FCFIFRAGA%CIUC2.UC.RCCN.PT@CUNYVM.CUNY.EDU (FRANCISCO AMARAL F FRAGA) (11/20/90)
The following notes were based in my experience in building an
OTL amplifier. I understand some tube basics, but I am not a tube
expert. Comments of people with more experience in do-it-yourself
electronics and tube equipment are welcome as any other form of
criticism.
Building an OTL amplifier (part 1)
1. Choosing the right output devices.
There are many circuits available for OTL amplifiers, each
claiming its virtues. The first big decision is to choose the
output tubes: pentodes or triodes? Experts say that the ultimate
in performance is obtained with triodes, due to their superior
linearity. Unhappily good power triodes with low output
resistance are quite difficult to get. The solution would be to
use many lower power triodes in parallel in the output stages,
but this implies lots of tubes and wiring. The pentode solution
is the easiest one: good power pentodes are easily available,
you can get a reasonable amount of power with 4 or 6 output tubes
per amplifier (about 30 ~ 60 watts, depending on load), and
besides Futerman amplifiers used pentodes so they can't be so
bad. So I decided to use pentodes (PL519), but I will change my
mind if some one tells me how and where to get the 6336A tube at
a reasonable price without much trouble.
2. Modifying circuits.
Available circuits of OTL amplifiers were designed in the 60's.
Power supplies of these circuits are quite simple usually LC or
RC filtered and decoupled with no regulation at all (Futerman's
are the exception with some elaborated regulators for the screens
of the output pentodes). My first idea was to built a super
regulated supply closely inspired in the power supply of Audio
Research preamplifiers (by the way, if some one knows which are
the MosFets they use as output devices in their supply circuits I
would appreciate to know). But further thinking about matters as
local feedback being introduced by power rails, and analysis of
the circuit kept me away from modifying anything in the original
circuit: I will built them exactly as specified 30 years ago in
order to have a sound reference. May be latter I will try some
mods and upgrades, eg replacing the tube voltage regulators of
the screens of the Futerman with solid state circuits).
3. Sourcing components
It is one the most time consuming parts of the project.
High voltage power capacitors are not to difficult to get due to
their use in switching power supplies (I have been told that
paralleling the type used in flash lamp power supplies is a good
and cheap solution). Computer grade power capacitors are much
more expensive but you can easily get capacitors as large as
4700uf 250V. Watch carefully the life expectancy versus
temperature of the capacitors you are using. Some capacitors with
a mean life of 200,000 hours at 40 degrees C have a mean life of
5000 hours at 85 degrees C! If you want to use cheap capacitors
keep them away from the tubes.
The power transformers are not easy to get of the shelf. I
ordered some custom units from a local supplier. In order to
simplify the design of the transformer I ordered three
transformers: one high power (500W) for the output stage,
another for the input, drivers and polarizing screens and one for
the filaments of the tubes. It is more expensive than a single
transformer, but if something is wrong in the design of the
transformer you only have to replace one of them. I asked to have
transformers with copper foil screens between primary and
secondary windings.
Good quality sockets and top caps for tubes are not easy to get,
but are quite important. I remember reading in this bulletin
recently about trouble with tube equipment due to bad contacts.
If the contacts are not thigh enough, they heat and then they
become loose.
(to be
continued)
........................................................
Francisco A. F. Fraga
Dept. Physics
Univ. Coimbra
3000 Coimbra PORTUGAL tel. 351-39-34668 fax 351-39-29158
FCFIFRAGA@CIUC2.UC.RCCN.PT