[AMPS] Big Tetrodes at UHF

[km1h@juno.com]

On Sat, 20 Mar 1999 21:41:45 +0100 Peter Sundberg <sm2cew@telia.com>
writes:
 
>
>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


Hi Peter,

Parasitics at 70cm ?  Is it possible that you were experiencing a cavity
effect internal to the tube?  I believe that has been documented on the
reflector in the past.  Also what is the self neutralizing frequency of
the tube? 

I'm curious since I intend to start the design on a 70cm amp using a GE 
GL-51064 tetrode which has a plate dissipation rating of 2600W and full
ratings to 1GHz.

I am not familiar with the YL-1050 but would appreciate any help in
placing this big  GE tube on 70 cm. Playing at that level power on UHF
will be a new experience for me....BANG!

73  Carl  KM1H


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