K5GW wrote:
>In a message dated 1/26/2006 12:59:49 P.M. Central Standard Time,
>Xmitters@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|