Denco 912-Plus (1950s)

DENCO / OSRAM 912 PLUS

With an EF86, ECC83, 2x El84 and EZ 81 rectifier and Partridge transformers.

This was in a right state when I got it. It was covered in soot from being in an attic for years, had only 2 knackered valves and wires and sockets either missing or hanging off. It was my first major repair project and I couldn't find any info on the web. I guessed the missing valves and thought this might make a nice AC 15 project.

When looking at the images below of the rag - tag assembly of replacement components and pen marks, understand this was never an attempt at sympathetic restoration. I was just seeing if I could get it to work, using my skills as a trained Engineering Physicist, and I didn't have a schematic.

Eventually, I found out it was a GEC design, a rival to the Mullard 5 - 10. One look at the top of the chassis tells you DANGER!!! Naked trannys!! I still haven't finished making a case for it, but one day....

Most of the components on and around the board needed replacing. Most of the coupling caps leaked DC, it needed valves, and there was evidence of more than normal heat generation around the valve bases. After replacing the faulty caps and a couple of burned up resistors (one of unknown value) it was time to give it a try;

I made a current limiter before tying any voltage on this Frankenstein monster, as I wanted to wake it up very slowly. I remember using a long wooden stick to switch it on and being ready to pull the plug!!

Well, there were no bangs or smoke, no blown fuse, I could see heater glowing faintly. The bulb was working, fear turning to excitement, a gentle hum from the speaker. I tried the volume control and it worked, there was a faint hiss then a few spits. The tone controls made a horrible noise even though I had cleaned them, but it was WORKING!

When I finally dared to apply full mains voltage the rectifier arced, but I managed to switch off immediately.

After eventually tracking down a schematic, I checked over every component value and connection and noticed there were some parts absent. A couple of high wattage wire wound 58 Ohm resistors on the anode pins dropped the current to an acceptable level and that cured it. I bought a replacement used EZ 81.

The next problem appeared shortly after warm up when one of the EL84s decided to arc. A closer inspection revealed a damaged valve base that could only be seen after it was removed.

The modern replacement base required the use of a reaming tool to make it fit. OK, I know, "Don't make holes in a vintage chassis."

It still surprises me that I got this grime ball of scrap working at all and it has a beautiful warm sound and very clear with no distortion. I learned a lot with this very cheaply acquired chassis and use it as a test bed for experiments. I have tried to "Marshallise" it, (sorry!), and disconnected the feedback and the ultra-linear connections, but it is still low output uncoloured and undistorted, probably due to all the components around the tone controls and input selector, and the hefty ultralinear, 7000 Ohm OT.

I fired it up again today and it still works nicely. Perhaps I will do a proper restoration on it one day, but really, it's guitar amps I'm interested in.