It sounds like the V2 tube (original one) is questionable. In the Caliber series, the channel switching adds V1 to the front of the pre-amp chain. Since it adds an additional stage of gain, you end up amplifying any noise from from V1 in the following gain stage. My theory on what you are experiencing is that while on lead channel, the noise wasn't enough to be noticable through the distortion the lead channel adds. Swapping tubes caused your noisy tube to now be amplified in that additional stage.
Noisy tube in V2, you could live with to a point. A noisy tube in V1, means all the noise it might generate gets amplified in those stages and then re-amplified again by the V2 stages. V1 does not get used in Clean channel at all.
If you have any low microphonic tubes (surrounded by heat shrink, and is a different part number than the "non-low microphonic" version) use them in V1 stage. Preferably, in both V1 and V2. These are the most crucial to have low noise tubes, as any noise they have will get amplified and signal to noise ratio goes down. The earlier in the circuit any noisy tubes may be the more prevalent the noise will be. And conversely, you can have noisy tubes later in the circuit (V3) and not affect the noise level much, due to the later stages not amplifying the signal (and noise) that much.
Had a similar problem in my .50 Cal+. Previous owner (or tech) put the low microphonic tube to V3, and put the normal, high(er) noise, tubes in V1 and 2. When I did tube replacement, I used the old tube installation locations as a guide, which meant my brand new tubes were noisy. I swapped the V3 (low microph) tube into V1 and magically my noise problem went away.
Stephen
