The 807 is a classic coke bottle shaped tube primarily intended as an RF amplifier. I decided to take on the challenge of building an audio amplifier out of them. The fact they have a top, external anode connection that would provide a cool look was all I needed to go ahead with the design.
Knowing I was probably going to have run the 807's at reduced gain to tame any high frequency instability issues that may arise I went with a 2-stage preamp design using both halves of an 6SN7 dual triode. A second 6SN7 acts as the phase splitter. The 807's are run in triode mode - by coupling the screens to the plates through a 100 ohm resistor. The plate impedance of the 807 can approach 10K ohms - so the output transformer selection was tricky. I selected a pair of Hammond 1620's - not optimal because they only offer an approximate 7K impedance - but they were within my budget and came close. I added a pair of millimeters and switching to allow measurement of the 807 cathode current to facilitate matching if needed as "matched sets" of 807's are hard to come by. Sadly two 807's rolled off my work bench during the construction of this amplifier and met an untimely end.
This amplifier sounds very open in the mid and treble ranges but may be considered a bit weak in the low end. Paired with a subwoofer you'd never notice! It provides around 25W and offers classic tube sound and unusual looks. The blue front, copper feet and heavy gauge anode wires make it a conversation piece.
Contact me at geoff@bandonelectronics.com if your're interested in this amplifier. I can build this design into any configuration - so you don't have to go with the look I chose if you'd rather have something a little more subdued.