There was a surplus of ECH81s which is a remote cutoff heptode and low mu triode with a common cathode (Datasheet). I spiced and breadboarded lots of ideas and came up with this:
[img
522]https://tubeamparchive.com/files/schem_191.jpg[/img]I 'pentode' strapped all the heptodes by using G1 as the grid, connecting G3 to the cathode and using G2+G4 as the screen. The preamp starts with a heptode that I scoped at a gain of around 40 while breadboarding, followed by the triode with a plate driven TMB tone stack. The Ra is only about 5k for the triode so it works ok to drive the TS, since a CF is impossible with the common cathode. I measured a gain of about 10 for it.
The common cathode is the biggest problem by far in the phase inverter and power section, because the triodes in the PI and the heptodes in the power amp all share a cathode connection, and the biasing for each is very different. I solved it by using a split cathode resistor where the PI triode grid leaks connect between the two Rks instead of ground. That way the bias for the triodes is set by the first Rk, and the bias for the heptodes is set by both Rks in series. A hefty bypass cap is needed to keep it from oscillating like crazy.
I used a 16 ohm speaker on the 4ohm tap of a 10k OT to get the 40k plate to plate impedance the heptodes needed. Output is somewhere around 3W.
I wanted to build it as cheaply as possible, because even after breadboarding I wasn't sure how it would all work. So 95% of the whole thing is recycled parts. Old SS practice amp chassis and cab, iron, sockets, tag strips, resistors from the radios and caps from junk PCBs. Only new part is a Weber ceramic Blue Pup, the only 8" I could find in 16ohm.
It needs some tweaking still, but sounds quite nice and very unique! The ECH81 is pretty microphonic in the preamp when the gain is cranked, which didn't show up breadboarding but does in the combo.
Not a pretty build at all, but goes to show there is a world beyond the handful of commonly used tube types.
