From: peterca@ento.csiro.au
Newsgroups: rec.audio.high-end
Subject: Request for tube article
Date: Thu, 5 Nov 92 11:06:08 EST
Organization: University of Wisconsin - Milwaukee
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu


John A. Shriver wrote the following comments in passing while discussing
another matter.  
>
>... know intimately
>the details of it's construction.  (The dead giveaway is the
>spot-welded little metal plate on the plate, I think it's a hunk of
>tantalum to eat the gas these tubes ooze.)... The
>ordinary ones ($14 when I bought them, now only fetching about $5;
>supply and demand) gassed out in no time flat.  Blue glow, soot on the
>glass....Some
>problems with cold solder joints in the tube pins, but that is a
>problem endemic to all EL34's in my experience.)  I've seen plenty in
>white boxes (military surplus)...Do note that only the Mullard EL34, and exact >clones (eastern european
>and Chinese) are true pentodes.  Both of the US-made 6CA7 styles are
>beam tubes.  That will have different characteristics.  However, the
>Mullards durability suffered from having a real third grid, it's very
>easy to have those wires melt and droop if you overheat the tube for
>any period of time, causing screen to suppressor shorts. 

I was born when valves (tubes) were already on the way out for general use.
 I have been using valve amps however for the last few years (made a preamp
and modified a power amp) and feel that their is  alot of helpful tips
which the older people regard as common knowledge and the younger are
haveing to rediscover.  For example, I have been living with an iritating
intermittent high pitched tone from my preamp for a while.  I tried various
anti-microphonics tricks and changed valves but neither helped.  finally
installling a grid stop resistor was suggested and this fixed the problem
and improved the sound generally.  I would like to request either John
Shriver or any one else to write an article outlining several aspects of
valves.  
Firstly, I would like to know more about the internal workings.  (I
understand the general principle of a hot cathode emitting electrons which
then flow to a more +ve anode and that this flow can be regulated by one or
more grids)  I would like to know things like: what is that metal disc
hanging off the side of my power valves (KT66)? Is it "the hunk of
tantalum" mentioned below?  If it prevents gas in valves isn't it a good
thing?  How does gas arise?  Can you do anything about it or to prevent it
(apart from a change of diet)?  What is the difference between a true
pentode and a beam tube?  when you look through the glass which bits can
you see?   Are the main sheets of metal the anode?  What are the
construction differences which result in different noise and other
properties of nominally the same valve type?  These questions would apply
equally to preamp types also.  
Secondly, I would like to know more about useful tips eg. the use of grid
stop resistors which I only recently heard of,  different approaches to fix
microphonics (I have some metal cans from PM components for my preamp, 
sorbothane melted on my 12AU7s).  Can the overheating John mentions above
occur in normal use and if so how do you prevent it?  One suggestion I
heard was to drill a circle of holes around the valve sockets on a chassis
to encourage convection past the valves.
Thirdly, A more ambitious article might discuss the merits of different
design approaches.  EG Pros and cons of triode vs pentode in
preamp/driver/output stages,  differential inputs vs feedback to single
input valves,  cascading valves eg shunt regulated push-pull circuit (what
I have in my preamp) vs triodes without cathode bypass caps (-> local
feedback) and loop feedback.  Different biasing approaches eg capacitor
bypassed cathode resistor vs DC added to the grid of output valves.   This
would be interesting and might dispel the simple minded vacuums-good,
silicon-bad ideas.  I'm sure it is possible to make a lousy valve amp if
you are either ignorant or try hard!
Perhaps if there is a collection of tips in addition to larger articles
these could be collated and put in the archive under "valve/tube tips" or
some such title.

----------------------------------------
Peter Campbell
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601
Australia  (peterca@ento.csiro.au)
Ph.61-6-2464158 (w), 61-6-2516213 (h)
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