AC feed-through in my tube guitar amp, what might be causing it?

I have a small guitar amp running through a step-down transformer (amp originated from the US and over here the voltage is 220V). I can see about 150mV rms 50Hz Sine on both tip and sleeve of the guitar's input.

This is a schematic of the amplifier:
https://robrobinette.com/images/Guitar/Special6Ultra/Ultra_Kleuck_Tweed_Mod.jpg

The amp is grounded (checked that using a DMM), lifting the ground connection does not make things better.

Any advice on what might be causing this? And perhaps how to cure it?

On the schematic I see that the chassis is connected to the earth pin of the mains socket, but I don't see any connection between that and the circuit ground.

I would suggest connecting the circuit "ground" (e.g. the sleave of one of the inputs) to the chassis, either directly or with a 10 Ohm resistor. A direct connection would be safer, but may cause a bit of hum due to ground loops, depending what other equipment the amp is connected to.

godfreyl said:

On the schematic I see that the chassis is connected to the earth pin of the mains socket, but I don't see any connection between that and the circuit ground.

I would suggest connecting the circuit "ground" (e.g. the sleave of one of the inputs) to the chassis, either directly or with a 10 Ohm resistor. A direct connection would be safer, but may cause a bit of hum due to ground loops, depending what other equipment the amp is connected to.

Click to expand...

If you'll look at the input jack (JK1 for example) you can see the circuit ground is connected to 'earth' through a capacitor C1 which is 10nF which is pretty high impedance at 50Hz. Why did they do it? I once saw a similar setup and it was due to the fact the circuit used PNP transistors and had a 'positive ground' and if the power supply feeding it was hooked up to feed another circuit with normal 'negative ground' the power supply would short out.

Shorting the ground to earth would effectively kill any voltage I can see on the sleeve because the short is 'stronger' than anything else on the circuit. However why did you suggest a 10 Ohm resistor? How can that help with the 50Hz hum?

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Update - I checked the resistance between the input jack sleeve and the wall's ground and it came out 3.8 Ohms.

In such situation I would start with battery powering the audio amplifier. If this kills the hum, then you should better filter the DC output from the power supply.
If the hum enters along the way pick-up (on the guitar), input cable, then you will have to try various schemes of grounding the pick-up and the cable to amplifier ground to reduce the hum.

The amp is about 40W, so that makes running it from a battery somewhat impractical...

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Update - Connected the input sleeve of the amp to the wall's ground/earth prong and the noise did not change at all...

mr_monster said:

Update - I checked the resistance between the input jack sleeve and the wall's ground and it came out 3.8 Ohms.

Click to expand...

Something's wrong here. Earlier you said you measured 150V rms on the sleeve, but there can't be that much voltage between the sleeve and ground if the impedance between them is only 3.8 Ohms. Something would have caught fire long before now.

godfreyl said:

Something's wrong here. Earlier you said you measured 150V rms on the sleeve, but there can't be that much voltage between the sleeve and ground if the impedance between them is only 3.8 Ohms. Something would have caught fire long before now.

Click to expand...

SORRY!!! That's 150mV - I forgot 10e-3

With a ripple on your DC power you cannot do anything. Replace the DC power supply with a good one. Using a DVM to measure AC is not a good idea, DVM alone and its lines can and do pick up hum, at 150 mV. Try to use a good oscilloscope and screened test lines.

Due to hum problems people learned to use transistors in audio amplifiers as well as optical fibers for signal lines. Tubes are lovely but introduce the hum.

I had a guitar amp (tube type) which had a similar high-ohm connection to mains AC.

If I plugged it into the wall the wrong way, I got a shock from touching any metal on the amp or guitar. Particularly when I was standing on soil barefoot.

The shock was alarming but it was only small current.

AC voltage was actually checked using a digital scope.

What I can not understand is how can it be that after shorting the sleeve the ground I can still see 50Hz on it and there was no attenuation of the signal?