anrque wrote:
http://www.freewebs.com/valvewizard/heater.html
The section on Layout / lead dress has something to say on single ended amps vs push/pull amps.
He says,
"Valves in push-pull or in balanced stages (such as long tailed pairs using separate valves) should have their heaters wired in phase. Any noise induced will then be common mode and rejected by the stage (mostly). Valves in parallel single-ended stages should have their heaters wires out of phase for mutual cancellation. Using two different colours for the heater wires will make this easier. " This is what I have been advising for some years, and even before Merlin came on the scene.
He then goes on to say,
"The common pre-amp valves (ECC83 / 12AX7 etc.) when run from a 6.3V supply, should be wired from one side only [see right], not by looping one heater wire all round the valve socket, which would create a hum loop and cause excessive interference noise (though many amp makers DO make this mistake and get away with it). The wire twisting must be kept very tight right up to the socket, where it matters most. Their pin arrangement is also deliberate, so that the main heater pins (4 and 5) can be orientated towards the chassis wall, allowing heater wires to be run along the wall away from any other sensitive signal wiring." This is total nonsense. Don't trust 100% everything you read from Merlin (or anyone for that matter), although most of what he says is really excellent information. The fact is that heater wires don't actually need to be twisted at all, as long as they run close together. Soldano's SLO 100 is a classic example of a very high gain amp that doesn't use twisted heaters - just a pair of parallel bus bars. Early Marshall 18W amps used flat twin cable (aka zip cord) for their heater wiring, and I've built amps like that myself. Same for the only wiring on one side of the sockets thing, along with sacrificing a chicken at midnight before you first fire up an amp. It's voodoo.
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