Sunday, 12 July 2015

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

Hello to everybody,

following on my restoration series about Leak TL12.1 amplifiers. After the general check out of the hardware, i.e. the test of continuity of all windings of each transformer coils and their check to shorts, it can be assumed that the expensive and irreplaceable hardware might work well. If it will operate flawless needs to be evaluated with precisely other tests running the units under current. We will see that later and take in mind that everything is ok.

A precise photo of the wiring details will help later to find the right connections.

Nice ensemble of expensive iron cores, the Leak TL12.1 mains transformer, the output transformer and the choke. Now they are ready for a careful cleaning.

Next step will be the solid cleaning of the chassis. For that reason it will be the best practice to disassemble the transformers and the PSU capacitor case from the chassis. But first I would suggest to make precise photos of the interior wire connections for a later reassembling guide. If all transformers and capacitor connections are documented, they can be desoldered from their wires. Next step is the unscrewing of parts from top. If all "black" components have been removed, normally a leave a "footprint" in form of dirt or wax edges will be visible. With a little effecting time, even with modest smooth household cleaner these edges can be wiped off easily. With a "Q-tip" all smaller edges (like slot screws, etc.) can be cleaned as well. If necessary the chromed screws on top of the transformers can be polished with steel wool to perfect shine. If rust makes more advanced labour necessary, these might be chromed again, in order to present the finished restoration of the amps as a detailed care mission. Such details are important optical signs of care.
I personally prefer a clean original look better than any complete new painting of the cases. The silk screen printing at the terminal side will make a new paint expensive and difficult to reproduce and finally I do prefer a clean authentic look. A 70 year old amplifier should shine with grandeur instead of a virtual competition with "chromed tube burgers from China", which are cheaper than a matched pair of NOS KT66s. The typical restoration of "like new" condition is actually very fashionable with the Hongkong based collector crowd. They restore all TL12.1s completely to new standard with reproduced main boards, new painting and silk screen printing, new covering boards and a new tar dipped paint for the transformers. Their results are looking expressive new, but lack clearly the superior charm of authenticity.


Typical dirt edges where the transformers were mounted need a good cleaning,
dish cleaner solvent and a little time might be good base to keep authenticity.

Mild cleaned chassis left comparing a untouched amp to the right
As already reported in part I, all broken bakelite parts like sockets, plugs, fuse holders, etc. can be found on Ebay in GB, when the main interest is original authenticity. Bulgin plugs and sockets are widely available in white and in black bakelite, as their matching modern successors from the same brand are supplied "new" from modern materials.

The main board might need necessarily to be desoldered completely as well. From the back side the soldering terminals are connected by wires. Isolated from the chassis it can be checked through for continuity measuring connection after connection. This process makes later disfunction problems obsolete, coming from the normally hidden wires. In general all electrolytic capacitors should be replaced definitivly, but after 60+ years of service even the oil paper coupling capacitors (5x 0.25uF/500V) should be replaced as well. Here the Russian caps are are a excellent and cheap value, much preferred above "boutique ware" for audio nuts. For the cathode bypassing (30uF at the input valve and 2x 25uF at the power valves) are several different types available today, a wide range from Black Gate Capacitors, over several audio optimized electrolytic capacitors with low ESR through classic bipolar tone frequency caps for crossovers, are a solution for replacement here. As well the cathode resistors of the power tubes should be replaced in general. They have heaten up a lot (burning marks at the board) might have got out of value a lot, so good 600 ohms wire types of 10 watts are a save solution.


Parts which should be removed after decades of service; red marked pio coupling caps (CC) , blue marked electrolytic cathode bypass caps (BC) and the cathode resistors of the power tubes (CR).


New stuffed board with new coupling and bypass caps (here 0.22uF/600V with porcelain tubes, the best non resonant material for coupling caps) and carbon composition restors.

Another big problem are the power supply capacitors hidden in the black box next to the output transformer. These are originally made by Plessey TCC, four units of 4uF/600V matched together in one can. It is widely common knowledge today that these will not fail, this is generally true. Paper in oil capacitors are sealed and so they don't dry out their coupling component like electrolytic capacitors do. But in real almost every set of Leak TL12.1s I have seen, one showed a leaking case. It might be the oil itself, the temperature or other chemical processes together which might solve the tin sealing the case for leaks. Beside such problems the used isolating mineral oil in 1950 was definitive far away from todays health preferences and did contain amounts of PCB. So this leaking oil is not should be avoided anyway, beside hefty hum problems the amp will show up with when leaking. The designed values here of 4uF for the loading cap, 8 uF for the filter cap and another 4uF for the smoothing cap are the lowest possible values for such a push-pull design and will only work if the used power valves are perfectly matched for symmetry. Any misbalance of tubes will show up with clear audible hum. If the cans leak, it will result directly as well in loss of filtering ability as audible hum. In almost all restoration descriptions in the web people use electrolytic caps with higher values, since they will fit physically inside the can and will "give a better filtering"-effect!? Some people don't realize that the most rectifier tubes cannot charge higher values than 16 to 24uF and it will quit service with short term live cycles. On the other hand are the huge metal-paper capacitors in the PSU rail is the key for the exceptional smooth sound attitudes these amps are known for. So the "black box" is a must in terms of originality and unmatched audible excellence.

I have observed the market for original and NOS paper capacitors since the beginning of Ebay in 1995, original replacements from TCC or similar manufacturers are almost impossible to find if they need to match physically the size of the original box. They are alway wider, higher or don't match electrically the preference.
The only company today able to produce perfect replacements in the western hemisphere might be Jensen in Copenhagen, but with considerable low amounts of units it might be impossible to start a realistic production. So here is now a option to keep both preferences up, originality and audible excellence.
I am happy to present here a general solution for all enthusiasts of these incomparable amplifiers for that problem.


Solid copper cases folded and welded from 1mm thick copper sheets. 
Exactly matching paper capacitors in terms of physical size and electrical preference

The unit will be potted with Carnauba-Wax against mechanical shock. They will get a primer coat and a finsh enamel coat in black to match the original cases.


I have made solid copper cases which match the sizes of the original Leak cases, which are filled with caper-in-oil-caps of the original values, 4-8-4uF/600V and will be potted with Carnauba-Wax for mechanical isolation. These will be available on request to match any Leak TL12.1 amp to resemble their original authentic appearance. The extreme smooth pio sound will be available again for the next 60 years of service...

Read on soon, Volker

No comments:

Post a comment