Discussion in 'Amateur Radio Amplifiers' started by VA2NA, Apr 26, 2019.
What I believe is happening is that you have inadvertently created a
push-pull resonant circuit between the filaments of the two tubes, possibly also with the filament choke involved.
This is a "sure-fire" way to make an oscillator out of a grounded-grid amplifier circuit.
Suggest bonding the filament connections of the two tubes together with the shortest leads possible, and putting a lower-value bypass (200 - 500 pF) across each filament. Also check the filament choke for any resonances in the ~100 MHz range.
I finally get the oscillation stop using Rich AG6K trick...not exactly the same but close... i getting a nice 150 mA resting current when i apply HV and bypass 33k resistor.
I cant believe the problem was so simple...
Will now try to load 6 meter pi network
Static 6 meters work with this parasitic at 2500 ohms.... next RF
Here some more picture of tubes running 150mA resting current ... meter pic to show stat.
Thank you so much for so informative Web page.
Like you can see in my reply i can fix the oscillation
I Take few more reading this morning using VNA on both parasitic ( mine and Rich technique one) to see what will be the difference on test gear.
The plotting talk by themself.
This is the first time i got an rf deck that go in oscillation as soon as i apply bias and it give me an testing platform for experiment.
I have work on the grid, plate choke, output pi-network and everything around the tube.. the only way to get it out is the rich paratic technic in this situation...i dont mean it solve all the parasitic situation...by in the present case...it does the trick.
I gona post more picture of the plotting.
How i setup my test gear :
- I feed the VNA PORT on cathode across 2 1000pf on each side of the fillament after the filament choke and PORT 2 on the output of the Pi network. This is probably not the best way but i want to see how the output pi network also acting in the circuit... there 2 traces one from port 1 to 2, and one from port 2 to 1.
VA2NA Build parasitic: pretty nice and sharp resonnace at 138-140 Mhz..cant bias the tube it go in oscillation and will destroy the tube in few secondes...
AG6K Method: No resonance beyond 6 meters, tube idle at 150mA and stable here.
Marked, notable improvement, if I read you correctly!
About a 20 dB change looking at the trace on the FSH8 screen, sufficient to be stable and not oscillate.
The real test will be with RF (this week) and not destroying the parasitic at 50 Mhz