Tone Knob Inducing Hum/fuzz?

[crunchman]

New build (18 watt) type had some hum which I resolved most off.I notice when the tone knob is rolled C.C. no fuzz is heard but the fuzz comes on strong at 9 oclock and gets progressively worse, I don't know what would cause this.......any ideas?

[pdf64]

Oscillation - got a scope?

[crunchman]

unfortunately, no scope available.It's a proven layout.What might cause the oscillation?

[R.G.]

Wire placement, and spacing between wires, even on a "proven" layout can do this, as can decoupling caps.

Get out some wooden kitchen skewers or chopsticks and move wires around a bit while listening to it buzz. If the buzzing so much as changes, you're finding some part of the problem.

[crunchman]

I'll give a try, I just re-wired all the board to tube-pin wires because some were overlaping now they are not, the amp is silent when V1 is pulled, very frustrating.I may have to re-wire w/ solid core so I have more control over wire placement.......Thanks

[pdf64]

What schematic are you using? Grid stoppers can work wonders for this type of issue.

[crunchman]

http://www.brownnote.net/forum/downl...=717&mode=view

This is the layout, there is a shared screen resistor for the power tubes, not sure about the pre and splitter, i'm open to suggestion.My lead dress from board to tubes could be improved, like I mentioned, pull V1 and it's silent.......Thanks

[pdf64]

Does the setting of the pre-set (that feeds the second section of V1) have any effect? It should stop it, at low settings, at the expense of reducing the gain, perhaps too much.
Re lead dress, there are 2 signal inversions between the input sockets and the vol/tone controls; that will make the stray capacitence between the wires/components prone to result in oscillation, so keep the wiring well seperated.
I'd fit a 10k-22k grid stopper on pin 7 V1. Pete.

[crunchman]

Thanks Pete, I re-wired V1 w/ rt. angles wherever possible, very little effect though.I ordered some mini-shielded cable from Hoffman and will re-wire

V1,tone pots and inputs using that, hopefully that will rectify the noise issue..........Thanks again.......crunch

[loudthud]

What did you do to remedy the hum issue? Any changes to the grounding?

[crunchman]

I TRIed everything I could think of, maybe my heater wiring in the vicinity of V1 is not quite right, there are 2 single wires that come over from the splitter that I did not twist, I will redo them.My other thought was although the volume/tone pots are grounded the chassis with a single wire, the chassis is powdercoated so the pot shafts themselves are not contacting metal, I'll fix that as well. I powerscaled the amp and at the volumes I usually play, it was tolerable.I'm waiting on a new scaling kit as I grounded on of the voltage regs. against the chassis,LOL, and could not replace it. This is my first major build, its been quite an experience, I'm learning though !........Thanks

[km6xz]

I noticed everyone giving advice to fix things but did not have enough evidence to suggest what noise was actually being heard, level, frequency components etc. All that would lead someone to completely different sources of hum/noise.
Can you describe the sound, what level compared to signal, and the relative frequency components to the sound. For example is it primarily a fairly pure 60hz note or 120hz, is it mostly indistinct noise with a bias towards high frequencies? Does it change frequency of the fundamental when any control value is changed? Does it predominately have a high frequency component with general noise underneath the stronger high frequency fundamental? Does the noise spectrum change along with the signal played through it?
If you are going to play with amps you really need some basic test instruments to avoid needless experiments in hopes of stumbling onto the solution, without ever knowing what caused it. That has a practical downside besides time and confusion, it messes up carefully wired assemblies so it never looks as good as if left working sections alone.
Usable test instruments that are a must to know what is going on in your circuit include an oscilloscope, a signal generator with relatively low distortion and a calibrated attenuator. An AC voltmeter which function can be the ac function of a digital multimeter but in most cases the general purpose DMM that are so low cost do not have much sensitivity for measuring low level signals. A variac. This is crucial to avoid blowing a lot of gear, but most people do not use one for some reason. They are not expensive if bought surplus as a bare variable autotransformer that you put into an insulated case with a AC voltmeter and AC ammeter, on-off switch, a fuse or circuit breaker and a AC socket. It will save your butt more often those without it can imagine. Later you can add details like resistance/capacitance/inductance bridge that is very useful if you ever reuse parts from old projects or find valuable older parts that you need to verify values. It is also very useful in determining the characteristics of an unknown transformer or inductor. A good dummy load is good, since it allows all sorts of tests and measurements without risking an expensive speak until you are perfectly satisfied that the amp is performing correctly with stability. None of these are expensive getting old used ones that might be larger than modern units but are well suited to working on tube amps.
A quick look with the scope would have told you the type of noise, which suggests it entry point, and where it enters. You have probably already spent more time than the $50 needed to get a decent 2 channel 30Mhz scope.
Good luck....