--- In ham_amplifiers@..., R L Measures <r@...> wrote:
RICH SEZ...When a high-R exists in the cathode circuit, as soon as
a little current begins to flow, the tube cuts off. My modified
SB-220 and TL-922 do this, and if a tube ever happens to short,
nothing is damaged.
#### Partially agreed. When a grid fuse [inserted between
grid meter and chassis... (or between grid shunt and chassis
on a multimeter)... AND the grid fuse is shunted with a 100 K
resistor..... AND the grid fuse opens, one would think the grid
current [600-900 ma on a 6000A7] flowing through the 100 K
resistor would create enough bias to cut off the tube ? It
sorta does..... fact is.... with 800-1000 w of drive..... you
still get 1/4 pwr output . We simply REMOVED the 100 k
resistor across the grid fuse.... and now all is well. Now
when grid fuse blows open...... zero DC grid current.. zero
watts outa the amp. [where the 800+ watts of drive goes is
beyond me. It either stays in the cathode.. or cooks the
grid.... or both ??? ]
### On one of my L4B's a few yrs back [this one was unmodified], It
was still using the +130 V scheme for cut off bias on RX. The
contacts on the 3PDT t/r relay were bad... and the +130 v was
still being applied on TX !! You could STILL get power
output... [now in class C] ! Fix was to replace relay.... then
rewire with a 100 K in the CT.... and relay contacts shunt the
100 K on TX.... end of problem. RL Drake's... and later
Heath's "brilliant idea" of using +130vdc for cutoff bias on
RX... was one of the stupidest moves ever...... 2nd only to
using 6 x 200 pf caps + 2 x rf chokes on the 6 x grid pins !!
### BTW, the 100 w + 1 kw contacts of the bad T/R relay were
perfectly intact. It's the center contact [with the +130 V}
that goes bad eventually. When this center contact is in mid
air... you have opened the cathode up for a split second.... and
the cathode will want to assume full plate V !
RICH SEZ... - note - In a stock SB-220 or 922, a shorted tube can
destroy the filament transformer in short order.
### say what ?? IF the fil xfmr draws too much current on
it's sec,...... the fuse/small breaker on the PRI of the fil
xfmr should blow ! Oh, I forgot.... in it's infinite wisdom...
neither the SB-220 or the Drake L4B even had fil xfmr pri
fuses........ they relied on the ..'big momma' 20 A slow blow ckt
breaker in the 240 V pri!!! Fix... install a fil xfmr primary
fuse.
## The L4B has an extra winding on the fil xfmr ... after being
run through the 1/2 wave rectifier [ one diode, one cap.... how
CHEAP could they have gotten ?] u get +27 vdc... to run the
T/R relay. Fix is.... either change to a FWB..[+27 vdc]... OR
change to a doubler. The doubler will put out +63 vdc.... and a
resistor is simply inserted in one leg of the stock T/R relay's
coil..... and presto.... you have a fast sped up mech stock
relay.... not qsk... but faster VOX. The + 63 vdc ocv can
[and is] be used also for a speed up circuit for a RJ1-A.
## Either way... this extra smaller sec... used for the T/R
relay... should be FUSED. Fix... install a 3agc fuse holder...
with the appropriate sized fuse. .... end of problems.... like as
in this small sec ever shorted out..... you don't melt the
fil xfmr [ "fused" with a 20 A breaker in the 240 V pri]
RICH SEZ... With 3-500Zs, 1k-ohm will limit anode current to about
25mA when the fuse opens.
### when WHAT fuse blows? A cathode fuse ? If so, I
agree.... and it should be increased to a 20-100 K unit..and
complete cut off will /shoiuld occur.
RICH SEZ - A 250v-rated fuse in a circuit operated by 3000v is not
good engineering practice.
### the 250 V rated cathode fuse is NEVER gonna see 3000 V
across it... and here's why. The worst case senario will be a
HV to chassis short.. or a anode to grid flashover, etc. With
reverse connected safety diodes [ I use a pair of paralled 6A
diodes pointed one way... and a 2nd PAIR, pointed the other
way] .. connected between CHASSIS and B-.... the fault current
simply flows from chassis through the diodes... and back to B-
... completing the loop... and presto... the sand filled fuse in
the B+ blows.. every time ! Also use reverse connected
6A diodes across BOTH the grid and plate current meter's. You
will NEVER arc across the 250 V rated fast blow 3agc CATHODE
fuse !!
##### Question is... what's an easy way to obtain a visual
indication /led/neon/etc.... to let us know said fuses are
blown ???
TONY SEZ... If the cutoff bias is high enough, a neon bulb with a
series resistor could be used as an indicator.
RICH SEZ. It takes c. 90v to light neon. A LED takes c. 1v. Also,
with 1k=ohm and 3-500s, 25mA means the fuse is open, so one does
not need an idiot light.
####### When the big amp is 30' away... in a garage or a
workshop, etc...."idiot lights" def are needed !
RICH SEZ...Also, when the grid-I meter deflects backward, a tube
has a filament that is shorting to the grounded-grid.
### agreed. And the fix is to install reverse connected diodes
across the grid meter/shunt.... then the diode will conduct....
instead of deflecting ur grid meter backwards.
#### For some added protection... we are thinking of an
adjustable spark gap between load cap ...and chassis..... set
to fire at a V higher than the normal low swr V.... and a point
well below the rating of the 5 kv vac load cap. I think
Rich may have done this ?
RICH SEZ ..Correct
### I don't want the expensive ceramic vac load cap to ever
internally arc.
RICH SEZ.... Spark gaps are good! However, adding a rugged low-ohm
low-L resistor in series with the spark gap helps to limit peak-I
during a glitch.
### Huh ? This is HIGH RF voltage across the spark gap [across
the vac load cap]. What do u suggest.... a 50-500 ohm 90/150
watt globar ??? There's already a heavy duty glitch R in the
B+ of the HV supply. Adding R in series to any spark gap
may well slow down how fast the grid/cathode/ sandfilled HV
fuses blow ???
Later......Jim VERF
