Jonny Tubetronics: amps and synth stuff now in one thread

If you go back a few pages, you might remember I failed to spot that the two halves of the capacitor were not separated and were grounded by the chassis, so it wouldn’t work in the circuit. Annoyingly, I’d already drilled the case before I realised and because it’s quite a big lump it could only really go in one place and dictated the layout of everything else. That means it’s a bit too late to go back on a couple of things without shelling out on a new case and starting again from scratch. It’s not a massive issue and I made up a bracket to fit the potentiometer where the variable capacitor was originally going to go:



I used the mounting holes I’d drilled, but decided I’d correct the position on the front where the shaft comes through the front panel. Because of the dimensions of the variable cap, the shaft was going to come out low in the chassis and slightly offset compared to everything on the top. It would have looked a bit weird but there wasn’t really much I could do about it. So I made the bracket offset so the shaft comes out in an aesthetically better place. I’ll open the hole up on the front with a hole saw in the pillar drill and make an offset 3D-printed inset to plug the hole.

Lunchtime today I decided to modify the preamp slightly with a 3.5mm jack in the back. I've been using some RCA doublers to send the signal to the VU meter, which are are huge and take up loads of space at the back. I tried some piggyback RCA leads but they weren't much better. Drilling in a 3.5mm jack was the way forwards.



Not completely straightforward, however. The wall thickness of the case was too thick for the threaded portion of the jacks I've got. So I had to drill them slightly oversize so that the alignment slot would also go through. Drilled without checking, of course, but then found out the locking ring was smaller than the hole. Not enough thread to use a thin washer, not even shim, so I've had to bodge it a bit and bond it in place. It'll knock back out with a bit of persuasion if I need to take it out.

Some synth progress. Where I left off I'd decided on a final layout and drilled the case to suit. The bit I hadn't done was worked out how the case could be assembled. I figured I'd work that one out later and it would all be fine. Well that was today so here we go.

I started by putting the valve bases in and drilling a couple of holes with grommets to take wires up to the top valves. This simply took a bit of careful measuring to avoid the PSU underneath and the backs of the valve bases above:



Next I decided on a sensible way to distribute the power. The PSU has to come completely out to get the top enclosure off. Hard-wiring everything would mean it would need long wires to stretch that far, so I decided to make the 0V, heater supply and HT disconnectable:



That basically meant I couldn't get to any of the screws that secure the top enclosure or the ones at the back of the PSU. I added a new bolt hole into the back of the case which secures the back of the PSU and now it all comes apart and goes back together ok.

With everything in place it looks like this inside, before I start wiring everything up:



With all the hardware now in place it also means I can plug the valves in and have a look at what it's going to look like:



Wiring-wise, I'll start by laying down all the heater wires, then wire up the main oscillator circuit. Then I can add the square wave section onto that (valve at the back) and work on designing the sawtooth section (two horizontal valves). I'll leave provision to add the voltage control and I'm thinking about maybe having a removable capacitor pack so I can swap in different frequency ranges into the timing circuit (e.g. swap from 10-100/100-1000/1000-10,000 to 20-200/200-2000/2000-20,000 etc, or even frequencies of actual notes). That would be pretty cool.

I think I can feel another rabbit hole coming on. I need 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 remember MiataMark mentioned his one on the first page of this thread and I looked it up at the time. Then recently I keep getting videos about the cheap Chinese 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 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: en.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: www.worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/60s/Wireless-World-1969-04-IDX-75.pdf



And there's a bit of early Lycos html webpage dedication here, talking about the transistors in particular: paul-kemble.tripod.com/sound3b.html

So to stop this getting ridiculously expensive if I do it, I could do with a power supply. About a month 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 this morning 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 has 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 is what I'm left with:



The transformer itself works fine. The two secondaries put out 33.7V AC which is ideal. I'm going to have a look at what smoothing capacitors I've got at home and if I need to recover the bridge rectifiers. There might be some fun electronics to pinch out the front panel and I might think about re-using this case.

I do enjoy reading this thred, even though, on a good day i understand about 10% of it since my electronics knowledge is basic to say the least.
A bit i think i understood was the swappable capacitor packs, might it be possible to include them in the main build, with a rotary switch to change settings. If this is not possible, how about a stand alone unit with some old school type latching push buttons?
Ttfn Glenn

I've got the sine wave part of the synth mostly wired up. It's just the timing circuit left (best till last, eh?) and I need some longer screws for the valve bases so I can attach the ground returns. Before I do the timing circuit I need to have a think about how I'm going to mount and wire the three pairs of capacitors through the rotary switch without ending up with a complete mess of wires, because it's getting that way already :/

Same here! I’ve not had one go that far though, it must have stank!

In the post:



5x output transistors. Motorola MJ481, as specified in the original JLH 1969 design, dated 1972. So I guess this build is on!

Been collecting some bits in the background for the amp and the synth VCO.

I've bought a couple of those cheap eBay JLH 1969 amplifier kits, cheapest ones I could find, and I'll be dumping all the components and just using the PCBs and hardware. I'm wondering if they'll actually turn up because I've heard nothing from the seller. No matter, it's eBay.

Capacitors, I had most of the big ones in my stash but not the 100uF and 220uF ones at high enough voltage, so I've been on eBay and found some. Will use a couple of Mullard tropical fish ones from my stash for the input coupling and apart from some 3906 transistors in metal cans, I've got pretty much everything.





I got a used 150x100x50mm heatsink for cheap (above) which sounded quite sizeable, but once I got it in my hands it didn't actually have that much more area than my MOSFET amps have. I think it's a bit too marginal for old transistors which are less tolerant to overheating. The power supply needs some heatsinking too, so I might use it for that. Instead, I'll stack up a couple like the ones below. That's how it's done on my bench supply, so that's legit.



No progress on the synth yet. I needed some longer screws and kept forgetting to order them. Done that now and also ordered some engraved tags for the jack plugs etc.

Suddenly realised I've got a few unfinished projects on the go. Need to prioritise...

Yes, there seems to be a fair number of them still about, which is quite surprising given their age (late 50s?). They do command a bit of a premium though, especially because they were used in old guitar amps.

On that subject, that does make them a generation earlier than this amp. I’m going to see if I’ve got a couple of the later type which had a beige resin casing. I think they’re basically the same construction inside.

I'm not an expert on this, but I've got a couple of generations of film caps.



Left to right is oldest to newest. Left hand one is a Mullard 'mustard' cap which I think are 1950s. Stripy one is a Mullard 'tropical fish' cap, which I think are late 50s into the 60s. Next one is the beige resin one I meant above, which I think is 70s but not 100% certain. Orange one was around in the 90s and the new ones are like a dark red colour.