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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Mon 10, 2020 11:45 pm 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1067
Location: Tokyo
Are you getting both 'oscillation' (at 455KHz or thereabouts) and 'motorboating' (oscillation at an audible frequency)? Motorboating is usually caused by a shared impedance in the power supply, hence the standard practice of decoupling. RF oscillation may be caused by a combination of stray capacitance (from the wiring, component layout, C between the windings of an IFT, etc) and internal tube capacitance; the most common case with two IFTs seems to be in the form of a TGTP oscillator. I suspect that IFTs designed for relatively low Z transistor amps have too much internal C (between windings, between windings and the metal case, between lead wires, etc) to work in a tube circuit.

Btw, how did you align the IFTs?

That was an interesting article you linked. However, choosing tubes for 455KHz IF amps and for high RF amps is a different game. The connection between AVC voltage and change of transconductance is very important for IF amps. That change will detune the amplifier, and that is critical given the narrow bandpass of a 455KHz IFT. The 6/12BA6 was designed to minimize AVC detuning. Hot RF tubes, even remote cutoff types, do not work as well as an IF amp.

Rob


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Tue 11, 2020 3:07 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
Concluded that only the secondary of the 1st IF (grid) and the primary of the 2nd IF (plate) have to be in tune to sustain the oscillation.

Checked the screen grid capacitor and made sure that the tube socket is actually connecting to the screen grid pin (there's voltage drop on the screen grid resistor, so screen grid current is going through it). Removed the 100nF cathode capacitor but it had no effect.

Then attached a 330k shunt resistor between the grid and ground to control the Q of the grid LC tank, which worked with the 12BA6 (cathode cap removed). I was able to tune the whole thing up with my homebrew sweep generator.

I'm now pretty convinced that the cause of my trouble is the combination of the transformers being too close to each other as well as the mica capacitors being outside of the shielding.

The 6Ж1П is still going into oscillation, probably due to a slightly higher transcinductance, but I guess a lower (220k maybe) shunt resistor will eventually help.

(Tried placing a steel plate between the transformers, it had no effect.)


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 Post subject: Re: Help needed with IF stage oscillation / motorboating
PostPosted: Aug Wed 12, 2020 12:49 am 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 155
I hope shielding will fix the problem it should. The only last thing I can think of is to wire it exactly like a typical AA5 IF. Do you have any schematic of a typical AA5? I can't upload schematics for some reason but perhaps someone else has one. AA5 has a resistor in the cathode like your circuit this gives a little negative feedback they don't have a cap across it. Maybe increasing resistance will decrease amplification until no oscillation


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 Post subject: Re: [Solved] Help needed with IF stage oscillation/motorboat
PostPosted: Aug Thu 20, 2020 7:15 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
After studying some theory, I searched for some IF transformers that have lower impedance (assuming same Q) on 455 khz.
The IF transformers I originally used had 100pf capacitors; I found a and installed a new set: the 1st transformer has 140pf caps and the 2nd has 400pf on both sides. All four LC tanks are now representing significantly lower impedance at resonance.
The IF stage is now stable and sharp with all degeneration removed (cathode bypass capacitor in place, no parallel shunt resistor on the grid) both with the 6BA6 and the 6Ж1П. I see some influence on the primary of the 1st transformer when I tune everything up with the 6Ж1П in place, but it's much easier to keep things under control now. The gain is more than 120x (42dB).
Lessons learned.


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