GrindingThud
1000+ Head-Fier
- Joined
- Dec 11, 2012
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There are a couple of noise issues typical of the amp (or any similar OTL). The output tubes are notoriously noisy. Some hum, some fizz, some very microphonic, and some arc over on rare occasion. It took me a long time to find tubes that I liked and are quiet. The drivers also can be microphonic, hum, or fizz. It's not so much an amp design problem but more a 6AS7 otl thing. Once you find good tubes, stop rolling. Now, the amp does have a design that is prone to very low level hum, that is magnified by bad tubes. I did obsess over the hum because I was using sensitive low impedance phones not recommended for the amp. I ended up modifying the power supply and heater wiring in the amp to quiet it all the way down (described in the WA3 mod thread in my signature line). As far as the PS xformer goes, the amp draws very very different current when swapping to 5998 or 7236 tubes....the plate current actually drops almost in half, as does the heat. Flip side, I'm running constant current sources (like the speedball from bottlehead) and hold the current high and sometimes 6n1p 6CG7 and/or other tubes with higher heater current in the driver section The transformer can easily keep up. The amp has good bones.