Trouble shooting scratchy pot on JCM800 style build

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JazzGuitarGimp
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by JazzGuitarGimp »

Why not AC couple the pot to the tube and be done with it?
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sluckey
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by sluckey »

Why not AC couple the pot to the tube and be done with it?
The pot provides the dc ground return for the tube grid.
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JazzGuitarGimp
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by JazzGuitarGimp »

sluckey wrote:
Why not AC couple the pot to the tube and be done with it?
The pot provides the dc ground return for the tube grid.
Correct, so a 1M resistor from grid to ground would need to be added.
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by pdf64 »

Such a high grid voltage seems more characteristic of grid leak bias; what is the cathode pin 3 voltage, and its measured resistance to 0V?
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

pdf64 wrote:Such a high grid voltage seems more characteristic of grid leak bias; what is the cathode pin 3 voltage, and its measured resistance to 0V?
I just measured the voltage on pin 3...or should I say I tried to measure it but got 0 volts. Switched to millivolts and still got zero. Pin 8 on the other side of the tube was getting -1.7 v or so. This amp was built with lower overall DC power for using 6V6 tubes rather than EL34. All cathode resistors measure fine and there is a 10K unbypassed resistor on pin 3. per the circuit schematic.
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pdf64
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by pdf64 »

mmmoser wrote:I just measured the voltage on pin 3...or should I say I tried to measure it but got 0 volts....All cathode resistors measure fine and there is a 10K unbypassed resistor on pin 3. per the circuit schematic.
The only way there can be 0V across a 10k resistorance is if there's no current flowing.
It looks like the cathode is shorted to 0V, hence the stage is running in grid leak bias, hence the grid voltage leading to scratchy volume control.
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

pdf64 wrote:
mmmoser wrote:I just measured the voltage on pin 3...or should I say I tried to measure it but got 0 volts....All cathode resistors measure fine and there is a 10K unbypassed resistor on pin 3. per the circuit schematic.
The only way there can be 0V across a 10k resistance is if there's no current flowing.
It looks like the cathode is shorted to 0V, hence the stage is running in grid leak bias, hence the grid voltage leading to scratchy volume control.
I am hearing what you are saying. I just cant for the life of me figure out how this is happening physically. I will need to get out my super high magnifying glasses and look very closely maybe under the board for a stay wire whisker or something similar to explain this.
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ToneMerc
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by ToneMerc »

Do you have a final as built pic so we can see all the board top side terminations on the area of v1 plate/input jack area? For grins,check the layout verses your build.

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Re: Trouble shooting scratchy pot on JCM800 style build

Post by lovetone »

Measure the resistance from pin3 to ground to check if. Oh can see 10k or a short circuit
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mmmoser
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by mmmoser »

I thought I would come back and post the conclusion of this issue. I hate it when someone has a problem but it just never gets finished in the stream of posts trying to figure out what finally solved the problem.
I have never actually seen this issue before but what I found was that the turret that the first preamp tubes first amplification side (Pins 1, 2 and 3) cathode resistor/cap was attached to was "bad". What does this mean? Well, it was not allowing current to flow through it. I thought maybe that there was some unseen contact under the board or something like that making a connection to ground but could not see any such evidence of this.
I merely installed a new turret in the empty hole next to the old one (I used that perforated board material so there are holes all over the surface) and once the new turret was installed and the components soldered to it, like a Christmas Miracle, the DC noise was gone and the cap/resistor was doing its job. Got the expected voltage on that pin and all was well. Was the old turret made from some strange material? I am not sure, all the other turrets were the same and they all worked fine- I mean were are talking simple passing of electrons here or lack thereof.
Anyways, rather than pull the board and do a microscopic assessment of the old turret, I just put in a new one next door and all is good again.
Thanks for all the suggestions. This was a new one for me.
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deeder
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Re: Trouble shooting scratchy pot on JCM800 style build

Post by deeder »

So it's fixed, thanks for sharing the solution ;^)
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