Fulltone TTE hum

[klooon]

I have a Fulltone tube delay unit and it hums badly. The strange thing is that is also hums in the True Bypass mode. Any clues what might cause this?

It's a unit from 2015. I didn't use it for a while. Next to the hum it's also noisy. I opened it up and there is an additional small board with two strange components that I haven't seen before. (I have built a dozen amps up to now). When moving this around the noise seemed to become ess. Hum is still there.

I think it's strange that it hums in true bypass, although ground goes through the chassis....

[klooon]

Anybody? Some clues would be welcome! Thx.

[Enzo]

Schematic?

[Mick Bailey]

Does it hum in bypass with the power disconnected? It depends on whether the bypass is JFET or relay controlled, but sometimes a piece of equipment with a ground fault (such as high resistance to the chassis or oxidized socket contacts) will cause hum even when not powered. If you still get hum then this points to a simple connection issue somewhere between the input and output grounds.

If the hum only appears under power, remove the tubes - any change? Make sure it's not a tube issue. If it still hums with the tubes removed, check any components or leads that are grounded to the chassis including pot nuts. Something that puts noise into the grounding scheme can still do this even in bypass. If the unit uses an earthed mains lead, make sure the earth connection is good - I've had equipment that hums because the earth terminal was loose in the mains plug (with screw terminal type plugs here in the UK).

[klooon]

Unfortunately, there is no schematic available. I searched on Fulltone's web page but there is now way of contacting them, unless you're a dealer.

Mick, thanks for your message! I will try that this weekend.

[Mick Bailey]

There's a useful view of the inside of the unit here at 4:50;

https://www.youtube.com/watch?v=D6S1ZTFSRbA

It's a really neatly built, solid unit and worth taking a considered look at. The aim should be to identify where the fault lies before replacing any components. The user manual describes the function of each tube but also outlines hum arising from ground loop issues, which should be checked.

[klooon]

I checked several things:

• Power chord disconnected:
  • True Bypass - no hum
  • "Effect On" - no sound

• Power chord connected:
  • True Bypass - very little hum
  • "Effect On" - very little hum
• Power On:
  • True Bypass - a lot of hum
  • "Effect On" - hum, but less as in True Bypass mode

Tubes out: no difference...
Spectrum analysis: peaks at 100Hz, 150Hz,...
Components with leads to ground checked, but no strange things...

What else can I do?!

[Mick Bailey]

With the unit in bypass, insert a lead into the input and another in the output and measure the resistance from tip to tip, and sleeve to sleeve. You should have low resistance with each reading. What is the bypass switching arrangement - is it a DPDT switch? More hum in bypass compared with 'effect on' suggests to me that there's perhaps something wrong with the switching - maybe oxidised contacts. If this is the case the resistance measurements should show this up.

You say it makes no difference with the tubes removed, is this compared with the Power On observation?

[klooon]

Thanks. I did the resistance measurements and in bypass mode (power on and off) the resistance was 0 Ohm.
Yes, with power on it makes no differences with tubes in or out...
Tough issue...

[Mick Bailey]

Did you eliminate the possibility of a ground loop hum?

[Silver Mica]

Did you find a solution for eliminating the ground loop hum. I have exactly the same problem?

[klooon]

Unfortunately, no solution found. I reached out to the Dutch company that imported the unit in the past, and apparently they are not able to contact Fulltone anymore about this.
I thought already that it must be a 'common' problem, so interesting to hear more people are facing the same.
It must be something straightforward, as it is giving the problem in already in true bypass mode.
Let me know if you make any progress towards a solution, or if you are able to contact Fulltone reps.

[Silver Mica]

Unfortunately, no solution found. I reached out to the Dutch company that imported the unit in the past, and apparently they are not able to contact Fulltone anymore about this.
I thought already that it must be a 'common' problem, so interesting to hear more people are facing the same.
It must be something straightforward, as it is giving the problem in already in true bypass mode.
Let me know if you make any progress towards a solution, or if you are able to contact Fulltone reps.

klooon, could you send me a PM?