magnus454 - 19-8-2011 at 22:24
I ran across an article on improving my EICO 666 trans conductance vacuum tube tester. In the article the author suggested installing a pair of 1n4148
diodes back to back across the analog meter to prevent damage from incorrect setting when testing tubes. I have a question, what exactly does he mean
by back to back? Does he mean anode to anode, cathode to cathode, or anode to cathode? Does anyone have any ideas?
bobm4360 - 19-8-2011 at 22:32
In parallel, anode to cathode across the terminals. Place 2 diodes side-by-side, with the large stripes on opposite ends, attach each pair of leads
to a meter terminal.
Regards,
Bob
[Edited on 8/20/2011 by bobm4360]
ok
magnus454 - 19-8-2011 at 22:36
Ohh, I see now. I didn't get it from the article, thanks, so wired in parallel in opposite direction across the meter terminals. I can cook up a
wiring harness for that, and bolt it .
IrC - 19-8-2011 at 23:54
Good idea as the meter coil is protected from any voltage in either direction higher than around 0.6 volt. However depending upon what may actually
occur 1N4001's would be a better choice. 1N914 or 4148 is 0.05 ampere max whereas 1N4001 is rated at 1 ampere. Much harder to blow open which would if
it occurred burn the coil out. Might as well go for maximum protection.
magnus454 - 20-8-2011 at 03:22
Ok, thanks. Apparently 1n4148 is plenty. I also have to add a simple pilot lamp in series with the filament power control pot to prevent damage to it
from excessive current. This is a trick used by the HICKOK line of vacuum tube testers for that reason, that this guy has imported into the EICO 666.
Vogelzang - 20-8-2011 at 05:20
Some links say the 1N4148 is a fast switching silicon diode, others say its a zener diode. If its a zener diode it would make sense. When forward
biased, it can act like a regular diode and when reversed biased its acts like a zener diode. He's an example with zener diodes placed across a
meter.
http://www.arkansas-aresraces.org/Antennas/HFAntennas/ZETAGI...
According to this, its a zener diode.
http://www.datasheetarchive.com/zener%20diode%201n4148-datas...
Eico 666 with diodes across meter.
http://www.fourwater.com/files/restr666.txt
Lab 8:� Diodes:� Clippers, Clamps, Shifters, and Rectifiers
http://www.nhn.ou.edu/~bumm/ELAB/Labs/lab08_Diode_Circuits_L...
[Edited on 20-8-2011 by Vogelzang]
not_important - 20-8-2011 at 06:55
As bobm4360 said - an old trick. The diodes must be low leakage below their conductance threshold least they 'steal' current from the meter. For
really good matching the meter's coil resistance can be used to calculate the voltage across the coil at full scale and diodes selected to match
that.
magnus454 - 20-8-2011 at 14:26
The link, http://www.fourwater.com/files/restr666.txt was the one I found about restoration and modifications. I was thinking serially not parrallel, that's
what was throwing me on the setup for it. thanks everyone.