LEAK AMP RE-ENGINEERING.
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General comment on Leak amps and 1955 design  considerations.
Fig 1. Leak TL12 on the work bench.
Fig 2. Leak TL12 upside down on the work bench.
Fig 3.
Schematic of the original Leak 1954 TL12 , carefully re-drawn for
everyone who can't read the old copies.
Fig 4. Schematic of the Turner Audio Reformed leak TL12. 
Fig 5. Graph of the frequency response for the TL12 with the circuit mods,
but without global NFB and with global NFB.
Fig 6.  Graph of power output versus RL, Leak TL 12.
Comment on set up, class of operation, UL or triode ?, speaker matching,
and the use of KT88, 6550, or KT90 preferably
in triode for the best performance. 
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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 definitely 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 compromises. 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 favorite 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 manufacturers 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
Leak amp on the
          work bench, 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. Reformed Leak mono bloc turned upside down on my bench, 2005.
leak mono amp
          underchassis wiring, 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 auxiliary 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...

1954 Schematic tl12
          re-drawn 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
unreadable 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.
Modified Leak
          TL12 schematic, 2005.

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.
Graph for Leak
          TL12 response.
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 rearrangement of the RC values
between V1, and V2&V3 in my 2005 schematic stopped that problem.
Fig 6.
Graph for Leak
          TL12 po vs RL.

KT66 cathode resistances were increased from the 1954 original 600 ohms to
750 ohms to reduce the idle Ia
to about 48mA because I found that I had a higher
B+ than in the original amp. The dissipated power in each KT66
is still only 21
Watts approximately, for 42 watts total and the class A maximum power is thus
about
17 watts in UL into 20 ohm load and for triode the pure class A maximum is about
12 Watts.
For UL, at 8 ohms only 7 watts of class A is available with the remaining
20 Watts in class AB
There is also only 7 Watts of class A into 8 ohms, and with
the remaining 7 Watts in class AB.


The amp as set up with secondaries wired as shown was meant for a customer
who said he had a 16 ohm speaker.
For 16 ohms there is 11 watts of pure class
A in triode or 22 Watts AB with including 14 Watts of class A.


The triode instantaneous AB1 maximum is 15 Watts into 5 ohms but with continuous
sine wave it is 13 Watts.
The use of loads less than 8 ohms isn't recommended.
These figures should not mean much unless you have speakers rated for only
85dB/W/M and you like huge levels.
Most people would find these amps to sound
very well with moderate levels and speakers above 90dB/W/M sensitivity
and
above 8 ohms.
About 7 years ago I did a similar modification to a pair of TL12
and my client and I tried one amp in triode,
and one in ultralinear using a good
source and fairly sensitive 15" dual concentric 1969 Tannoy speakers.

Not the slightest change in sound could be detected between triode or
ultralinear
, and more tests since confirmed that I can safely conclude that either
modes of output tube operation makes not the slightest difference in sound quality;
either way its excellent, well detailed, smooth, dynamic, creamy, warm and never
dull, frumpish, or euphonic.
The pair of leaks I altered 7 years ago eventually wore
out their motley collection of 40 year old aged KT66
and the owner had me change
the KT66 for Sovtek 6550 which I wired in triode. The triode connection of either

6550, KT88, or KT90 gives about the same class AB output power as the KT66 in
UL, and may be plugged into
the KT66 sockets without any circuit alterations.

The leak TL12 OPTs are very fragile, with very thin wire in the 4,000 primary turns
and hence my use of an active protection circuit board to warn an owner of bias
balance problems.


Fig 7.
PP amp protection
          circuit.
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