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In Reply to: I believe that the design of the Moondogs was DC on the heaters......... posted by Cut-Throat on November 10, 2006 at 12:34:22:
No, the Moondogs used an AC heater. I built mine from the kit and there is no rectification on the heater supply. It's just a tap off of the power transformer.If someone "customized" your amps to run with the AC heater, you were lied to. BTW - concensus is that the AC heater sounds better.
Follow Ups:
Well, can you tell me the purpose of "BR1" on my Moondog schematic?This was not here when I bought my Moondogs. (BTW - I was not lied to at all, I was just trying to tame the hum) So I added BR1, as outlined in my instructions and schematic.
Cut-Throat
Look at the green 2.5v leads. They lead (with a paralelled hum pot) directly to the heater of the 2A3 via pins 1 and 4. This is the AC heater supply I am referring to.The rectifed signal is used for the 6sn7s, but not the output tube.
I've got 2 schematics (separated by 2 years) for the Moondog and both show the rectified low level supply (BR1), so I am curious why someone thought removing it would be a good thing. Whenever I've seen reference to an AC heater, it always been discussed in context of the output tubes.
Yes, the 5.5VDC supply would be for the 6.3V tubes, not the 2.5V tube. But it is most assuredly a heater supply. 6SN7s have heaters, which is the proper terminology for the filament inside an indirectly heated tube that is separate from, but heats the cathode. DHTs like a 2A3 have a filament which also function as the cathode.AC heaters and insufficient B+ ripple reduction of driver and input stages can both cause hum that is often mistakenly blamed on AC filaments of DHTs.
There are too many different applications and gain structures possible to suggest only AC or only DC supplies as a definitive method for all cases. And there are a lot of different ways to build each.
Well, that's what I was told. Anyway mine came without BR1 installed. So I hooked it up and it tamed quite a bit of hum.BTW - If yours still hum, they can be made 'dead silent' - I use 104db speakers too. Try lifting the ground on your Moondogs with a cheater plug and see if the remaining hum disappears. If it does, you'll know you have a ground loop problem and can work on that to remove the remaining hum. That was the last source of hum on mine.
I was convinced that it was not ground loop hum until I tried it. But indeed it was, slight but it was there!
Cut-Throat
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