On Sep 29, 2006, at 4:56 PM, Tony King - W4ZT wrote:
pentalab wrote:
Fellows
I have several fuses installed in most of these linear's.
Like a fast 750 ma 3agc style grid fuse... located between
chassis.. and negative terminal of 0-1 Amp grid meter.
I'd protect this with diodes and leave that fuse out. It wouldn't be a
good idea for the cathode to ground circuit to open and essentially
float the grid.
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.
- note - In a stock SB-220 or 922, a shorted tube can destroy the filament transformer in short order.
Also have a 3 A cathode fuse [also a 3agc] in the center tap of
the fil xfmr circuit [ along with the 60 x 6A10 6-A , rotary
switched bias diodes. ] This 3 A cathode fuse has a 1k-25
watt ressitor across it.
1k is a little low, use something like 15K to 20K across the fuse. When
the fuse blows you will have an increase in bias that will cut the tube
off and will develop a voltage proportional to that needed to cut it off.
With 3-500Zs, 1k-ohm will limit anode current to about 25mA when the fuse opens.
- editorial - A 250v-rated fuse in a circuit operated by 3000v is not good engineering practice.
Question is... what's an easy way to obtain a visual
indication /led/neon/etc.... to let us know said fuses are
blown ???
If the cutoff bias is high enough, a neon bulb with a series resistor
could be used as an indicator.
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. Also, when the grid-I meter deflects backward, a tube has a filament that is shorting to the grounded-grid.
... ... ...
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 ?
Correct
I don't want the expensive ceramic vac load cap to ever
internally arc.
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.
Sri for the drivel....... does anybody actually read any of my
posts....... or is this all old news ???
Later... Jim VE7RF
I enjoy reading your posts Jim!
Best thing I have seen is to use one of the triode control boards and
use the protection circuitry they provide. You will find there's hardly
any failure that will can not be detected and reacted to almost
instantly. You might have to make a few adaptations to meet the need for
QRO++ but the principle is the same. This is especially good because it
can trip the amp off line so you don't have high drive going into a
malfunctioning amp.
73, Tony W4ZT
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