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.
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.
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.
continued below...
Most of the "visual indicating" type fuse holders I have seen... all require use of the line voltage being fed into em... ie: 120 vac, etc. I was thinking.. perhaps a 1-2 ohm resistor across each fuse holder... such that the v drop across the resistor [with fuse open] will trigger a led/etc. [this would amount to installing a typ grid/cathode electronic overcurrent device..... except it would only work IF the grid/cathode fuse opens]. Origionaly, we installed a 100 K- 2w-MF across the grid fuse.... believing if the grid fuse blew.... the resulting bias developed.. would cutoff the tube... and tube couldn't be driven. In actual practise... with the typ globs of drive... you can still drive the tube... albeit.. in class C ! So we subsequently removed the 100K resistor. Now,,, if the grid fuse opens... zero output... input swr rises from flat.. to 2:1. ..... No DC grid current. Here's the real concern. With the grid fuse blown open.... there is NO return for DC grid current... fine so far. But what happens to the drive RF [800 ++ watts worth] ?? Are we burning up the cathode.... and /or the grid ..or both ???? In all cases these RF decks/ power supplies are remote located 30-
50' away from the station. The station can however... read all 4 bird line sections... and also continuously monitor the PEP refelcted pwr [part of the high swr shut down circuit]. On a related note.... IF either HV fuse blow open..[1st one in one leg of sec of plate xfmr... the 2nd in the B+].... and with tube driven... ALL the electrons flow to the control grid... instead of the anode... and you guessed it... the grid current will skyrocket.. [read it will blow the grid fuse asap]
We have seen cases whereby the HV fuse + grid fuse will blow open... cuz of an antenna problem... and high swr [trips on high reflected pep power] circuit has NOT tripped the amp offline. We have also seen cases, where the high reflected power circuit trips 1st.... and trips both linears offline.. [which also applies -10 vdc to the ALC of the xcvr. ]
All depends sometimes... whether it's an arcing ant/badly installed connector. [resulting in an INSTANTANEOUS wide open/dead short... infintite swr] OR just a high swr... between 2:1 and 5:1
In some cases the B+ sandfilled fuse blows..... some times BOTH sandfilled fuses blow. 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 ? I don't want the expensive ceramic vac load cap to ever internally arc.
Spark gaps are good!
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
