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

Hello to everybody,

interested following our remote restoration project of the Leak TL12.1's in Tjörnarp near Malmö. As one of the first steps after general optical inspection of the amplifiers it is necessary to work out all defective hardware. The most important parts are the transformers, so here a first concentration of interest should take place. A good start for safeness is to check the dc resistance of any single coil wound with a meter for continuity and dc resistance higher than 0 Z. Here the schematic (published already in part I ) is a great help to identify the numbered contact terminals. Beginning with the primary side of the mains transformer, each section of the input voltage selector between 0, 200, 230 and 250 Volts should be checked for continuity (here indicated in order as no. 1,2,3,4). As well a still working fuse is a good indicator for intactness of the amp, even when its last running service was some decades ago and typically during storage some parts may have deteriorated? So don't connect it to ac power before all checks might have taken place, when you don't know about the history of your amp.

Continuity check with analog meter set to ohms, here shown testing at the first half of the two heater coils between 5 and 12 as grounding center contact (12 and 6 build the other half )

Next check will follow to contacts no. 9, 10 and 11 for the high tension coils with 10 to be the grounding center contact. Final check of the mains transformer (part number TL12/T3, this here is the closed type where /T2 is the older open frame type). Final check of the mains transformer is the heater winding for the rectifier tube between no. 7 and 8.
Next check will follow on to the choke (contact terminals are not indicated with figures) and it is just a single coil. If it shows continuity the check will be forwarded on to the most difficile coils, the output transformer (indicated as here TL12/T2/2). Here two different transformers can be found, the TL12/T2/1 shows a higher output impedance value as option.
Beginning with the primary side of the transformer, it shows mirrored symmetry between terminals no. 9,13,and 10 as first half and 11, 14 and 12 as second half. Continuity should be seen between each contact to the next. This means beginning with 9 to 13, than from 13 to10. The same with the mirrored other side. As well the secondary side will show up with four different coils to be checked: terminals no. 1 and 2, 3 and 4, 5 and 6; as well 7 and 8.  If all contacts are checked at both amplifiers without any detected failure a first deep breath can be made, the continuity check was successful and everything irreplaceable seem to be intact. But this does not say that all isolated coils will work well with high voltages and realistic currents in the circuit?!

Here a 500k potentiometer was customized built in to keep the high sensitivity of Leaks input design variable. The high sensitivity was necessary in the early 1950's with low output components like tuners, tape recorders etc.. But today it is not any longer useful, since it makes a lot of unwanted side effects Today it might me more reasonable to go for a 100 to 250k stepped attenuator for the best sound.

One amplifier has got additionally equipped with some old style input socket and a attenuator for the input signal. Both parts are not original and might be rejected for this reason and for perfect matching of both amps. A much better way is to design such a attenuator for the input grid in a separate housing with octal plug in order to match physically the input socket (octal) of the amp (originally used to match the preamplifier until RC/PA/U. Here the common octal plugs (as used with Leaks connecting cables) or octal cases as used for relays are good components to build up for such a attenuated input circuit in combination with a rca socket. A excellent value here will be a stepped attenuators made from switched resistor pairs in order to keep only one exactly defined value in the signal loop instead of lossy carbon potentiometers. Another benefit will be that you can do different input modules for different reasons,  like one with symmetrical input transformer to match signals from studio equipment. So both variants will be plug&play!

Standard input module with RCA socket and 100k stepped attenuator with bulgin bakelite knob in separate octal bakelite housing, originally dedicated for relays.

For perfect care of the chassis it might be necessary to dismantle the hardware of both amplifiers further. If you want to go the whole way you should do perfect photographs from all sides in particular from all cable connections of the transformers (with flashlight) to identify later all connection again before you desolder these.
If you really have dismantled the chassis from all transformers, it can be cleaned perfectly. I am not a proponent of a complete dismantling with all parts like tube sockets, component board and cable tree,  or even further with new enamel and silk screen printing, because it might happen that working parts might brake and it will get difficult to find replacements matching the chassis. Octal sockets for example have to be the same type used by Leak itself, otherwise the sizes of holes will not match, very difficult to be found todays. Component boards need to be made completely by hand if necessary, but it is manageable. Aging parts need definitely to be replaced, so all electrolytic caps and lots of resistors need replacement, because their values have got out of range. As well here originality will be first choice and in almost all cases a better solution than modern "improved parts".

After dismantling the transformers and the capacitor housing the chassis need a good clean to get exempted from dirt, dust and grease of 60 years of service.

As already mentioned Leak did use for the smoothing and filtering rail of the power supply huge paper capacitors which were made to order by TCC in the 1950ties. Four capacitors of 4 uF/600V got sealed in one box with grease and oil as isolating liquids. The first one is the loading capacitor following the rectifier tube and heading a smoothing choke, followed by two of them in parallel to form a 8 uF filter unit with another 4 uF capacitor at the end of the rail. These paper capacitors normally will not fault, but after 60 years of service, storage and move, after heat, coldness and physical punishments almost all of them show oil at their base. This is a indicator of "leaks" which commonly will be accompanied with hefty audible hum. These leaking capacitors can be measured for capacity, but more important will be their esr (equivalent series resistance) factor. The esr can be measured only with disconnected capacitors with a special instrument, the most will not have such an option. The pure presence of grease might be reason enough to exchange the blocks, because the leak of oil will dry them out. And it will be pure pcb poisoned oil used within these days, so take care with such leaks, for yourself and for the environment!
At the well known heritage site for Leak amplifiers is shown how to replace the outworn capacitors with modern non lasting electrolytic types. By the one hand it is as already said extremely unpleasant to open up the original cans and clean them from all the pcb poisened grease and capacitor components. On the other hand the exchange of the exceptional sounding oil paper caps to modern electrolytic types degrade the sound characteristics to a mediocre level. I can not recommend using such components and would strictly rate to use paper capacitors again, if you want the full advantage these amps can show up with. But you will not find the matching sizes/values in needable amounts to drop into the cans. You might order them customized made with Jensen Capacitors in Copenhagen. They will be able to make them for you, but don't ask for costs. So what to do?

Capacitor block dismantled, showing clearly lots of leaking oil and grease as indicator of
a defective irreplaceble component. 

I hope I can show up within the next article of this thread about a authentic solution for all lovers of these amplifiers. A new set of perfectly matching cans made from sheet copper with paper capacitors built in perfect values of 4uF/8uF/4mF/600V might be ready to be shown than. Even better, I might be able to deliver these on request to people which only can accept a optical perfect original looking replacement with perfect sound characteristcs for their beloved amplifiers in order keep the desperate climbing values of these amps in mind.

Read on soon, Volker

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