> 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?
Depends on the input power.
If the DC input power is only 300 watts with 250 watts out, the tube
will not glow red.
If the dc input power is 500 watts with 250 watts out, the tube WILL
glow red.
Glowing red is not a sign of a defect in the tube. The only problem
you seem to have is low RF output.
> How can two different new tubes behave in exactly the same way?
Because the tubes may not be the problem. It might be the tank is
on the edge and not tuning, it might be something else. It might be
the tubes are bad, having no peak emission.
> And if the tubes are weak as the owner says, they should draw low current
> and not glowing at all?
As little as 200 watts dissipation will make the anodes glow a dull
reddish color. It is the low output that is the issue, the anode color
is normal and should show even when the tubes are putting out
ZERO watts as long as they have idling current.
> 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?
811's are not designed to "glow red" on the anode. If they are
glowing red, you are hurting the tubes.
Non-graphite 3-500Z's MUST glow red or the tubes will fail from
outgassing and gas ingress.
> 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?
The parasitic system DOES NOT include the filament choke. The
problem is the control grid is not grounded well at some frequency
up above 150 MHz. At some frequency up there, the 3-500 PA can
behave like a TPTG oscillator if you have no suppression at all in
the anode.
It's actually kind of difficult to make the 3-500Z in that amplifier
oscillate, unless you start disconnecting grid leads.
To check for a parasitic, look for grid current when the amplifier is
keyed and no drive is applied. Rotate the tuning capacitor through
its range. If you do not see a change in grid or anode current, the
amplifier is almost certainly stable.
> 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.
All this stuff about tube gain is silly. The gain in a parasitic mode
hardly changes as long as the tube is biased into conduction. The
gain does change slightly in a GG mode because the driving
impedance of the tube is reduced with less cathode bias applied.
There is very little likelihood that changing from an old working tube
to a new working tube will cause a VHF stability problem, or that a
bias change would do the same unless you are moving the tube
from out of cutoff into quiescent current.
I'd look other places first, after doing a one minute stability check
by keying the amplifier with no RF applied and rotating the controls.
73, Tom W8JI
w8ji@contesting.com
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