New Build With a Problem

[JKRamsey1991]

So I have built my first tube amp and it is up and running! There is a problem though I can't figure out.

When the volume control is at 0 turned completely off there's a loud hum. Bump it up just a bit and it's quiet. The amp is based on a Single channel bassman with some mods from the trainwreck express. She sounds pretty good overall but I haven't had a chance to crank it yet.

After the noise started I completely reworked the eq control wiring and made it match a fender twin normal channel. Noise still persists. Tonight the only thing I have planned is to run shielded cable from the tube to the volume control but I doubt it will fix it as it's only noisy when the volume is on 0.

Any guidance guys?

[sluckey]

Sounds like a defect volume pot. Just replace it.

An easy test would be to connect one end of a gator clip test lead to chassis. Set the volume pot to zero. Now connect the free end of the gator clip lead to the center lug of the volume pot. If the loud hum goes away the pot is defective.

[JKRamsey1991]

I'll test that when I get home tonight then as well!

If that is the case I guess it was luck I can't count and ordered an extra 1 meg pot lol.

[sluckey]

If that is the case I guess it was luck I can't count and ordered an extra 1 meg pot lol.

Always nice to have a little extra pot lying around. 

[JKRamsey1991]

 

That was a good one!

I am going to have to put a master volume in this thing. It's pushing 50 watts but seems a heck of a lot louder than my AB165 Bassman. Not sure why could just be perceived volume due to eq I guess.

Hopefully it is the pot that's bad and I can rewire the controls like the trainwreck schematic.

[JKRamsey1991]

Well that didn't work. Here's a video of the noise.

[sluckey]

Well that didn't work. Here's a video of the noise.

Replacing the pot didn't fix it?

[JKRamsey1991]

 Nope. It passed the test replaced it anyway and the noise persisted.

[sluckey]

Nope. It passed the test replaced it anyway and the noise persisted.

Did the noise go away when you had the gator clip test lead on the pot?

[JKRamsey1991]

Nope changed the timbre of the tone of the noise a bit but didn't decrease it any.

[sluckey]

Show us your as built schematic. Also post some hi-rez pics of the inside of the chassis.

[JKRamsey1991]

Here's the pics. Give me a minute to upload the schematic

[JKRamsey1991]

The build is basically an AA864 bassman phase inverter and power amp with the preamp of the trainwreck express from the triode layout.

[JKRamsey1991]

I FOUND THE PROBLEM!!!!!!!! (actually two problems).

1. I did not have a resistor going to ground for the negative feedback circuit. Installed a 100ohm resistor to ground.

2. The main culprit was the little 2in run of wire from the 0.1 cap to the entrance of the phase inverter. I pushed it as far towards the phase inverter as it would go and BAM no more noise. I can crank it now and its quiet as a church mouse.

I am not going to admit how many hours I have spent trying to fix this.