Tnx everybody who contributed in this discussion. But I am still mixed up:
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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