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Re: [Amps] How to read the 3-500Z spec sheet?

To: "Joe Subich, W4TV" <>
Subject: Re: [Amps] How to read the 3-500Z spec sheet?
From: Larry <>
Date: Sun, 23 May 2010 13:49:50 -0700
List-post: <">>
On 23-May-10 12:42 PM, Joe Subich, W4TV wrote:
> WSJT may be one tone at a time but that still does not eliminate the
> need for reasonable linearity unless one wants to generate "clicks"
> every time the tone shifts. With class C any attack/decay is made
> much more sharp ... you can even hear it on CW with a class C amplifier.

Wrong again Joe. Joe, K1JT, has real insight into this area. He has 
programmed the DSP in the soundcard tone generator to produce a smooth 
phase transistion between one tone and the next. The result is complete 
freedom from the type of clicks you hear when your receiver noise 
blanker is driven nuts by the strong RTTY signal 200 Hz off your 
frequency. If you don't believe me, go ask Joe. He will most likely be 
happy to point you to the documentation.

>  > I have run a SB-220 converted for 6 on EME at 800 watts output (CW
>  > position). It worked OK as long as I operated it with the outside
>  > cabinet removed.
> Since that was two tubes, I suspect (hope) you were staying under
> 800 mA (400 mA per tube) - you should have been able to reach 800W
> out with two tubes although running on the high tap would have
> been easier.

2x3-500Z will run 1500 watts output on HF (and probably 6 mtrs) carrier 
100% duty cycle for a very very long time (years) IF you run the HV high 
enough to keep the anode current below 350 ma. per tube. And run lots of 
air past the seals in the manufacturer approved manner (no like SB-220) 
At least you can with Eimac and Amperex, no telling what you might get 
from the chinese tubes.
>  > BTW, WSJT JT65 modes as used for EME are true 40% duty cycle, IE 48
>  > second out of every 2 minutes full carrier power followed by 72
>  > seconds of standby and/or RX.
> They may be 48% duty cycle on a long term average but they're
> 100% duty cycle while transmitting ... and as bad/hard on the
> amplifier as RTTY contesting. They're not 50% duty cycle while
> transmitting (like CW) or even less like moderately clipped SSB.

Last I looked Duty Cycle was defined by the ratio between ON time and 
OFF time. No time limit specified. CW is not "real" 50% duty cycle 
unless you are broadcasting 24/7. One minute of TX at 50% and one minute 
of RX is more like 25% duty cycle, probably less. When I design my amps, 
they are for real 100% duty cycle. I can take 15 or 20 minutes to tune 
them up without resorting to any stupid pulser crap or test dummy loads 
if I want, they don't care. If they break, it's because I screwed up 
someplace and that only means I need to fix the weak areas.

> I still would not push a single 3-500Z past 400 mA "DC" plate
> current and would replace the SB-200 transformer with an AL-80
> transformer (with additional filter caps) if it were my amp.

I won't run mine over 350, just like the "recommended" operation in the 
tube specs. I guess some folks just like to buy tubes.....

Antique tubes like 4-400A, 813 and 4CX250 can be pushed way above normal 
ratings if you know what you're doing. Modern junk like 3-500Z and 8877, 
well just look at how many people on this reflector are always looking 
for new tubes to replace the ones that mysteriously went bad.

73, Larry
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