From: neal@cmptrc.lonestar.org (Neal Howard)
Newsgroups: alt.guitar
Subject: Re: Built your own amp?
Date: 19 Nov 92 01:41:55 GMT
Organization: CompuTrac Inc., Richardson TX

In article <TOM.92Nov17210326@bailey.cpac.washington.edu> tom@cpac.washington.edu (Tom May) writes:
>In article <12NOV92.19291772.0050@UNBVM1.CSD.UNB.CA> META000 <META@UNB.CA> writes:
>>Many mid-power tube specials
>>use a pair of 6L6GC power tubes, and these can sometimes be rebiased
>>(depending on design) so that EL34 ("Marshall" tubes) can be
>>installed in the same sockets.
>
>My Marshall has 6550 power tubes.  Does anybody know what the difference is
>between these guys and 6L6 and/or EL34 tubes?  I'd never even heard of
>6550s until I got the amp a couple years ago, and I've never seen them
>mentioned in this newsgroup.

Most Marshalls made from the mid 1970's to recently came equipped with
6550's.  6L6's and 6550's are both classified as "beam power" tubes which
have a specially shaped plate electrode and a "beam directing" electrode
(in lieu of a 3rd grid) which focuses the electron beam onto 2 extra heavy
areas of the plate. The 6550 and 6L6 have almost the same pinout but the
6550 can handle a little higher current (i.e. higher power rated) than a
6L6 and likes to have a little less 1st grid bias than the 6L6 in typical
guitar amps (i.e approx -40 to -50 volts for 6550's and approx -50 to -60
volts for 6L6's). The EL-34 is a "power pentode" which has a true 3rd grid
and a more conventional plate. It likes lower 1st grid bias also (-35 to -45v).
The "desireable overdrive" characteristics of these three tubes in order
of most-to-least (this is my opinion only, no flames please) is
EL-34 followed by 6550 and lastly 6L6 (cleanest sounding of the three).
The 6550's will put out more raw power than the EL-34's in a typical
Marshall and they will last longer but most people that want to get some
power amp tube overdrive (as opposed to preamp tube shred/crunch distortion)
prefer the sound of the EL-34's. Be aware thet here in the US there are (or
at least used to be a couple years ago) two different  tubes being sold as
EL-34/6CA7's. The American made GE and Sylvania 6CA7/EL-34's were beam
power tubes and not power pentodes. They looked big and rugged like a 6L6
on very mild steroids but sounded like crap in guitar amps and needed about
-65 volts of 1st grid bias to keep them from melting their innards in
most guitar amps which reduced power output and made the amps too clean
sounding. The "European" style EL-34's are kinda skinny looking compared
to an American made 6L6 and are what you want for power amp overdrive.  
There were also two different sizes of 6550's made. One was appreciably
larger than the other and is the desireable kind to find also. Both GE and
Sylvania sold both sizes and sometimes whenever I ordered a dozen of them
I would get some of each even though I specified I only wanted the big 6550's.
Nowadays, I'm glad to even be able to find the small ones. I am glad I stocked
up on a few of the good American made 6L6GC and big 6550 tubes for my own amps
while they were still easy to find a few years ago... and NO I won't sell any
of them. They're mine MINE all MINE !!!!! hahahahahahahaha...... oops, sorry,
I must've twisted off a bit there.
-- 
=============================================================================
Neal Howard   '91 XLH-1200      DoD #686      CompuTrac, Inc (Richardson, TX)
         "Let us learn to dream, gentlemen, and then perhaps
          we shall learn the truth." -- August Kekule' (1890)
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Newsgroups: alt.guitar,rec.music.makers.guitar