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Valiant hum




 
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K2GTM
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« on: March 22, 2021, 11:35:11 PM »

I'm working on a Valiant that has hum in the audio.  I went through the entire audio chain because someone had done some mods in the past and the wiring was a mess.  I used the original schematic along with Timtron's Valiant mods from this site to repair the wiring and perform some upgrades in the process.  (Clipper removed, etc.)  Every part has been either replaced or tested as needed.  The power supply filter caps have been replaced/increased in capacity.  Voltages test good.  The audio tubes test good, but I did try known good ones with no improvement. 

I have a low level AC hum on the carrier with the audio gain turned all the way down.  (No hum in CW mode.)  The hum gets worse when the gain is increased.  The hum is markedly stronger at the output of the 6C4. 

I've heard this is a common issue, but does anyone have any ideas what is causing this?  I was thinking it might have something to do with the filament lines not being run in a balanced (twisted-pair) style, or maybe it's a ground loop issue.  Maybe the audio section needs more attention to detail - something similar to a tube guitar amp? 

I don't want to completely gut the audio and start over.  I also would rather have the audio remain inside the rig - I don't want to run an external audio amp to drive the modulators.

Any help is greatly appreciated! 

73,
Greg K2GTM
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Detroit47
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« Reply #1 on: March 23, 2021, 12:19:48 AM »

Just for fun pull the 6C4 and see if it goes away. Divide a conquer pull the 12ax7 and see what happens. This should narrow things down.

N8QPC
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DMOD
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« Reply #2 on: March 23, 2021, 12:27:28 AM »

The best way to trace problems of this nature is to use an oscilloscope and trace backwards from the the plate of the 6C4, then to the second audio plate, then the first audio plate, then to the input to see if any AC is present at these points.

Phil - AC0OB
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Detroit47
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« Reply #3 on: March 23, 2021, 12:36:53 AM »

this  is true about the scope but some folks don't have one or know how to use one.
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K2GTM
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« Reply #4 on: March 23, 2021, 01:10:00 AM »

I actually did trace it out with the scope and there is a little bit of hum in the 12AX7, but a lot more at the plate of the 6C4. Problem is, there is a negative feedback loop from the plates of the 12BH7 (where the 12AU7 would be) back to the 6C4 plates (part of Timmy's mods), so the hum might be mostly coming from the 12BH7. I'm going to try removing that resistor and tracing it again tomorrow.  I do remember trying removing the tubes one at a time, and the only one that made the hum go away completely was the 12BH7. 

One other thing I noticed, the 12BH7 (or a 12AU7) gets SUPER hot. The plates aren't red, but I'm thinking it gets much hotter than it should. I think I should take a closer look at the wiring in that area...
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Tom WA3KLR
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« Reply #5 on: March 23, 2021, 02:01:01 PM »

Do you have other tubes to try? I'm wondering about unbypassed cathode circuits with heater-cathode leakage as a source.
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K2GTM
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« Reply #6 on: March 24, 2021, 02:08:20 PM »

Hey Tom - I did try replacing the 12AX7 (1st and 2nd audio), 6C4 (3rd audio) and the 12BH7 (what I used in place of the 12AU7 audio driver).  None of that helped, but I did notice that the 6C4 seems to have quite a bit of hum on it's plate and the tube itself is microphonic.  I tried swapping the 6C4 with a good spare but it was also sensitive to tapping on it.

I can't see any wiring issues.  I also checked all of the ground connections.  The hum is definitely present from the 1st audio section, but the ratio of audio-to-hum is low there and at the 2nd audio.  It really jumps up in the 6C4, but I'm at a loss as to why.  I think the 12BH7 is getting hot because it's amplifying the hum from the 6C4 all the time.  It just seems a lot hotter to the touch than it should be. 
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Tom WA3KLR
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« Reply #7 on: March 24, 2021, 04:05:30 PM »

Tube heat - The 12BH7 has a 2X higher than usual filament current draw for a peak cathode current of 300 mA! for each section since it was designed for horizontal deflection duty. 12.6 V @ 0.3 A = 3.8 Watts, then each plate could be dissipating about 2.9 Watts (depending on biasing) for a total of 9.5 Watts for the tube.  Seems like a fair amount of power for an object the size of your thumb, so I would expect it to run hot.  Do you have a handheld IR thermometer? I can't say what max. temp. to look for though, and I think the glass itself is low emissivity?

Hum - Is the other external input unconnected?

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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
K2GTM
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« Reply #8 on: March 24, 2021, 07:04:05 PM »

Ah, that makes sense.  The only thing is, the 12AU7 that was in there before I swapped it out also gets really hot.  I don't remember the 12AU7 being a "hot" tube.

I did look at the tube with my Flir, but I don't remember the reading.

I get the hum when I feed audio into either the mic or the phone patch inputs.  I have only one connected at a time.
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W1NB
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« Reply #9 on: March 24, 2021, 08:27:26 PM »

You might consider lighting the tubes temporarily with a DC supply and see if the hum disappears.
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DMOD
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« Reply #10 on: April 01, 2021, 01:17:01 PM »

I actually did trace it out with the scope and there is a little bit of hum in the 12AX7, but a lot more at the plate of the 6C4. Problem is, there is a negative feedback loop from the plates of the 12BH7 (where the 12AU7 would be) back to the 6C4 plates (part of Timmy's mods), so the hum might be mostly coming from the 12BH7. I'm going to try removing that resistor and tracing it again tomorrow.  I do remember trying removing the tubes one at a time, and the only one that made the hum go away completely was the 12BH7.  
...


What happens when you remove that feedback loop? A removal of any feedback circuit component would be a good start in order to get an unmodified circuit for problem tracing.

What does this feedback circuitry look like and what components does that feedback loop contain? If it is an RC circuit, what are the values?

If it is an RC circuit the circuit component values may not be giving negative feedback but the phase change may actually be enhancing some low frequency component.

I have seen people attempt modifications with this kind of thinking: "If a 0.001 uF is good them a 0.01 uF should be better."

Phil  
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K2GTM
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« Reply #11 on: April 01, 2021, 05:28:18 PM »

Whelp, I feel a bit silly.  I should've looked closer at the B+ going to the audio section.  The AC ripple level was enough to introduce the hum.  I found that the 5V4 LV rectifier was the culprit. 

Thanks for everyone's help!
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