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Originally Posted by yestertech
Lovely amps the leaks, and the TL12+s look like quite a late production, as the usual McMurdo valve bases have been changed to a different supplier.
1. To ensure the project is worth continuing with, run a continuity test ( with ohm meter ) on all the transformers, as any failure here will be possibly render the amp 'BER' ( rewinds can be done, at a price ! )
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Will do I found this handy piece of information in the LEAK Book
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2. Replace only the essential parts to begin with - ALL the TCC 'metalpak' and 'metalmite' caps have to go. The main electrolytic can is paint matched to the chassis but if you're lucky and its powered up SLOWLY, it may reform. In the absence of any suitable tester, Hum on the output will give you a clue that is hasn't
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When you say power it up slowly, do you mean by using a current limiter such as a light bulb?
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3. Get rid of all the old EL84s and buy some decent new ones - old ones are notorious for 'grid current' which can destroy the amp over time. Do not go down the 'NOS Mullard' route as these are likely to be just as bad.
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I was hoping to get these valves checked by an AVO valve tester, but I was also thinking of buying fresh new valves from Watford Valves, they supply a range called the Harma Retro.
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4. Once the amp is running safe and cool, you should be able to measure the various voltages, against the circuit diagrams to determine anything else which may need changing. This is a good exercise for finding your way around the circuit. The measurements will not be exact, but valve circuits are generally very component tolerance immune.A listening test here will show up any noise ( old EF86s are notorious for 'rustling' ) or hum. Resistors in certain positions can cause noise and if the anode loads ( usually 100K & 91 K ) on the ECC81 are a long way out it can cause increased distortion.
4. Ignore all the articles on how to 'tweak' and 'improve' the amplifier, how xyz valves and caps ' improve the soundstage' and firm up the bass' This is 99% fantasy. The only mod. which is probably worth doing is that of gain reduction around the first stage to get rid of some noise, as both the TL12 and the ST20 are over-sensitive for today's sources. A volume control can be interposed to reduce the level into the amp, but, of course the noise remains.
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Leak knew what they were doing with these amps and the under chassis wiring is a joy to behold. For not a lot of money they can give many years more service.
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Looking foward to it A, thanks for the input. I am positive with everyone's help I will choose wisely and myself and the family can enjoy.