[Amps] GE EF-5A VHF Amplifier

[Ian White GM3SEK]

K5GW wrote:
>In a message dated 1/26/2006 12:59:49 P.M. Central Standard Time,
>Xmitters at aol.com writes:
>
>Hello  Again :-)
>
>I have a General Electric EF-5A VHF power amplifier. I think  it is Progress
>Line or Mastr Progress Line vintage, not sure.
>
>Every  one of these amplifiers I have seen at hamfests a few years ago, all
>had  the plate current meter blown open. And with mine, I found out why. As
>you
>get close to tuning the thing up, you get a big nasty high voltage bang  from
>inside the unit, the windings on the RF coke on the plate lead get  squashed
>into
>the center of the RF choke form and quite often, the 4CX250B  tube gets
>damaged. I was very careful to properly neutralize the amplifier  and careful
>not to
>overdrive the thing. I got to the point where it  appeared stable for 10
>consecutive keyups. OK, I fixed it, so I thought.  On the next keyup, she
>arced over
>again! I had a simpson panel meter in  use to watch the plate current, and
>that
>meter just like all the others,  blew open.
>
>So are there any old timers in the group that has some  flying time with this
>amplifier that might know how to calm this thing  down? Maybe some
>strategically placed Zsorb blocks inside the plate  cavity? The amp has been
>in my shack
>collecting dust, for about ten years  because of this issue.
>
>This really looks like a good amplifier and I  have a good supply of 4CX250B
>pullouts, so I would sure like to fix this  amplifier if at all practical to
>do
>so.
>
>And BTW if there are any  boat anchor collectors out there that would like to
>have some GE Progress  Line gear, we really need to get together :-) Swap
>gear
>for intellectual  property maybe? :-D
>


>Is the required 25 to 50 ohms current limiting resistor installed in series
>with the B+ to the tube?
>


Is the screen voltage of this amplifier stabilized against both positive 
and negative screen currents? It could be screen voltage runaway caused 
by negative screen current, to which the 4CX250B is very prone. A very 
large anode current would explain why the RF choke has collapsed under 
its own magnetic field.

The current limiting ("glitch") resistor will help limit the damage, and 
obviously the meter also needs some protection too; but neither of these 
is treating the root cause.

Another possibility is an LF parasitic oscillation, which can happen 
when some inductance in the power supply is resonated by the VHF 
"bypass" capacitors. If this tuned circuit is completely unloaded, the 
oscillation can be very violent. The glitch resistor can help here too, 
by damping the resonance so that oscillation becomes impossible.




-- 
73 from Ian GM3SEK