#1
Hello,
To start, after a move I noticed my reverb was not working. I applied pressure to the upper RCA connections and it worked again. The next day it was out again. I just took the tank out to check and clean connections. They are okay. Maybe the RCA cable or pre-amp tube? I will see if this works here shortly..

However, when looking at the power tubes, I notice that 3 are GT-6L6B and one is a Fender 5881.

Is this okay? factory?

I expected to see 4 x GT-6L6B

Thanks

1990 Fender Twin Reverb Re-Issue
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#3
Reverb cutting out could be a bunch of things, often it's the jack itself. See if you can get in there with a cotton swab and some contact cleaner to clean it out. Reverb cables and tubes are also worth a look, as you suspected.

Mismatched power tubes aren't optimal. It's unlikely they'll cause any damage, but to run the amp most efficiently you should have a matched set. I wouldn't worry about it too much, get a full set when you retube. If they're mismatched the power tubes will probably start making noise earlier than usual.
#4
It is possible that the 5881 has the same bias rating as the other tubes (a matched set electronically, not visually) which would be totally fine. I had 2 gt6l6 rated at 7 and 2 gt6l6 rated at 2 (groove tubes approximate their bias rating with a number for easier matching)in my 65 twin ri, and after about three years of playing at a moderate volume my filter caps melted. My amp tech said fender ri amps often have cheap filter caps anyway so now I have some nice meaty ones in there.