[AMPS] IC-706 output power spike: AMP KILLER???

Thu, 3 Jul 1997 07:50:29 +0000

> 	My delightful new '706MKII exhibits that problem: I don't have a 'scope,
> but there's no question a huge over-power spike appears on my PEP-reading
> Yaesu YS-60 power meter on initial key-down. Meter characteristics cause
> the power measurement to be elevated for about a second, after which the
> meter drops down to the real output. (When I switch to "average" power
> reading, of course, the spike disappears.) I have *NOT* noticed such a
> spike with my IC-735.

If you think it's bad on the slower response Yaesu meter, you should 
see the spikes on my sample and hold storage power meter that only 
requires a few RF cycles to read peak power!

My 706, with a 15 volt supply, spikes up past 150 watts. That's 
enough to trigger tank circuit arcs, and blow FET's in power 
amplifiers. The bad part is the spike remains at 150 watts or MORE 
even when the power is set at ten watts!   

> 	I'll accept that this brief excess of power could definitely damage
> delicate-grid ceramic tubes, but should I worry about driving (a) my
> solid-state IC-2KL? and (b) a 2x572B SB-200?? In the case of the '2KL,
> which only requires ~25 watts drive, I *could* put in a 3dB attenuator to
> buffer the spike; 

I'd put in a six dB pad. Then you'll only be smacking the transistors 
with a 150% overshoot instead of 300%!

>I do not have such drive overhead with the SB-200.

But the overshoot can trigger a tank arc, unless you load the PA for 
150 watts plus of drive. 

One way to minimize this spike is to feed the ICOM from a 12 volt 
supply, but then the IMD will get much worse. A better way is 
to applied fixed external ALC voltage and use that as a power 
control.

I like my 706, but I keep it miles away from FET and low 
drive tube type amplifiers. The only major problems are CW operation 
on the alleged "QSK" mode and that darned spike.
mode. 

73, Tom W8JI 

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