Build Report: Kay 720 - inspired

[passaloutre]

I picked up a pair of Magnavox 169 power amps from an antique-store record console. These are push-pull 6V6 power amps with 6AN8 phase inverters meant to be driven by a stereo ceramic phono cartridge. I've had my eye on the Kay 720 circuit for a while now, as the pentode/triode phase inverter seemed unique enough in the amp world to be worth a try. So when I saw the 6AN8 in here, it seemed like a good enough candidate. Edit for the keen-eyed: I should say here that I realize the Kay amp used 6L6s and mine is 6V6s, but it is otherwise inspired-by the Kay. I rarely have a need for more than ~15 watts.

Since the Magnavox chassis lacked a preamp tube, I didn't know if the power transformer would be up to heating another bottle, so I decided this would be a good opportunity to try out another thing I've been curious about: the LND150 MOSFET. My simulations showed the LND150 should be able to make the same gain as the 5879 in the Kay design, so I forged ahead.

I stripped out the original circuit and laid in my trademark what's-the-point (WTP) layout. It was pretty straightforward; as someone here once mentioned, this circuit seems to be an exercise in building an amp with the lowest parts count. This is actually the third amp in a row I've made that just worked on the first try, no wiring errors or troubleshooting (pat myself on the back).

I'm very impressed with the tone. I've read on this forum several times that the best place for a pentode is not the first stage, but further downstream, where it can be driven by what comes before it and show more of its character. I assume that's what's happening here: the MOSFET preamp provides lots of clean-ish gain (I haven't scoped it yet) to slam the pentode in the phase inverter. The result sounds, to me, a lot more like a power amp distortion than a fizzy 12AX7, not entirely unlike a cranked Champ. My only complaint is the taper of the volume pot. I used audio taper pots, but this thing jumps from zero to nearly-full-volume within a few degrees, and the rest of the dial just adds more distortion. I think I might build out the other of the stereo pair from the Magnavox console in a similar fashion, but with a post-phase-inverter master volume.

I put this one in one of my trademark plywood-over-spraypaint (POS) combo cabinets with the Alnico 15" woofer and grillcloth from the Magnavox console. I think I might add an L-pad attenuator to the back of the cabinet and call it done.

[passaloutre]

Here are some photos of the finished product

[dogburn]

Looks like a fun project. You probably could have added another preamp tube - I found specs for the transformer at the radiomuseum website (my go-to place for specs when scavenging bits).

[passaloutre]

Radiomuseum is one of my most frequented sites, as I get most of my parts from old hifi. People usually just give this stuff away, and most of these consumer console amps are right in that 5-15W sweet spot for guitar (SE or PP 6V6, 6BQ5, 6BM8, etc).

Anyways, I do believe I checked the transformer specs, but this was already a borderline case (6V6, 6V6, 6AN8 need 1.35A, greater than the transformer 1.2A spec), and I wanted to try the MOSFET anyways.