Valiant Modulator Bias Problem

[AB2EZ]

Dick

(I apologize for calling you Mike in a few of my earlier posts)

The two capacitors in series are there to block the DC.

The two 47k ohm resistors in the resistor chain are part of the voltage divider. I'm not sure what the function is of the 4.7k ohm 5W resistor and the 680 ohm 1/2 watt resistor. I speculate that someone was trying to make these all add up to 100k ohms in order to literally match the recommended value.

The 1.2k ohm resistor from cathode to ground is in parallel with the impedance looking into the cathode of the tube (1/the transconductance)... which is roughly 1.2k ohms in parallel with 400 ohms = 300 ohms. This 300 ohms of resistance,  in conjunction with the 100k ohm resistor (above), is intended to form a 300 ohm : 100.3k ohm = .003:1 voltage divider (which is the feedback ratio, "beta", in the feedback formula)

If you want to keep the feedback, I would suggest that you try adding a 0.1uF capacitor from cathode to ground (in parallel with the existing 1.2kohm resistor). This will result in a 3dB reduction in the feedback ratio (and also the gain around the feedback loop) at audio frequency 5.3kHz. The feedback ratio will drop 6dB per octave after that.

If that doesn't work, you can try further reducing the feedback ratio by adding another 47k ohm resistor is series with the two 47k ohm resistors that are already there.

Stu

[AB2EZ]

Note  I have modified my post, immediately above... to correct some errors

Stu

[W1KSZ]

I re-connected the feedback loop and added the .1 mfd cap. I could
adjust the bias to the proper value but as soon as I started talking
into the mike, the Modulator Plate current just about pinned the meter.

So, I dis-connected the feedback loop and listened to myself on another
receiver. Not the best test, but it didn't sound bad, not the quality of
WQXR, but I am not in the broadcast business !!

I'll try a couple of on air tests and see what others think of the sound.

Thanks for all the help,

73, Dick, W1KSZ

[AB2EZ]

Dick

okay...

Good luck!

Best regards
Stu

[AB2EZ]

Dick

One last thought:

Since the Valiant went from working to not working (due to too much feedback):

Check the actual resistances of the two 47k ohm resistors in the feedback path. Since they are each dissipating power equal to about 50% of their rating on audio peaks (maybe more)... one or both of them may have decreased in value, over time, to the point where the gain around the loop is too high.

If so, try replacing them with a new pair of 2W (or 5W) 47k ohm (or 56k ohm) resistors... or a single 100k ohm 10W resistor. Wire wound is okay.

Also consider replacing the two 0.22uF capacitors. If they are leaking (even a little), that will significantly increase the power being dissipated in the resistors, and also change the biasing of the 6C4.

Stu

[WD5JKO]

An interesting discussion concerning HF oscillations in a guitar amplifier when GNFB applied:

http://www.audiobanter.com/showthread.php?t=138909


quote from discussion, "But what I done loads the 6V6 with 9.4k a-a above about 30kHz, thereby stopping the oscillations"   (note a-a means anode to anode or plate to plate)

Perhaps the Valiant needs a series R-C Zobel network across driver or modulation transformer primary?

Jim
Wd5JKO

[VE3AJM]

Could you put up/post/ draw/ a schematic of these W6BM? mods. It would make things a whole lot clearer as to whats going on with your Valiant.

Al VE3AJM

[W1KSZ]

The W6BM mods were in Electric Radio some time ago. I had to
draw it out by hand to see what weas going on.

As it stands today, it's back in the cabinet and I will do some
audio checks with the locals today. If it sounds OK (or even
passable), I'll leave the feedback loop dis-connected.

If I find a quality 0.1 mfd 1 KV cap, then perhaps I'll re-do it.

There's a feedback mod by 'LHR that uses the vacated Clipper
pot that might be worth doing.

Thanks again to all that responded,

73, Dick, W1KSZ

[AB2EZ]

Dick
All

If I were going to try to reconnect the feedback, here is what I would do:

Step 0: Make sure the Valiant's power is turned off, and all power supplies are discharged

Step 1: Leave the feedback disconnected from the cathode of the 6C4. I.e. the (approximately) 100k ohm resistor (i.e. two 47k ohm resistors in series) that is in series with the (approximately) 0.1uF capacitor (i.e. two 0.2uF capacitors in series) is disconnected from the cathode of the 6C4.  Add a 0.1uF capacitor from the 6C4 cathode to ground (i.e. in parallel with the existing 1.2k ohm resistor). You may already have this capacitor in place as a result of our previous exchanges.

Step 2: With the power still off, and the B+ supplies still discharged: use a 330 ohm resistor in parallel with a 0.1uF capacitor to connect the open end of the 100k ohm (i.e. the two 47k ohm resistors in series) feedback resistor to ground.

Step 3: Connect one probe from a dual trace oscilloscope to the cathode of the 6C4.

Step 4: Connect the other probe from a dual trace oscilloscope to the 330 ohm resistor || 0.1uF capacitor combination added in Step 2.

Step 5: Connect a low-level, 1kHz sine wave to the microphone input of the Valiant

Step 6: Making sure that everything is properly connected, and that nothing is inadvertently shorting to anything else: turn on the Valiant

Step 7. Observe the two waveforms on the oscilloscope: The voltage across the 330 ohm || 0.1uF capacitor should have zero DC value. If it does have a DC value, greater than 100mV... then the .1uF capacitor (actually two 0.2uF capacitors in series) is leaking too much.

Step 8. The two waveforms on the oscilloscope should be close to 180 degrees out of phase. The waveform across the 330 ohm || 0.1uF combination should be larger than the waveform at the cathode of the 6C4... maybe 2 times as large... but no larger than that. If the waveforms are not approximately 180 degrees out of phase at 1000Hz, then there is something introducing excessive phase shift in the 6C4=>12AU7=> driver transformer=> 6146 amplifier chain.

Step 9. Increase the frequency of the sine wave toward 5kHz. The phase shift between the two waveforms on the scope should increase by around 45 degrees (i.e. to around 225 degrees), and the amplitude of the sine wave across the 330 ohm||0.1uF combination should be closer to the amplitude of the sine wave at the cathode of the 6C4 (maybe 1.4 times as large, or less).

Step 10. Increase the frequency of the sine wave beyond 5kHz. The amplitude of the sine wave across the 330 ohm || 0.1uF combination should drop to less than the amplitude of the sine wave at the cathode of the 6C4 at a frequency at which the phase shift remains less than 270 degrees. I.e. As the audio frequency is increased, the gain around the (open) feedback loop must drop to less than unity before there is enough phase shift around the (open) feedback loop to cause positive feedback.

Step 11. Decrease the audio frequency of the sine wave to 200 and also to 50Hz... to make sure that, as the frequency is decreased, the phase shift around the (open) feedback loop remains well below 360 degrees until the gain of the (open) feedback loop drops to below unity.

If the gain around the loop remains above unity, when the phase shift increases toward 360 degrees... then increase the value of the 100k ohm feedback resistor.

Stu

[W1KSZ]

I noticed in another post mention of the REA Modulation Monitor.

Does anyone use this ? How does it compare to say an SB-610 or 614 ?
I realize the REA does not show received signals.

73, Dick, W1KSZ