Unusual Hum/oscillation

[RockinRocket]

When no cable plugged in to my newly built Express it has a loud hum that's full volume.
This ONLY happens when the VOL is all the way DOWN and TREBLE all they way UP. No other pot TMBV settings will do this.
Bring UP the VOL by 1/10th it goes away or bring DOWN the TREBLE by 1/10th it goes away.

With a cable plugged in all this is non existent and works as it should.

Is this normal or inherent of plate feed Fender Tone Stacks/front ends?


PLEASE NOTE my vol potentiometer doesn't bring on vol until about 1.5 - 2. Possibly a shorted trace causing this?
It would be a shame if I have replace it as this build is clean and wired spot on.

[Mark]

That certainly is an odd fault. Do you have an oscilloscope?

I think that will help you with this issue. I assume you have shielded cable on the input and from the wiper of the volume pot?

I have noticed with amps that have earthing issues, that there will be audio going through the pot when on "zero" and the audio is then "nulled" out and the volume pot then acts as it should. The problem in these amps is signal travelling through the power supply nodes.

What frequency is the hum?
I usually assume that the hum is a low frequency when it is referred to as hum.

[RockinRocket]

Thanks, Yes its a loud low frequency hum at full vol. That was a unpleasant surprise!

I only have shielded wire from input to the grid. No grid stoppers. I test the amp with out any grid stoppers and the amp is almost stable with everything on 10. This amp has the least amount of hiss for a Express Ive built. This being # 5. Has a NOS Stancor A3801.

Unfortunately no oscilloscope.

[pdf64]

In the absence of a scope, you'll have to experiment.
Do 10k grid stoppers, mounted at the tube socket terminals of the 1st 2 stages, fix the issue?

[Littlewyan]

Check your ground connections and check the volume pot using a multimeter. Make sure when it's turned down that it's actually going to ground. Then keep measuring it as you turn it up to see what happens. All with the amp off of course.

[RockinRocket]

Thanks fellas

Will I be able to measure a pot in circuit?

Im really starting to think it is the dead spot in the pot not making ground at full countercockwise?
Once I bring it up to where it will make vol its fine.

Im going to try the grid stoppers first though I don't think its the problem but easy to test. Amp is completly stable everything dimed.

[Littlewyan]

You could test the ground connections with the pots in circuit. In this circuit you might be able to test the taper of the volume pot, I can't honestly remember as it's been a while since I had my Express out!

Check the grounds of your filter caps too.

[RockinRocket]

UPDATE This only happens with the bight switch on.
Nothing plugged in and Vol on zero and Treble dimed.

[pompeiisneaks]

That sounds like a ground loop, even with the volume at 0 there is still a 'slight' resistance, but with the bright cap engaged it has 0 resistance, and the ground loop hum bypasses the cap and goes into the tube after the volume pot wiper.

~Phil

[Littlewyan]

I think you're right Phil

[RockinRocket]

I can confirm 99% its not the pot as the 3 1M audio CTS pot in this build have that dead spot in the Vol, Treble and Bass spot. Its not completely silent in the "dead spot" but it comes on abruptly. Guess I'm just noticing this looking for any kind of fault.

Can any one confirm with there express before I go hacking away at finding a problem-

With no cable plugged into a Express it has a loud hum that's full volume with vol all the way down and treble all the way up with a bright switch on.

[Littlewyan]

I might not have time this weekend to check, but I'm fairly certain that this shouldn't be an issue. Have you checked the grounds? Made sure the input jack shorts to ground when nothing in plugged in?

[Stevem]

I would like you to better define what you are hearing that we are trying to help you fix because so far it does not sound like you have a oscillation going on as in the title of your post.
1) A hum is a near pure tone and if your amp is powered off of a 120 volt wall outlet then you can have either a 60 hz or 120 hz hum taking place.
2) 60 hz hum can only come from the primary side of the PT , or on the secondary side in regards to the filiments.

3) 120 hz hum is a bit higher in pitch then the low E on your guitar and can only come from the D.C. Power supply section of the amp.
Are you hearing either of these , or is it a multi frequency buzz you are hearing?

[dbeasley]

Definitely sounds like ground loop hum to me.

Where are your main reservoir cap(s) physcially grounded at?

Another thing that it could be is unbalanced heater hum. Does your PT have a Heater center tap? If not, use an artificial Center tap with a pair of 100R resistors to ground.

A good trick for locating hum is to clip a roach clip to ground and ground out the grid input of each stage starting from the input. If you ground the grid and the hum goes away, chances are it's coming from the stage immediately preceding it.

Hope this helps!

Sent from my Moto G (4) using Tapatalk

[RockinRocket]

Ok its fixed.. simple as a 10k grid stopper.

Didn't sound like any oscillation ive heard. Low frequency continuous honk is the best word to describe it.
That bright switch network seems likely to be a good source of oscillation.

Still don't understand why in these unusual particular settings of nothing plugged in, and a bright cap bypassing a off Vol pot with treble maxed made this sound?
I made a video ill have to upload it to a hoster.