From: kludge@grissom.larc.nasa.gov (Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: Tube Amplifier Design Questions
Date: 5 Jan 1993 18:39:40 GMT
Organization: NASA Langley Research Center and Reptile Farm
NNTP-Posting-Host: grissom.larc.nasa.gov

In article <1993Jan05.141200.24931@uhura.neoucom.edu> wtm@uhura.neoucom.edu (Bill Mayhew) writes:
>Instead of worring about the tubes, the better question is, "where
>can I find output transformers to handle 200 watts?"  From
>experience, I know those will be difficult to find and expensive
>when you do.  I found a few UTC "Ultralinear" transformers several
>years ago at Fair Radio Sales.  I like the Williamson ultralinear
>design with tetrodes.

You lucky dog.  I go out of my mind trying to find decent output
transformers.

>  6L6 derivative tubes are pretty good, as a
>lot of output trnasformers are desinged with those in mind.  That
>includes 6L6, 1614 metal envelope and KT-88s.  A pair of 6L6s is
>good for driving about 30-40 watts.  Going up to 200 watts is probably
>better done with industrial type tubes such ass 6550s or 6080s.
>I picked up some Amperex 6L6 tubes at a hamfest last year to
>replace the 1614s in a Macinthosh MC-30.  The Amperex tubes have a
>very nice sound in the MC-30.

You won't get 200 watts out of the 6550s by any means, unless you
parallel a bunch of them.  Even the 4CX150 isn't going to give you
that much power when you start running it in class A.

>You'll have to go back to the 1950s and early 1960s to find much
>useful literature.

The Radiotron Handbook is nic to have, and the RCA tube databook is
absolutely essential.

>I always thought it would be fun to make an audio amplifier out
>of something like Eimac 4CX150 tubes.  The execution would be awful
>since those tubes need somewhere around 1800 volts on the plate.
>That would reqire a pretty exotic output transformer and forced air
>cooling.

No problem.  I have an 833-based amplifier at home right now, and it's
fairly impressive, although the output transformers had to be custom-wound
in order to get a high enough input Z.  The 807 is another nice little
transmitting tube that does a good job with relatively low power audio,
but you aren't going to get 200W here.

>  In fact, we used to have a 60 Hz to 400 Hz power
>converter rated at several KW that used 4CX150s.  I think it had 8
>tubes and a GIANT transformer.  It dated to about 1964, before
>semiconductors that could handle those sorts of power levels were
>affordable and/or reliable.

They still aren't affordable or reliable.  That's what motor-generator
sets are for!

>Unless you have tube nostalgia and/or want a PA for music performance
>applications, I'd forget about making a tube power amplifier and
>design with transistors.

I agree that transistor design these days is a lot easier, mostly 
because you spend less time fighting with the power supply and trying
to get decent output transformers.  When tube amps are good, however,
they are very good, and they can be quite rewarding to build.  The
project of the month is an amplifier built around an 829 tube, believe
it or not.  It's definitely been a learning experience....
--scott
Newsgroups: sci.electronics