--- In ham_amplifiers@..., "pentalab" <jim.thomson@...>
wrote:
### Here's the deal..Now this IS important. . A buddy of mine
brought over a QST article a few yrs back... in the stray's
dept... it depicted ur typ "electronic grid overcurrent"
protection scheme.... seen in many handbook projects... and also
used in many commercially produced linears. In many handbook amp
designs.... you will see a pair of diodes REVERSE connected...
between the chassis and B- Only one diode is really needed...
the worse thing that can happen usually.. is a B+ to chassis
short.
### The tech note went on to show what really happens... IF
reverse connected diodes are installed from chassis to B- AND
electronic grid protection is used. They re-drew the entire
schematic.... and what is now depicted is the 2nd diode [not
needed].... ends up DIRECTLY in parallel... with.. u guessed
it....
the small value resistor installed between grid shunt and
chassis. This small value resistor, [typ 5-15 ohms] has DC grid
current flowing through it all the time. It also has a V drop
across it.. typ 1-3 volts... which is fed to the wiper of a
pot... used as a V divider... then applied to a transistor...
activates a relay.. etc.. kicks amp offline, lights LED...
latches... etc. Trbl is... If the un needed 2nd safety
diode
is used.... the V drop across the resistor is directly in parallel
with it... and turns on the diode !!!! DC Grid current is then
partially or fully diverted through the un-needed safety
diode....
and u guessed it... the dc grid meter is now reading too low !
### The fix is... to remove the un-needed safety diode.
########### Here's the MISSING note. Even IF the un -needed diode
is removed on an electronic grid over current scheme.... and the
remaining WANTED safety diode [between chassis and B-] ever
shorts out [typ on ALL Ameritron amps..cuz they use TOO small a
surge rated diode], your fancy electronic grid overcurrent
protection scheme is NO longer gonna work until that safety diode
is replaced ! With the diode shorted, the small value drop
resistor that's part of the above mentioned grid overcurrent
scheme... is now directly in parallel with the shorted diode !!
ALL DC grid current will simply flow through the shorted
diode.. through the PLATE meter... back to cathode.
### Wait, it gets worse! With the safety diode shorted
out... the plate and grid meter are now directly in parallel with
each other. The grid meter however, will still have a 5-15 ohm
resistor in it's neg leg... the plate meter does NOT. The Pos
terminals of both meter's are tied together of course..... so plate
meter will now read the SUM of the plate current AND grid
current.
### Left in that above config... the amp will still work... except
the grid meter will not function... and the grid overcurrent will
not function. IF a HV fuse is used.... and say blows at the
same time the safety diode shorts... you are now in bigger trbl !
With drive applied, and NO plate V.... you will send the grid
current into orbit... and with NO functioning grid over current
protection.... u can kiss the tube goodbye.
### On my buddy's Ameritron amps..[he has since gotten rid of
em].... the manual tells you to .. "check for a shorted safety
diode"... "after an event" !! Now if the bozo who designed it had
used a bigger surge rated diode.. and a glitch R... this would never
happen.
### Beware... some pundits on the "other reflector" have
suggested using a standard pre-packaged bridge rectifier 4-25 A
50V diode assy... like the typ sealed type.. with 4 x wires
protruding out.... as a "safety diode". Now this will work....
PROVIDED u strap the TWO AC input's together.... THEN grnd em to
chassis.... and then wire the + output of the rectifier assy...
to the B- of the HV supply. The neg output of the bridge
rectifier assy is NOT used.
#### On the other reflector.. it was stated to simply wire the
AC input's between chassis and B- !! Draw that config out on
paper.. and u will see that u end up with opposing pairs of
diodes.. and the PIV of 2 of the diodes will be exceeded... and
those two diodes will blow clean in 1/2..... leaving two
paralled open circuits... and now an open loop. Unless u got
more safety diodes installed across each meter.. AND 3-4
more...wired in series in the grid overcurrent circuit.... u will
smoke the meter's and the grid overcurrent assy.
### PIV rating for a safety diode is not an issue... it's
strictly the SURGE rating. 6A diodes are cheap... so are
paralled 1N5408's [3A] .
Later... Jim VE7RF
