[AMPS] SB1000 parasite question for Tom and Rich

Tom Rauch W8JI@contesting.com
Thu, 3 Aug 2000 18:09:27 -0400


> I just got a mail from a SB1000 owner who gets the normal 750 -900 watts
> out with his old used tube. But wth two different new EIMAC tubes he only
> gets around 200 -250 watts out and both tubes tubes get red. 

Sounds like bad tubes.

> parasite suppressor that changed from original value and now with new
> tubes with higher gain at VHF the linear is parasiting. Right?

Very very doubtful. The way you can find a parasitic is to key the 
PA without drive so it has idling current, and rotate the tuning 
control through it's range while looking for a sudden appearance of 
grid current or an increase in anode current.

You could look on a spectrum analyzer also, but the easiest test is 
looking for changing current with no excitation when the amp is 
placed in conduction and the tuning control is rotated. 

> sure,  but I think he said the ceramic switch needs replacing and he tried
> to get a replacement from Tom? A burned switch is "normal" in a SB1000
> that has parasites if I remember right in earlier SB 1000 discussions on
> the net.. I know Rich knows a lot of this fenomen, dont you?

No, the burned switch comes from the fact Heath did not include 
an anti-corona washer on the high voltage lug, and the relay 
sequencing may be wrong. They also used a transformer with a bit 
poor voltage regulation, which doesn't help.

A parasitic will virtually never damage a bandswitch. Underloading 
the amplifier will, as will driving it with an exciter with spikes when 
the loading control on the PA is too far closed.

Change the switch, adding the anti-corona washer, and check the 
relay sequencing on the TR relay. Make sure the PA is loaded 
heavy enough, especially if the exciter has transients.




73, Tom W8JI
w8ji@contesting.com

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