On Mon, 20 Apr 98 23:30:51 -0800 Rich Measures <measures@vc.net> writes:
>>>The way I was taught is that you key the amp with no drive and vary
>the
>>>caps all around and look
>>>for a kick in the plate or grid current.
>>
>>Also decrease the bias so the tube is running a fair bit of plate
>current -
>>say 1/3 to 1/2 rated dissipation so the gm is high.
>>
>However, unless the anode current is being pulsed, the VHF-resonance
>in
>the anode circuit will not be generating damped-wave VHF signals that
>could possibly lead to an oscillation.
I forgot to include that step in my prior post. Keying the amp on and off
with no RF is an important part of the debug process but I do find that
more critical in a grid driven high gain tetrode.
>
>>Then if it's stable unloaded, as Carl says, it should be OK.
>
>At VHF, with a conventional Pi-output, there is No VHF load on the
>amplifier, even if a 50 ohm VHF-rated termination is connected to the
>output.
In theory but not in practice. There are numerous paths thru the tank
circuit to the load that will pass VHF/UHF energy. This is not too common
with the 572B but as the tube gain and its frequency capability increase
the paths will become more evident.
One way to test is with a high output tube GDO such as the Measurements
59 and a HP or similar microwattmeter on the output. Couple the GDO at
both the plate strap and also the tank components and watch the meter
kicks.
When using a high gain tube such as the 8877, 3CX800, etc it is often
more cost effective for the home brewer to use the full grid grounding
ring instead of depending on just the socket pins.
>
>In a grounded-grid amplifier, how could an anode arc to the
>grounded-grid
>damage the filament?.
Why are you so positive it is strictly an anode path arc?
If the anode effectively goes to the B- rail in that first instant but
the gas arc plasma is still there then would not the path go filament to
grid or vice versa depending upon your view of current flow. This would
then tend to explain the damage to zener diodes, grid chokes, tubes, and
suppressors.
IMO the only thing that could bend a filament is the almost instantaneous
discharge of the unprotected PS. There is absolutely no way ( at least
for today) that I will believe that a parasitic can generate that much
energy.
Even an out of control parasitic will not cause a bang initially. The
tube will run away, the plate current will soar and the tube may fail but
this takes several seconds from start to finish and is usually a silent
process in the vacuum of the tube. The bang in this case is usually a PS
path failure.
73 Carl KM1H
I have never autopsied a tube that had a bent
>grid. Bent filament, yes. Gold-sputtered grid, yes.
>- later, Peter
>
>cheers
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K
>
>
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