Bugera used a weird numbering layout.
The first tube that the signal sees is labeled V8.
So working from the input jack end Clean & Dirty use V8.
At that point Clean goes to the FX loop.
Dirty uses V7 & 1/2 of V6.
So if you have a spare 12AX7 you could try swapping out V7 & 6 one at a time.
Short of that, this amp utilizes relays to switch all of the separate channel features.
Tech time if that is the case.