AB763 - Deluxe Reverb Build - Hum at Idle - Solved

[ToneWitch]

Built this deluxe reverb from mojotones spec and layout.

My power and preamp grounds are seperated.
I have a long ground buss wire tucked up above the pots. Using the PT center tap.

I went through the normal check. Pulled one tube at a time. The hum stops when you pull the PI tube. The hum does get louder as you turn the volume up. No other control effects the hum. The hum is there even with no guitar plugged in.

So I checked everything at PI section of the board. It’s all good. Nothing is sensitive and my voltages look fine.

The amp sounds incredible. But this hum is killing me. I am aware that fender used a large brass plate behind the pots. Idk. This has got me stumped.

 

[ToneWitch]

Solved it.

 

[Springfield Scooter]

Great! What was the issue?

 

[Len]

Solved it.

How?

 

[neikeel]

On my build I spent a while checking for ground loops and power supply filtering. I got the scope out and discovered it was inadequate filtering on the bias - hum was coming through the grids!
Would be very interested to know solution here.

 

[ToneWitch]

How?

I forgot to put in the low voltage center tap lol

It’s dead quiet now. Just simply overlooked it.

 

[william vogel]

I forgot to put in the low voltage center tap lol

It’s dead quiet now. Just simply overlooked it.

The heater center tap? That will cause serious hum. Glad you found it.

 

[ToneWitch]

The heater center tap? That will cause serious hum. Glad you found it.

Yeah, for some reason I thought this transformer had a low voltage and high voltage center tap. So I had convinced myself it was already in. Then realized the transformer only has a high voltage tap. 2 100ohm resistors off the bulb and it’s so good now.

 

[Im247frogs]

I recently bought a Silverface Bassman that some genius had desoldered and taped off the 6v center tap, for some unknown reason. Maybe they did it and then forgot to add a virtual ct? Hummed like a power station. I could see why the recording studio I got it from couldn't use it. Awesome amp, shoulda kept it.

 

[Dblgun]

On my build I spent a while checking for ground loops and power supply filtering. I got the scope out and discovered it was inadequate filtering on the bias - hum was coming through the grids!
Would be very interested to know solution here.

neikeel, did you increase the bias cap over the 25uf used originally? If so, how much did you have to increase it to eliminate the hum?

 

[neikeel]

I cheated and rebuilt bias circuit like a Bassman/JTM45!!

 

[ToneWitch]

Well, it’s got another issue. I finally got a tank and pedal for it and I have really low volume out of the vibrato channel both inputs. Hmm. Any ideas? I retraced everything and it all looks good. Anyone got another place to check for that condition before I tear it out and redo it? Lol

 

[ToneWitch]

Solved this issue also. Went through and traced everything for a 4th time and found that I had V4 pins 6-7 swapped with each other. Things sounds incredible!