Vintage John Linsley-Hood ‘1969’ amplifier build

I started this a short while back. I’m currently partway through the build of a John Linsley-Hood ‘1969’ amplifier. This is a seminal amplifier design that defined all future class A designs. It was published in 1969, hence the name, and has been difficult to surpass in terms of simplicity and sound quality.

First a bit of background. I have some previous with vintage amplifier design and build, and I’ve built a number of valve amps and other bits of kit going back to when I was a teenager. I work in a lab which has a fair turnover of old electronic kit and regularly manage to pull things like scopes and power supplies out the WEEE bins. Some works, some doesn’t, but this stuff tends to donate a lot of parts to my builds.

So, I needed a small amp for upstairs and I was going to build something. Easiest way would be to use one of my many power supplies and a stereo class D module from eBay. This one would be ideal:

Fix it at the 15V setting and use the hole for the volume pot. Add inputs and outputs, job done.

But how about a new rabbit hole instead, because that would be far too easy. I’m thinking what about something iconic like the John Linsley-Hood 1969 class A transistor amp, using new PCBs but built with vintage transistors and the Philips capacitors from my stash.

Why complicate it and add loads more expense? Well, I kept getting videos about the cheap Chinese JLH amp clones with fake 3055 transistors popping up on my YouTube feed. They are available pretty cheap as kits with all the components, PCBs, heatsinks and the hardware for mounting those T03 transistors. I can't buy a pair of PCBs, heatsinks and hardware individually for the price the kits, so I figured I could just bin off the cheap components and swap in better quality ones, and basically replicate the original sound by using old transistors and capacitors.

Resources:

Wikipedia article on John Linsley Hood: https://en.m.wikipedia.org/wiki/John_Linsley_Hood

It says in there that it was published in Wireless World in 1969 (April 1969 issue, p. 148).

ORLY? So it is: http://www.worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/60s/Wireless-World-1969-04-IDX-75.pdf

So to stop this getting ridiculously expensive, I needed a power supply. A couple of months ago I pulled a 2x 30V @3a digital bench supply out the WEEE bin. Only one channel was working so I popped it open to see if I could fix it. It was a bit frazzled on the dead channel so I pulled off a couple of heatsinks for a pal and tossed it back in the bin. Amazingly, when I looked a couple of weeks later it was still in there so I pulled it back out to recover the transformer. Here it is:

When I said it had got a bit frazzled, I mean it had cooked itself. Properly. You can see it got a bit hot just from the circuit board:

But with the heatsink pulled off I reckon it must have been smoking a bit when it popped the fuse!

This all happened because the fan had packed up and jammed. And the thermal cutout clearly worked well on that model.

Anyway, with the rectifier board etc pulled out the case, this was what I was left with:

The transformer itself worked fine. The two secondaries put out 33.7V AC which is ideal for this project.

I started this at the end of Jan but I’ve got a decent chunk of the build to post up to get us up to date. The thing that kicked this off for sure was I found some unused original spec output transistors on eBay. Motorola MJ481, as specified in the original JLH 1969 design, and dated 1972. So that meant a full vintage build was possible.