> 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.
I don't care what kind of transition Joe has programmed into the
DSP. The rise time/wave shaping makes little difference with a
class C amplifier because the sharp turn on/turn off characteristic
and saturated operation will always square up the pulses.
A Class C amplifier requires a certain level of drive to "turn on"
and operates in saturation ... the turn on threshold will remove
much of the low level ramp up and the saturation will create sharp
"corners" at the top of each pulse. Even the most basic measurement
with a dual trace oscilloscope will show the difference between a
linear amplifier and one run in class C. Run a string of dits
through a class C amplifier and compare the difference between
the drive signal and the output.
> 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.
The clicks are not receiver generated ... they are generated in
the transmitter/amplifier by pulse squaring and eliminating the
wave shaping created in the exciter. W7AY has done extensive
work with wave shaping and can show the "damage" caused by non-
linear amplifiers in FSK and AFSK systems.
> Last I looked Duty Cycle was defined by the ratio between ON time and
> OFF time. No time limit specified.
Any calculation of "duty cycle" must include a time limit ...
otherwise even full carrier AM can have an extremely small
duty cycle if the transmission period is short compared to
the measurement period.
Simply put, when one limits the calculation to transmission
periods, AM, FSK/AFSK, WSJT, etc. are 100% duty cycle modes.
CW is a 50% - 60% duty cycle mode (depending on message
content) and SSB is up to 60% (depending on compression or
clipping levels). If you start adding "receive time" all
of those numbers will decrease but it still does not eliminate
the fact that AM/RTTY/WSJT/etc. are as much as four times as
taxing on an amplifier than CW at a similar peak power level.
> I won't run mine over 350, just like the "recommended" operation
> in the tube specs.
Good for you ... however, the Eimac data sheets clearly show 400 mA
single tone (CW) plate current in AB2 service at all plate voltages
from 1500 - 3500 (http://www.g8wrb.org/data/Eimac/3-500Z.pdf).
On the other hand if you are running high duty cycle modes (e.g.
FM or plate modulated AM) the recommendations are lower - as low
as 275 mA for plate modulated AM.
73,
... Joe, W4TV
On 5/23/2010 4:49 PM, Larry wrote:
>
> 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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