Thursday, 7 May 2015

Vintage Treasuries – The Restoration of Leak Valve Amplifiers, TL10.1

Hello to everybody,

today I want to start with a long planned series about vintage valve power amplifiers and their restoration for everyday use. I want to give some advanced help for enthusiasts following here to preserve some rare and incomparable products representing audio history with tremendous skills to perform music in many respects a lot better than later made products. Cost cutting issues were within audio products likely the same than in any other industrialized mass production the decisive argument killing any individual contribution towards quality.
I will start with one of the most famous european brands from the golden age of tube amplification, the H. J. LEAK & CO. LTD.. I want to introduce a bit history of this manufacturer and try to show the adequately organized preservation of early examples with well selected parts and components. Aim hereby will be the historical correct interpretation of the final result in perfect balance with the best possible audible ability in mind. Almost all Leak power amplifiers are excellent audio performers independent of any later built technology (transistor>integrated>classD). They don't shy any comparison with more modern examples of their bread, their exceptional realized superiority has made them to icon of the audio age. So a careful knowledge based restoration make them to be a lifetime investment with exceptional natural capabilties and as lively sounding instruments too, when properly implanted with efficient and expressive transducers.

Leak TL10 "point one", with their typical straight golden appearance with black transformer covers.
I will start the series of entries with a quite common example of the early 1950's Leak models, their last called "point one" amplifier, the TL10 point one. When Harold Joseph Leak started his business in 1934, he first manufactured several amplifiers as contractor for the cinema and recording industry (like Geaumont Kalee and General Electric Co. ). He introduced his first amplifier series, the early triple loop Type 15 four stage KT66 amp in 1945. In 1948 Leak released his most famous model the TL12.1 with 12 watts triode push-pull power and its bigger brother the TL25. The TL12.1 was the first model named "point one", because it was the first amplifier in history with such a low distortion figure as  0.1 percent. This codicil was proudly used for a period of time to demonstrate technical advancement against other competitors in a fast raising business. The TL12.1 was a very successful amplifier in the broadcast and recording business, since the BBC ordered a noticeable amount for their studios around the world. This success did consolidate the name Leak in the professional business as first grade supplier of amplifiers. The TL12.1 will be covered quite soon with a own multipart restoration documentation, shown here online as a work in progress.

The later TL10.1 amplifier was brought into the market to get as well access to the new home audio market, a well decreased lower cost version of the professional outrider TL12.1. These amplifiers are  in every respect a downscaled version of the former heavily oversized designs. Today the TL10.1 models are highly underrated, seen from the today pricing results of auctions. These are from time to time still available in pairs for quite low costs, comparing their built value of quality. Their predecessors are hardly found anymore for sale anymore as single, have almost completely disappeared as later coupled pairs. Their production counts never have been high, in monophonic times less than 2000 pieces, where the TL10.1 has been sold in a lot bigger amounts, even still as monophonic units.

Their later successors from the mid to late 1950ties, the monophonic TL12+, TL25+, TL50+ and their stereo counterparts made after 1958, the ST20, ST50 and ST60 have been produced in tremendous quantities (each model 10.000 to 100.000 expl.) due to their well establishment in the audio market (Leak did make on special request for the BBC special versions of the TL25+ as back up units or as replacement for the TL12.1 in the late 1950ties). All these late types as to been from today represent a wide spread treasury of the best times of audio products made in Britain, the only country in Europe with a similar prosperity and a common targeted market like the US in the 1950 years for home audio products.
Any of these amplifiers is still today a good valve amplifier to show the exceptional audible inherent qualitiy of solid manufacture standards, contrary to the modern built samples of the "tube amplifier revival since 1980". Any of this models is with a little bit care able to fill even highest expectations and in a well suiting surrounding, able to show the foremost qualities of tube amplification in general. So all Leak tube amp models have been collected since their disappearance in the early 1960 years. Their typical gentle style with various luxurious metallic golden paints and their perfect technical realization within open cased designs, have made them together with a whole lot of other British brands, to some sort of incarnation of "the valve amplifier" (AEI, Soundsales, Pye, GEC, Armstrong, etc.). All of them have been dedicated to reach the same luxury consumer at home and as well abroad. Major US brands like Marantz, HH Scott, Heathkit, Fisher, Eico and others shared the same strategy with golden paints, only a view tried different concepts and designs. In the foremost stage here again a Europeen company, Peter Walkers Quad is an exceptional example of a individualized design culture with a typical modernist understanding of formally reduced appearance. Some other US manufactures did try the strategy towards the chromed jukebox appearance to match the taste of a younger generation, like Macintosh or Radio Craftsmen.

Close view of the valve line up and between output and mains transformer a Siemens MKP oil cap for the PSU filtering adapted with golden paint.

A must for this amplifier: british NOS KT 61 valves 

A restauration project always starts with a first check of the capacitor conditions, typically electrolytic capacitors have been dried out after several years of use or even in storage. A long term use might have shifted the resistor values within permanent physical warming, tubes might have run out of order or might show other types of failure (like "gassing", loose parts or broken sockets). For such a first check all valves should be plugged off and the secondary readings of the power transformer might give a first check. If these are in a expected tolerance (without load the figures are higher), the rectifier tube should be plugged back into place. But now, even without load the PSU capacitors are seeing high voltages, so it will be helpful to use a rheostat variable transformer, which gives the opportunity slowly to increase the incoming voltage. Starting with 60 volts for a while, it can be set higher in 20 volt steps every 10 minutes till the final voltage has been archived without failure. Now the other valves can be plugged in with a first test to read the voltages at certain points of the design. First it needs high power resistors of 10 watts dissipation in values between 8 to 16 ohms to give the amp a dedicated load at the exit terminals. Tube amplifiers need a load and should not be used without a speaker or such a dummy load connected.
A first voltage reading should be made behind the last PSU filter cap, than all anode voltages beginning with the first driver stage, second the phase shift stage and finally both push-pull power stages. In the same order all cathode voltages should be checked as second test, as third test ac leakage of all coupling caps might be checked with a multi meter at all secondary terminals of all coupling caps. If all readings are in a acceptable tolerance for a 60 year old amplifier (20%) it might be checked now with a real signal and with loudspeakers connected (check to use the right speaker impedance terminals or the right setting at the output transformer with later Leak models). Hum and hiss are common signs that parts have got out of tolerance and do degrade the performance or may be  falling completely. Normally burned resistors, leaking capacitors can be detected visually by deformations, burning scuffs or wet surfaces surround capacitors. I always would recommend after sixty years of service a complete exchange of all resistors and liquid conducted capacitors.

The TL10.1 needs to be used with the original KT61 valves for best performance, all 6L6 types or similar types like EL33 even with dedicated changes at working points do not match the sonic excellence of the early and rarely used tube due to their higher internal impedance ratio and this matching output transformer.

Rarely seen in real, perfectly matching paints of two originally monophonic amplifiers. Normally they differ a lot since the production fluctuations were high.


For resistors the classic carbon composition types from different brands are available from different sources are the agent of choice for such classic amplifier. They sound generally better (i.e. more musical) than cheap metal types and are historically seen the right decision. Here the higher power resistance of 2 watt is to be preferred, since these don't show the typical hiss, known from low power types.

Silver mica caps in the feedback line, oil paper coupling caps, 2 watt cc-resistors from Beyschlag and bipolar cathode capacitors from F&T beside Rosenthal resistors for the cathodes.

Coupling caps are widely available now at almost any price and origin. Here plastic foil types are in competition with oil paper insulated types. Generally I do like "pio" or "mp" better than more expensive plastic foiled types. With "oil-paper" you have as well a wide choice to be selected. You might get hand rolled types. made "under new moon" conditions from of long term stored copper foil, or you might find several so called audio caps made from exotic materials like bamboo fibers in your audio boutique available from different brands and with "definitive different sound attitudes",  or you simply go for former Russian military graded pio caps –  nor of them will make you a different amp...

MP capacitors for the smooth filtering of the anode supply are the aim, even if they are a lot big

I was always a believer of long term usability, so I always preferred Metal-Paper-Capacitors (MP) over unreliable electrolythic types as well in the the power supply stage. But the oil-paper types are always bulky and hard to find, in particular if you want to restore a amplifier to look like the original. So it makes the process difficult, to find space underneath to install some extra caps to get it made without compromise. Her in my example the big golden painted type on top of the chassis is such a compromizing type. It is a Siemens 8mF type which resembles the original black container in-between the two transformers, containing three electrolytic types which are after 60+ years leaking anyway. There is no way to get MP-types in that small box with similar values, so I decided to change it and mounted another two caps underneath. The boxes are still in storage for physical completeness of both amps in theory.
The only position in such a power amplifier where I hardly accept electrolytic types is at the cathode of a valve. Here I did use bipolar tone frequency capacitors as to be used in crossover networks with smooth anodes, in these days one of the only opportunities at such delicate position. In the meanwhile several special audio products have established like the Black Gate types and several other brands which show up with a improved abilities, so it depends a lot at the taste of the owner which capacitor here might be used.
Leak amplifiers normally do use silver mica capacitors in the feedback line to match closest tolerances. These are very good and do normally not fail, so they can be reused or if they need to be exchanged, styroflex types are the value if no other mica type is available.

I did restore my pair around 1990, a time when it was already difficult to find the right spare parts for such a project. Carbon composition resistors, oil paper capacitors and nos tube stock had already disappeared within the traditional electronic supplies in 1990. So it needed good researched informations to find New Old Stock parts or even specialized side business products, like Danish Jensen Capacitors or the British Billington Valves.  For example a 2 watt type Beyschlag carbon composition resistors was still in sale back these days, but it needed a minimum order of 10 for each value costing some 30 DM each. For a typical Leak amplifier some 20+ values were necessary, which ended in a 600 DM region just for the resistors. Another 200 DM for the Jensens pio caps (each around 30 DM), some switches, cables, tubes (even than a set NOS KT61/EF86/6SN7/GZ32 was 250 £) together with new contacts and switches it did lead roughly up to 1500 DM base cost for such a restoration. If you take the basic costs of a pair of TL10.1 in GB in account, in these days 450 £, a equivalent of 1400 DM, the cost did add to 3000 DM, which is almost 1500 EUR. This amount will hardly cash back for such a good pair in this restored condition. So it is another example of  typical hobbyist projects, where costs are no option. Today though the easy access to NOS spare parts via internet such a project has got a lot cheaper. As well you can find the original resistors and caps to make it perfect, or alternatively with some former Russian NOS parts you have just to pay the shipping costs...

In order to improve the sound the TL10.1 a smoothing choke (10H/100mA) resembling the original 1K filter resistor is a must. Oil caps for improved smooth filtering in the power supply have been installed for completeness.

Schematic drawing of the Leak TL10.1 together with dedicated "point one"-preamplifier module, supplied from
the power amplifier through the dedicated octal socket.

Read on soon about the famous TL12.1 amplifiers, Volker