Re: tube choice for Leslies

[GOFFProf@aol.com]

Ben - You don't need to 'subconsciously' ask --- The tube issue is a sore
subject with all of us!  Lots of questions, so my post will be long.  Sorry!

We have experienced so much trouble with 6550's failing prematurely with my
touring equipment that I finally requested manufacturer samples of all
available 6550-series tubes and tested them myself in Leslie amps.  

SURPRISE!  - the most expensive (gold line & GE 6550A's) were not the best!
 They are nothing like the old real-GE tubes!  The Ruby tubes were repackaged
Chinese tubes. 

A Chinese 6550 tube in any label should NEVER be used in a Leslie.  Although
they are inexpensive, they are the WORST choice.  They are gassy, poorly
constructed and do not stand up to the vibration from the woofer and rotor.
 After testing European and other tubes, I settled on a solidly-made Russian
(not-SOVTEK) tube manufactured by Svetlana, the main Russian-military
supplier of tubes.  I have them made to our drive and audio specifications,
specially tested and privately labelled with the gold "GOFF Professional"
logo specifically for use in Leslie's.   

(If anyone is interested in a set of GOFF Professional 6550B tubes,  Email me
and I'll make them available at Wholesale prices to board members.  We sell
them RETAIL for $60. per pair, and they can cost even more from dealers who
buy them from us.)

As far as when to change tubes-  it's difficult to say.  Tube performance
degrades steadily with prolonged use. The original bottle-type 500 hour-rated
6550 tubes and the later 2500 hour-rated 6550A tubes really don't work too
hard in a Leslie amp, as the tubes are actually capable of providing close to
100 watts EACH in a different amplifier environment.   In a Leslie, they are
almost idling along and don't degrade as quickly as in a guitar or other amp.
 BUT, they receive constant vibration and mechanical stress from the woofer &
rotor, although they run cooler due to the rotor spinning so close by.

Our new tubes sound great, have a hot output, and I expect at least 500 hours
out of them, possibly more.  We have not had a set fail yet in service other
than two sets on tour with a very big band which were hit and smashed by
microphones that had come in contact with the Leslie's spinning lower rotor.
  (needless to say, even easy-going-Al didn't replace those under warranty)

TUBES - AMP REBUILDING  --  When a Leslie 122 or 147 amp is rebuilt (normally
$199. but $189. this month as our Mail-Order Special in Keyboard Magazine),
many tests are run to insure that the original factory voltages, bias
conditions, and other age & component related failures are found and fixed.
  Installing a set of tubes in an original "grey painted" amp chassis without
having all of the capacitors replaced, the can cap replaced, the relay
changed, and some critical resistors replaced is just plain nuts!  Many
times, the reasons why a Leslie amp sounds, switches, or otherwise performs
poorly have nothing to do with tubes.
   
Just changing a set of tubes in an older amp does NOT require rebiasing of an
older amp if it has been rebuilt and performs well.  As info, some components
in the newer "gold" colored amps do not last as long as the earlier amps,
especially amps from the 80's.  The electronic component styles were
constantly being changed in production.  Due to high failure rates, the amps
with light-blue 150vac-rated suppression capacitors (in the upper left hand
corner as viewed from the bottom) MUST have these caps replaced!  If they
fail "open" the amp will snap loudly in the audio.  If the caps fail
"shorted" then BOTH motors will be energized simultaneously, probably causing
one to burn out!  The best money you can invest while restoring / repairing a
tube amp Leslie is to have the amplifier rebuilt!  New speakers and other
things should come later.  

My brother Bill rebuilds every mail-order tube amp we get (more than 12 per
week from around the world).  After rebuilding, most  "40 watt" 122 / 147
amps actually will deliver from 63 to 68 watts, depending upon the
transformer conditions.  The added power difference is not so important in
audio output as it is in providing headroom for the amplifier to run without
digesting itself!

TUBE MATCHING - is relatively important in a Leslie.  It is NOT so critical
to have a perfectly matched pair of 6550's as it is to NOT have a set with
large differences in output at a given drive level.  As the tubes age, they
will degrade at different rates anyway, so a "perfectly matched" pair soon
becomes "perfectly unmatched".  The secret in replacement 6550 tubes for a
Leslie amp is to get tubes that are good, consistent performers, and to
ALWAYS change the 6550's in pairs, and always from the same manufacturer.  

For this reason, our 6550B tubes generally fall within 10 - 15% output of
each other due to our design specification from Svetlana.   In a batch of 500
tubes, I reject around 12 or 15 and send them back for relabelling and resale
to "someone else".   In a recent batch of 100 GE (Owensboro, KY) tubes, we
flat rejected 25, and had shorted-tube failures in almost 25 more.  For
almost double the cost in tubes, this was completely unacceptable!   As much
as I dislike buying non-American replacement parts, the Svetlana tubes made
to our specifications are working well.  (some of our bands also use them in
guitar amps due to their warmth and smooth performance)  

We still test and hand-match the pairs to fine-tune the amp performance.  We
charge no more for this since if I'm going to put my name on them, they had
better perform as intended.   Sometimes, an extremely high-output tube set
will sound harsh (almost solid-state) in a Leslie, so output is NOT the
critical factor in tube selection!   

SOLID-STATE RECTIFIER ---  The VR-105 (OC3) rectifier tube in a 122 / 147
Leslie amp may not a suitable candidate for a solid-state replacement.  It
may work, but be sure that the replacement is exactly for an OC3, not a 5U4
or other rectifier tube.  If you're going to go to this trouble and expense,
why not get rid of the tube entirely and replace it with a good quality 105
volt Zener diode?  (Don Leslie did not have such an animal available when the
amp was designed.)  The zener diode is not as rugged as the OC3 tube in the
event of tube failure, etc., but they can be easily changed, and they are
cheap.  Some other components can also be removed if the change to a zener is
made. 
I do not necessarily recommend that this change be made, only that it is
possible and will work.  I personally install the tube rectifier as per the
original design.  

Ben, I hope this helped.  Maybe others have some additional opinions or
experience.  

Al Goff