Richard W. Ehrhorn wrote:
>Hi Ian...
>
>I have no doubt that we saw many "Rocky Point effect" flashovers in the
>3CV1500A7 (vapor cooled 3CX1000A7) tubes we used in our first amps - the
>ALPHA SEVENTY c. 1970. The BANG occurred much more often in standby (tube
>biased beyond cutoff) than when drawing plate current, and it was LOUD.
>Reason? The arc was in a vacuum. But until we got smart and installed
>surge-current suppression resistors ("glitch resistors") in the HV leads,
>the 25 uf filter cap charged to ~4 kV discharging through the tube arc
>often blew the plate choke into 200+ neat half-turns of magnet wire!
>
>Eimac engineers referred to the cause of the flashovers as "barnacles," but
>your description is much more descriptive. These flashovers usually
>occurred very early in tube life - more often than not during factory
>checkout of the amplifiers - and were self-clearing after two or three
>flashes. We also experienced frequent new-tubes-in standby flashovers in
>early 8877s and 8874s through the late 70's, but in the last 15 years or so
>they seem much less common.
>
>But the BANG from those RF chokes disintegrating during a Rocky Point arc
>during checkout of early '70V made several of us pretty jittery about
>pushing the power-on button the first time! Proper fault current limiting
>resistors tame the phenomenon to the point that many times the plate
>overcurrent relay or breaker doesn't even trip.
>
>73, Dick W0ID
My homebrew 432 MHz EME amplifier uses a Siemens YL1050 tetrode in a
grounded screen configuration with the screen voltage B+ grounded.
This tube is famous for it's arc-overs and mine was not any different. Just
as Dick pointed out, the big BANG often came when the tube was biased off
rather than during transmit or idle. Many other EME operators have seen the
same thing, and often the BANG comes after a 2.5 minute EME transmit
interval, just after the amp had been biased off.
I tried two tubes, one old and one new and they both behaved the same way.
The screen protection circuit kicked out but the resistor in the HV line
prevented the fuse from blowing all the time. I started thinking that the
problem might be something else than an internal arc-over, maybe a
parasitic. So, I totally rebuilt the plate bypass capacitor (sandwich type)
and the grid/cathode feedthrough. Also installed a series
resistor/capacitor from the the cathode to ground. G1 B+ , G2 B- and HV B-
go to the cathode that is lifted above chassi ground and also holds the
input circuit.
My "arc-overs" dissapeared! The amplifier now behaves nicely and I never
have the BANG's any more. Since then I have put the old tube in a cavity
for 1296 MHz and having been careful with decoupling there too it has never
once tripped the protection circuits.
I know there has been a lot of problems for EME'er with these so called
arc-overs in their YL1050 amps, it is my belief that most of them are
actually parasitics.
Now, here's my question to the experts on this reflector;
If the tube is "gassy" and has internal arc-overs, will they also occur
without screen voltage, but with all other voltages, HV, G1and filament on ?
My amplifiers have NEVER had this type of arc-over WITHOUT the screen
voltage, that's what lead me to believe that I was dealing with parasitics
rather than gassy tubes.
The amp always comes up with the screen voltage off (protection circuit
needs to be reset to start operation) and in that mode it has never ever
shown any "bad behaviour".
So, does a tetrode require all voltages on to show that it is gassy ?
73 de Peter/SM2CEW
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