>I question the resoning on a high value such as the 50ohm 50 Watt
>resistor in their amp. Seems a little off to me.
>
>Comments?
>
>Chris WA6SUY
Svetlana recommends 10 to 25 ohms. The HV glitch resistor must be
capable of discharging the HV filter capacitors without breaking down.
My guess is that limiting the peak fault current to roughly 200A is a
reasonable precaution. The average ESR in an anode supply is around 8
ohms, so that helps a bit.
>
>TUBE PROTECTION
> The EMTRON DX-2 has a very sophisticated tube
>protection circuitry, designed to protect against excessive plate
>current, screen grid current, screen grid voltage, bias voltage,
excessive screen and bias voltages? I do understand how such a condition
would be possible.
>overdrive, SWR, etc. The tube is also protected from excessive
>temperature by continuously monitoring the airflow temperature. In case
>of any fault, the amplifier is automatically cut off.
>
>A 50 ohm, 50 watt resistor in the B+ circuit further protects the tube
>and the power supply from a possible internal arc of the tube.
>
I do not believe that a healthy tube can have a temporary internal arc
(caused by temporary gas) which fatally damages the tube.
- The three most common types of tube damage I see are:
1.. a bent filament helix in a 3-500Z. . . . Consider that, according to
centrifuge tests, it takes ~10G for about a minute to produce enough
filament bending to cause a grid-filament short. I have disassembled a
number of such tubes. I have never seen an arc-mark on the anode in one
them.
2. a shattered filament in an 811A or 572B, which occurred during during
transmit, in tubes that were not subjected to mechanical shock.
3. gold-sputtering from the grid in 8874s, 8877s and the like. In
February, 1986, an Eimac tube specs. engineer told me that gold
sputtering was determined to be the result of an 'oscillation condition'.
IMO, damage allegedly caused by an 'internal arc' -- in tubes which
subsequently prove to have good vacuums -- is yet another 'old wives'
tale'.
Rich---
R. L. Measures, 805-386-3734, AG6K
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