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#1
The volume pot in my amp is acting strange.

It's quietest at 3/10. When I turn it down towards 1, the volume increases. Above 3, it works like it should.

I've taken the pot out of the amp, taken it apart, not found anything, and put it back into the amp. The problem was still there.

It's a 1M log CTS pot. This particular pot is 44 years old, as is the amp it's in. I'd strongly prefer repairing it (if possible) instead of replacing it. This amp isn't a museum piece, so I'll replace if it's really needed. I've considered temporarily replacing the pot, just to see if the problem lies with the amp; but I haven't tried it yet.

Does anyone have any idea what the problem is?

EDIT: I checked the code: It's not a CTS. It's a Stackpole, made 4th week of 1964.
Last edited by mr_hankey at Feb 17, 2008,
#2
You tried blowing a bit of compressed air and contact cleaner into it? Always worth a try.
The Laney Thread are big and clever. No exceptions.
#3
Quote by MrCarrot
You tried blowing a bit of compressed air and contact cleaner into it? Always worth a try.


No. I took the pot apart, and I may have wiped the carbon track clean, but I can't remember (it was a while ago).

Thanks, I'll see if I have any of those things.
#4
check it with a multimeter and see if the resistance changes as it should
#5
Quote by s.r.v.
check it with a multimeter and see if the resistance changes as it should


I took the pot out of my amp. According to my DMM, there's nothing wrong with it.
#7
Quote by s.r.v.
thats odd. maybe you're crazy?


How can I measure that with my DMM?
#8
****. I took the pot apart, wiped away some of the excessive grease, then put it back in the amp. The problem persists.
#9
just try another pot in its place. it doesn't have to be perminent and see if the problem stays.

and im not sure about the dmm testing
#11
Are you sure there isn't a dead spot down near 1? The only thing that comes to mind is that the carbon is worn away, or has been damaged down there. Since you haven't found anything mechanically wrong, you're probably going to have to replace it ;-/
#12
I replaced the pot! I put a brand new, tested Alpha pot in.

The problem is still there, but it has changed a bit. The quietest (but not silent) spot is now at 2.
#13
if i was you...i'd be startled :s

Get it fixed, or im forced to adopt your amp for mistreatment
#14
Here are the schematic and the layout, hoping that someone can come up with something:



#15
Quote by mr_hankey
The volume pot in my amp is acting strange.

It's quietest at 3/10. When I turn it down towards 1, the volume increases. Above 3, it works like it should.
Quote by mr_hankey
I replaced the pot! I put a brand new, tested Alpha pot in.

The problem is still there, but it has changed a bit. The quietest (but not silent) spot is now at 2.
Okay, so you know the pot isn't at fault. And the difference between the knob position is just the difference in taper.

Is your wiring really like the layout in the Bassman drawing? I'm inclined to think maybe it isn't.

If the "Middle" resistor (820 for the Bass channel, 6800 for the Normal channel) is connected to the ccw lug of the volume control, I can envision a problem.

To reduce noise, sometimes these amps are wired just that way, for a reason. You eliminate ground loops. Then you only have 1 place where the tonestack and volume control are grounded ... at the eyelet board. A break that ground wire, I think, will show the same symptoms you've described.

... and the "quiet" spot, is still very loud.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#16
Quote by SomeoneYouKnew
Okay, so you know the pot isn't at fault. And the difference between the knob position is just the difference in taper.

Is your wiring really like the layout in the Bassman drawing? I'm inclined to think maybe it isn't.

If the "Middle" resistor (820 for the Bass channel, 6800 for the Normal channel) is connected to the ccw lug of the volume control, I can envision a problem.

To reduce noise, sometimes these amps are wired just that way, for a reason. You eliminate ground loops. Then you only have 1 place where the tonestack and volume control are grounded ... at the eyelet board. A break that ground wire, I think, will show the same symptoms you've described.

... and the "quiet" spot, is still very loud.


You're very right thinking that it might be different from the layout. This was one of the last 6G6-Bs, and some of the components values were already from the next model (AA864). 'Unfortunately' though, the mids resistor is grounded against the back of the bass pot, as it should be.

'Very loud' is taking it a bit far. The lowest volume I can get on the normal channel is equivalent to 2.5 on the bass channel (a pretty useless statement, seeing as the treble setting affects the overall volume nearly as much as the volume knob itself, but you get the point).

I've done a bit of 'chopsticking' (poking at wires) while it was on to see if I could find anything; but I didn't. I did, however, notice that tapping on the eyeletboard is well amplified.
#17
Hey mr_hankey,

Sorry I was away for a while.
I have some strong inclinations about the source of this problem.

If we look at the schematic and try to find a way to make it fit the symptoms, it seems impossible, doesn't it?

If the ccw lug of the volume control is at ground, minimum MUST be the the quietest.
But it isn't. So it seems there must signal of some sort on that lug. I think you'll do better to power off, and put the chopsticks away. For this one, an ohmmeter will be your best friend.

I don't know exactly HOW, but there seems to be a connection from the mid resistor where it's soldered to the back of the Bass pot, and the ccw lug of the volume control, where it's connected to the back of the volume control.

It also would seem that this connection can't really be at ground. This is implied, because there is signal at this point. Somehow, some way, there must be a ton of resistance from these points to real ground. We wouldn't be talking a few ohms. We're talking thousands. That will be easy to find with an ohmmeter.

The other thought I had, was there could be signal getting through to CATHODE of the second half of that 12AX7. This seems less likely, though. It would probably have to be in the same phase as the plate of the first half. And the cap on the cathode should shunt the signal to ground. I dunno. Maybe if the ground connection of the cathode resistors of those two stages were tied together, but had a large resistance to ground, you might be able to make your symptoms happen. Again, an ohmmeter will be your friend.

2 plus 2 ALWAYS equals 4. The only way to make it equal 7 is to add 3 to the equation somewhere. So all we need to do, is find that somewhere.

Good luck,
SYK
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#18
What's the latest on this one, Hankey?
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#19
Quote by SomeoneYouKnew
What's the latest on this one, Hankey?


I wasn't going to post until I had tried everything I could come up with, but yeah; the problem is still there.

I checked every ground connection in the normal channel. All good, except one of the cathode resistors, which was showing a 0.4 ohm resistance. I was going to redo that connection, but I didn't get to that yet.

Anyway, I thought it would be a good idea to make sure all the pots were properly tightened against the grounding plate/chassis (despite my DMM telling me it was fine); so I took them out and had a close look at at the connections. I found nothing, so I put them back in, but well-tightened this time. Measured again; same result.

Another idea I had was to check for shorts at the tube sockets, but I haven't done that yet, either.

Do you think there could be some sort of cross-talk between wires?
#20
Quote by mr_hankey
Do you think there could be some sort of cross-talk between wires?
I'd think any crosstalk would be from melted insulation and a resistive path. Capacitive coupling seems like it wouldn't be enough to make that large of an effect.

The connections at the tube socket seem too far away, on the opposite side from each other, for carbon tracking to cause this. But maybe on the bottom of the tube itself, or even internally.

Have you tried swapping tubes? The 3rd 7025/12AX7 would be where the problem would be. I'd be especially mindful of anything that looks suspicious around pin 8 (the cathode) of that tube.

Also, next time you power up, check the DC voltages at both cathodes of that tube. They should be around 2 volts.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#21
Thanks!

I'll take a look tomorrow; 1 AM is a bit late for that kind of stuff...
#22
I checked the cathodes: first triode 1.7V, second 1.9V. Looks fine to me. I scared the **** out of myself when the probe touched the plate pin.

I've swapped tubes before, so that isn't the problem. I did notice that the tube in socket 3 was the loosest one, so I tightened the socket. Now it's the tightest one. It made no difference.

I checked for carbon traces, and made sure none of the pins were too close together, but that all looked fine as well.
#23
Some pics:


Controls: presence, bass, treble, volume.



Normal channel inputs; normal channel's tube socket in the middle.



If you need pics from a specific angle, just ask.
#24
I have a theory!

Because the volume is at minimum, when the control is rotated slightly clockwise from 1,
some of the signal must be getting to the second stage.
AND it has to be out of phase with the signal getting there from the volume control.

I think it may be getting there through the anode circuits of the 2 sections of the tube.


In the schematic, I've drawn an arrow to one of the filter caps.
This cap primarily affects the Normal channel.
The Bass channel has another cap that de-couples its stages from each other.



The input signal has Phase A

The output of the first stage has Phase B. It's inverted.

That goes through the second stage and is inverted again.
We'll call that Phase A', because it's almost the same phase as the input signal.

"b" is a small amount of signal that doesn't belong there.
It's supposed to be shunted by the filter cap.
But if the cap is open, it's not doing it's job.
Some of the Phase b signal is getting to where A' is marked.
When the volume is at full ccw, there is no Phase A' signal,
only Phase b.

When we rotate the volume control clockwise, there is a point where
Phase A' is large enough to ALMOST cancel Phase b.

Why doesn't it cancel completely?

Actually it does.
But not for all frequencies, at the same rotation of the volume control.
The tonestack affects the phase and frequency response of the Phase A' signal.
The mids are scooped.
So the mids will cancel with a higher setting of the volume control than the Bass and Treble.

Am I 100% certain this is the cause of the problem?

Hell no. But it does seem to fit the symptoms.



I've drawn a Red arrow to indicate the part of the circuit where power supply node C is, on the original schematic. If you connect a 20uF cap between that point and ground, I think your problem will go away.



44 years is a looooooong time for an electrolytic cap.
I don't think they owe you anything, at this point.
Those boys have served their duty.
If you determine this is the source of the problem,
maybe it's time for a cap job?


Good luck,
SYK
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#25
That's the thing. I did give it a cap job. Replaced all of them with shiny new Sprague Atoms. I didn't test them, so it's very possible that one wasn't working from the start, or a solder joint is bad. I can't remember if the problem started immediately after the cap job (don't think so), but definitely within a few weeks.

Everything you're saying makes total sense. Again, it's the middle of the night here, so I can't check things out now.

I was actually reading about the de-coupling function of the filter caps yesterday, I should have thought of this myself!

Anyway, even if this isn't the problem (but I get a strong feeling it is), thanks a lot for your help.
#26
The culprit!



You were totally right. The cap you pointed out is faulty. There is no continuity across it.



So, if I were to fire up my amp without it, I should get exactly the same result. I'll just order a new one, and fix it.

Thanks again, I owe you a beer!
#27
Quote by mr_hankey
You were totally right. The cap you pointed out is faulty. There is no continuity across it.
Well ...
As much as I'd like to crack open the beer and celebrate with you, we should hold off a bit longer. A proper cap won't show any continuity after the first second or two. The resistance should look moderate at first, but quickly move to infinite, as the cap charges up from the current supplied by the ohmmeter. The time it takes to charge to the same voltage as the meter can be really short. It all depends on the design of the meter.

So let's wait until you get the new cap in, to be certain. I still feel confident, though.

* SYK buys pretzels *
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#28
Quote by SomeoneYouKnew
Well ...
As much as I'd like to crack open the beer and celebrate with you, we should hold off a bit longer. A proper cap won't show any continuity after the first second or two. The resistance should look moderate at first, but quickly move to infinite, as the cap charges up from the current supplied by the ohmmeter. The time it takes to charge to the same voltage as the meter can be really short. It all depends on the design of the meter.

So let's wait until you get the new cap in, to be certain. I still feel confident, though.

* SYK buys pretzels *


You may be right, but this cap definitely measured differently than the other five. Once the reading settled on the other caps, it measured around 15M; while it never showed any continuity at all on this one.

Either way, I've taken it out, and ordered a replacement. It should get here in a few days.
#29
Quote by mr_hankey
but this cap definitely measured differently than the other five.
That's very encouraging. We'll know in a few days. If it works, then crack out the cold ones, and we'll start tossing around ideas for the Silverface. I already have a few ideas.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#30
Is the amp a tube amp?
Have you replaced the tubes at all or biased them at all if so?
#31
^Right...


I got a new cap today; installed it; and the problem wasn't gone.
I was just about to get very pissed off, when I noticed how the caps were grounded: underneath the board. I lifted the board, and there it was: a break in the wire. Soldered in a new wire; and it worked.

So, it seemed the problem was caused by two things (since I am 99% sure the cap was faulty). Thanks again to SYK.
#32
woohoo, party!!!

I brought some hot, soft pretzels.



Pour the beer, brother.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#35
Quote by mr_hankey
*pic so glorious that reposting it just seemed wrong*

I can taste it from here. Cheers!

What's the topic of conversation? 70's Bassman rebuild or Franken-amp?
Quote by kurtlives91
Glad it is working....SYJ your a fuc king genious......you too Hankey heheh
That's SYK. Did you get a head-start with the beer? lol

Pace yourself, man.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#36
this was like house. but with vintage fender amps, less drugs and less meanness.
my name is matt. you can call me that if you like.
#37
Quote by SomeoneYouKnew
What's the topic of conversation? 70's Bassman rebuild or Franken-amp?




Have you managed to overlook my sig?
#38
Quote by mr_hankey
Have you managed to overlook my sig?

Not at all. I'm aware of brownie.

I remembered you saying you were going to get a troubled '70s Bassman.
Is that deal still in the works?
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#39
Quote by SomeoneYouKnew
Not at all. I'm aware of brownie.

I remembered you saying you were going to get a troubled '70s Bassman.
Is that deal still in the works?


Oh right, yeah. It's little more than 'troubled'.





It's already been hacked up, so I don't have to worry about that. I think it was used for a Vibroclone.
If I'm lucky, the tube sockets; jacks and switches might still be usable.
#40
Quote by mr_hankey
Oh right, yeah. It's little more than 'troubled'.

It's already been hacked up, so I don't have to worry about that. I think it was used for a Vibroclone.
If I'm lucky, the tube sockets; jacks and switches might still be usable.
Holy Moly!

Hacked doesn't begin to describe it. More like gutted. No iron? I'm thinking the channel on the left could be an Ampeg with a Bax tone network. The right, more like a Fender or Trainwreck and the power amp something like a Deluxe. But that really depends on what kind of iron you might have available. Any of that sound interesting?
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
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