bias vdc may be screwed up..and changing with a load applied.
Even this doesn't make sense.   Typ, when zeners fail, I believe they typ
fail shorted....which would result in sky high idle current. Which would
make it easier to drive the amp.

## But in this case, it's as though the zener V starts off normal, then
decreases, and I find that hard to believe.  Something else is amiss, and
it maybe one or both tubes has gone wonky.

##  do u have a dead on wattmeter to test with....or a scope ?

## The zener has to handle cathode current....which is the sum of the plate
and grid current.  I'd replace the zener with a string of  1N5408's.   I
have had no luck with zener's.

Jim   VE7RF
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