SV: [AMPS] SB1000 parasite question for Tom and Rich+

[Ian Roberts]

Gentleman, could Han's problem not be a huge SWR on the output of the
tube, unrelated to anything more exotic?

Ian ZS6BTE

sm5ki wrote:
> 
> How can a new 3-500Z tube, pushed to around 250 watts output, power glow?
> 
> How can two different new tubes behave in exactly the same way?
> 
> And if the tubes are weak as the owner says, they should draw low current
> and not glowing at all?
> 
> Is this not an indication the problem is in the amp not the tubes? My
> swedish made copy of the COLLINS 30L-1 had glowing 811s  on 80 when the
> tubes were at the same time selfoscillating on around 50 MHz. Could this SB
> 1000, if it has  parasitics,  behave in the same way?
> 
> The reason could be the parasitic choke changed value due the temperature
> effects or capacitive coupling beetween the filament bifilar choke and anode
> on the higher bands?
> 
> If the zener is shorted, the tube draws more zero signal current and has
> higher small signal gain. The chances for a parasitics to start is thus
> higher especially when the new tubes have higher gain at the parasitic
> frequency.
> 
> That his SB 1000 has a broken ceramic switch may, of course, be an
> indication that something is wrong causing the problem?
> 
> Maybe he did not burn-in the tubes long enough?
> 
> How are the chances the parasitic has now ruined the new tubes cathodes?
> 
> This amp is in another city some 500 kilometers from the home QTH and I had
> no chance to see the amp and help my friend with the problem.
> 
> These two EIMAC tubes were taken out of sealed cartoons and they were thus
> new and looked very much unused.
> 
> Tnx in advance fellows and 73s de Hans SM5KI
>

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