Any risk in swapping preamp tubes?

[jakelly]

I want to try some old 12AX7's in my Mesa Mpulse 600. I have a bunch of old, used GE and RCA 12AX7 tubes, but they are untested so I have been hesitant. What if a tube is shorted or otherwise no good? Could that mess up the amp, or will it just not sound good if the tube is no good? I can see that the used tubes look OK with silvery gettering, that they haven't lost vacuum, etc.

It probably has Mesa tubes in it now.

[SactoBass]

You won't hurt the amp. Just use your ears for testing what sound you like best. I'll bet those older tubes you have will sound better than what came in the Mesa. I could be wrong, but I believe Mesa just takes Groove tubes and re-labels them. They are average quality tubes. Your GE and RCA tubes will likely sound better.

Have fun. Tube rolling is fun to do.

[iiipopes]

Not necessarily. The 12AX7A tubes that are designed for low noise can sound better. The Tube Store has a chart about the currently available preamp tubes and their comparative performance.

http://thetubestore.com/12ax7review.html

Also, it is common to get a low-gain ultra-clean tone by swapping out a #1 preamp tube from a 12AX7 to either a 5751 or a 12AY7 (the original Fender tweed preamp tube). But be careful, as the internal impedance of tubes can affect the circuit. For example, you may not want to use a 12DW7 in the #1 stage, but it can be good, depending on the amp circuit, to use it in the #2 position to buffer the internal impedance going to the phase inverter.

http://thetubestore.com/gainfactor.html

And yes, tube rolling is really fun. For example, on my BillM modded Blues Jr guitar amp, I use 3 X 12AX7 for a "mini-Marshall" tone, and 5751, 12DW7 and 12AT7 for "vintage tweed" tone.

[jakelly]

Thanks folks.

The old 12DW7 must be pretty rare. I used to sell tubes and maybe came across just one, out of numerous batches. But it was fun to go through bunches of tubes to find the desirable ones.

[coreyfyfe]

Originally Posted by jakelly

Thanks folks.

The old 12DW7 must be pretty rare. I used to sell tubes and maybe came across just one, out of numerous batches. But it was fun to go through bunches of tubes to find the desirable ones.

They're around, just not used as much. Plus a few companies have started making new ones (JJ for instance). They're used in vintage SVTs, V4, as well as some other amps.