[Amps] sb-220 tuning question - closing in

[Gary Schafer]

The plate tune capacitor should be the LAST thing that you tune.
It should also be the first thing that you tune.

When you start out apply a small amount of drive and dip the plate with 
the plate tune cap. Or peak it for maximum output of the amp. Then 
increase the drive slightly and increase the loading for maximum output. 
Retune the plate tune again. You can do this in 3 stages as you mention, 
each time doing it with more drive until you get to full drive.

  You want to end up with full drive applied, tune plate tune for 
maximum output, tune load control for maximum output and then tune plate 
tune for maximum output again. If you get much of a change on the last 
retune of the plate tune then readjust the load control again and the 
plate tune again.

By the way, keep one hand on the carrier level on the exciter as you are 
tuning up. Only insert carrier for the short time that you are actually 
turning the knobs. Then reduce the carrier until you get your other hand 
on the next knob to adjust. This way you will not have excess current in 
the amp for very long while it is not in tune.


It sounds like you are not loading the amp heavy enough and the plate 
circuit is arcing when you apply drive again.

Are you turning the exciter on the second time with full carrier applied 
at turn on? If so the relays on the amp may not have enough time to 
connect the antenna before power is being generated causing arcing.

73
Gary  K4FMX



Dan Sawyer wrote:
> All,
> 
> I am closing in on 'tuning' an sb-220. To make a long story short I was 
> tuning at low power and running it at a higher power. Now I am tuning up 
> incrementally in 3 steps from 10 watts to 100 watts CW. This works 
> smoothly on both 80 and 40.
> 
> However on 20 when I finished the a 3rd tuning cycle of optimizing 
> 'tune' and then 'load' I turned off the exciter drive. Upon re-engaging 
> the exciter the amp went ballistic; the sound was a loud continuous 
> spitting. It was continuous, not a single snap or pop. I backed the 
> power off and re-tuned following the same steps and after turning the 
> exciter off and then on it did the same thing. I waited about a second 
> looked around the side of the amp. The tube plates were glowing dull 
> red. I was able to prevent this by carefully de-tuning. However this was 
> more by feel then by a plan.
> 
> What is most likely happening??
> 
> How can it be prevented??
> 
> Thanks,
> Dan
> 
> 
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