[AMRadio] GK500 OTA almost

Jim Candela jcandela at prodigy.net
Tue Jan 3 07:22:39 EST 2017

It's been  while since I have owned a GK 500B. The RF PA stage is not neutralized, and therefore it tends to be slightly unstable from some regeneration. On the upper bands, plate dip and screen current max does not coincide with maximum output. That said, it is not prone to parasitics either. With many class C RF amplifiers, opening up the loading capacitor in the Pi net allows a greater amount of harmonic energy to reach the RF output, and also might push the final RF amp closer to instability.

I would try to go simplistic and use a crystal instead of the VFO. This eliminates one form of RF ingress. 

Many GK owners use a copper braid chassis to chassis, one piece, straight up and down.
If the RF PA tube has been flashing inside, the tube might be toast...but not necessarily the cause of the problem
A neon bulb fastened to a popsicle stick, and placed near the PA tube plate will glow orange when transmitting. Whenever there is a parasitic, the bulb color will shift to violet. Don't hold that stick...just place it nearby, hot melt it in place.
Is the instability there when transmitting into a good 50 ohm dummy load?
Also see the post from K1JJ here:"on air tuning causing stability issues"


    On Tuesday, January 3, 2017 4:57 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:

 Don't give up--I almost did a few times with a rig here but when it
finally got on the air it was like when I was 5 years old at

I've never owned a Globe King but it sounds like the final PA breaks
into parasitic oscillation.  Get a cheap crummy battery powered
transistor FM broadcast receiver, something with a telescoping whip
and 9 v. or 2 AA batteries and put it between stations and next to or
on top if the rig.  Don't know the parasitic frequency for the tube
but my experience has been if the final oscillates you hear a lot of
hash on the cheap FM set.

The parasitic choke can be fine but if there is a grounding problem,
rusty rivets, poor bonding between chassis and panel, tube socket and
chassis, and most important, the bonding between the output pi  net
caps and ground (chassis) that can set things off.

If the output pi net caps are connected to the chassis via the front
panel that could be your problem.  Keep in mind the problem could be
something else entirely--I'm just an armchair troubleshooter.



On 1/2/17, Collin via AMRadio <amradio at mailman.qth.net> wrote:
> Dear AMers,
> I am having a time with this Globe King 500B. I have it ready to make RF but
> no modulation yet.
> Something is just not right and I can't quite put my finger on it yet. Maybe
> some of you have had an experience like this and might be able to lend me a
> hand.
> Tuning the final goes fairly well until I take it out of tune and the full
> screen voltage is applied to the final. Tuning is very touchy acting and
> reaches a point where plate current suddenly pegs the meter and some
> arcing/flashing happens inside of the tube. I can tame this and get it to
> stay stabilized but increasing loading and the plate current pegs. Any
> ideas?
> I have cleaned and secured grounding of tune and load air variable
> capacitors and also disassembled each air variable and cleaned and buffed
> all contact surfaces. I have cleaned the final tank band switch. I have
> inspected every connection and contact between the antenna connector and the
> plate cap on the final amp.
> I will not give up!
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