Vintage Treasuries – The Restoration of Leak Valve Amplifiers, TL12 "Point One", Part I

Hello to everybody,

today I want to start with a first part of a step by step restoration project. The restoration of the well known monophonic tube power amplifiers made by the H. J. Leak & Co. LTD, first issued in 1948 and called TL12 "point one", will be covered with this series of entries. These amplifiers do have reached cult status for vintage audio aficionados within the last 60 years. Some would rate these the best vintage amps made ever. Direct opponent for the throne for the best audio amp of all times will be the Western Electric 91A with single direct heated triode 300B as power tube. A whole crowd of enthusiast for early US american cinema equipment in the western and eastern world will rate these legendary monitoring amps (6 watts directly heated triode) made by Western Electric Co. in the 1930ties, the WE 91A, to be the most favorite amp of all times. They have set the pace for the forty year old single ended audio virus to a hunt to find the right speaker matching such amps. But specially with professional broadcast designs, there is a whole lot of not so well known amplifiers, which work exceptional well too.

My own pair of restored Leak TL12.1, here shown with plugged octal input adjustment modules and NOS tubes. Unfortunately I own a mixed pair of both versions, one early open frame mains transformer type and one later closed mains transformer type. Technically seen identical options of the same design made by the same manufacturer (Haddon LTD.). The closed type came into order for improved humidity protection.

Such comparisons are for real life conditions as useless, as comparing fish with beef, specially if you are already on a vegan lane... Both designs can show tremendous advantages against each other when combined with the right matching components, in particular if the important fact of well matching efficient speakers will be part of the test. The most of us will not be able to make such a comparison anyway with a first hand impression, since both monophonic amplifiers are almost lost without trace as find for a consumer market, since their low productions figures and age have set them extremely rare.
The push-pull Leaks are by far more universal to use and they are exceptional transparent sounding examples of their bread (Williamson design derivate with 12 watts in triode mode KT66). I do personally know no other pp-tube-amplifier which I would prefer in any discipline better than the old Leaks TL12.1.

First I want to do a bit of historic digging about these famous amplifiers. In 1948 these amplifiers were released by Harold J. Leak in order to match as first audio amplifier in history with such low distortion figures of 0.1% as selling argument. Together with ultra conservative specifications for the established transformers (Haddon LTD.), a complex case protection (4 layer zinc grounded enamel to withstand any humidity stain) and elaborated manufacture specifications for long term operation in difficult conditions. Like glass sealed paper capacitors and sealed cathode bypass electrolytics are signs of a purely professional intended market introduction. Harold Leak did travel Britain doing live - versus - reproduced sound comparisons with his new TL12 and RC/PA (pre-)amplifier set. The sound was reproduced via the Leak system using an STC4033 microphone (also known as the WE639A in the USA) and a 15'' Tannoy Dual Concentric Speaker (Monitor Black) in a 280 liter sealed enclosure behind black curtains in order to show that there will not be any difference to the live signal!!

He published a expensivly printed triple tone color (metallic gold as separate tone) 28-pages sales brochure to inform his aimed customers about the realized qualities of these amplifiers, different available versions and optional preamplifiers. The BBC was one of the first customers who made the success of these amplifiers possible and with their continuous orders over two decades they manifested the name for the coming "world class supplier ".  The BBC got the first active speaker units equipped with Tannoy Dual Concentric speakers, where the amplifiers are positioned at a drawer in the base of the enclosure in 1950.

"The New B.B.C. Monitor Loudspeaker,
incorporating a Leak TL12 "Point One" amplifier and a 15'' Tannoy Monitor Black speaker to form a active unit. Underneath it says 731 amplifiers were ordered by the BBC! (mono!)

Till today it is hard to find another push-pull amplifier in history able to fetch up with similar audible qualities. Normally this today more common topography of tube amplifier design (push-pull, global feedback, etc.) establishes in a foremost way a clean technical defined soundstage. With a glass clear superficial aural attitude of more technical expression these designs are not known for their natural harmonic aural performance, not so the Leak TL12.1. Generations of US-american or European KT88, 6550 or similar tubed power horses did set the pace in terms of power, control or drivabilty, but never showed the slightest chance to demonstrate a bit of Leaks natural harmony. Instead they flooded the market in the late 1950ties with cheap EL84 PP designs in tremendous counts, again fast rejected by the even cheaper power ratio of transistor designs in 1960. It needed the delicate intimate knowledge of individual masters from the eastern world in the early 1970ties like Anzai, Shindo, Kondo, Sakuma and lots of others to give path to the understanding of the refinements as stairway to such natural harmonic soundstage of early vintage tube amplifiers like the We's, Leaks, Brooks, RCA's and others.

The Leak TL 12.1 are exceptional well made and harmonic sounding power amplifiers worth to be restored not only because of their increase of value, due to their low production figures and a well named reputation. When I bought a first pair in London around 1990 I had to pay 850 pounds, which was a good amount of money for a 40 year old tube amplifier wrack. A Leak Stereo 20 in these days was 85 £ just in comparison. Today a good stereo pair of TL12.1 is a rare find mostly coming from collectors or dealers which want already 7500 € and more, a single unit might be found for 2500 to 3000 €?
A well restored pair of such amps might give a whole lifetime audible satisfaction and musical performance, so such amounts are still paid by really interested people, who know about alternatives available for such rates. But the high boost of prices came with the early 2000 years, when Honkong based Chinese audio collectors did try to get any offered single Leak amp at Ebay for any price. Some of them are proud to show a whole line of these amps as some sort of new living standard...

Hongkong based line up of restored Leak TL12.1, you will never see a amount like this in real life...

A friend of mine in Sweden has got his pair decades ago, but he never used them or did restore them for everyday use. So now I will take the chance and try to do a experiment and guide him here online through a restoration of his pair as a remote project, so that everybody of yours interested to follow can participate.

This pair will be restored through the next articles in parallel as some sort of a remote experiment?!
The left one is missing its ac plug, no problem to become.

A list of the most necessary parts:

A real problem will be solid hardware defects like broken windings of transformers and chokes, missing parts of the amplifier, like the component board, shields or outer case parts. Original replacements are impossible to find, it needs another amplifier for spending its parts in transplantation. If you know a winding company, it might be possible to rewind the original iron cores (if you need further informations about winding details visit here). The company Sowter offers replacement transformers which are sold as technical copy of the original, but look and feel very different. Bulgin plugs can be found at Ebay-UK. Good Luck!

Exceptional rare tube shields for the input tubes, difficult to find two-pin Bulgin socket for the speaker terminal, the common three pin AC-socket and dedicated plug, both latter are still available as new parts.

First steps

1. First all electrolytic capacitors need exchange, after 60 years of use/storage they are dry out and need replacement. Within these amps they are only used as cathode bypass of the input and power tubes:
C1 = 30mF/32V
C7, 8= 25mF/100V
Several new capacitor topologies and materials (Black Gate, Bamboo fiber, etc. ask your audio boutique dealer) have been established in the audio market resulting in very different sound structures. A classic, cheap and authentic decision would be electrolytic bipolar tone capacitors for crossovers with smooth anode.

2. All coupling caps are oil impregnated paper types. These last a lot longer than any electrolytic designs, but after 60 years it might be time to reset as well here:
A wide range of types is available from NOS suppliers or new brands. Standard is the use of aluminum foil, Jensen and Audio Note are able to supply additionally copper and silver foils with oil impregnated paper. Modern substitute are polypropylene foil types as advanced isolation material for the one who likes their typical sound. Sound wise, smooth and cheap and a good all round decision are the Russian PIO types:

C2, 3, 4, 5 and 6 are originally stated to be 0.25mF/500V, even if the highest voltage at any of them will not be higher than 300 volts.

A range of usable capacitors for the restoration, on top:
Russian 4mF/600V PIO caps for the PSU filter stage;
from left: Russian 0.25mF/1000V PIO with glass sealed wires; two vintage silver mica types;
there different actual Jensen PIO coupling caps

3. In the power supply line the Leak does incorporate as well oil impregnated paper types, which were in these days extremely expensive (and bulky). These are hidden in the black cover left of the output transformer. They are welded as 4 pieces of 4mF/600V in that case. For transistor aficionados the unbelievable low values of 4mF as charging type and as well 8 mF plus 4 mF as filter capacitor rail is set here. The push-pull topology make such low values possible, but the efficiency of such design depends on perfectly matched power tubes for hum free performance.
These capacitors normally last a lifetime, but I have seen several amps with typical oil leaks. Here a replacement has got necessary. It is almost impossible to find matching types and sizes in such amounts (8 pieces for a pair of amps) from NOS stock. The originals are welded into that can, it is a lot of work to dismantle these to keep the metal boxes for some modern replacement capacitors. Several users do refill them with smaller sized electrolytic types, and in a foremost manner with higher values to improve the filtering effect. The result might bring optical authenticity by the reuse of the original metal case, but will be accompanied by technical and aural detoriaton from the electrolytic capacitors.
The common electrolytic capacitor rail will come along with a lost of the incomparable smoothness in sound typical for these amplifiers. It is a must to replace them all with oil-paper types to keep their sound qualities. Another problem is the capacity of the first loading capacitor, which will be charged from the rectifier tube to its ripple limitions and might do complex damages when set to a higher values than 16mF (depending on the tube type) and a short life time period will result together with noticeable sound deformation. I hope to present here soon a perfect replacement solution for Leak TL12.1 users within the next entries...

C10, 11 (two of them in parallel), 13  are each 4mF/600V. These need to that high!! Big cans, look at the image!

The small values C9, 14 are welcome to be replaced as well with highly selected and measured paper oil types for the shown values and perfect soundstage, but silver mica types (original) are another option to keep the tolerances in a low range.

4. All resistors are carbon composition types of high wattage (1 to 3 watts), the so called british bone types have been used. It will be almost impossible to find NOS replacements of that brand. But several other brands are available from different countries like US, Europeen countries or Japan to match here. To my understanding it needs to be carbon composition types of high wattage to prevent hiss and show up with exceptional tonal delicacy.

Look into the schematic for all built values.
The two KT66 cathode types R16, 17 of 600 ohms should be 5 to 10 watts wire wound.

5. Tubes should be NOS types, British brands. Otherwise with modern Chinese or Russian replacements the amplifiers will perform with some sort of withdrawn performance of its original sound abilities. Yes they MO/GEC KT66 are expensive! But to come back to the beginning of this article, a (one) NOS WE 300B tube with engraved base will cost the double amount of two complete sets for stereo use of NOS British tubes for two Leak TL12.1's.


Read on soon,
Volker