BACK FROM THE SIXTIES

Production chassis will probably have one of three board layouts; S-10-1, S-10-2 or S-10-3 / 005 0051. All three share essentially the same circuit, they just have track reworks to get rid of wire links. There are also some pre-production or prototype units about that are different. 'MM Musician and Redmere Amplification' magazine of summer 1978 says;

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'Another prototype was built and fitted with the very first prototype amp that Leo had been hand building meanwhile, and this was shipped off to this year's Frankfurt Trade Fair. We had not had time to finish the brand new 200 watt power stage for Redmere and so we used half of an AP360 P.A. amplifier.'

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The publicity material and a magazine review by Mark Sawicki in September 1978 claim an output power of 210watts and these versions obviously did exist. Measurements on production models from late 78 and early 79 produced 156watts into 4ohms, 112watts into 8ohms and 73watts into 15ohms. (the combo version used two 15ohm Celestion G12-80s wired to 8ohms). All measured via the AUX input at the onset of clipping.

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A note of caution when testing S-10-1 boards. Don't try to use the two mounting screws as a ground reference. They're bolted straight to the transistor collectors on the AP2/2 boards. The top one's got +50volts DC on it and the bottom one is virtually shorted to the positive speaker output. S-10-2 and S-10-3s were redesigned with four screws fixed to the grounded heatsink.

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Wire colours to the AP2/2 boards seem to vary so check before disconnecting them.

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The quiescent current through the output transistors is set by R17. There's no fixed value for Iq, just high enough to get rid of crossover at cold and low enough not to cause thermal runaway.

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TR1 and TR2 need to be fairly well matched to minimise the DC offset at the output but unfortunately, like a lot of the semi's in this amp, they're obsolete. Current production Fairchild KSC1845FTAs work well. They're very cheap and it's no problem to buy ten at a time and select them for hfe. Beware buying obsolete parts (or any parts come to that) from eBay, they're quite often not what they seem.