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Discrepancy in one channel...

knockbill

knockbill

Lunatic Member
This is on the 6L6 SE amp I'm building...
Have a scope connected to the speaker leads for a few days, to watch the waveforms... Put terminals on the bread boards today and when I hooked the scope back up, I noticed one channel has a solid "filled in" waveform, that appears as the VC is advanced, starting around 10 o'clock and then, disappears at around 1-2 o'clock.... (sorry for the tech talk, best way I can describe it!!)... I can't hear any distortion at all, the amp is dead quiet...Also noticed, if I put my hand between the output tube and the VC, the wave goes back to normal...
First thought it had something to do with the old VC I used to mock this amp up, but does the same thing with a new one....it does it with or without an input connected... Probed around, checked joints, changed tubes channel to channel,, changed CDP inputs, swapped scope leads, and can't find the connection to the "discrepancy"...
Voltages are good, there's no noise or static...
I'm sure the scope is telling me something, but I don't speak scope,,yet!!!!

Any ideas are welcome...

Regards,
John
 
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Couple pics...

These are with no input, can't get one of the waves,,, I have a 2 second clip of it playing, with and without the "distortion", but can't load it here...

Regards,
John
 

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Volume Control...

Just put the scope on the input cap, and its clear on both leads,,,, starts getting ugly on the wiper (grid tap} of the VC)...

Regards,
John
 
Does the scope a a 3 lead cord and is the grounding pin connected , at the receptacle , same for the amp .
 
Yes, and yes,,, the scope has 2 inputs,X1, X2... the result is the same using either input, swapping leads, etc...the other channel reads fine on either X1, X2...
I had the scope hooked up to the amp for days,,, just started doing this...
Regards,
John
 
Sounds like a description of ultrasonic noise/oscillation. I would look into some of the standard fixes for this type of problem - grid stopper resistors on the preamp tubes, and possibly a low pass filter somewhere to kill response above 20 kHz.

Another possibility is your negative feedback circuit isn't designed properly for the output transformer you are using, and there's enough phase shift it oscillates under some conditions. This can also be cured by judicious use of some low value capacitors.
 
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knockbill said:
Volume Control...

Just put the scope on the input cap, and its clear on both leads,,,, starts getting ugly on the wiper (grid tap} of the VC)...

Regards,
John
Click to expand...

Doh! For some reason my smallish brain kept going to voice coil, and I knew that couldn't be. So it gets ugly at the wiper on one channel or both? I got to where I started taking these old pots apart to clean them. It's surprising what you can find inside, like old grease and such. After doing it with them apart, it's tough just to squirt some de-oxit in and call it good.

OK, so you have a new VC, hmm.
 
maxhifi said:
Sounds like a description of ultrasonic noise/oscillation. I would look into some of the standard fixes for this type of problem - grid stopper resistors on the preamp tubes, and possibly a low pass filter somewhere to kill response above 20 kHz.
Click to expand...

OK,,, why did it just start, only do it in one channel? I'll go back and match voltages side to side, and check each component and see if anything changed value...
I don't believe in coincidences, something had to change...

Regards,
John
 
The fact that you can stop it by placing your hand between the VC and an output tube is a real clue, I just don't know what it's telling us.
 
jaymanaa said:
How about a pic?
Click to expand...

New VC on this side... only thing different about the channels are the OPTs,,, different makes, but measure pretty close, and I had it running for weeks,,, listen to it while working out teh PS situation...
I guess I could swap Output Tx, but everything else is the same...

Regards,
John
 

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Its gotta be some kind of oscillation?? Can't be a terminal problem, the amp sounds too good...
We'll get it!

Regards,
John
 
yeah, looks like oscillation to me too. I was dealing with this on the Phase Linear before I got that hammered out, and it made "full" waves just like that.


Try maybe 100pf or so from the driver plate to ground to see if that kills it off.

Happen to have a schematic?
 
Tried 100pF cap on driver plates,, no change... Touching the driver makes it stop, tho???
Guess this is how you get experience!!!

Regards,
John
 
Sounds like the clip leads were giving it enough parasitic capacitance to make it quit acting up. Lead dress and other really minor details can be the difference between good stability and a nice low power radio transmitter.

link won't link, so I'm not sure what you're using for a driver. If it happens to be a metal tube, try grounding pin 1 on the octal socket. Thats typically the connection for the metal can to use it as a shield.

Could also try a shield on the tube, or maybe try the capacitor to ground on other pins of the tube to see if one makes it quit. I've also seen cases where it needs 100 or so ohm resistors with a short lead between the coupling capacitor and the grid of the output tube, or the same thing with the screen on the output tube.
 
I just noticed the wood chassis! Is there not any shielding on the top side?

As a troubleshooting aid, put a 47,000 ohm resistor between the grid of the the driver tube, and that heavy copper ground bus, and see if it helps. Make sure to solder it in place, don't attach it with long clip leads. I am thinking it is possible that high input impedance plus no shielding at all is making it sensitive to oscillation.

Also check to make sure your volume pot is wired properly. Middle terminal to grid of first tube, other two terminals to ground and signal IN.
 
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