Help with reverb issue on my Fender Twin

[TheMadCap]

Hello everyone, I tried to search for this issue, but did not find one that could help me out, so here goes:

I have a Fender Twin from ~1997 or so. This amp is the one with the three channels and the pull knobs. Anyways, the reverb works fine on the gain channel, but I get nothing on either the clean or the semi-gain channel. Any ideas on what might be wrong? I did move this amp for a jam session I had recently, and the reverb was working find then. Thanks, and let me know if more information is needed...

[Gutmann]

Is it THIS ONE ore the RedKnob Version?

There was another reverb-problem with a twin, HERE - maybe you can find some help there, but the problem was a bit different.

[TheMadCap]

Is it THIS ONE ore the RedKnob Version?

There was another reverb-problem with a twin, HERE - maybe you can find some help there, but the problem was a bit different.

Yes, it is a redknob version.

I did see that particular post, but it's not helping with my present issue. Thanks for your response...

UPDATED: Looks like the culprit is a bum tube. It appears that #4 might be barely alive, but definitely on it's last legs. so this poses a new question, I've actually never replaced tubes on my amp before, do they need to be replaced in pairs?

[markb]

Only the power valves need to replaced as a set (4 on your Twin). Preamp valves can be swapped at will.