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I am working on friend's vintage 1960s tube guitar amp and I have repaired several problems and upgraded some parts (mainly capacitors). I have a persistent buzz that goes up and down with Vol control. Doesn't that suggest the problem is before the inputs, input tube and before the Vol pot? (I don't usually experience this with the many audio amps I have worked on). Buzz drops to zero w/ Vol turned down. Maybe the pot itself?
Thanks for advice
Peter
Follow Ups:
1. The buzz goes away when 1st input tube removed. But, of course, so does the signal.
2. The buzz goes away when treble control turned down (not shown in schematic, just "tone" - a model variation apparently).
3. No change in buzz when you go back to 2-prong plug. And then you measure about 64VAC on chassis.
4. I tried testing small capacitors to ground thru parts of the circuit with no effect - except adding noise.
5. The extra 1/4" inputs shouldn't have to be grounded - and it doesn't get rid of the buzz when they are.
6. My touching the chassis lowers the buzz considerably. Maybe just stand there with my hand on it while playing. Just kidding. This is starting to be like phono grounding buzz (something we are all familiar with, esp in DIY.)
Thanks, friends.
Usually, when trying to track down this sorta thing, you find the magic 'trick' that solves it. Not this time, but it is better. There are about 25 ground points, circuit to the chassis. And I think this is endemic to all the cheap parts in this amp: power transformer internals, open-frame OPTs, pots, etc. I tried all the suggestions, thanks. As long as you don't turn up to full volume the buzz isn't really noticeable.
Edits: 01/12/16
If it is not noticeable until you turn up the volume to full, as you say, the buzz could very well be inherent in the design. If you can enjoy it as is, I say go for it.
Dave
So that almost always rules out a filter cap..I had this hum one time after recapping a Fender bassman I think it is,anyway what was causing the hum was being out of the case because there was a shielding in the case and as soon as I put it back in,the hum was gone.
This also could be filament hum related being you have AC filaments in high gain stages.What you can do is raise the filament above ground with a couple resistors like this thru the center tap with one of the DC sources.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Edits: 01/12/16
Thanks, but there is no CT on the heater windings. I think filament AC has something to do w/ the buzz. But I tried resistors to ground and capacitors on the heater circuit to see if it changed. It didn't.
Yes, installing back in the cabinet helped diminish the buzz a bit.
Does the heater line have resistor at the ends, to make an artificial center tap?
Fender usually uses 100-ohm resistor tied from each heater end to ground. At the power lamp.
The schematic he posted shows a filament center tap to ground.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Hi Mike,
Would switching to 12AX7LPS tubes help in this situation?
Dave
That is a miracle tube so it may help.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
There doesn't seem to be a CT for the heater, despite what the schematic shows. I tried adding 200ohm resistors to the heater cables to ground. No effect, so I removed them. I think there is an internal leak in the PT.
Now I wonder about the fact the manufacturer used the unused pins on the 5Y3 rectifier socket as tie points for the heater cables coming out of the PT. Kinda close, causing EMI noise? Probably not, since we have determined the noise is likely in the signal path.
Thanks for suggestions.
Peter, You did try shielded line from input jacks to the input stage? Grounded at jack side, only?
Thanks!
I did but it's a moot point since EVERYTHING is grounded to the chassis, including the input jacks. There's no signal ground. Adding a 3" shielded cable from jacks to the input resistors and then 4" to 1st tube grid had no effect on the unwanted buzz. I expected some result or fix. Guitar amps are odd beasts; crude compared to what I usually work on.
Edits: 01/14/16
okay. are you trying the amp with a guitar with humbuckers? Single coils will pick up some noise. if it buzzes with the guitar unplugged check the input jacks to see if they are wired correctly/shorting.
Fenderlover's suggestions are all good ones. One other thing, when you went from two prong AC to a three prong, did you remove the death cap?
Dan Santoni
I don't recommend removing the so-called"death cap".
There are times when a three-wire grounded outlet may not be available, and it's needed to remove noise from the line.
As long as the plug and outlet are correctly polarized, and the cap is connected from the neutral side of the line to ground, it cannot cause "death".
What if the receptacle is not on the neutral side.
I've seen this cap cause exactly the type of problem the OP is having.
Dan Santoni
Fit a proper three-prong line. Make sure the "hot" or black line goes to the fuse. White to PT. Green grounded to chassis, right after entry into the chassis. I have not had hum issues, when doing this setup to the vintage amps I've worked on.If the amp has an auxiliary AC socket, make sure wiring to it is correct.
I think the OP amp's hum is in the gain stages and/or PSU of the amp. Not AC wiring. At least, I'd rule these out, first. IMHO.
Edits: 01/11/16
That's what I was getting at.
Dan Santoni
Yes, and with this arrangement you can get rid of that "death cap." Which usually is a nice vintage cap, that can be re-used.
test
I'd concentrate on this section. I still think the problem lies between the jack and the first stage. You may need to go to shielded input wires. Ground one end (I like grounding the jack side).
Moving the leads going to V1 socket may also help. Be sure heater lines are wound tight together and lifted away from the other leads and the socket, as much as possible.
May also want to re-flow the solder points. Esp those to ground. All jacks and pots have good contact and clean grounding points.
Good luck! Keep us informed of updates.
8^)
That amp has 2-terminal, open circuit input jacks. Try grounding the hot lead of one of them to see if that reduces the buzz. If so, installing three terminal shorting jacks will help.
Also, try reversing the AC plug in the wall outlet. It shows a 2-wire, un-grounded AC connection.
Is a guitar plugged into the amp when it makes the buzz?
ET
Yes, there are 3 input 1/4" jacks. Did the obvious things like switch out tubes. The noise lessens when I touch the chassis, but it has been removed from the cabinet for service. The chassis is not 'hot' - no voltage measured there. 3 prong plug added. Lifting power cord ground makes buzz louder.
Are all three input jacks grounding out, when nothing is plugged in?If you pull V1, does the hum go away?
Edits: 01/09/16
The schematic shows two terminal jacks, not grounding when unused. Shorting jacks would probably fix it.
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