Fender 6G4A Super / Intensity Pot Pop!

[plexi50]

I have a 1961 Fender 6G4A Super amp that has an Intensity Pot issue. The pot is a new 10MRA i got lucky enough to find.The Vibrato works great. But as the intensity pot stops in either direction CW or CCW their is a loud Pop! The Tremolo continues to function fine. It's just a very loud pop. As you can see i had a dozen+ bad leaky Astrons and replaced them with Mallory M150 yellow capacitors. I am not reading any stable voltage on the Intensity pot as it fluctuates. I bought a second pot for stock. What in the H*ll is this? Oh and also their are no grid resistors on the power tubes which makes me think that may be an issue.

[Latole]

  I don't think the Altros capacitors you replaced are a good idea. It's impossible for them all to be problematic.
You've probably compromised the sound of this great amp.

It's the electrolytic capacitors that should be;
Bypass and filter caps.

[glass54]

Hi Plexi
Very neat chassis 

I am not reading any stable voltage on the Intensity pot as it fluctuates

You WON'T ie NO DC! The intensity Pot is AC coupled. Maybe some sort of sine/triangular wave with long timebase on oscilloscope. Do you have access to an oscilloscope?
On a side issue, did you change any Electro's in Doghouse (PS decoupling)?
Are the 25uF electro caps on 12AX7 and 7025 Cathodes in Vibrato components all good?
Regards
Mirek

[plexi50]

  I don't think the Altros capacitors you replaced are a good idea. It's impossible for them all to be problematic.
You've probably compromised the sound of this great amp.

It's the electrolytic capacitors that should be;
Bypass and filter caps.

I replaced all of the electrolytics 2 weeks ago. Cathode Caps as well. I do a full cap job. The astrons were leaking badly like 3-14 volts. One on the end near the PI was burnt.The amp sounds fantastic now. I just have a problem with the pop of the intensity pot.
The Preamp tubes are old Telefunken & Bugle Boys. I found a microphonic pair last night moving them around. Time for newer tubes. They all test strong but that don' mean they aren't oscillating here and there. The board is clean. No wax at all.

[plexi50]

This is the issue i am having with this 1961 Super 6G4A Amplifier. All new Electrolytics. The Astrons were BAD and i had to replace them. 3 of them had leads that were corroded away at their solder points and crumbled apart on contact. I Am a firm believer in (NOT) just replacing old coupling capacitors because they are old. They all were leaking voltage. One Astron near the PI was toasty looking and semi melted.Otherwise the Super sounds fantastic including the intensity function and Vibrato.

[Calboy]

Try pulling preamp tubes one at a time and see if it stops?

[AlNewman]

Kinda just sounds like a signal thing.  Like maybe a bad/dirty pot.

[plexi50]

I tried moving tube around already but have a full new set of tubes coming tomorrow. I say it a dang tube. All these tubes are old Telefunkens & Bugle Boys.Great tubes but i have seen great tubes over the years have voodoo going on inside of them and honestly this is all i think it could be at this point. You know how it sometimes turns out to be the most simple thing. I will know tomorrow. Just frustrated at this point and now it's time to leave it alone until tomorrow. Otherwise i am very happy with how the amp sounds.

[plexi50]

Kinda just sounds like a signal thing.  Like maybe a bad/dirty pot.

When i have a real dirty preamp socket i use a D string from an acoustic guitar to actually file or scrap the socket pins. I learned that one on a 1959 5E3 that would not make contact with pin 8 cathode years ago after all the cleaner in the world did nothing to physically clean the socket pin.

[glass54]

So far you have won the "2025 crazy fault of the year"   
If you had one area ONLY making pops, I would suspect the substrate on the Intensity pot.
Silly question (forgive me). Did you change the cap as indicated by attached photo?
It still sounds like a sudden DC discharge (almost "static" electricity).
Could you try a known 1MA (vol type pot?) And YES I know its the wrong taper etc, But it might prove something?
I agree with Allnewman BUT its a "Pulse" rather than a "dirty patch" of noise.
....and nice cleanup/recap job
Regards
Mirek

[Latole]

  Time for newer tubes. They all test strong but that don' mean they aren't oscillating here and there. The board is clean. No wax at all.

Tube testers miss at least 50% of all potential defects.

[plexi50]

So far you have won the "2025 crazy fault of the year"   
If you had one area ONLY making pops, I would suspect the substrate on the Intensity pot.
Silly question (forgive me). Did you change the cap as indicated by attached photo?
It still sounds like a sudden DC discharge (almost "static" electricity).
Could you try a known 1MA (vol type pot?) And YES I know its the wrong taper etc, But it might prove something?
I agree with Allnewman BUT its a "Pulse" rather than a "dirty patch" of noise.
....and nice cleanup/recap job
Regards
Mirek

The cap you have circled in red is the Presence cap. Their is 12.20VDC on that pot.

[Latole]

You must not read any volts on pots
Reading 12 volts is not normal

[plexi50]

You must not read any volts on pots
Reading 12 volts is not normal

Right. I was going to pull that cap 3 days ago and change it. Doing it now.

[Latole]

You must not read any volts on pots
Reading 12 volts is not normal

Right. I was going to pull that cap 3 days ago and change it. Doing it now.

It is not this cap the culprit !
This cap can't send voltage on the pot, it wired to ground.

Chance it is one or more caps in the PI circuit the culprit.
 

[plexi50]

12.20vdc Circled in red.  Presence pot wire.

[Latole]

12.20vdc Circled in red.  Presence pot wire.

Show where in schematic.
Easiest to see and trouble shout

[plexi50]

12.20vdc Circled in red.  Presence pot wire.

Show where in schematic.
Easiest to see and trouble shout

Doesn't show any voltage on schematic. The 6G4 Schematic has a lot of differences. Let me look at that one.

[Latole]

12.20vdc Circled in red.  Presence pot wire.

Show where in schematic.
Easiest to see and trouble shout

Doesn't show any voltage on schematic. The 6G4 Schematic has a lot of differences. Let me look at that one.

Showing the right schematic you use is a must to help you
The Presence Control is same on the schematic I show:  6G4A the one you built

[plexi50]

This is  6G4A amp schematic i am using.

[Latole]

This is  6G4A amp schematic i am using.

Same as the one I show on answer #1
Show on schematic your answer #15

[plexi50]

Minute. Having computer problem. right back

[plexi50]

I got a problem here with my computer trying to crop the schematic. This is the layout.

[plexi50]

I am just going to wait until the Preamp tubes get here. This is crazy 2025 for sure!                                                                     Edit: Do i win the Lottery for 2025?

[SEL49]

Having DC voltage on that presence pot is absolutely normal. And this has nothing to do with the pop on your intensity pot.

[plexi50]

Having DC voltage on that presence pot is absolutely normal. And this has nothing to do with the pop on your intensity pot.

Yes i know this. I find it hard to believe i got a bad pot but i bought (2) of them and measured them both. The same. I will kick myself after i find it to be a bad tube which i suspect can be the only thing else it could be. I mean this is pretty elementary. Believe me the last thing i had to do was replace all those Astrons. They were all that bad and corroded on the terminal solder connections. Sucks!

[Latole]

Having DC voltage on that presence pot is absolutely normal. And this has nothing to do with the pop on your intensity pot.

I may be wrong, a voltage may come from negative feedback resistor

[plexi50]

Having DC voltage on that presence pot is absolutely normal. And this has nothing to do with the pop on your intensity pot.

I may be wrong, a voltage may come from negative feedback resistor

Yes i was going to mention earlier that the Presence pot is tied to the Negative feedback resistor.The main thing is the amp sounds Stellar minus that wisp when turning the intensity pot.

[SEL49]

Having DC voltage on that presence pot is absolutely normal. And this has nothing to do with the pop on your intensity pot.

I may be wrong, a voltage may come from negative feedback resistor

Wrong again. The 56K NFB resistor is connected to the OT secondary/speaker. Only AC signal voltage coming from that direction.

The DC voltage is due to the cathode current through the tube. The current path is through the 820Ω, 6800Ω, and 4700Ω to ground. There is 32V on the cathode. Doing the simple math shows there will be 12.2V on the top side of that 4700Ω. The presence pot is also connected to that 4700Ω so the 12.2V will be felt on all three lugs of the presence pot. The .1µF cap blocks dc current from flowing through the pot, so there is no pot scratchiness when the pot is turned.

[SEL49]

The main thing is the amp sounds Stellar minus that wisp when turning the intensity pot.

What happened to the loud pop? That pop is caused by a faulty pot. The wiper is losing contact to the pot resistor element at each end of rotation. A squirt of DeOxit may fix it. Weber sells a 10M-RA pot.

[plexi50]

The main thing is the amp sounds Stellar minus that wisp when turning the intensity pot.

What happened to the loud pop? That pop is caused by a faulty pot. The wiper is losing contact to the pot resistor element at each end of rotation. A squirt of DeOxit may fix it. Weber sells a 10M-RA pot.

The 10MRA pot came from Weber a few days ago. I bought (2) of them. I thought no way could a new pot be that bad off the shelve. So i sprayed the pot with deoxit and it made no difference. So let me change out the pot for the other new pot from Weber and i will get back in about 30 minutes.

[plexi50]

Second Weber 10MRA pot installed. No difference! Dang!

[plexi50]

Dam i see what i did wrong. The caps polarity is wrong. If it is polarized that is.  Got to swap ends to ground. Will see in a few minutes.

[plexi50]

Unreal! No difference. Same old whisping!

[AlNewman]

Check the DC voltage at the top of the cap, as well as the top of the pot.  If the cap's bad there will be a voltage drop across the pot.

[plexi50]

Check the DC voltage at the top of the cap, as well as the top of the pot.  If the cap's bad there will be a voltage drop across the pot.

Intensity pot in Off/ CCW Position  -22.2mv top of cap. -22mv pot ground DC setting.This is really irritating. I have tried other caps and another 10MRA pot with no difference.

[plexi50]

Put new tubes in the amp and no change with the intensity pop. What else could this be? I have never run across something like this all parts being the correct values? I have triple checked every cap,wire, voltages etc. I can see the power tubes light flicker for a second when it pop's.

[glass54]

This is "WAY OUTSIDE THE BOX"
If you can put your Oscilloscope on the output, Vertical scale <1 Volt, Horizontal 1mS, try the turning of the pot. Could it be oscillating in PA at many 100's kHz?? (even low MHz) and you're interrupting and thus power tubes flicker. You may have to try it a few times while extending the Timebase out to 1uS/div.
As I said, its way out there!
Your Crazy fault is starting to stress us out, we need this conquered!
Don't give up
Kind regards
Mirek

[plexi50]

This is "WAY OUTSIDE THE BOX"
If you can put your Oscilloscope on the output, Vertical scale <1 Volt, Horizontal 1mS, try the turning of the pot. Could it be oscillating in PA at many 100's kHz?? (even low MHz) and you're interrupting and thus power tubes flicker. You may have to try it a few times while extending the Timebase out to 1uS/div.
As I said, its way out there!
Your Crazy fault is starting to stress us out, we need this conquered!
Don't give up
Kind regards
Mirek

I understand. I am myself beyond stressed out. Cant' give up. That's not in me. All is always revealed.

[punkykatt]

Does the pop start at the wiper of the intensity pot? or is it created downstream? upstream?

[plexi50]

Does the pop start at the wiper of the intensity pot? or is it created downstream? upstream?

The Pop is created only when the pot is at it's full end of travel. CCW or full CW. Anywhere else in between it functions fine and their is no pop or noise of any kind. I have discovered something really strange. 300+ volts that are joined by a single blue wire from the PI area to the 1st .05 cap on V2. WTFIGO?????  A Plate wire feed for Edit: V2????????????????????????????????????????????

[punkykatt]

according the layout that blue wire from the 220k at the PI is supposed to be connected to V1 pin 6 down stream of the .05 cap marked A on the layout.

[SEL49]

I have discovered something really strange. 300+ volts that are joined by a single blue wire from the PI area to the 1st .05 cap on V2. WTFIGO?????  A Plate wire feed for Edit: V2????????????????????????????????????????????

Nothing unusual here. Look at the schematic to understand why there is 300V on that blue wire.

[plexi50]

according the layout that blue wire from the 220k at the PI is supposed to be connected to V1 pin 6 down stream of the .05 cap marked A on the layout.

Yes that is correct and clearly marked (A) on the layout.I'm just getting frustrated by having a new pot changed out twice now with new coupling caps etc, and where is this voltage on the intensity pot coming from? Resistors in the entire amplifier are in spec. I have used my Hoffman noise detector for fractured resistors and possible noise from them. This is a whole new ball of real Intensity! When a pot make noise it's either dirty and can be cleaned. Or it is replaced. The Cap on the pot is Good! I haven't used my Tektronix 465B in a decade.

[shooter]

CCW or full CW.

autopsy the pot, bet ya find poor Quality control, welcome to the 21st century

and where is this voltage on the intensity pot coming from?

Nothing unusual here. Look at the schematic to understand why there is 300V on that blue wire.

[plexi50]

CCW or full CW.

autopsy the pot, bet ya find poor Quality control, welcome to the 21st century

and where is this voltage on the intensity pot coming from?

Nothing unusual here. Look at the schematic to understand why there is 300V on that blue wire.

Ya know you are probably 100% right on the pot's build quality. I will take the first one i installed and then removed to do an autopsy on. Good thing i bought 2 of them but then again not really maybe. BACK IN A BIT.

[plexi50]

Looks funky to me. Their is a center contact ring that runs around the wiper surface but their is a small gap on the wiper surface that seems to be missing. Like 1MM at both CCW & CW end of rotation. Man all this Hoopla over a part that is junk. I am glad you pointed this possibility out because i am not yet in full DIMENTIA!!! Talk about your mind being pushed to a level of madness and in the end? Always turns out to be something very obvious or simple.

[SEL49]

I have that same cheap Weber 10M pot in my revibe. Weber was the only choice I had. Luckily my pot works OK. I suggest trying this CTS 3M-RA pot. I don't think the value difference will be noticeable. The CTS should be much higher quality.

You may want to try this simple, easy band aid trick to see if it kills the pop. Worth a try. No guarantees. Solder two 10M resistors across the pot like this...

[plexi50]

I have that same cheap Weber 10M pot in my revibe. Weber was the only choice I had. Luckily my pot works OK. I suggest trying this CTS 3M-RA pot. I don't think the value difference will be noticeable. The CTS should be much higher quality.

You may want to try this simple, easy band aid trick to see if it kills the pop. Worth a try. No guarantees. Solder two 10M resistors across the pot like this...

AM I STILL GOING TO HAVE THE LEFT TERMINAL GROUNDED? I will try a 3MRA pot and see what happens after the 10M r trick.
At this point i have nothing to loose. I have never had a problem before like this with a pot other than it being dirty or plain bad. That's usually a quick fix. It's problems like these that drive me INSANE!!!!!!!!

[plexi50]

I took the original 10MRA Pot apart and cleaned it with 91% rubbing alcohol using Q tips and my air compressor hose outside. There is 98% improvement. I sprayed it before but that did nothing. It needed a physical wiping  / cleaning lightly. Very careful as to not wipe the wiper carbon path away. The tiny metal tang that rotates near the middle of the pot is what i was mainly trying to get as clean as possible.It works well now. Their is no maker name on the pot i bought from Weber..