>Forwarding this on to the list. Maybe some of you experts can help this guy.
>
>
>
>On Fri, 8 May 1998 12:02:57 +1200, in rec.radio.amateur.boatanchors you
>wrote:
>
>>Need Help on curing problem with SB-1000. Output stage is arcing badly, even
>>on very low Ip [around 100mA]. Have tried replacing decoupling caps on
>>either side of series choke but to no avail. Tube appears OK. Only other
>>clue appears to be that low drive current is twice what it should be. Have
>>heard that it may be parasitics and this can be cured by rewinding portion
>>of series choke in opposite direction.
I doubt it. A better suppressor, yes.
>>This sounds a bit serious
Indeed.
- The SB-1000 is virtually identical to an Ameritron/MFJ AL-80B. You
can see some vhf-parasite arced bandswitches from them at:
www.vcnet.com/measures
The good news is that the incidence of vhf parasitics in either version
appears to be below average. According to the designer, Mr. Tom Rauch,
W8JI, the amplifier's Tune-C has a resonance just above the amplifier's
anode resonance. It is my opinion that this makes designing a vhf
suppressor a bit of a sticky wicket since the suppressor is not looking
at a low Z path to ground where it needs to . . If you couple a
dipmeter to the dc blocking capacitor, you should be able to observe the
anode-circuit resonance of approx. 155mHz. . The stock vhf suppressor
has a measured Q of 1.3 at 150mHz. It is fairly simple to build a vhf
suppressor with a Q of 0.90 by using Ni-Cr-Fe wire instead of Cu wire.
Doing so should decrease the vhf gain of the stage by c. 40%. .
However, the problem of a C-Tune resonance near the anode-resonance still
needs to be addressed. This subject was recently discussed here on
[AMPS]. I would like to explore a means of addressing the Tune-C
resoance problem in the SB-1000/AL-80 by connecting a 10pF fixed,
RF-rated , "dooknob" cap. in parallel with C-Tune. Since such a 10pF
cap. is self-resonant above 400MHz, it should be useful for addressing
the resonance problem in C-Tune. Unfortunately, I do not own a SB-1000
or AL80. . . A Proposal: If you have an inquiring mind, and you can
solder, I would like to send you everything you need for making the two
aforementioned modifications. The price, including airmail, is reporting
your findings here.
- thanks
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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