could possibly be the tube, but it seems like it would do the same thing in parallel mode too if it was the tube, but couldn't hurt to swap it. also, i'd check the wiring inside the unit to make sure it is all kosher and check all the solder joints. I have heard of a few of the units breaking wires and joints loose. only a few, though, most of the long term reviews seem favorable. check all the solder lugs on the pots and jacks as well. as far as i know it is a very simple circuit just like a c-lator, with the addition of the reverb, so it shouldn't be too hard to chase the problem down.

Also, i snagged this from this instructions, in case ya didn't have em

When the Verbrator is used as a loop, the pedal is plugged into the amplifier patch jacks. The output from the preamp (usually labeled send, or preamp out) is sent into the jack marked “From Preamp”. The output to the effects is taken from the jack marked “to effects”. This is a low impedance send which can run pedal or rack level solid state or tube equipment easily. The send level is adjusted by the send knob.
The return from your effects chain plugs into the jack marked “From Effects”. The return level is adjusted by the return level knob. This knob may be pulled to get additional return gain for lower output pedals or rack devices. This is adjusted to taste.
When the pedal is used in parallel (normal) mode, the send and return controls are used to adjust that the input to the first pedal is not overloaded, and the return level may be adjusted for how much effect you desire. When the pedal is in series mode, the return control will set overall level of the system, and the Verbrator should be adjusted so the level of the amplifier (your system level) is approximately the same as when the unit is not used. The intent of a properly functioning effects loop is to insure proper level and impedance matching from the amplifier to and from the effects loop. While the unit can add gain when required, it is usually not recommended as it increases the chances of extraneous noise.
When using the unit as a reverb, it can be used a number of ways: Plugged directly between the guitar and amp, at the end of a pedal board, plugged through your amps effects loop (which may be a passive or active loop).
When the unit is used as an effects loop, the reverb mixes into the dry signal (or the effected signals returning from your effects chain, in the output gain stage. Levels must be carefully adjusted for the proper mix during use.

Gregg

sounds like it could be a bad pot, or maybe the connection to it.

Gregg