This is one explanation: Microphonics. Try tube dampers.
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Assuming you mean the hum is coming through the speakers, rather than directly from inside the preamp, and that you've somehow determined that the power amp is not responsible, I'd guess a filament-to-cathode short in a tube. Another possibility, more difficult to isolate, would be a failed capacitor, such as an electrolytic in the power supply section of the preamp.
Another possibility -
The original hum may have been from a loose RCA cable to the amp. If the center pin was making contact, but the outer outer ring was not, that would cause a (very) loud hum. Are you sure all cables are firmly reconnected? Could be a bad connection inside a cable, also. Try swapping in some other cables.
Loud him in mine was a bad 40 MF section of main power supply dual filter capacitor. Antique Electronics has a CE Manufacturing 40/20/20 MF, 500v can that will work fine. You will need to drill additional hole in circuit board, but it fits fine. It is 2 1/2" tall. Original is 2" tall. New one fits fine.
Even after replacing filter caps, I had hum even with the volume turned down. I swapped tubes. No change. I checked voltages on the zener diode string and each was reading 36 volts instead of 33 volts. The regulator transistors Q2 & Q3 were bad. I used a MJE15032G for Q2 and MPSA42G for Q3. I replaced Q1 also with MPSA93G though it checked ok. Quiet as a mouse now! Ground rings on many jacks are bad. Next job is replacing them with jacks with nuts on inside of jack panel.
One RCA socket lost ground from the cable yanking.
This was my first thought as well. I lived with CJ preamps in the PV-5 through PV-8 series and the internal RCA wire connections tend to weaken over time with just normal use, partlyh due to the less than robust pop rivets holding the phenolic that is the RCA plug support attached to the back panel. Plus, modern RCA cables tend to have heavier plugs and cables which may put strain on the ports. Beautiful vintage CJ sound from these units.