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.
A surge of say 50A comes up through the grid meter shunt, and even if
the shunt resistor is only 0.5 ohms, a quick calculation of I^2*R shows
a very severe risk to the resistor and the meter, unless they are
protected by a diode.
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. If the diode fails, your last hope is that it fails
short-circuit, which the manufacturer doesn't exactly guarantee.
Considering the cost of components that the diode is protecting (meter,
zener, tube) it seems like good design practice to use a much bigger
diode. Packaged bridge rectifiers are cheap, and easy to find as
surplus.
>
>- 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!). Also, arcs can take place outside the
tube; 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.
>
>It would seemingly not be possible to damage the cathode bias zener -
>from an anode to ground arc - D.a.02.GIF if the builder had the forsight
>to put glitch diodes across the grid current meter shunt R. .
...and uses a surge limiting resistor in the B+ supply, and rates
everything to survive the fault condition with plenty in reserve.
>
>>[Note to the reflector: this is a good case for a small GIF file to make
>>the circuit more clear. If you want, and with the lsit-owner's
>>permission, I could post one.]
>>
>I have used illustrations/photographs before on [AMPS]. I post the JPG
>or GIF illustration on my Web site, provide the URL, and proceed with the
>discussion. [AMPS] archive readers see the URL as a Link on their Web
>browser. If they want to see the illustration, one click does it. For
>example, to see how glitch diodes are connected to meter shunts:
>http://www.vcnet/measures/D.a.02.GIF
>
Good point, thank you - I'll try that in future.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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