Single-Ended 807 Amplifier
This amplifier was built on-the-cheap. I used salvaged transformers purchased well below online prices and 807 tubes which are still reasonably priced. 7 watts per channel. I won an impromptu local “shootout” with this. Recently added speaker/headphone switch and headphone jack. Now it’s personal!
How it was done: Single-ended pentode mode, with primary-side feedback, and fixed bias operation.
A look at the construction:
Evil Science Audio 
Just wondering why you didn’t use a bypass capacitor on the cathode of the 807.
Ah, look again! It’s a fixed bias circuit, not cathode bias. The 10 Ohms are present for allowing the current level to be measured (50ma = .5V). Very little degeneration at 10 ohms, bypass isn’t needed. Ignore the schematic comment, it’s .5V not .05V.
most of my parts are ordered for next round of amp builds and I cant wait. I am building this 807 version, Blueglow’s 807, a 7C5 SE and should only take me the next few years to finish 🙂 Its the OT transformers that there isnt many options on budget wise. Edcors will wind up being the winner I am sure but I was wondering what are these OT’s you have in this 807
The OPTs were found on a shelf at an electronics surplus store and are of unknown origin. My guess was from a reel-to-reel deck.
Wondering what the FWB is? Also, can the feedback be eliminated with minimal affect? Russie
Full Wave Bridge. That power transformer had a bias winding. If you use one with only a bias tap, adjust accordingly. Feedback is really necessary for this, or it will sound BAD. If I were ambitious, I might change the output to triode-connected 6DQ6 and redesign the driver. That could work without feedback, kind of a poor man’s 300b amp.
Any actual distortion measurements? THD? IMD?
No. With something like this, there is no upside to knowing. It was fun to make and sounds good (not single-ended good, but not bad either). Measurements will just degrade my perception and ruin the magic of the hobby. Now if I spend real money, I might feel differently!