From: neal@cmptrc.lonestar.org (Neal Howard)
Newsgroups: alt.guitar
Subject: Re: Power Tubes (long) was Built your own amp?
Date: 21 Nov 92 01:46:19 GMT
Organization: CompuTrac Inc., Richardson TX

Sorry for the long post, email to David bounced, besides other tube-heads
out there may like to read my blabberings.

In article <By0F1H.8E1@dcs.glasgow.ac.uk> dam@dcs.glasgow.ac.uk (David Morning) writes:
>
>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
>
>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.
>

Screen grid connected to Cathode ????? HuH ???? Here's the terminology 
I'm using (this is from ARRL books, these are the US terms. Your UK terms
must be different from what we're familiar with over here):

    Triode = cathode, control grid, plate  (let's exclude filaments
         				    as cathodes in directly
					    heated types.)
    Tetrode = cathode, control grid, screen grid, plate.
    Pentode = cathode, control grid, screen grid, suppressor grid, plate.
    
    We must be disagreeing due to a language barrier, in US tube terminology,
    the screen grid is the second grid from the cathode going outward to the
    plate.
    That means both "power pentodes" and "beam power tubes" (and tetrodes) have
    screen grids, usually connected to a high positive voltage  at a little
    lower potential than the plate.
    
    All the 6550's I've ever seen (or taken apart) have no third grid 
    ( suppressor grid is US terminology). In it's place, they have a much more
    solid piece of metal, like a plate, (the "beam" electrode) but with holes
    cut in it as little "windows" thru which the electron beams pass. This
    electrode is usually connected internally to the cathode like the 3rd grid
    in a lot of pentodes. Having this kind of electrode (instead of a real 
    "grid") is why we refer to it as a "beam power tube" even though it is
    technically a variety of pentode. The same description applies to the
    "real" 6L6's we used to be able to get here and also to the US made 
    6CA7/EL34's that used to be available here (but are no more). The European
    style EL-34's (the only ones available now) that I've taken apart
    have all had a real third grid, made of wire wound around forms, similarly
    constructed to the 1st and 2nd grids. If there are 6550's made nowadays 
    that have a real 3rd grid, please post the brand names, I'd really like
    to get four of them for my amp since the only ones I've got left are a
    pair of the small GE's and a pair of the big GE's (all are beam power type)
    and  my best friend has been begging me to relinquish the two big ones for
    his 50w Marshall and I'm afraid I'm gonna have to give in since his amp
    sounded so bad the last time we played. Right now I'm currently running
    6L6's (the real ones) and occasionally switching to some Sovtek 5881's
    for a little variety. I've thought a lot about just using the EL-34's but
    I'm hitting the output tubes with nearly 725 volts DC and I was afraid
    that'd be a bit rough on EL-34's. (I can vary the 1st grid bias from
    anywhere between -25 to -75 volts). I guess I could cut the B+ down to
    half and use the EL-34's on 350volts since I really don't need all the power
    output.... I'd rather sacrifice some raw output power for better output
    tube overdrive that wasn't as harsh as the 6L6's since I've got plenty
    of volume and can always mike the guitar cabinets through the PA system if
    I ever need the extra volume.

    Ob KT-88's> I've got access to four ancient KT-88's and a pair of KT-66's, I
    wonder what they'd sound like in a guitar amp???

    Whew !!!!

-- 
=============================================================================
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)
=============================================================================
Organization: Sophomore, H&SS general, Carnegie Mellon, Pittsburgh, PA