I used a 220K-100K voltage divider to give the heaters a ~50VDC lift, which completely fixed the noise issue (note the B+ in my circuit is 180VDC as opposed to ~150VDC in the original design, so adjust voltage divider resistor values accordingly).
The second change that must be made to completely cure the noise issue in these tubes is to elevate the heaters above ground. The hum is present on some input tubes, not all, due to heater-to-cathode leakage. Not sure of the time period or value of this Dixie bedroom suit.Any help would be appreciated. This improves the hum but does not eradicate it, I suspect due to significant primary-to-secondary leakage currents in the transformer which is of poor quality. Dixie Furniture Vintage Dresser, Mirror and Nightstand EXCELLENT. A virtual center-tap can be made using two 100ohm 1/2W resistors. The heaters in this amplifier are not center-tapped, the designer has grounded one leg of the heater at the power LED ground point.
The widely reported hum issues in this amplifier can be corrected by making two circuit changes. I'll post something in the DIY thread to show it off when it is done, but wanted to point something out here. These are not hocus pocus modifications, actual circuit design changes based on sound tube amplifier design. I am doing a total circuit overhaul of this amplifier to improve the design and also correct many of the mistakes of the original designer, just a fun DIY project for someone on Head-Fi.
