hum on stand by, any ideas ?

[FatGaz]

Hi,

I have a small amount of (constant volume) hum I'd like to try to reduce on my newly built -not sure if this will be possible ? I'd like to record with the amp, so less noise the better. it's a low frequency mains-like sound.

It's a SE 6L6 combo, the hum is present when power is applied (and thus heaters turned on.)
Turning the standby (B+) switch on/off ( lift's the ground connection on the bridge rectifer) doesn't change things.
Pulling the 6L6 tube doesn't change things either.
The total heater current is at the max rated limit for the PT. the PT and OT are standard laminated frame transformers.

So, guess this is possibly the OT (& OT wiring) picking up hum from the PT / heater wires / internal mains wiring ?
I have the PT and OT on opposite sides of the chassis and orientated differently (90 deg). I also have a choke, but this is after the BR of course, so shouldn't be a cause ?

Any ideas as to what I could do / try to reduce this hum ? what tests might I try ?

I did try to improve the twisting of the AC wires leading to the BR, and moved the BR so the connections are closer. This didn't noticeable change things.

All of my AC wires are twisted together and mostly placed next to the chassis, apart from the ones for the pre-ap tube heaters with is elevated away from othe bits fo the circuitry.

thanks All !

Gareth.

[Tom Phillips]

Gareth,

Based on the described symptoms I would initially suspect that the OT is picking up inductive coupling from a strong stray magnetic field, most likely from the power transformer. However, your description of your build sounds like you did everything correct to prevent this. It would take an unusually strong field from the PT to couple to the OT mounted at the other end of the chassis. Could there be something else near by causing a strong line frequency field? Does moving the whole amp around or into another room change the hum level?

As to further tests that you might try I’d suggest removing the OT from the chassis, attaching long flying temporary leads and then moving it around to determine how its position (Spacing from the PT and relative orientation) affects the hum. Then post the results.

Tom

[FatGaz]

Hi,

I'll consider moveing the PT, but with the way the thing is built, it will be a bit of a pain to move it as the bolts to the chassis are by now close to some other bits of circuitry.

On thing I'll do is disconnect the heater wire to see if that changes anything.

also I did take the ground off the OT secondary, but this didn't change anything.

The other thing is that (due to huge lack of hind-sight), the speaker magnet is very near both the PT and the OT - I just thought now if I've managed to steer flux from the PT towards the OT ? I can take the speaker out and see if that changes anything.

G.

[Tom Phillips]

...The other thing is that (due to huge lack of hind-sight), the speaker magnet is very near both the PT and the OT - I just thought now if I've managed to steer flux from the PT towards the OT ? I can take the speaker out and see if that changes anything.

Worth a try. Or just dis connect existing speaker and hook the amp to another remote speaker.
Many amps have a PT very close to the speaker without apparent issues but the concern has been raised before.

[FatGaz]

shocking, literally - ticker still ticking luckily !

Tried the amp with the speaker moved out and using a separate cab, didn't seem to change much, so probably OK to have the speaker that close.
Also tried disconnecting some of the heater wires that are towards the side of the chassis with the OT - no difference.

Thanks for the help and advice Tom.

------
Anyhow, I decided to just have a bit of a play to get more of a feel about how the amps sounds; lazily I left the top off the chassis while playing. Even more lazily, I decided to swap from the combo speaker to the ext cab speaker, so I flip the standby switch - but leave the mains switch on......

holding the neck of the guitar with one hand, i grab the metal jack of the speaker lead, and try to put it into the socket

and... SHOCK << electric kind - straight across the chest from one arm to the other - eek.
Glad I had the guitar on a strap, otherwise I'd have thrown it across the room...

Not sure how this happened - the jack was metal and I think that it basically only should have touched a ground potential only, BUT maybe with the chassis open I managed to touch something.

OR, maybe there is a fault with the amp and a voltage from the strings (so the grid of the first stage tube?) to earth has occured ????

anyhow, stopped testing and playing amps for the evening....

Gareth.