>Rich Measures wrote:
>
>>K5GW (reproduced here with permission).
>>>>
>>>>I have lost many cathode zener bias diodes in my 2m 8877 amp over the years
>>>>(none since installing the 50ohm plate resistor). I always assumed that
when
>>>>the plate arced to ground and by way of grounded grid, perhaps plate to
grid,
>>>>that the HV power supply b plus was almost instantly placed at ground
>>>>potential. If this is the case, the B minus lead must also nearly instantly
>>>>rise to a negative value close to the full HV dc voltage less whatever
amount
>>>>has been lost during the nearly instant period of time. This being the
case,
>>>>the cathode is now negative several thousand volts compared to the still
>>>>grounded grid. Looks to me like a large current would flow in this case,
>>>>possibly damaging either/both cathode and grid. I know enough current
flows to
>>>>blow the smithereens out of the grid meter!
>>
>>However, most builders protect the grid current meter shunt resistor with
>>=>200a glitch diodes -- which also protects the grid current meter. With
>>glitch diodes, the peak potential across the grid current shunt R during
>>an anode/grid arc would be roughly 1.5V -- which is not enough to blow
>>the zener bias diode.
>
>OK, we're nearly in agreement. However, if the grid current meter is
>damaged, it is clear that the shunt and/or the diode didn't do the job.
>
I have never seen 3A/200a-pk diodes fail to protect a grid current meter.
If the designer used flimsy diodes, which failed to protect the meter,
that is a separate issue.
... ... ...
>Agreed that 1N540x diodes will usually work, but their 200A surge rating
>is strictly time-limited... and we don't actually know how long the
>surge will last.
sure we do. T=R*C.
>>
>>- However, there seem to be some problems with the grid/anode arc
>>theorum:
>>1. I have never seen a kaput 8877 grid bearing an arc mark.
>>2. No amplifier builder has reported finding one.
>>3. The reported arcs/'big bangs' sound like an arc in the room. Arcs
>>in a near vacuum do not make a 'bang' sound. For instance, an arc in a
>>vacuum variable capacitor produces a faint 'tink' sound. Bangs need an
>>atmosphere.
>>4. Roughly 2/3 of the kaput 8877 grids I see have fairly uniform gold
>>blistering around the circumference of the grid.
>
>I agree about the 'tink' inside the tube, but the bang can come from the
>consequences in the circuit outside - like a meter being blown to
>smithereens (love that word!).
- The voltage across the tube is reportedly 50v. It seems a bit of
stretch to imagine that this could produce a secondary arc outside the
tube that would produce a loud bang.
- People who use glitch diodes report 'big bangs'. The meter is
undamaged. The diodes survive.
>Also, arcs can take place outside the
>tube;
Surely, which seems to be the case during some intermittent VHF/UHF
parasitic oscillations.
>they are very common in VHF/UHF amps, probably because the output
>circuit often has the full DC on it as well as RF voltages.
>
>This side-topic is not so much about the causes of near-crowbar shorts
>from anode to chassis, but about design techniques for preventing damage
>when they happen.
>
There seems to be scant evidence to support the theory of an anode/grid
arc causing damage to the grid. Too many cathode bias zeners are
shorting. Too many parasitic suppressor resistors are being damaged -
and not during 15m or 10m operation.
... ...
>...and uses a surge limiting resistor in the B+ supply, and rates
>everything to survive the fault condition with plenty in reserve.
>
- As I recall, there was an article in the January, 1994 issue of *QST*
that proposed such things. However, in the September, 1994 issue of
*QST* there were some "experts" who did not agree.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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