The schematic below shows the tube amplifier circuit (click to enlarge). The items in red are the changes that are required for the conversion to UL mode.
To change the output section to UL the screen grid resistor (R107, 1k) was removed and the UL tap (tap 3) from the Edcor audio output transformer (OT) is wired directly to the screen grid (pin 10). Other changes I made to the amplifier circuit were to replace the input capacitor C101 (and C201) and the coupling capacitor C102 (and C202) with 0.22 uF metalized polypropylene types. This helps improve the bass response slightly.
The power supply schematic is shown below (click to enlarge). On the power supply I made the following changes which are shown in blue on the schematic. 0.01 uF polyester capacitors across the rectifier diodes. 0.1 uF polypropylene capacitors across all the electrolytic capacitors. One additional 100 uF electrolytic capacitor in the power supply.
Tube Amp Construction Notes
I wanted the amp to look cool (very black) and minimalist with its gold connections hidden and all hardware neatly tucked under a black enclosure. Two coats of Epoxy Enamel spray paint was used on the case once all drilling had been completed.
The amp is designed to deliver music well, to be unassuming in appearance and have a high wife acceptance factor (WAF). The OTs are slung under the top plate where the power transformer is mounted. Once again the power transformer and the OTs are rubber mounted. Multi-core shielded cable is used to connect the rear gold insulated RCA connectors to the 50k potentiometer and to the printed circuit board (PCB). The power switch was conveniently placed at the front of the amp and a captive power cord was used.
The potentiometer should be connected to the signal ground. Also keep the chassis earth and signal earth separate. Use the shielding on the input wiring to distribute the earth to the input RCA connections, potentiometer and PCB.
6T9 Tube Amp Measurements
The photographs below show the sine and square wave response of the amplifier. There is little to no change in the response compared to pentode operation.
Sine Wave Response - 1kHz @ 6.2V P-P, 8-ohm load
6T9 Tube Amp Sound
I played the new amp without NFB and with to determine the best sound. I found the amp sounded much better with NFB. The UL version of the 6T9 tube amplifier sounds better than the Pentode version. Because so many of the components are the same in both amps the improvement in sound is likely due to the UL operation mode.
Additional 6T9 Tube Amplifier Links