LectroLab R-500 60 Cycle Hum

[misterc57]

I installed a 3 prong AC cord, removed death cap and installed 3 new 22uf/500V filter caps. This is a point to point and there is some final trimming of wires that I have not completed yet.

After testing the amp I noticed the amp has a 60 cycle hum which is noticeable when dialing up either of the two volume pots past "3" and it gets louder as you increase the volume. With or without an instrument plugged in.

I tried swapping the two preamp 12AT7 tubes.

I always find 60 cycle hums elusive to diagnose and could use some suggestions please.

Lectrolab_r500.pdf

[Helmholtz]

A scope pic of the hum could help.

[The Dude]

I can't tell from the picture. Is one side of the filament winding grounded as the schematic shows?

[35L6]

I hope the inputs are not wired as drawn .

[g1]

I hope the inputs are not wired as drawn .

Are you referring to the contact bias, or something else?

[Jon Snell]

Mains hum, 50 or 60HZ, is always a poor ground. Usually on the input jacks.
You are using 12AY7s, not 12AT7s?
Also this is using the leaking grid method of biasing with extremely high valued grid leak resistors for the first preamp stages.
Meg Ohm resistors are notorious for going very high resistance.

Check your earth on the three pin plug lead is connected to a chassis point;- the point where ithe old death cap was grounded is best for that.

[misterc57]

Thanks everyone. Problem turned out to be no ground at the input jacks and resistors/caps before V1. It was using the center metal hole of the V1 socket as ground, which was not grounded. I ran a ground to one of those wires and the hum is gone.

To answer some other questions.

The filaments are grounded via a cement 5W 250 ohm resistor. Not as shown on schematic.

The input jacks are wired per the schematic.

The amp cranks and does sound good.

Also attaching a scope image I took of the hum. No input signal. volume 1 set around "5".

Thanks again!

[Helmholtz]

Are you referring to the contact bias, or something else?

I'd say low input impedance.

[35L6]

33k to ground no matter which one you plug into .

[g1]

The filaments are grounded via a cement 5W 250 ohm resistor. Not as shown on schematic.

I think this will be the cathode resistor for the power tubes. Referencing heaters to this elevated DC point is sometimes used.

[misterc57]

Mains hum, 50 or 60HZ, is always a poor ground. Usually on the input jacks.
You are using 12AY7s, not 12AT7s?
Also this is using the leaking grid method of biasing with extremely high valued grid leak resistors for the first preamp stages.
Meg Ohm resistors are notorious for going very high resistance.

Check your earth on the three pin plug lead is connected to a chassis point;- the point where ithe old death cap was grounded is best for that.

12AY7 and yes on the ground point.

[Helmholtz]

33k to ground no matter which one you plug into .

There's a 33k series resistor and a 33k shunt resistor.
This means voltage dividing by two.
Input impedance is 66k either input.
The input wiring is a poor attempt to isolate 2 simultaneously connected instruments/sources.