fuzzy sound when playing notes

[casssax]

This is about the Realistic AF-12 mono block amp that I converted to a guitar amp.

It's a 12 watt, push-pull 6V6 with a very similar circuit to a Fender Princeton amp. (but with no reverb or vibrato)

I've been playing it for a while (couple of months) without problems but recently something strange started to happen.

When the volume controls are both set to about 4 or 5 it has almost no noise at all when not playing (good).
(I have one volume control after the tone stack like the Princeton, and then another before the 3rd gain stage)

But when I play a note there is a sort of fuzzy sound behind the note. Like a very slight distortion. The note itself does not sound distorted, it's more like a ghost behind the note.
Sometimes it is barely there and other times it gets more noticeable.

I tried removing the tubes and cleaning the sockets but that didn't make any difference. I also tried using different tubes (I was able to change all the tubes except the rectifier tube) and that didn't make a difference either.

The strange thing is that after I've played for 5 or 10 minutes it almost completely goes away.

I thought I would post this to see if someone has had a problem like this and/or knows what the most likely culprit might be.

I'll try to get a sound clip up of the problem soon and can post more specific info about the amp if needed.

Thanks. 

[sluckey]

We need to see a schematic of your amp. A Princeton doesn't have a third gain stage. A Princeton Reverb does have a third gain stage ---BUT, it also has a 3.3M resistor just before that third stage.

If you have your second volume pot connected to the grid of the cathodyne phase inverter, that could be a problem. The pot would need the regular blocking cap from the previous tube plate and it also needs another blocking cak between the wiper and the grid of the cathodyne.

A look at your schematic will quickly show if the extra volume pot is connected correctly.

[Willabe]

But when I play a note there is a sort of fuzzy sound behind the note. Like a very slight distortion. The note itself does not sound distorted, it's more like a ghost behind the note.
Sometimes it is barely there and other times it gets more noticeable.

The strange thing is that after I've played for 5 or 10 minutes it almost completely goes away.

How old are your filter and cathode (K) bypass caps?

Try playing a B and Bb on the low E string when you 1st turn on the amp, do you hear the ghost note?

The caps might be dying and at 1st turn on their leaking and as they warm up they stop leaking so much and the ghost note goes away.   

[casssax]

Here is the schematic. I should have said it's like a Princeton Reverb without the reverb.

All the filter caps are new, but the coupling cap on the output of V1B is an old round TDK .01 cap.

That sounds like a good place to start. I'll replace it with a new one and see if it fixes the problem.

[Willabe]

All the filter caps are new,

Depending on where you bought the E-caps they might be unused and still old/dried up.

I don't think a leaking coupling cap will cause the problem you have. If it leaks dcv then there will be dcv on the next tubes input grid and mess up it's bias. This won't change after 5 to 10 minutes. To test if the cap is leaking dcv, unsolder/lift the grid end of the cap and hook up your meters red lead, set for dcv, to the caps free end, USE the test leads ALLIGATOR CLIP, black gator clip to ground, you want less than 1 dcv.     

Did you try playing a B and Bb on the low E string when you 1st turn on the amp, do you hear the ghost note? If the B+ filter caps are dying it will be heard pretty clearly on those 2 low notes.

[PRR]

Fuzz could be poor solder joint. If solder does not *WET* the metal, it can still give a 99.9% good mechanical joint. This will oxidize into a less-good connection. Sometimes a loud signal will burn-off the oxide for a while. And I would not bet that Realistic's supplier got every joint "perfect" (I have seen bad joints in fancier gear). There's also your mod joints; not saying your soldering is bad, but soldering to very old parts is hard to do, and it takes a few builds and fuzzes to get the proper aggressive attitude.

Jacks are also prone to oxidation fuzz. Speaker jacks will usually clear the oxide (temporarily) with some strong signal. 1/4" plug tips are classic. The switch-finger in insert jacks can be problematic.

[casssax]

I replaced the old coupling cap with a new .022 last night. It seemed to be better but the problem is not consistent so I'll have to play it a bit and see if it comes back.

I didn't hear the fuzz on a low B or Bb, but I do still hear a little bit of it when I play a low D or Eb on the E string or the A string.
(That seems to be the only frequency where it's happening now)


All the E-caps came from Mouser except for the can cap that is a JJ Electronic 40/20/20/20 µF. I'll have to look back to see where I bought that from.

I'll have to take a look at solder joints too. I've become progressively better at soldering while I was working on this so I'll check the oldest work I did first.

It seems like this problem became much more apparent after I turned every thing up to 10 the other day. 
I usually play with a pretty clean sound and use pedals for distortion, but I wanted to see what kind of distortion I could get from the amp itself.
Maybe I should be a little gentler with a 56 year old amp. 

[casssax]

I still need to go through and check the solder joints (hopefully this weekend). But something different happened today.

This morning I turned the amp on and it was quiet with no fuzz in the background, even on the D/Eb notes. It then started making a noise on it's own without me playing any thing. It was like a squelch sound that built up from barely there to quite noticeable. It lasted for about 30 second to one minute and then abruptly stopped. After it stopped I could hear the fuzz when I played the D/Eb notes again.

 

[John]

That almost sounds like a microphonic preamp tube. Can you have someone grab the small tubes while you play to see if that helps? They usually don't get too hot to do that (for a while)

[jojokeo]

I didn't hear the fuzz on a low B or Bb, but I do still hear a little bit of it when I play a low D or Eb on the E string or the A string.
(That seems to be the only frequency where it's happening now)

This oscillation can change freq when the tone and/or vol controls are changed.

I'll have to take a look at solder joints too. I've become progressively better at soldering while I was working on this so I'll check the oldest work I did first.

Chop stick every connection and don't be soft about it. If you hear anything at all, hit it again.

It seems like this problem became much more apparent after I turned every thing up to 10 the other day.

Yes, it can and will get more noticeable (and more irritating).


*always try easiest thing first - swap tube(s), then chop stick all connections. If not fixed then you have component, layout, shielding, or grounding issues.

[casssax]

I finally got around to opening the amp up to try to find the problem.

It has hardly been noticeable until recently. When I would turn the amp on the volume would be much lower than it normally is. I'd start to play for a few minutes and then I would start hearing the fuzzy sounds again. After a few more minutes the problem would be gone and the amp would be back to full volume without any fuzziness.

I started poking around with a chop stick until I found one wire that would cause the amp to make noise.
It was the center wire on the hum balance pot. It goes from the center lug to a 50uf cap which goes to ground.

This cap is one of the electrolytics that I replaced so I checked the connection where I soldered it and it seemed good. I ended up replacing the wire from the pot to the cap because that is where it seemed to make the most noise when it wiggled it.

After that it seemed better but later started to make noise again so I opened the amp back up and took a closer look at the pot itself. I found that the center lug on the pot is loose and can be moved around a bit. The other two lugs seem solid.

So next time I get a chance I'll pull the pot out and take a look at it to see if it looks like it can be fixed or needs to be replaced.

At one point I tried to wiggle the entire pot to see if it was loose and I found that the back cover of it came right off. The inside is interesting, instead of having a plate with a wiper like most pots it has what looks like a spring that the wiper makes contact with. (OK, I just looked online and found out this is a 'wirewound' pot.) It's a 100 ohm 2W pot.

[Paul1453]

Since you had the pot open, I'd make sure to thoroughly clean the insides with DeoxIT.

If that doesn't work, I'd replace that pot.   

[PRR]

I'd just replace the pot with two 100 Ohm resistors.

Unless the wiring was crap, the best tap position is always 49%-51%.

If it does make a difference, the "lowerst hum" setting will vary with Volume control. (There is an element of cancellation between stages, different as you change gain between stages.)

Nearly ALL guitar amps get away with simple CT or 2*100r.

[casssax]

I think I've got this all sorted out now.

I removed the hum pot and cleaned it up and then re-installed it. I seemed to be good and I didn't hear any noises when moving the connecting wires around.

But, after I started playing the amp again another problem occurred. The volume started fading in and out. The volume would drop to a very low level, but if I played a loud strum the volume would come back up to full volume for a second and then fade away again.

So I poked around some more and found a problem with the signal wires going to the first volume control (after the tone stack). I had done a sloppy job of connecting the shielding to ground. I thought the problem was some loose strands of the shielding shorting out the signal. So I cleaned up the wiring to make it nice and neat but the problem persisted.

Finally I decided to replace the volume pot and that seems to have fixed all remaining problems. (at least for now) I've been playing the amp for a few days and there are no strange noises or fuzz sounds.

[jojokeo]

I had done a sloppy job of connecting the shielding to ground. I thought the problem was some loose strands of the shielding shorting out the signal. So I cleaned up the wiring to make it nice and neat but the problem persisted.

Finally I decided to replace the volume pot and that seems to have fixed all remaining problems. (at least for now) I've been playing the amp for a few days and there are no strange noises or fuzz sounds.

It is very possible that if even one of those small wiring strands found it's way inside the pot when trimming the stranded ground wires or even neighboring wiring also - that it could affect things. Especially the symptom being random as it gets moved around internally while you are changing the pot's settings.