112 noisy amp issue - AMP MODEL SAYS 112A

[Whalerman]

I've replaced the filter caps to no avail.   I've watched Uncle Doug amp videos (I've even built my own Fender tube clone from scratch).   I've wiggled a few things, touched a few things to see if noise was dramatically different.

I know this isn't going to be easy.    I've replaced (temporarily) all the tubes.   I'm using two prong cord, but have swapped polarity.   Neither impacted the problem.

Noise increases with volume - I believe that's an indication of the issue being in the 12AU7 area of the amp (before the 6V6 tubes).

Power wires are twisted, as are speaker.   Tried touching the 2.1" input - louder.   Touched the pickup bar - modulated the hum.   

Tried increasing and decreasing the volume with the on-board trim pot - just lowered or increased the overall volume, still has the hum (goes up and down with volume).

It's pretty significant - it's very prevalent.   Key sound is probably 20-30% louder than hum.

Any cool thoughts?   I've read a lot of Jezza's posts on his amp, and others as well... 

- Paul

[pianotuner steveo]

*Check for "death cap" FIRST.(See below.)

A very simple thing to try, cut a length of wire, IE speaker wire, slightly longer than the power cord. Connect one end to the chassis (ground).connect the other end to the plate screw of an outlet that you know for sure is grounded. This should reduce hum in most cases.


* This is a large disc cap that is connected between the AC power jack input and chassis (ground) on some early models. If it is there, remove it. Do this before connecting a wire to the chassis. A leaky cap will send stray voltage into the chassis,which is not good.

I'm not sure if the 112 has this, but 120's and 700's do.

[Whalerman]

Will try both in the next couple of days, and will report back.  THANKS!!!

[pianotuner steveo]

If the ground wire doesn't work, it likely needs a new power switch/volume control. I've seen this happen a couple of times.

[DocWurly]

Later 112's use amps labelled "112A."  That's fine.  Schematic attached.

I'm not a tube amp expert.  I recently toiled a whole lot on a 120 amp.  But there may well be others on this forum who know a lot more about the 112A amp (and tube amps in general).  It's one of the better-designed Wurlitzer tube amps--more sophisticated than the amp in the 120 and 700.

It could easily be a bad solder joint somewhere.  A resistor not doing its job.  I recommend checking your amp against the schematic and measuring the DC voltages at all of the test spots (the voltages in the rectangles).

Are you using all-new tubes?  Do.

Keep checking the whole system with the hum shield in place--without it, you will have a lot more hum.  And what happens when you detach the reed bar from the amp? Same hum, or less?

Voltages in this tube amp can be lethal.  Follow standard electrical safety measures....study up on them if you don't know them.  Never have both hands in the amp, etc.

Steve-o is correct --remove that death cap if it's there.  You should also 3-prong that baby ASAP.

[Whalerman]

Thanks Doc and Steve.  No time to test yet - will report back when there's progress!

[pianotuner steveo]

I can't stress the following info enough. Before spending any money, first check for and remove the death cap. Temporarily adding a ground wire from the chassis to the screw on an outlet cover (as long as that outlet is grounded ), will tell you if adding a 3 prong cord will be worth the time and effort.
If it still hums with the ground wire attached, I really think the volume pot should be checked. Sometimes the carbon wears away so much, that it causes hum.

[Whalerman]

Sorry for the delay, was only able to get time away from other stuff this weekend.   I ran a wire from the case to the metal on the case of the amp - no change.   But...   When I touched my hand to the amp case, or touched my hand to the 1/4" speaker jack, the volume of the hum went down considerably. 
Because access to the innards of the amp case require a good deal of dis-assembly to analyze, I did not make any steps in that direction.  I was VERY surprised by how dramatic the reduction in hum was as I touched the grounded areas.   But at the same time, it definitely did NOT cure it - there is still hum there (and it will be hard to play if I need to keep one hand on the speaker connection... :-)
I've swapped out the 12AX/AU7s, as well as the rectifier tube.   Next time I open it up I'll check all the voltage measurement points.
- Paul
 

[pianotuner steveo]

I think the next step would be take the amp out and remove the death cap if it is there. This will cost nothing except a little time. It still could be volume control related. I tried EVERYTHING once in a humming Wurli.New caps, new tubes, ground wires... I just about gave up, then changed the volume pot and that was it. (The carbon on the inside was very worn out)

[4kinga]

You could try by-passing the volume pot all together.   Simply unsolder the wires (audio, not AC!) and twist them together.

[Jezza]

I'm not here often anymore so I didn't see this for a while, but I'm happy to hear that my old posts helped a bit. I made a few more little tweaks to my amp since those posts, and one of these days I'll update my old posts with all the specifics.

I got my amp to a place where it was good enough and I stopped worrying about it. I usually run it through a guitar multi-effects pedal on the way to an amp, a noise gate conceals a lot of the amp noise, and noise reduction software does the rest. It still bothers me when using tremolo effects though, because that makes any noise floor obvious.

Easy stuff:

• Remove the phono input for reduced noise.
  
• Bypass the internal pot for slightly increased gain.
  
• Test your main volume pot by bypassing it. Move the center wiper wire over to the non-grounded outer lug or to the tube. This cuts out the pot and puts the volume at max. If the hum is still there, the pot isn't your problem. If the hum is gone, change the pot. (My pot was not the problem source.)
  

Deeper rebuild ideas:

• My power transformer hums audibly and may be the source of my remaining noise. Vintage Vibe sells it for $200.
  
• James from Vintage Vibe suggested rewiring the amp to use balanced power for the 6.3v heater supplies might help (with artificial center tap to ground). I haven't tried it yet.
• I suspect a bus bar grounding scheme would be a big improvement over the haphazard chassis grounding the amp uses.

Here's an article that may help:
https://www.tropicalfishvintage.com/blog/2019/7/4/how-to-fix-hum-in-your-wurlitzer-electronic-piano-or-other-vintage-amp-part-ii

You should get basically the same noise floor with the reed bar connected, disconnected, or a different audio source connected. If your reed bar is contributing lots of noise, shine up all the connection points as best as you're able.

Like you, I replaced the filter caps first, but it wasn't good enough so I replaced all the caps.

Like you, touching anything grounded reduces the power noise a bit. (This suggests ground loops are the problem.) Using the pedal barefoot works for me.

Like you, grounding the chassis did not help me. I chose not to ground it with 3-prong power, because inside the amp the power and audio grounds are the same. I didn't want to add more risk of ground loops when connecting to other equipment.

If your amp is open and also on, you'll probably find that putting a finger near any of the capacitors in the input section makes a lot of noise and perhaps gives you radio. It's just not a great design.

Good luck with it!