Fender Studio Bass Amp- Hum only when the master is at zero

[pbman1953]

The amp has been great but I've noticed something for while.


There's a Master and a Volume (Gain). The volume has no effect over this.
There's a hum only when the Master is from 1-2 1/4. after that it's silent.
Any thoughts?

[rumpus]

I assume what you're calling "Master" is the one labeled "Output Volume" on the schematic, right?

How does that pot connect to ground? Not how the schematic shows it, but the way the amp is actually laid out? Could that ground have some hum on it relative to the PI input ground?

Does that pot control the volume smoothly, or is there a discontinuity in the level at low settings? Does it go all the way to zero volume or just "much lower"?

[pbman1953]

Yes, to being the Output control.


I'd have to open it up to check the ground


Control wise seems to be very smooth and runs in either direction equally

[tubeswell]

Could be


1) Bad MV pot


2) bad ground connection for the MV pot (oxidation between ground plate and whatever)


3) Bad/hummy preamp tube (maybe a stage before the MV?) - easy to try tube swaps first


Or something else like that


Some testing will be helpful

[pbman1953]

I'll pull it out of the case tomorrow and have a look around and report bsck


Thanks

[rumpus]

Pictures would be nice, if you have a way to do that. Someone who's seen the inside of more of these amps than I have might spot something.

[pbman1953]

Will do!

[pbman1953]

I may have found an issue

2 pictures sent-

A- pot rear connections-

Light green wire to center

Black coax wire center to right  lug

Black coax's shield to left lug- white wire goes to the 100 ohm resistor > ground


B. Coax connection to the board-


The white center is soldered to the board


The shield was snipped off  because it was grounded to the 100 ohm resistor

[rumpus]

Connecting the shield only at one end is correct (assuming the end where it's connected is a good ground). If it were grounded at both ends, there would be a ground loop.

It looks to me like there's a white wire connected to the same lug of the pot as the shield. The other end of that wire connects to the board at the end of a 100 ohm resistor along with some other things not visible in the picture. That should be ground (look at the schematic). How does that ultimately get grounded?

Measure the resistance* from that pot lug to other grounds in the amp. In particular, measure resistance from there to the ground point of the PT high voltage secondary center tap ground, and to the input jack ground.

Also, find the ground for the last preamp stage (V3A, the ground for the 1500 ohm cathode resistor). Where does that get grounded?

The point of all this inspection and measurement is to find a way that noise could be introduced at the ground reference for the master volume pot, relative to the signal ground somewhere in the power amp.
* I'm relatively new here, and don't know people's skill levels well - I'm assuming that you know how to make sure the amp is safe to work on. Probably a good assumption, since you've opened it up!

[pbman1953]

It looks to me like there's a white wire connected to the same lug of the pot as the shield. The other end of that wire connects to the board at the end of a 100 ohm resistor along with some other things not visible in the picture. That should be ground (look at the schematic). How does that ultimately get grounded? There's a black wire going from the resistor to a power tube's ground lug, That lug runs to a welded factory chassis ground point

Measure the resistance* from that pot lug to other grounds in the amp. In particular, measure resistance from there to the ground point of the PT high voltage secondary center tap ground, and to the input jack ground. That ground end of the 100 ohm resistor zero's out all over the chassis. The input jack grounds out with that resistor

Also, find the ground for the last preamp stage (V3A, the ground for the 1500 ohm cathode resistor). Where does that get grounded? To Input #2's ground lug

The point of all this inspection and measurement is to find a way that noise could be introduced at the ground reference for the master volume pot, relative to the signal ground somewhere in the power amp.
* I'm relatively new here, and don't know people's skill levels well - I'm assuming that you know how to make sure the amp is safe to work on. Probably a good assumption, since you've opened it up!

[rumpus]

Well, I'm running out of simple, easy explanations.

Tubeswell mentioned one thing that's easy to check: a possible bad master volume pot. What's the resistance from the wiper to ground with it all the way off, and at the point where the hum starts to go away? Does that resistance vary smoothly? Could it be that turning it all the way down doesn't actually ground the wiper like it's supposed to?

Also, does adjusting the bias balance affect the hum at all? I don't see how that would cause hum that's affected by the master volume, but identifying the source of the hum might give a clue about the problem you're trying to solve.

[pbman1953]

Well, I'm running out of simple, easy explanations.

Tubeswell mentioned one thing that's easy to check: a possible bad master volume pot. What's the resistance from the wiper to ground with it all the way off, and at the point where the hum starts to go away? Does that resistance vary smoothly? Could it be that turning it all the way down doesn't actually ground the wiper like it's supposed to?


Let me refresh by saying that the hum is from 1-2 1/4 when 1 is the all the way down. After 2 1/4  the hum goes away.

At 1, there is zero reading on the meter.

Resistance starts right before 2 at .3 on the 200K scale. At 2 -1.0 , at 3 -10.9 , at 4 - 20.9, at 5 - 32.6, 6- 42, 7-52.8, 8-63.8,9- 71.5, 10- 73.2

Does this look to be smooth?



Also, does adjusting the bias balance affect the hum at all? I don't see how that would cause hum that's affected by the master volume, but identifying the source of the hum might give a clue about the problem you're trying to solve.
counter clockwise, only, on the bias pot does reduce the hum somewhat.  But still not gone like at 2 1/4.

Thanks for your help!

[tubeswell]

May be take the knob of the MV pot and tighten the nut ever so slightly.

[pbman1953]

May be take the knob of the MV pot and tighten the nut ever so slightly.

I did, but it was very secure. No change. I did some poking with a wooden chop stock and this thing seems solid. No reactions

[tubeswell]

Maybe take the MV pot right out and inspect behind all surfaces for oxidation and clean up with isopropyl etc

[pbman1953]

I'll do that but while it's out I could test with another pot but I don't have another 100k Linear. Only 250K linear or a 25k

[tubeswell]

I'll do that but while it's out I could test with another pot but I don't have another 100k Linear. Only 250K linear or a 25k

250k will be fine. With attenuation in a volume control, it’s a lot more about the voltage divider than about the overall resistance.

[pbman1953]

Sadly no change with the new pot

[rumpus]

It's hard to tell for sure from a single picture, but back in #7 it looks like the pot doesn't depend on its mount point on the chassis for its ground; there's a wire to the ground side of the 100 ohm resistor and it doesn't look like the ground tab of the pot is soldered to the case. I think this should (in theory, for what that's worth) be better than Fender's old assumption that the entire chassis was a good ground.

With the master volume pot unmounted from the chassis, it would be interesting to check resistance between the pot case and ground. If there's an unintended connection there, it would create a ground loop. In fact, the MV pot should work as intended while it's unmounted because of the ground wire.

[pbman1953]

It's hard to tell for sure from a single picture, but back in #7 it looks like the pot doesn't depend on its mount point on the chassis for its ground; there's a wire to the ground side of the 100 ohm resistor and it doesn't look like the ground tab of the pot is soldered to the case. The lug is not grounded to the pot case




I think this should (in theory, for what that's worth) be better than Fender's old assumption that the entire chassis was a good ground. So you suggest to ground this like a guitar volume pot  with the lug to the chassis?

With the master volume pot unmounted from the chassis, it would be interesting to check resistance between the pot case and ground. Unmounted , no reading from pot case to ground


If there's an unintended connection there, it would create a ground loop. In fact, the MV pot should work as intended while it's unmounted because of the ground wire. Unmounted the pot acts the same

[rumpus]

It's hard to tell for sure from a single picture, but back in #7 it looks like the pot doesn't depend on its mount point on the chassis for its ground; there's a wire to the ground side of the 100 ohm resistor and it doesn't look like the ground tab of the pot is soldered to the case. The lug is not grounded to the pot case

I think this should (in theory, for what that's worth) be better than Fender's old assumption that the entire chassis was a good ground. So you suggest to ground this like a guitar volume pot  with the lug to the chassis?

Sorry for expressing this point unclearly. I was trying to say that the way it is currently wired should be better than Fender's old way of soldering the ground lug of the pot to the case and depending on the chassis ground. I don't suggest changing the way the pot is currently grounded.

Turning the pot all the way down means the coupling cap from the pot to V4B grid is grounded to the same ground point the PI uses. There should be zero signal at that grid. I'm casting about for a new hypothesis... anyone have any ideas?

[pbman1953]

It's hard to tell for sure from a single picture, but back in #7 it looks like the pot doesn't depend on its mount point on the chassis for its ground; there's a wire to the ground side of the 100 ohm resistor and it doesn't look like the ground tab of the pot is soldered to the case. The lug is not grounded to the pot case

I think this should (in theory, for what that's worth) be better than Fender's old assumption that the entire chassis was a good ground. So you suggest to ground this like a guitar volume pot  with the lug to the chassis?

Sorry for expressing this point unclearly. You were not unclear, I'm being careful. I was trying to say that the way it is currently wired should be better than Fender's old way of soldering the ground lug of the pot to the case and depending on the chassis ground. I don't suggest changing the way the pot is currently grounded.

Turning the pot all the way down means the coupling cap from the pot to V4B grid is grounded to the same ground point the PI uses. There should be zero signal at that grid. I'm casting about for a new hypothesis... anyone have any ideas?




After installing the Output 250k  pot, with the volume/Gain all the way down, I noticed that the hum was still there. At full rotation, there was some hum starting around 9 1/2-10. So between 2 1/4-9 1/2 it was whisper quiet. After re-installing the 100k back, the hum was still there but the full rotation hum was less that the 250k.

The question is if I have a problem that deteriorates the amp? The amp already sounds amazing and I guess I could live with the idiosyncrasy of the hum. I'd simply stay away from the problem areas. If you feel that this could  have or lead to problem then I'd keep going.
Any thoughts on that?

[rumpus]

I don't see any reason to think there's anything going on that could damage the amp. It's just... odd.

There might be a clue in your latest result - hum when the master volume is all the way up. Of course, this could be hum from a completely different source in the preamp. It didn't do this before (with the 100K pot)?

Is the hum 60 Hz (power line) or 120 Hz (B+ ripple)?

[pbman1953]

I don't see any reason to think there's anything going on that could damage the amp. It's just... odd.

There might be a clue in your latest result - hum when the master volume is all the way up. Of course, this could be hum from a completely different source in the preamp. It didn't do this before (with the 100K pot)?

Is the hum 60 Hz (power line) or 120 Hz (B+ ripple)?  120

[pbman1953]

A new discovery-

I decided, since the amp was already accessible, that I'd spot check the bias. First I wanted to pull tubes in certain orders to see what would happen with the hum.

I think the bias is running a bit hot, hence the hum. From the forum's advice years ago I installed a 10k pot with a 15k resistor to ground in able to adjust the bias feed to the output tube matching pot (balance pot). What should I change the resistor to in order to cool back the setting? I'm at the max counterclockwise on the pot. It came down some from the max setting , but not enough

[rumpus]

Can you clarify where you added the 10k pot with a 15k resistor to ground? Is that from the wiper of the bias balance pot to ground, in parallel with the 2.7k 2W shown on the schematic? If so, that adjustment can make the bias hotter (more idle current) by decreasing the bias voltage, but not cooler; and it will make a rather small change in any case. That's a pretty stiff bias supply! I'd like to make sure I correctly understand what you have done already before I recommend any modification.

BTW, this schematic seems just a little bit more legible.

[pbman1953]

The addition was made here in the forums months back. Inserted right before the Output balancer pot




Is there a way I can tweak the 220 ohm resistor to get closer to the -67 volts that shown coming from the PT?

[rumpus]

So the 10K pot + 15K is from the center tap of the balance pot to ground? That places it in parallel with the 2.7K from the wiper of the balance pot (when balance is centered) and can only decrease the negative bias voltage (increase idle current, making the bias hotter). I think this should result in bias adjustable about -52V to -55V, hotter than the -61V on the schematic. Of course, that's in theory, what do you measure in practice?

-61V is pretty cold - take a look at the plate curves for a 6L6GC. Even -52V should put the output tubes at only 30-35% plate dissipation.

Rather than changing the 220 ohm resistor, consider increasing the 2.7K from the balance pot wiper to ground. 3.3K 2W ought to give you a range of approximately -61V to -65V. A 3.0K 2W would bring hotter bias into range.  Those are very rough approximations, if you do this please post the actual measured bias range.

I'd be interested to see the discussion thread for this change, got a link to that handy? There might be some point about this I'm missing.

[rumpus]

Oh, haha, you started a new thread and posted the actual schematic there. I didn't understand your modification correctly (I thought your 10K pot and 15K resistor were in series to ground), so my previous post was all wrong 

[pbman1953]

Oh, haha, you started a new thread and posted the actual schematic there. I didn't understand your modification correctly (I thought your 10K pot and 15K resistor were in series to ground), so my previous post was all wrong 

So changing the 220 to 100 was ok in the end , for you? I did get the results I was looking for

[rumpus]

I can't think of a reason why there would be a serious problem with it. It does raise the corner frequency of the last RC filter (the last 70µF cap) from ~11 Hz to ~22 Hz. Since it's filtering 120 Hz ripple, that shouldn't make much difference.

Did it affect the hum when the master volume is at zero?

[pbman1953]

Not sure but you may be mixing up the controls.


On the face of the unit, on the left next to the input jacks, there's one labeled "Volume". It's also shown at the beginning stage of the schematic. This control is after the first half of V1 a 12ax7.This is a gain control and has no bearing on the hum.
The "Output" volume  control is the master and shown in the middle of the schematic. This is the only control that exhibits the hum when all the way down.

[tubeswell]

If you take the MV pot out altogether and ground the wire that otherwise would go to the pot wiper, do you get the hum? (Because if you do, the hum won’t be being caused by the MV pot)

[pbman1953]

If you take the MV pot out altogether and ground the wire that otherwise would go to the pot wiper, do you get the hum? (Because if you do, the hum won’t be being caused by the MV pot)

Not knowing if you read back, but I replaced the MV pot and there wasn't a change. Do you feel I should till do your test?


After some listening, and paying attention to when the hum starts or stops , it sounds like switch being activated. So going up from 1 (all the way down) to 2 1/4, at 2 1/4 it's like switch sound to when the hum stops. Then same in reverse. 

[tubeswell]

If you take the MV pot out altogether and ground the wire that otherwise would go to the pot wiper, do you get the hum? (Because if you do, the hum won’t be being caused by the MV pot)

Not knowing if you read back, but I replaced the MV pot and there wasn't a change. Do you feel I should till do your test?

After some listening, and paying attention to when the hum starts or stops , it sounds like switch being activated. So going up from 1 (all the way down) to 2 1/4, at 2 1/4 it's like switch sound to when the hum stops. Then same in reverse.

I’m not sitting where you are with it in front of me, so I’m suggesting a process of elimination rather than changing parts. So yes, disconnect the wire from the pot wiper and ground it to the signal ground return, and see what happens.
And while you’re putting the wire to ground, try grounding it at several different points on the ground return to see if that makes any difference- not suggesting it will. Its an issues-discovery method