Buzz/Hum on V2 of SLO mod.

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VaporDemon

VaporDemon

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I recently powered up and did preliminary checks on my Jet City 50 which is modded to SLO spec. Everything is great except for some noise related to V2. While searching for the source of the buzz/hum, I noticed getting close to V2 made it louder. I swapped the tube out and it is 80% better but as I was searching I realized anytime I got my meter probe near the components of V2 it got super noisy. If I touch the grid stopper on pin 7 it’s like touching the tip of a guitar cable. The resistor tests fine as far as the value but could this resistor be causing the noise for some other reason? Someone reminded me that film caps can be sensitive depending on which side is attached to the outer foil. What else can I try to remove this noise? Thanks in advance!
 
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glpg80 said:
Going to need pics to be able to help.
Click to expand...
R37 is the grid resistor on the input of V2. If I touch that with anything I’m holding in my hand or in the center of the pins on V2 it makes a ton of noise. This isn’t repeatable on any other part of the board.
 

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Do you have a scope? Can you probe the stage before it and see if there’s noise there? Could be starting at your input. Check your wiring and routing. What components have you replaced around V1 and V2? Have you checked for continuity from V1 to V2?
 
Do you have a schem for the mod?
I would do two things, see if the gain pot/ other pots affects the buzz. And second pull v1, fire up the amp and see if the issue persists.

Also is it a low pitch buzz or high pitch whistle? Anything like that
 
glpg80 said:
Do you have a scope? Can you probe the stage before it and see if there’s noise there? Could be starting at your input. Check your wiring and routing. What components have you replaced around V1 and V2? Have you checked for continuity from V1 to V2?
Click to expand...
Unfortunately I don’t have a scope yet but that’s on my list of things I need asap.

Wiring is pretty straightforward and power supply leads all are routed and attach at the opposite side of the pcb. I’ve moved them around and cleaned that up as much as possible with no audible change in noise level when shifting them. Definitely improved over the factory wiring if nothing else.

All components on the board were replaced with better quality and spec. and all values confirmed prior to installation. There is literally only 7 values that change in the amp to bring the circuit to SLO spec aside from filter caps. I quadruple checked everything and all components are 100% within tolerance and value.

This noise is audible regardless of the preamp gain control. This is 100% on the overdrive channel and isn’t present when on the 1st channel. I actually pulled V1 eliminating the input stage and the first channel and it’s still present and it just gets louder anytime I get near V2 with my hand or a probe and is ridiculously loud if I’m near or touching R37 which is the grid of the first half of V2 and I’ve tried repeating this with every component on the board but it’s only in the area of the components near R37 and the solder joints on the tube socket pins. Touching the tube itself doesn’t affect it at all. I did improve the noise yesterday by swapping V2 for a new tube.
 
Chubbs said:
Do you have a schem for the mod?
I would do two things, see if the gain pot/ other pots affects the buzz. And second pull v1, fire up the amp and see if the issue persists.

Also is it a low pitch buzz or high pitch whistle? Anything like that
Click to expand...
I have the Schematic for the original Jet City 50 & 100 and the slo schematic and the mod just changes the values of 7 components to bring the preamp and phase inverter sections to SLO spec. I do have the schematic I edited showing the new values in place if you’d like to see it.

None of the pots change anything related to the noise other than the master which increases the level of the noise. When I am near the V2 section of the board if I touch the back of the gain pot for the OD channel the noise improves a bit.

I actually did pull V1 to eliminate the input and channel 1 and the noise is still the same and still gets louder the closer I get to R37 and the grid on V2(pin 7).

The sound is like a hum and buzz kind of like an ungrounded guitar or at its worst is like touching the tip of a guitar cable. Definitely not high frequency oscillation or squealing.

I’m wondering if the grid resistor is just bad creating a condition where it’s picking up external noise. I’m in a somewhat noisy environment where there’s a fair amount of ambient EMI.

I’m thinking of jumpering R37 to see how that affects it. And if necessary I can swap it to see what happens.
 
VaporDemon said:
Unfortunately I don’t have a scope yet but that’s on my list of things I need asap.

Wiring is pretty straightforward and power supply leads all are routed and attach at the opposite side of the pcb. I’ve moved them around and cleaned that up as much as possible with no audible change in noise level when shifting them. Definitely improved over the factory wiring if nothing else.

All components on the board were replaced with better quality and spec. and all values confirmed prior to installation. There is literally only 7 values that change in the amp to bring the circuit to SLO spec aside from filter caps. I quadruple checked everything and all components are 100% within tolerance and value.

This noise is audible regardless of the preamp gain control. This is 100% on the overdrive channel and isn’t present when on the 1st channel. I actually pulled V1 eliminating the input stage and the first channel and it’s still present and it just gets louder anytime I get near V2 with my hand or a probe and is ridiculously loud if I’m near or touching R37 which is the grid of the first half of V2 and I’ve tried repeating this with every component on the board but it’s only in the area of the components near R37 and the solder joints on the tube socket pins. Touching the tube itself doesn’t affect it at all. I did improve the noise yesterday by swapping V2 for a new tube.
Click to expand...

Chubbs said:
Do you have a schem for the mod?
I would do two things, see if the gain pot/ other pots affects the buzz. And second pull v1, fire up the amp and see if the issue persists.

Also is it a low pitch buzz or high pitch whistle? Anything like that
Click to expand...
Actually I wasn’t completely correct the gain control for V2 slightly increases the noise if you go high enough. I just wasn’t turning it up enough. Also I must have missed something but getting close to the grid on the input stage of V1 also adds a slight bit of noise but nothing close to the issue on the grid of V2.

And I turned off a led light in the room I had forgotten was on and it reduces what I’m hearing. May just need a noise gate lol
 
glpg80 said:
Do you have a scope? Can you probe the stage before it and see if there’s noise there? Could be starting at your input. Check your wiring and routing. What components have you replaced around V1 and V2? Have you checked for continuity from V1 to V2?
Click to expand...
Actually I wasn’t completely correct the gain control for V2 slightly increases the noise if you go high enough. I just wasn’t turning it up enough. Also I must have missed something but getting close to the grid on the input stage of V1 also adds a slight bit of noise but nothing close to the issue on the grid of V2.

And I turned off a led light in the room I had forgotten was on and it reduces what I’m hearing. May just need a noise gate lol
 
Try putting a piece of metal over the chassis while it’s upside down and see if that removes it. Could be EMI with the chassis flipped upside down.

Definitely use a power conditioner when debugging noise to remove external appliance sources as a potential cause.
 
glpg80 said:
Try putting a piece of metal over the chassis while it’s upside down and see if that removes it. Could be EMI with the chassis flipped upside down.

Definitely use a power conditioner when debugging noise to remove external appliance sources as a potential cause.
Click to expand...
I run my amps through a voltage regulated furman power conditioner. And actually that shielding with metal sounds like a great possibility and I wouldn’t have ever thought of that so thank you! I’ll give that a shot tomorrow and I’ll let you know what happens.
 
glpg80 said:
Try putting a piece of metal over the chassis while it’s upside down and see if that removes it. Could be EMI with the chassis flipped upside down.

Definitely use a power conditioner when debugging noise to remove external appliance sources as a potential cause.
Click to expand...
The sheet metal doesn’t seem to affect it. I did however notice the power transformer I just installed seems to have very slight physical humming when it’s on and kind of matches the frequency of the noise in the signal. The other noticeable thing is that from the factory the PT and OT are oriented in the same direction at opposing ends of the chassis. I’ve seen some amps that have one turned 90 degrees in relation to the other. Maybe worth the time to rotate the OT and see what happens.
 
Well the noise issue is gone! We were having power issues in the neighborhood and after talking to my neighbor apparently we had low voltage which I somehow missed on my power conditioner input meter. Today the voltage came back up to 117 and the noise is gone. Not sure how or why that would make buzz/hum but it’s gone.
 
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