Does Adjustable Bias Mod Introduce Hum?

[apehead]

I have performed the "1 ohm resistors on cathodes" modification on two formerly fixed bias amplifiers, a Peavey Butcher and a 1974 100W master volume Fender Silverface (Quad Reverb). It became noticable on the Fender (the Peavey always had a little line hum to it). After the mod, I started hearing an extremely quiet hum at normal audio levels (about "5" on the channel vol., and "2 1/2" on the master). This is how I preformed the mod:

Disconnect ground from the cathode pins on power tube sockets

Place 1 Ohm resistors in series from ground to cathode on all power tube sockets

Solder leads in parallel across one of the 1 ohm resistors. Connect to installed test points.

For the Peavey, I just removed the 47K resistor in the bias circuit, and replaced with a 50K trim pot.

The Fender already had the "Balance" pot rewired to be the Bias pot when I started working on it, I just wanted a way to measure the current.

Is this an inevitability when installing this mod? What would introduce hum? The ground lift? Poor solder joints? Would a cap in parallel help?

[Bruce / Mission Amps]

I have performed the "1 ohm resistors on cathodes" modification on two formerly fixed bias amplifiers, a Peavey Butcher and a 1974 100W master volume Fender Silverface (Quad Reverb). It became noticable on the Fender (the Peavey always had a little line hum to it). After the mod, I started hearing an extremely quiet hum at normal audio levels (about "5" on the channel vol., and "2 1/2" on the master). This is how I preformed the mod:

Disconnect ground from the cathode pins on power tube sockets

Place 1 Ohm resistors in series from ground to cathode on all power tube sockets

Solder leads in parallel across one of the 1 ohm resistors. Connect to installed test points.

For the Peavey, I just removed the 47K resistor in the bias circuit, and replaced with a 50K trim pot.

The Fender already had the "Balance" pot rewired to be the Bias pot when I started working on it, I just wanted a way to measure the current.

Is this an inevitability when installing this mod? What would introduce hum? The ground lift? Poor solder joints? Would a cap in parallel help?

Inevitable? No.

However, it is remotely possible that your speaker output transformer ground point is now 1 ohm different then the power tube's cathodes new ground point and maybe your test lead wires are adding some more resistance too.
I have never run into this myself and can't see your workmanship so I can't say what the real problem is.

[stokes]

Where did you ground the 1ohm resistors?Try it with the leads to the test points removed,maybe they are routed such that they are the cause.

[apehead]

I think I discovered the cause of the hum. An unrelated issue explained here.