Date: Sun, 17 May 2020 05:15:11 +1000
From: Adrian <vk4tux@gmail.com>
To: Gary Schafer <garyschafer@largeriver.net>, 'Jim'
<jimw7ry@gmail.com>, amps@contesting.com
Subject: Re: [Amps] SB220 Meter blown
## IF say the anode arcs to the grounded grid in a GG triode amp,
thats called a ..HARD fault. Even with 2 x diodes in series..so meter
accuracy is not affected in normal operation, the diodes will be into
conduction so fast, the meter wont even begin to deflect. Been
there, done that.
## IF say you apply way too much drive..and bury the plate
and grid meter past the end, thats called a ...SOFT fault. In the
case of the ..soft fault, the diodes, even 1 or... 2 diodes in series
will not protect the meter. Correct protection for..soft faults
would consist of a high speed protection circuit, aka .. plate
overcurrent,
grid overcurrent. Electronic Grid overcurrent is common in GG triodes
with delicate grids.
## Correct place to wire the diodes is between chassis..and B-.
IE: B- right at the cold end of the string of HV lytics. Done that
way, BOTH plate and grid meters are protected against hard faults.
## Triode Tubes like a 3-500Z, 3CX-3000A7, or any other GG triode
with a HD grid structure, only require protection for HARD faults. Triode
tubes like a 8877, 3CX-800A7, etc, require protection for SOFT
faults...fast electronic protection... AND also HARD faults.
## I usually parallel 3-4 6A10s so end up with a 1600A surge rated assy.
Then a FAST HV fuse in series with the B+...followed by a 50 ohm glitch
resistor.
Also a 2nd HV fuse..in series with just one leg of the secondary of the
plate xfmr.
## anode to grid arc, glitch R LIMITS the fault current to a safer
value.<1msec
later, HV fuse opens up.... event over. No meters damaged, tube not
damaged.
No diodes in HV supply damaged. Cro-bar anything you want..and nothing
bad
happens..except a blown HV fuse. Been there, done that. The concept
works good
and I have tested it many times. Paralleled 6A10 protection diodes
always stay intact.
## 50 ohm glitch resistor has to have its wattage rating sized
correctly..and ditto
with any HV fuse. Don’t use a 3A fuse on a 3-500Z amp. Use a 1A rated
HV fuse.
3000 V / 50 ohms = 60A fault current. 60A fault current will open
off a 1A rated
HV fuse in < 1msec.
## Ebay is a cheap source for 5000 V rated fuses, and fuse container
assys..used
in microwave ovens. Typ available in 750/800/900/1000/100 ma ratings.
2 x HV fuses, wired as I described above..+ a 50 ohm glitch resistor, will
ensure your HV supply is fully protected. In my case, only diodes
are used
for hard faults..to protect the grid + plate meter.
Jim VE7RF
The diodes(s) should be direct across the meter, and enough in series as
needed to excedd the full scale deflection
voltage required before forward bias is achieved in the diodes. It's all
very simple, as stated here a few times now. re ;
I would rather follow Rich's advice on the subject as per my previous
link. contained withjin ;
"It may take more than one diode to protect a meter shunt resistor. A
silicon diode begins to conduct at a forward voltage of about 0.5V. To
avoid affecting meter accuracy, the operating voltage per glitch
protection diode should not exceed 0.5V. For example, a 1 ohm shunt, at
a reading of 1A full-scale, has 1V across it. Thus, two protection
diodes in series would be needed to preserve meter accuracy. Similarly,
if the shunt resistor for a 1A full-scale meter is 1.5 ohm, the maximum
shunt voltage is 1.5V--so three diodes are needed.
Glitch protection diodes should not be petite. Big, ugly diodes with a
peak current rating of 200a or more are best. Smaller diodes--and the
meter they were supposed to be protecting--can be destroyed during a
glitch. Suitable glitch protection diodes are 1N5400 (50PIV) to 1N5408
(1000PIV). In this application, PIV is not important. The 1N5400 family
of diodes is rated at 200a for 8.3mS.
During an extremely high current surge, a glitch protection diode may
short out--and by so doing protect the precious parts. Replacing a
shorted protection diode instead of a kaput meter is almost fun."
ref; http://www.somis.org/D-amplifiers2.html
Once a petite signal diode blows apart it is no longer protecting the meter.
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