MOD 102 kit

[Scooby9261]

I got the kit rebuilt and it works. But there is a buzz in the background when playing thru it. Ive tried different tubes and that is not the issue. The buzz does not come in and out messing with the eq. Its not the cables im using. Not the guitar. The buzz is there without a 1/4in cable plugged into the amps input. Ive really really double checked all my connections and I am not sure where this issue is coming from. I didnt know if anyone else has built one of these and maybe the issue im having there is a easy solution that im not seeing ? Seems like a ground issue.

Thanks for any help

 

[Scooby9261]

Heres a video. As i said you can plug in a guitar and jam thru it, but this buzz is there.

 

[dankilling]

Likely a layout/lead dress issue. I built one and had the same issue, but since it’s point to point, troubleshooting is more difficult. Also, single ended amps do hum more than P/P

 

[Scooby9261]

Yeah but this buzz is louder than the normal noise in the background. I built a few of these years ago and this one was like a parts one and ive rebuilt it, they others did not make this loud of a buzz. The Transformer was faulty so i bought a new one and that was the original issue with it blowing fuses. Bad transformer. It powers up and does as I said work. But this noise seems louder than normal. Ive checked all my grounds to see if that was the issue and nope. Maybe shorten all the wires in it even more ?

 

[King Fan]

Video sound here is a good idea -- I *think* that's a buzzy or raspy hum? Hum has so many causes, it really helps to narrow the suspect list before trying different fixes. You've already done that some, noting that tube swaps didn't help and the vol/tone controls don't alter it. Even the raspy nature is a clue.

Standard hum hunting questions:

Did this amp work quietly before this revision? If so, something we revised is suspect. But maybe that's not helpful -- like if the amp never worked right, or we revised *everything.*

Is that 60 or 120Hz? Typically 60Hz will be well below the lowest E on the sixth string (at ~82Hz); 120Hz will be lie between Bb (~117) and B (~123) on the same string. Other frequencies are possible, too, as when OT hums from PT interaction.

If it's 120, we're basically looking at B+ hum. Are these new filter caps? If so, cap failure is unlikely, and a filter/ground wiring fault is suspect (though occasionally good caps will hum if current draw is too high). Are voltages all good?

 

[King Fan]

OK, this may be TMI, some of it not likely to apply to your case, but R.G. Keen wrote a good short piece on hum. I'll paste the text here and the PDF below:

Other sources of hum

Some tubes simply hum on their own. Always try replacing the tubes FIRST.

Remnants of old “line reverse switch” must be totally removed; no cap from the AC line to chassis, either side.

Heater CT:
a. If it’s left open you WILL get hum. It has to be connected to some voltage.
b. Ordinary connection is to ground. This works most times.
c. Two 100R 3W 1% resistors make a more accurate CT than most transformers.
d. Fender used a 200R pot with the wiper grounded. This not only let you balance the two sides, you could null out hum from elsewhere. It works, but it’s a pain.
e. In hard cases where the heater CT is known to be the problem, it can be tied to a voltage 5V - 50V positive with respect to signal ground. This will not help the balance issues much.

Heater wires should be twisted as shown. For super quiet performance, use shielded twisted pair and ground the shield to star ground point.

PT, Inductor, and OT can radiate to each other and to sensitive wires.

SS rectifiers which are standard rectifiers (i.e. not fast and soft recovery) can cause RF ringing at 2x line frequency. This is a buzzy sounding “hum” like fluorescent light hum.

Note the PT center tap. Because of the high pulse currents in it, this wire will cause 2x power line frequency hum if it’s not hooked ONLY to the first filter cap minus.

It helps some if the PT CT and the other two PT secondary leads on the HV are all three twisted together, as the CT and one of the others will be carrying balanced currents as the current pulses alternate between the two secondary leads.

Switch and control knob bushings are “grounded” by connection to chassis. Use a star washer to bite into the bushing and the chassis for firm connection. Do not solder a bus wire to the backs of the pots and ground signals to it.

To those who’ve seen a contradictory instance of this setup on an amp without hum:
Yes, I know that various amps violate various ones of these items and don’t hum. This
is not the only possible way to not get hum. But it is the only way which is known ahead of time to prevent hum. Every amp is a special case and may get away with any one of these issues violated, maybe several. But you can’t know that until you find out that amp’s special cases.

R.G. Keen July 2008

 

[Scooby9261]

Its more of a buzz than a hum. I remember this amp humming but this is almost like a 60 cycle hum. If you watch the video youll see what i mean and the noise is as loud as the note being strummed on the guitar. So this is a terrible noise ratio going right now haha. Ahh haha. Im just gonna go backwards again, then go thru it one more time. Weird.

 

[schmee]

s

make sure your input jack is physically grounded to the preamp part of the circuit. Is that chassis painted? it doesn;t look like bare metal. try running a jumper wire from the input jack ground lug to a known good ground point and see if it helps.

 

[andrewRneumann]

Did you solder the positive lead of the cap through pins 4 and 3? Easy to miss.

 

[Scooby9261]

Def did the above ^.

Im gonna run a ground from input to somewhere I know theres a ground and see if that helps like someone mentioned above.

Thank you folks

 

[Scooby9261]

Ran a ground to the input and its still got the buzz. Im going to keep messing with it, when i find the issue itll prob be something very simple and obvious.

 

[Scooby9261]

The sound reminds me of having a strat on 1-3-5 settings on 5 way switch with that 60 cycle hum, but could it actually be 120. I have bad ears. But ive watched a few videos showing how to identify 60 and 120. Not sure, if anyone watched the video I posted above and can identify if its 60 or 120 that help

I def have the grounds properly done. I am reusing some caps, im curious if this sound could be a bad cap. Or caps.

 

[Lowerleftcoast]

Being one octave apart... sometimes it is very difficult to hear the difference. What I hear seems like 120Hz or even 240Hz.

Please provide pictures and the layout if you have one.

120Hz suspects (including buzzy):
I am very interested in the grounding, so include pictures of the connections from the PT, rectifier, and filter caps. Iirc, the Mod 102 uses the chassis for ground return to save on wire. I would like to see how yours is wired.

60Hz suspects:
Try to get good pictures of the input and output jack wiring.
Is there a CT or artificial CT on the 6.3VAC heater secondary connected to ground?

 

[Scooby9261]

^ I sent you a PM with pictures

With the volume control set to 0 there is no sound. But as you turn up the volume this buzz gets louder and louder as you go up. As I said the EQ pots dont alter the buzzing sound. Tubes are not the issue. I tried a different Volume pot just for s**t's and giggles. Is a brand new transformer. Im down to maybe its a bad cap, input jack is bad or speaker out is bad, resistor is bad. Theres not much to these amps. But the noise almost has to be a ground, i dunno ive been looking at this thing all day and i have yet to find what is going on. I just was jamming thru it trying to ignore the buzz, but the buzz is about the same volume as the guitars notes. Horrible noise ratio going on. I built some of these years ago and they did not make this sound. I have reused some parts and im wondering if one of the caps is bad or something along those lines. I made sure there was bare metal access for all the grounds.

I think thats everything hehe

 

[andrewRneumann]

Is there a CT or artificial CT on the 6.3VAC heater secondary connected to ground?

These amps don't have a balanced heater supply. The heater elevation should swamp out the imbalance, but maybe in your case you got an unusually noisy tube / socket. Would you be interested in trying out a humdinger or balance resistors on the heater supply?

(I always go for the heaters, and it usually turns out to be a grounding issue. One of these days it will be the heaters!)

 

[dan40]

With the volume control set to 0 there is no sound

So when the volume is at zero, the buzz is gone? Knowing this will help to narrow it down to parts of the circuit before or after the volume pot. Sometimes it's something as simple as the shunt switch on the input jacks. You did say that the noise occurs with no instrument cable plugged in so that seems to point to the jacks.

 

[andrewRneumann]

Keeping in my quest for finding a problem with the heater circuit--check the pilot lamp. Make sure there are no inadvertent shorts there. Sometimes they come from the factory with a short. If you check the resistance from each side of the pilot lamp to chassis, it should be right about 200-Ohms. If it isn't, something is funky with the heater circuit.