Sridhar "Ganti" Gantimahapatruni wanted a bit more power than the Randall gave, plus wanted to experience the "big sound" of directly-heated transmitting tubes. Using a circuit derived from Nobu Shishido's designs, Ganti built a single channel class A2 amp on a Randall amp chassis. In class A2, the output tube is drawing grid current. In conventional output tubes this is troublesome, but by using the ultra-high mu 811-A triode, grid current is drawn all the time, so the driver doesn't have to make the transition from current to no current. The 811-A used here is not to be confused with the Svetlana SV-811 audio tubes; the 811-A is the original American transmitting tube with a plate cap. (Svetlana does make a transmitting-type 811A).
Here is the schematic in pdf format: (55K)
The power supply is derived from Ganti's Randall amp, with 6AU4GT damper tubes used as rectifiers. The signal flow is from right to left: the first stage is a triode-connected 7788 with battery bias, the second stage is a triode-connected 6W6GT and the 811-A is on the left. The output transformer is a One Electron UBT-2. Electra-Print interstage transformers are used between each stage.
This is the left side of the amp. The UBT-2 output transformer is at the left. The wire and connectors at the lower left are for the external DC filament supply. To the left of the 811-A is a bias supply transformer.
Here is the right side. The 7788 looks very low since it is mounted on the internal sub-chassis. At the right is a filter choke.
- John Atwood