Tubes for my F-2B that don't fit!

[hammondd]

In trying out different tubes for my F-2B pre, I bought a couple of Tung-Sol 12AX7s.  Lo and behold, they wouldn't fit within the metal casings holding the tubes--just a smidge too wide...
 
Is there any reason I couldn't take the casings off?  Can they be removed? and is there a down side to doing so?  (I'm currently using electro-harmonix tubes, which I'd like to improve upon.)  Any suggestions?  How does one know which tubes will fit in the casings before spending $30-40., like I did???
 
Thanks--

[o1967442]

YOu know - I had a similar issue with my F-1X. I actually haven't been able to remove the tube that is in the unit to see what I have.  
By the replacement books it should be a 12Ax7, but that tube seems too big to replace whatever is in the shielding (metal casing) around it.  
Any info?

[keith_h]

12AX7's can have different envelope sizes. There is no specification so each manufacturer can be different. The best way to handle this is take the shield with you when you go out looking for a new tube. This way you can be sure the new 12AX7 will fit inside.  
 
Keith

[hammondd]

Is there a problem in removing the shields and using the F-2B without them?  You don't see shields on all other preamps.  Thanks.

[o1967442]

Thanks Keith,  
Good 'sound' advice...
(Pun intended - sorry)  
The real issue is getting it out without breaking the socket, etc....

[bsee]

Well, the way the shields anchor, they hold the tube in place.  I think you could go without them from an electronic perspective, but playing vibration and travel shocks could allow the tube to work free.  
 
If you want to try those tubes, you'll just have to keep popping the cover to make sure they're well seated all the time.  
 
Another thought might be to get your hands on another set of the tube sleeves.  I suspect that if you used the right tool (dremel cutting wheel, maybe?) and sliced the tube 80% or so of the way up the sides, you might be able to get them to support a tube that is slightly too large.  Don't know for sure as I've never tried it, nor do I know if it would be a problem for the tube to be held that snugly.
 
Finally, I haven't tried it in the F-1X yet, but the JJ Tesla long plate tube (ECC803s) sounds very round in my Fender pre.  I think I have the short plate in my SWR Baby Blue II right now and it also has great tone.  These tubes are in the $10-12 range at many dealers.  In each case, they replaced a GT tube and I preferred the result.
 
Good luck!

[keith_h]

It has been many years since I worked on tube radios and TV but as I recall the shields are grounded to protect the tube from external interference. Whether the F-2B or F-1X really need to be shielded in all cases I cannot say. This would be a question for Mica (or Ron). In the old radios I inherited I can say the low power tubes do need to be shielded from the rectifier and power tubes.  
 
Keith

[mica]

The shield helps the tube stay in with vibration. You want to keep it if you can, though it can be removed (carefully!) without causing any change in sound.  
 
There are a wide range of tube diameters, even from teh same manufacturer. We do not know of a supplier that will ship only one size, so we too have several boxes of perfectly good 12AX7 tubes, but they are too fat for our use.  
 
If you are shopping in person, bring your old tube that fits with you, and compare sizes - you can even just use a piece of string. Or, if you're mail ordering, maybe they will be kind enough to measure the circumference for you.

[o1967442]

Thank you Mica!  
Seems like we need to incorporate better standards on the Tube industry...