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Re: [Amps] Why does power output increase?

To: Victor Rosenthal 4X6GP <>, <>
Subject: Re: [Amps] Why does power output increase?
From: gudguyham--- via Amps <>
Reply-to: "" <>
Date: Sun, 5 Jun 2022 11:05:27 +0000 (UTC)
List-post: <>
If you rx filament voltage is around 5.25 that’s ideal.

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On Sunday, June 5, 2022, 6:56 AM, Victor Rosenthal 4X6GP <> 

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 

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
> increase.
> ##  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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