Over the past 10 years I have had a
few Leak amplifiers brought to me for repairs or for some slight
re-engineering.
Like many amplifiers made in the 1950s the parts within such amps are
now 50 years old and unless carefully serviced
each year over all that time then there definately will be faults with
capacitors and resistors, and it is prudent to
completely rewire the amps at least if nothing has been done when you
find an old one that has been parked
in an attic or basement for 40 years. However, like many amps 50 years
old, the functions of the amp could easily
be improved to take advantage of the improvements in quality and
reliability of modern metal film resistors,
capacitors, and many other parts.
Original Leak amp quality was subject to bean counter restraint and
thus sustained serious design compromizes.
Its easy to look back at what could have been made, but in 1955, Harold
Leak must have been worried
about what Quad or Radford might be trying to produce to dominate the
market, and accountants in all
companies had a favourite word to design engineers....."NO!".....
Modern expectations of tube amps
include that they be reliable, need little service, and that they
produce
accurate dynamic hi-fi second to none and with all warmth in the
recording carefully preserved.
Many tube and solid state amps don't quite meet all those expectations.
Many old amps such as those from Leak, Quad, Radford, Dynaco, Luxman,
etc, were simply never originally
engineered to the high standards possible in 1955. For example, little
regard was given to the recommendations
within the Radiotron Designer's Handbook, 4th Ed, 1955, about output
transformers.
Just about all the major maufacturers skimped on output transformer
quality.
In the early 1950s, Mr D.T.N.Williamson spoke a lot about what was
required in a good output transformer and made public
his landmark design for a push-pull output transformer which had really
high bandwidth, low phase shift,
freedom from saturation, and low winding losses. The exact
details of his OPT design is within the RDH4.
Nearly all the mass makers ignored Mr Williamson.
Anyone who didn't ignore him and persisted with with trying to sell 16
watt amps using KT66 in triode all went broke,
because the post WW2 marketeering insisted people buy high power, which
they mostly didn't use
because the speakers of the era were typically 95dB/W/M.
One may wonder why bother re-engineering such ancient old junk but
in fact it is worth it, and the lack of output transformer quality does
not prevent good music being heard from these old amps.
To buy a pair of new tube amps now may be quite expensive, and
sometimes the new quality is no better internally
than what was used in a 50 year old Leak, despite the printed circuit
boards and fancy appearance.
Good listening is the product of the well done circuitry, and isn't
improved by good looking cosmetics.
Leaks can be rewired to include a few appropriate techniques to
address the original shortcomings in the original
amps and if used sensibly can provide sound quality second to none for
most listeners wanting a few watts at home.
In 2005 I re-engineered a Leak amp for a customer and I will relate
what changes I made and why.
Fig 1, Leak TL12 , a classic
mono bloc amplifier, 2005
Here we have a famous Leak amp from around 1955 sitting on my
work bench and as you can see it has
two leds located in the side of the chassis below the two KT66 power
tubes.
All major parts on top of the chassis have been retained to keep the
retro look.
The empty tube socket to the left side is where the preamp plugs in for
its power
and the feed signal for the power amp.
Fig 2. Leak mono bloc turned
upside down on my bench, 2005.
The image shows all the replaced R&C components. The white
active protection board is at the rear of the picture,
and I have included a schematic at the bottom of this page. There is an
additional 7VA auxilliary power transformer to the rear left, and
additional filter caps on the rear right.
Fig 3. The 1954 original
schematic of Leak TL12 mono bloc amp, which I re-drew in feb 2006.
All the digital file copies I had of the original schematic from the
Net were unreadable on the screen and useless if printed.
My own original 1954 paper copy became un-readable if anyone tried to
copy it, mainly because Leak refrained from using large
clear lettering on their information sheets. I can only suppose
that Britain was still rather short of ink after WW2,
or that british bean counters restricted the size of their lettering
since small letters are cheaper to have printed than large ones.
So I red-drafted the original schematic as exactly as I could so you
can read it and it may print out OK. This was during
the learning process with MS Paint, which is what I now use to prepare
schematics.
The original schematic was a little too simple for my liking, since
this amp was not unconditionally stable. Modern people now expect all
amplifiers to never oscillate or behave badly.
Here is the Reformed leak TL12 schematic......
Fig 4.
The above schematic has all the listed mods noted on the schematic.
The numbers I have used for components
are not the same as in the 1954 original schematic.
The improvements made this Leak amp unconditionally stable and
completely unlikely to oscillate at any frequency under
any load condition, or when turned on without any load connected.
The amp can have the KT66 either Ultralinear or Triode connected
without changing the NFB arrangements.
Fig 5.
The above graph shows the effect of applied NFB on the amp. The HF
response is shown with various capacitance
values used for a load along with the equivalent load which mimics
Quad's ESL57.
Notice the peaking in the sine wave response above 20kHz with
capacitance loadings.
This shows that the amp is barely able to remain stable. Any square
wave will also have considerable over shoot and ringing cycles before
settling with capacitor loads. But with all the new networks used in
new schematic I managed to make the the margin of stability sufficient
and the amp will NOT oscillate at HF like it surely does with the
original 1954 schematic.
The 1954 amp also oscillated at LF without a load but the re-arrangment
of the RC values between v1, and V2&V3
in my 2005 schematic stopped that problem.
Fig 6.
