Video of hum when I switch on the reverb of my Twin Amp

[tochiro]

Hi,

Here is a video of me switching on and off the reverb of my Twin Amp: YouTube - twin amp.wmv

The amp is only 4 years old and in very good condition, tubes are new and correctly biased, apart from the noise the reverb is working normally. The reverb tank cables are Ok. The tremolo also exhibits a similar hum on top of its modulation.

Do you experience the same problem?

Thank you.

[booj]

Wierd! Are you using a fan in the back of your amp? Sounds like motor hum. In any case it shouldn't be doing that. You say the amp is 4 years old. When did this start happening?

[knausz]

Hi There,

Can you switch the reverb on/off with the pedal unplugged using a switch on the control panel? A lot of amps put the ground path through the pedal and it could act as an antenna.

[tochiro]

There is no fan at the back of the amp.

I do not remember when it started happening or if it happened progressively. I don't know.

There is no button for the reverb apart from the footswitch but with the reverb switched on if I unplug the footswitch the hum remains the same so I do not think the footswitch is defective.

[booj]

I've got the PDF on your amp (Pro Tube), but its too big to download. If you need the scat I can scan it and post it.
The switching circuit is clever but everything depends on everything else to work right. (complex)
May be a power supply issue for the switching circuit

[knausz]

I looked at the schematic off the company's website:

Amplifier Schematics

Definitely more than the simple footswitch - the reason is that they need to sense 4 switches over 2 wires. The jist of what they do is to put an AC signal down the wire to the footswitch. The footswitch then rectifies it to a certain magnitude based on what switches are activated. The circuit in the amp figures out what happened based on what the waveform looks like. Control signals are generated and go to shorting devices to turn the features on/off. I would guess these control signals are not DC for some reason or another. That could be the power supply or a defective component. The clue is that both channels are affected. That points to the power supply or the opamp which generates the trem and reverb controls. The same opamp chip handles both functions.

How handy are you with electronics and a meter?

[tochiro]

Thank you all for suggestions.

knausz, I am not handy with electronics but a friend of mine is and could try to check different things. What do you suggest? Thank you.

[knausz]

First thing I would do is to check the power supply to the relevant parts of the footswitch circuit. Measure both AC/DC. The correct values are on the schematic. For example, the supply to the opamps is listed as a DC supply. It should have the correct DC supply and no AC component. Print out the schematic from the website for your friend and relay this information. Test points are labeled in the PCB drawing.

-Imre

[52 Bill]

If you turn the reverb control on the amp to zero and switch on the footswitch does it still make the hum?

[pecorporation]

Perhaps a ground is lifted on one of the wires to the reverb tank?

[tochiro]

If you turn the reverb control on the amp to zero and switch on the footswitch does it still make the hum?

If I turn the reverb control to zero, the hum is gone and I can switch on or off the footswitch there is no hum.

[52 Bill]

Then the problem is that the tank is picking up the hum and it is being amplified by the return circuit of the amp.

Make sure that all of the cables connections are clean and tight, and be sure that the output side of the tank is located away from the power transformer.

[Dave Curtis, dB AudioTech]

Before you go tearing into it, is it under warranty? Fender is 5 years, right?

[tochiro]

No it is not under warranty (it was only 2 years here when I bought it). I am really disappointed by Fender because I ask them questions many times about my problem but never got any answer.

I checked the reverb tank but could not find any problem there.

[Enzo]

Fender doesn;t have the manpower to provide repair lessons to the consumer. They would normally refer you to the nearest repair center.

Humming reverb can be so many things. Bad reverb pan, bad cables, poor connection, wire off inside amp, bad IC in reverb circuit, bad tube, etc, etc.