On Mon, 01 Sep 1997 12:18:30 Pete Smith <n4zr@contesting.com> writes:
>This morning, while tuning up my SB-220 on 15 meters, I did the
>flash-bang-bad smell number, followed by a sudden flare of white light
>from
>one of the 3-500Zs in my SB-220. As long-time readers may recall,
>this amp
>has the AG6K parasitic suppressors.
Mistake # 1 Pete! GRIN...
>After the event, the amp showed about 1/2 normal resting plate current
>and,
>when driven, approximately half normal output.
>
>Opening the case, I found that the low-value resistor in series with
>the
>anode of the other tube (the one that did not "light up") is
>shattered.
What is the condition of components at the grid and filament pins? Did u
install Rich's stuff there also? Check for open resistors or shorted
caps.
If it is original, check the RFC's for opens, they should be about 25
Ohms. Look very carefully at the grid caps at all 6 pins. The older style
dark red rectangular ones have a history of cracking open, etc.
Did you remove the VHF choke that goes from the ceramic HV feedthru to
the main plate choke? It should be replaced by a wirewound resistor of
roughly 10-20 Ohms 20W.
I have also developed a much improved parasitic suppressor for that amp,
it uses a pair of 5W non-inductive resistors in each leg.
The combination of the correct components to suppress parasitics and
absorb surges is important.
While you are at it, check the condition of the bandswitch, I wouldnt be
at all suprised to hear it arced. Also check the antenna safety choke for
an open; caused by a defective plate blocking cap.
It certainly sounds like one tube blew, try Ohming it out for starters. I
hope you didnt blow the Zener or the multimeter also. The meter should
have back to back diodes across it, 1N400x series are fine. They will
short during a glitch and save the meter.
I have lots of parts for that amp.
GL and 73 Pete....Carl KM1H
>
>As I understand it, the resistors are there as a fuse against plate
>glitches. My post-glitch experience suggests that the other tube is
>OK -
>normal resting current and output. Should I just replace the series
>resistor and try the amp again, or should I assume that the 3-500 on
>the
>damaged side is kaput? I ask because I'd rather not kill the
>remaining
>tube, if there's any risk of doing so.
>
>Also, any idea what caused the episode? The SWR seen by the amp was
>quite
>low (about 1.3:1). Loading was (I think) set just on the high side of
>maximum output, and tuning was at resonance. I was in CW position,
>driving
>the amplifier with full output from my transceiver (about 110 w.).
>Output
>power seemed abnormally low, which is why I was tweaking it when it
>blew.
>
>Any advice can only help this amplifier neophyte.
>
>73, Pete Smith N4ZR
>n4zr@contesting.com
>
>
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