Low Hum in AB763 Reverb Channel

[The Walrus]

I built a Deluxe Reverb AB763 using a Hoffman Board, mercury magnetics transformers, and a standard deluxe reverb clone chassis.

It works great, and channel one is darn near noiseless. However, channel two, without the reverb on, is only close to being noiseless. It has a slight hum above 7 or 8 (on the volume). But when the reverb is on and the reverb knob is turned on, a hum starts that gets louder and worse as the reverb knob is turned up. It is a low background hum. I have tried replacing the V4 tube, as I thought that was where the reverb circuit was and thought that this might have caused the problem.

I have checked all the groundings. I am a novice and this was my first build. Setting the bias was a big event for me. Everything works fine other than this hum. I could live with the hum, but I want to get it perfect.

[booj]

A low hum when the reverb is turned up is normal. You're driveing the reverb pan with a class A amplifier. There are ways to get it to quiet down, but that would be a mod.
One thing you could do is disconnect the power supply (red wire on the reverb transformer) connection and add say, a 100 ohm resistor in series with the transformer and the supply connection. Then add say a 20uf/500 filter cap to the red wire/100 ohm connection. Positive to the connection and negative to ground.
I haven't seen too much about such a mod. I live with the hum on mine.

[The Walrus]

Thank you

Thank you. I will just live with it. It isn't that bad. I didn't know that a low-level hum was normal in the reverb channel.

[Diablo]

Make sure you have "good" shielded cables going to the reverb pan. I had some shielded cables in mine that weren't the best quality and added some hum. Better cables dropped the noise by a good bit.

[The Walrus]

Thanx

Thank you. The cables are new. Also, the hum is identical whether the cables are connected or not. I appreciate your input.

[TheTinMan]

Using shielded cable for the runs to and from the Reverb pot will help reduce hum. So will shielded leads from the input jacks and from the Volume pots to the first two preamp tubes' grids (pins 2 & 7).

Is the "output" end of your reverb tank as far away from the power transformer as possible? It should be.

It's important to know where the weakest signals are in your circuit and then try to protect them from interference. For example, the Reverb pot is way down at the poweramp end of your chassis and those are relatively long runs.

Also, try moving the reverb send & return cables around. Does keeping them 2 or 3" away from each other help?

A separate power supply node for the reverb transformer also helps, but you have to pay attention to grounding issues.

Are the heaters for all of your preamp tubes in phase? In other words, each pin 9 tied together and the 4/5 pairs tied together? I'm wondering if V4's heaters may be connected "backwards" compared to other preamp tubes.

Cheers,

Chip

[The Walrus]

Shielded Cable

Just for fun I think I will replace the wires from the reverb pot with shielded cable.

All the heaters are in phase. I think. I remember being careful when I did that, but I will check.

Thank you.