Push-pull EL802 spud amplifier
I've been playing with a push-pull "spud" amp using EL802 tubes. EL802's are high Gm pentodes. For this experiment I have them connected as triodes.
Here's a schematic (click for full size):
I used some output transformers pulled from an old console, which I think are 8k:4. The input transformers are a pair of Sescom MI-19's a found cheap on E-bay. The breadboard looks like this...
It actually sounds very good, and measures well also.
Here are some numbers:
B+ = 300V
Vplate-cathode = 287V
Vcathode = 3.27V
Rcathode = 72 ohms
Icathode = 45mA
Ptube = 7W
Ig2 = 4.2mA
Pg2 = 1.2W
Pplate = 5.8W
1W THD < 0.18%, as low as 0.05%
6.78W @ 5% THD
The downside is that the EL802 has quite high input miller capacitance. Driven with a 600 ohms source it's -3dB at 20kHz. Driven with a 50 ohm source it goes out past 50kHz.