HIWATT T40 hum

[Palantine]

Probably not a widespread amp but quite interesting.....with an attitude. Mine is the 40/20watt (four EL84) but I guess the most part of the attached schematic is the same.
Mine HUMS a lot in DRIVE mode.
Everything's stock except for the pre and power tubes (which I swapped in favour of new ones) but it HUMS also with the stock ones (I bought it SH).
Any suggestion?
Thanks

PS: power transformer and the OT are mounted VERY CLOSE.

[mhuss]

Perhaps check the filter caps in the first stage and in the FET overdrive sections?

[Jon Snell]

What is the hum frequency?
If it hums with no EL84s installed, it is likely to be transformer coupling issues.

[Palantine]

Perhaps check the filter caps in the first stage and in the FET overdrive sections?

Can you please number them?
What do you mean with "check"? Voltage?
Thanks

[Palantine]

What is the hum frequency?
If it hums with no EL84s installed, it is likely to be transformer coupling issues.

I didn't check the amp with no power tubes installed.
I Know for sure the hum decrease if I sit in front of the cabinet with the guitar in a certain position...no one inch left or right, above or below.

[mhuss]

Can you please number them?
What do you mean with "check"? Voltage?
Thanks

C8, C89

[Helmholtz]

I Know for sure the hum decrease if I sit in front of the cabinet with the guitar in a certain position...no one inch left or right, above or below.

That indicates (pickup) hum.

Only hum showing without anything connected to input is produced within the amp.

[Palantine]

That indicates (pickup) hum.

Only hum showing without anything connected to input is produced within the amp.

No one of my thirty tube amps (I know, I'm crazy) acts like this one with the same guitar

[Helmholtz]

No one of my thirty tube amps (I know, I'm crazy) acts like this one with the same guitar

Makes no difference regarding my statement.

Only hum with shorted input is generated within the amp.
The effect you describe indicates hum picked up by your PUs.
Probably due to stray field of the power transformer (PT).
PTs vary regarding stray field.

[Palantine]

Makes no difference regarding my statement.

Only hum with shorted input is generated within the amp.
The effect you describe indicates hum picked up by your PUs.
Probably due to stray field of the power transformer (PT).
PTs vary regarding stray field.

Do you think a mumetal foil (around the PT) can fix or tame the hum?

[Helmholtz]

Do you think a mumetal foil (around the PT) can fix or tame the hum?

Might help. No own experience, though.

I like PTs having endbells and preferably a copper flux band.

But typically it's sufficient to increase the distance between guitar and amp.

[nevetslab]

Do you also have any RF buzz on the output when you turn the input volume and Master up?

If you have a scope, take a look at the AC Heater Voltage at Pins 4/5 & 9. Is there a clipped sine wave with a step-off on the falling side to the sinewave at the positive and negative peaks? If so, that step is an RF source, and I've had issues with the J/J ECC83S Preamp tubes on several of the Hiwatt amp heads. I've had to add a hum balance pot to null that RF buzz, as well as changing to a different tube mfgr. I found the 12AX7A's that Mesa supplies perform far better than the J/J tubes as far as the RF buzz goes. Usually the amount of hum goes down with that nulling, though not always at min buzz is also min hum.

Now, I haven't yet had the T40 on my bench to see if it has any of the RF buzz issues I've seen on the HLR Little Rig, the Stage/Studio 20/40, the 50W and 100W Custom Heads.

[J M Fahey]

What happens to hum when a shorted plug is inserted at the input jack?

[Palantine]

Today, in the late afternoon, I'll go to one of my best friends and a real amp guru to experiment all your advices, then I'll report.

[Palantine]

So, after debugging the amp with an oscilloscope, Andrea found DC voltage in the Volume pot (VR6) so he decided to put a bypass cap across the wire that comes from the wiper of the volume pot to the second 12ax7 (called T4B); also he replaced the non shielded existing wire wrapping it wih a ground wire (see the pics).
I did a minor part by just shielding the cab with a copper foil making sure the copper touch the chassis near one of the four screw holes.
I can't say the amp now is dead quiet, but the sum of these things tamed a lot the hum and now the amp is more usable even at high volumes.
Thanks to all who contributed trying to figure out what would be the cause of the hum. This is a nice amp after all.