Tracking down a hum.

This time the noise doesn't dissipate. The noise pulses. get louder then softer louder then softer. I have no idea what the cause is now. I'm thinking a faulty rectifier.

Noise presented on both channel?

For a tube preamp, the power supply seems very complicated! Curious what's inside the external power supply and did you send it in for repair along with the preamp?

This is frustrating and starting to get expensive.

Send it back! Rogue Audio should not charge for a failed repair. 

Hum is present in both channels and pulses. Failing rectifier maybe?

Sounds like low frequency oscillations associated with humming! There could be many things causing this problem, faulty rectifier, diode, regulator IC, capacitor or bad solder joint... Hopefully Rogue will be able to fix it this time. Keep your fingers crossed!

I noticed a hum from my tube pre amplifier, a Rogue Audio Model 99 (line stage only)... I'm using the Rogue to send a signal to a Musical Fidelity X-P200.

BTW, the problem may be with the power amp, how do you come to the conclusion that the preamp is the culprit?

If hum occurs only in one channel, swap tube between channels and see the hum follow. if hum stay in the same channel, tube isn't the problem.

If there is hum in both channel, probably a power supply issue.