From: dam@dcs.glasgow.ac.uk (David Morning)
Newsgroups: alt.guitar
Subject: Re: Built your own amp?
Date: 20 Nov 92 10:12:52 GMT
Organization: Glasgow University Computing Science Dept.

neal@cmptrc.lonestar.org (Neal Howard) writes:

>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:
>>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 EL34 is a beam pentode. Originally Marshalls came equipped with Mullard
versions. When they were shipped to the US some bright spark had the idea of
pulling the EL34s and replaceing them with 6550, also a beam pentode. 6550
give a little extra power but with a slightly harder edge. The reason for the
change was one of reliability however, initially they didn't rebias for the
6550 giving early imported Marshalls a *bad* sound. It was rectified later.
Marshall eventually got round to fitting 6550 at the factory for the US market
but apparently Jim Marshall didn't like the sound much. Current Marshalls are
now fitted with EL34s again, probably of Czechoslovakian or Chinese manufacture.

The purpose of 'beaming' is to improve the low level linearity of a multi-grid
valve. Most pentodes do it.
It's fairly common practise in the US to connect the extra screen grid 
internally to the cathode whereas in the UK all the grids are brought out to
the pins and the cathode connection made externally. That's the only reason
an EL 34 appears to have a true screen grid, a 6550 has one too, it's just
connected to the cathode inside the valve to save you the trouble.

Maybe your thinking about a tetrode like the KT88.
.

>EL-34/6CA7's. The American made GE and Sylvania 6CA7/EL-34's were beam
>power tubes and not power pentodes. 

I don't know about the US firms but I'm sure Mullard would be extremely 
surprised to discover that their EL34 wasn't a pentode.

Technically it was a beam power pentode.
You can't always work it out by counting pins.

Dave

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