My OP kinda got highjacked… maybe I can still use it:
Between trips to central TN to build a house (I’ve spent 3 out of the last four months there doing landscaping, electrical work, tongue and groove interior paneling, tiling, framing my future audio/man cave and more, oh, so much more…)
Came home a few days ago, fired up the Tektron 300b (it’s a TK2/50S 2A3 model switchable for 2A3, 211 (?) and 300b tubes) and that hum finally got me motivated to throw the amp on my “bench” (coffee table) while listening to Kenny Dorham, Shelly Manne, Miles through a Willsenton R8 tube amp with all the Skunkworks modifications I could justify and…
I couldn’t figure out months ago what was causing the hum. worse in left than right side tried different tubes (original Sophia 2.5V, EMLs, NEW Electro harmonix gold grid something or others), all hum. No hum when I listened to the amp at the guy’s house. I forget the speakers he used, but they were four figures more than I’ll ever afford.
The only schematic I have from Tektron is a general design from that era, some things are different, but the only ripple I could find measures in the microvolt range, like 0.8 mVAC. unhooked several capacitor leads (yes, I used a chicken stick to discharge them down from 450 volts first) and measured ESR’s in the sub-ohmic range on the 300b bypass caps. I poked around a bit, rerouted some wires, resoldered a couple of connections, fired it up and HMMMMMMMMM
Still.
In a moment’s frustration, I twiddled the voltage dial down to 2.5 V.
NO HUM.
Now THAT is interesting.
The amp is designed for 115 VAC. At 120 VAC wall voltage I get something like 5.9VAC filament voltage at the 300b tubes, so I run a variac between wall and power filter/conditioner to cool things down. Actual voltage from the 2A3 setting now is really closer to 3 VAC, and NO HUM.
Sure, I CAN run the 300b tubes on that voltage, but can someone throw me a bone here, advise on what most likely culprit may be and where to apply my - limited - time?
I have ALL the components in one large box ready to begin building the Skunkie Designs 300b amp once I’ve cobbled together the chassis - maple sides, steel top - which I’m also working on.
Thanks in advance.