Yes it is fairly straightforward, but you will need to pull the cover and ID the tubes for us.
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I have a couple of CJ preamps and have bought tubes from them before.
Minuses: You will pay more for the tubes.
Pluses: You will be getting an "A" level tube. They have been tested in their circuits (not a tube tester) and they go thru a controlled burn in (on/off cycles) that they say will give them a longer life.
Worth it? Your call.
How to? If the unit is cold then just wiggle the old ones out gently. If the unit was on, let it sit for 20 minutes before pulling them so the caps can drain and so you won't burn your fingers. When you insert the new ones, line up the pins carefully, you shouldn't have to force them in.
No big deal as long as you don;t need to bias the tubes. If small signal tubes you can do it yourself. Just make sure to use same type tube in that position. I have a VTL 5.5 which i have cover off and tube role onece in a while, i leave top off. Like you said just wiggle out tube and reset it in tight. Make sure pins are in exact place. What type tubes does it take? I have some you may need.
All I could find on the tubes used in the CJ 14L preamp are that it has 2 valves. V1 and V2 and both are 8080s. I can't say that I have much familiarity with 8080s so I will leave it for those that do. I think I also read that these are called 6C4s. Good luck my only comment is that factory tubes are very very expensive and I personally can't see the difference between buying from a knowledgeable and careful dealer who screens out bad tubes and the factory.
Generally, Europe made some very good sounding small signal tubes. Since an EC90 is a European equivalent for an 8080, and depending on the brand of the tube, I would venture to say a Telefunken EC90 may sound a bit better or be quieter then their American equivalent. JAN tubes (joint Army Navy) U.S. made tubes can be very good as well. The problem is, the military spec for each JAN tube model was different. Was it a quieter tube or less microphonic? It was difficult to find out. In addition, consumer type RCA, GE, Sylvania etc. were some exceptional tubes. Many of them are highly prized, and this depends on the tube type. Did you know that some U.S. manufacturers branded their name on other manufacturers tubes?. Sometimes an RCA branded tube was actually made in Europe by Amperex. It wasn't uncommon back in the day. Johnmc87, not a dumb question at all.