Noisy effects loop on Road King head

[beebout]

My send level on my effects loop creates a hissing/crackling sound when it is turned up. It does this with no effects even being plugged in or either effect loop being turned on.

Anyone know which preamp tube runs the effects loops?

 

[tele_jas]

I couldn't find a RK tube chart, but the Dual Recto it's the V4 tube. Here's the Dual Rectifier tube layout: