I had an unusual valve kicking around – an ECH83. It’s a triode-heptode designed for use in car radios, with an anode voltage of only 12.6V.

I also had a couple of the ubiquitous Park G10 practice amps from my early guitar-playing days waiting to be recycled into something better, so I thought I’d see if I could concoct some sort of bastardised hybrid.

The G10 actually has quite a nice chip amp for it’s final stage; the TDA2030A. What a pity about the JRC4558 preamp stages. I really hate the sound of that particular 80’s opamp.

Inspired by this wanton ECH83 abuse by Engels, I decided to use both the triode and heptode in the ECH83 as preamp stages and salvage the silicon from the G10 for a power amp stage.

This is the result (click to enlarge):

Read on for explanation, layout and construction notes.

Sorry for the lo-fi schematic; it’s scanned straight out of my notebook. I’d rather build stuff than spend ages making a pretty CAD schematic!

The TDA2030A part of the circuit is pretty much straight from the datasheet. Here’s my stripboard layout, in case has someone wants to build one (strip layouts I do use my PC for):

Note the cut at 5a, under C1, and the closeness of C3 and C4 to the chip. This is essential with the TDA2030A, otherwise it’s unhappiness manifests as parasitic oscillation. Also essential is the heatsink (also salvaged from the G10 board).

I kept the transformer, rectifier and smoothing caps from the G10 to give +20/0/-20V, and used a seperate wall adapter to give 6V for the heater. I could have used a regulator and tapped the main supply instead. It fits into the chassis of the G10 perfectly but I didn’t put it back into the G10’s cabinet as this thing really deserves a better speaker. IE, one that’s not made of cheese.

The lashed-up amp itself:

The valve socket’s led to a terminal block, with another terminal block leading to the power amp board in parallel (geographically, not electrically!). The components configuring the preamp stages are connected in between the terminal blocks.

Feeding 80’s Japanese silicon with 50’s British glass turned out a lot better than I expected. Feeding a 15″ Celestion I get an amazing warm bluesy sound with an undertone of gorgeous valve breakup, even at low volume. If you want soundclips, let me know and I’ll try and record something!

It goes much louder than the G10 did; this design runs the TDA2030A a good bit hotter than in the G10 and pushes the chip amp to give it’s full 18W. Really it could use a bigger heatsink than the one from the G10.

It makes a lovely practice amp, and if I could get rid of the small amount of 50Hz mains hum I can hear it would be a pretty decent studio amp too. Don’t know if I’d be able to replace the valve if it goes; a cursory glance at eBay shows a single ECH83 for sale at the moment. However this rig is perfect for experimenting with different strange-valve-as-preamp ideas, so I’m sure I’d come up with something.

A dream is to have a completely homemade signal chain from axe to amp; I don’t insist on making the actual guitar but I do want to try winding some pickups. Now I have an amp, and I’ve already got various homemade effects units that I’ll document here soon.