[Harp-L] old princeton preamp swap?

[Greg Heumann <greg@xxxxxxxxxxx>]

Hey, Blake -

Thanks for the plug! Glad you like the mic.

Here's the deal with tubes in the 12AX family - there are a bunch of  
choices, all with the same pins and operating voltages, that have  
different "gain factors". Here they are:

12AX7 = 100
5751  = 70 (not a true 12AX but a substitute that works and provides  
yet another option)
12AT7 = 60
12AY7 = 45
12AV7 = 41  (I've never seen one of these)
12AU7 = 19

Guitar strings don't "couple" with the output from your amp's speaker  
nearly as easily as a microphone, and guitar pickups' output is lower  
than a good hot mic element - so more gain is needed for guitar than  
for harp. The goal is to be able to turn your amp up louder so that  
the power stage (the bigger tubes in your amp) is driven harder,  
resulting in some of that desirable warm distortion. You'll have to  
play around a bit, but I would suggest starting with a 12AU7 in the  
first position  and a 12AY or AT in the second. Depending on the amp,  
swapping positions may make a big difference or a small one. Try  
both. You should immediately notice you'll be able to turn up a lot  
louder before you reach feedback, and the sound will be warmer. If it  
is really muddy, go to a higher gain setup. If it is still unruly,  
try 2 12AU's. There are, of course, a huge number of possible  
combinations, but that should give you a good place to start.

/Greg

http://www.blowsmeaway.com
http://www.bluestateband.net

> I just received a beautiful new mic from Greg at blowsmeaway.com,  
> with a hot
> vintage CM element in it. Greg is as advertized - very patient,  
> helpful,
> and professional. (and I'm a picky pain in the...) The  
> craftsmanship is
> goregeous, and the mic has a wonderful warm crunchiness with great  
> bottom -
> it simply growls with my big chromatics and low diatonics.
>
> I'm considering doing a preamp tube swap. I am a tube / electronics  
> idiot (I
> was raised by a pack of wild academics)  My amp is a '62 Princeton.  
> The
> problem is that I'm getting a lot of feedback at relatively low  
> volumes.
> Like, say, anything above a 2. I believe that the amp has two  
> preamp tubes.
> Both of them are 12AX7s. My understanding is that these are very "hot"
> tubes, and that I may wish to consider another tube.
>
> O.k., assuming I can get different tubes, which one or ones do I  
> switch? How
> is this done? What tubes should I look for to get more volume without
> feedback?  Any other feedback advice with my configuration?
>
> Thanks in advance.
>
> - Blake