Hello Doug!
On 03 Feb 98, Doug Southworth wrote to Lee Macmillan:
 DS> I have a question.  I have a Randall Pro-Tube 1000  head (55 watts, 5 12
 DS> ax-7's and 2 6Ll6's) with a slight problem.  After it heats up, it cuts
 DS> out, ie there is no sound at all unless I flip it into standby and then
 DS> back up to full on.  I am inclined to think that it is a capacitor that
 DS> has dried up, but I am not sure.
Doesn't sound like it.  That'll usually cause humming.
 DS> It has only started doing this after
 DS> I replaced the 6L6's.  I replaced them with some Groove tubes
 DS> from woodwind/brasswind, and they seem to be working fine.
Well... SOMETHING isn't working fine ~8-).  Honestly, if it started when you 
put the new tubes in, I'd pull them out and put the originals back in (you 
DID keep them for spares, right?).  If the problem goes away, then the source 
of the problem is either the tubes themselves, or the way they fit into the 
socket (unlikely, because merely flipping the standby switch "fixes" the 
problem). The contacts in the tube sockets may have widened a bit and aren't 
holding as tightly as they should be.  With the amp off, unplugged, and power 
supply caps discharged, you may be able to go into the sockets with a 
jeweler's screwdriver and carefully bend the contacts into a tighter grip.
Again, I'd put the original tubes back in first... if the problem still 
exists (and I'm betting it won't), then attack the sockets.
 DS> The only
 DS> reason that I mention this is because there is no Bias adjust screw on 
the
 DS> Randall (maybe they took a Mesa-Boogie approach to amp building), so I 
m
 DS> not sure how correctly they are biased.
It may also be cathode biased.  Cathode biased amps are, to a great degree, 
self biasing and you really don't have to worry about them.
 DS> They don't seem to run too hot or
 DS> too cool, but I wasn't sure if this would have anything to do with the
 DS> problem I am having now.
I'm almost positive that it wouldn't.  Improper bias will either kill your 
tone or burn the tubes out.
 DS> Also, is there some way to adjust the bias on Randall amps that I just
 DS> don't know about?
Bias can alsway be adjusted, just not always with a screwdriver.  Often, 
you'll have to change one or more resistors.  That's how my Carvin is set up. 
 The schematic says something like "Select R22 for -28vdc from TP-5 to 
ound"
 DS> If not, where can I get Randall tubes?
Good luck.
C-ya! Rich
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