Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on January 18, 2012, 11:15:26 pm
-
I'm not sure when it started, but i have a buzz that sounds very similar to a single coil pickup buzz, but it happens with any guitar even with humbuckers. I don't have a scope so i was hoping someone might have some suggestions as far as how to trace it. Or the most likely things that might cause it. Maybe a bad plate resistor or something. I just cleaned all the solder joints up by wicking the solder and reflowing new where ever needed since my recent tweak fest the last few weeks. So most joints were at least reflowed and i suppose something may have gotten heat damaged.
-
The old chop stick treatment is a good start. Touching those wires and components moving things around with a chop stick while the amp is live, if the buzzing get quieter of louder when you touch a peticular point sometimes is a good indication that your getting close to the problem to check out further. Platefire
-
What it is the amp ? Your marshall style ?
You may have problem in the wiring bad lead dress ; You probably need some shield wires on each tube grid inputs ? Hard to tell without seeing the amp. You have photos ?
You have the shorting input jacks working ?
Plate resistor don't buzz, they don't do this kind of noise, they "crack "
-
For all intents and purposes lets just say it's a 2204. I chop sticked it a bit even thot i'm sure thats not it since the lead dress hasn't changed. I do have shielded cable from the jack and also from the gain pot. Input jack shorting works fine, but it does this with guitar plugged in not w/o.
Sounds to me like a bad ground but i reflowed most of them last nite. Today i'm going to sub different filter caps because thats one thing that took a lot of heat recently when i resoldered it. (4 cap can) Also the 220k's at the bias point took a good bit of heat so i'll replaced those unless anyone can tell me those couldn't cause it. But i really have to take a long look at the grounding because thats what it sounds like to me, tho it's not intermittent wgeb i push on the board and all around the amp.
Really irritating because i just had the amp sounding so good i actually had no desire to do any more tweaking at least for a long while and I and zipped it up to just play and then this.
-
first identify if it is 60Hz or 120Hz
60 is heaters
120 is the rest.
if 60, chopstick around the sockets.
look around the leads coming off pin 2 and 7 of your 12xx7's
OH!
is the hum affected by any of the knobs?
if the tone controls affect it, it is before the tone stack, same with volume. and any other controls you may have. if you can only hear it when the amp is turned down or doesn't respond to the vol pot, its likely after the 1st gain stage or in the PI
EDIT
a quick way to make a 60Hz tone is to plug a cable into the amp and touch the tip of the plug. That is 60Hz, 120 is and octave higher.
-
Did you build this amp? Has it always buzzed?
-
first identify if it is 60Hz or 120Hz
60 is heaters
120 is the rest.
if 60, chopstick around the sockets.
look around the leads coming off pin 2 and 7 of your 12xx7's
OH!
is the hum affected by any of the knobs?
if the tone controls affect it, it is before the tone stack, same with volume. and any other controls you may have. if you can only hear it when the amp is turned down or doesn't respond to the vol pot, its likely after the 1st gain stage or in the PI
EDIT
a quick way to make a 60Hz tone is to plug a cable into the amp and touch the tip of the plug. That is 60Hz, 120 is and octave higher.
Good stuff, thanks. Yes, the tone controls do affect it, tho i figured they could also just be aggrivating a issue after that point. They don't remove it, just reduce. But I will try the things you say.. Thanks much.
Did you build this amp? Has it always buzzed?
Built it, and no, it just recently started. I've been trying to recall what i did just before i recall it starting this. problem is, because i get some form of buzzing randomly due to other things plugged in at the house, i didn't think anything of it at first because i thought it was that sorta thing. But then it didn't stop and i soon realized it's a issue. So the exact point it started is bluury. But sometime withing the last week or so for sure.
-
Isolate and conquer.
-
Isolate and conquer.
Thats what i'm trying. So far the gain at zero eliminates it completely even with the master on 10, so i'm assuming it's before that point. problem is, i tried most everything before that i could think of and nothing seems bad. tried checking the input jack's ground isolation to see if it was causing a loop by removing the hardwire ground to find there was no longer a ground. So the jack isn't making contact so thats good. Checked the coupling cap and a 470k/470uf filter and thats good. Tried new 12ax7, tube sockets pins, grid stopper. So far al looks good up to the gain pot between the 1st and second stage. Since no buzz is getting past there i would think it should be simple, but no so. Still a few unlikely things to check so i'll do that. But not looking like an easy fix. Odd...
-
Got it. Thanks for all the replies. I replaced the V1B plate resistor and coupling cap and it's now quiet. I know someone said it wouldn't be the plate R, but i lifted one end of the cap earlier and put another cap there and it still buzzed. So unless that cap could cause buzzing when out of circuit (one side still attached) then it hasd to be the resistor. Anyways appreciate the help, thanks.
EDIT: by the way, i should mention how i found it. I have a tracer i made thats a 1/4" jack with a probe coming off the hot and the ground has a wire with an alligator clip. I clip the ground to the amp, put a guitar cable in the jack and the other end into a P.A. or 2nd amp, andyhing to amplify the signal. Then i probe the signal path starting at the input in the amp listening for that buzz. there was none on the first stage but once i got to the signal coming off the 2nd stage coupler the buzz was there. So i just replaced both cap and resistor. I suppose i should have done one at a time to see which was responsible, but like i said earlier i had already tried lifting 1 end of the cal and clipping another in there to no avail. even tho i figured it was the resistor because of that, I didn't want to resolder those turrets twice in case i was wrong and thats why i replaced cap and resistor at the same time. I had already checked for DC at the coupling caps before i did this but they showed fine which was another reason i suspected the plate resistor.
-
Got it. Thanks for all the replies. I replaced the V1B plate resistor and coupling cap and it's now quiet. I know someone said it wouldn't be the plate R......
It is I who told you that.
It is always difficult to repair an amp without having in front of you.
What you call yourself "buzz" can be called a different kind of noise by another person.
Resistance plate is not noisy unless it is a very old and I caught it from moisture.
Which does not help is that you do not give details on the amp.
We had to insist that learning is an amp "home made" which never work well. This is change how we look at the amp.
Again, even after learning that, no photos, no details, nothing.
Ais you managed to repair it with the advice of people on the forum is more of chance, like your story master volume knob.
In this instance it is possible as you found that it is the resistance of the plate real problem. It is also possible that replacing the resistance plate, you have not arranged identically to the original and it did contribute to the problem. Or you accidentally moved another part which helped resolve the buzz . Anything is possible in an amp "home made" that never worked well.
A photo of the circuit should perhaps have helped to better assess the causes of your problems. Despite some claims, no, nothing.
You'll always be welcome to the other problems, but please more informations first.
-
Anything is possible in an amp "home made" that never worked well.
I'm not sure why you assumed that, but i never said that. It's always worked right always except for this issue and one or 2 others that happened along the way over the years since i built it.
-
Anything is possible in an amp "home made" that never worked well.
It's always worked right always except for this issue
That is what write no ?
Repair an amplifier that has working very well is not the same approach to repair an amp that has always had this problem.
-
I think we have a communication issue here, as you don't seem to be hearing what i'm saying.
-
I do not bother you more, I give up