F2-B preamp channel 1 problem

[analogger]

Hello - I recently purchased an F2-B on ebay that had an Op Amp installed.  It worked ok for a short while, but now the channel 1 pots don't work at all, but it still produces a low-level signal.  Channel 2 works, but cuts out from time to time and gets static, esp. at higher volume.  Can anyone suggest a DIY fix here?  I'm OK with a soldering gun and have very basic electronics experience.  I've already spent good money on this pre, and not looking to spend another $150-200 to get it fixed.  Thanks ~ Scott  

[JimmyJ]

Hey Scott,
 
Welcome to the forum.  These units are generally built like trucks, extremely solid and reliable.  The circuit is not terribly complicated and the schematic is available so you may be able to get it running yourself if you have some basic skills.  But be careful in there, if you don't understand the dangers then don't go poking around.  
 
I am not familiar with the op amp mod.  If that's a factory installed option then the tech support you get here will include it.  But if it's not factory work then your best bet might be to remove it and get the unit back to stock design.  (Perform an opampectomy!)
 
Good luck!
Jimmy J

[analogger]

Hi JimmyJ,
Thanks for your reply.  It's looking like surgery will not be required!  The problem might just be some old, worn out tubes.  I switched their sockets and the problem went to the other channel. Can anyone recommend a good 12au7 for the F2-B?
Regards,
Scott

[keith_h]

I noticed you said 12AU7. This is not the standard tube. The standard tube is a 12AX7 which has a little more gain.  Any tube that matches the 7025 spec should be fine (I think most do these days). Some tubes have a larger envelope than others so be sure to take the shield with you to ensure the tube fits.
 
Keith

[sonicus]

Yes make sure that it fits.  As far as newly manufactured tubes you will currently find mostly tubes from Russia and China. It has been discovered through test studies that the Russian tubes have a higher or harder envelope vacuum and the Chinese tubes a lower or softer envelope vacuum, .  A vast difference in sound has been reported ; the Russian tubes render a harder more  defined transient response and the Chinese a more mushy softer easier overdriven response. The Russian tube perhaps being more disable for Bass playing. Many Guitar players who like the over driven sound have liked the Chinese tubes,
 
            Among tube sound connoisseurs  many will strive to find new old stock  RCA  Red Label tubes . In Audio Engineering circles new old stock  TELEFUNKEN tubes are highly prized for their characteristics . All tubes do not render the same timbral and responses characteristics and sound and therefore one can pick and choose until you find the one that FLOATS YOUR BOAT .
 
(Message edited by sonicus on January 23, 2010)

[analogger]

Thanks a lot for all the great info.

[slawie]

Why not make your own?
 
http://www.youtube.com/watch?v=gl-QMuUQhVM
 
Check out this skilled artisan who has posted how to make
your own vaccuum tubes. The glass blowing part is particularly interesting.
Quite frankly though I would leave it to the experts.
 
slawie

[gtrguy]

Get a JJ, I think Alembic might even use them now. Make sure whatever you get will FIT into the tube cover thing, some don't.
Dave