>
>> I just got a mail from a SB1000 owner who gets the normal 750 -900 watts
>> out with his old used tube. But wÌth 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.
>
Yet another old wives' tale. This is how you test for instability at the
operating freq.
>.....
To generate the requisite damped wave anode circuit ringing, it takes
transient anode currents.
>> 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.
>
Owners who have replaced burned bandswitches and added the anti corona
hardware have reportedly continued to experience intermittent open
contact bandswitch arcing. The fix that seems to work is: reduce VHF
amplification.
>A parasitic will virtually never damage a bandswitch.
Please see the collection of toasted bandswitch photographs on my
Website. Is the Internet the best thing since sliced bread, or what?
>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.
>
? Tom recommends over 500mA of anode current -- no fooling. When is Tom
going to show us the Eimac letter which supposedly gives the ok to 500mA?
>
later, Tom
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|