Dynacord "Eminent" Valve Amplifier Restoration And Conversion

May 03, 2020

The Dynacord "Eminent" is a nice old German made valve amplifier that I was asked to restore. It was originally designed as a small mono PA. It has two instrument and two mic inputs, simple bass and treble tone controls, and 4, 8 and 16 ohm output taps. The owner wanted to repurpose this valve amplifier for recording guitar. This is an account of the work involved.

Dynacord "Eminent" Power Amp (before restoration)

(Interestingly enough, Dynacord still exist and still make audio gear!)

To get the Dynacord "Eminent" ready for use as a guitar recording amp, we decided on a few mods:

Disconnect the mic inputs so that they don’t add any noise (simply lifting one end of a resistor, so this mod can easily be reversed in future).
Create two direct outputs using the effects send and return connectors - one from the output of the preamp, the other an attenuated version of the speaker signal.

The unit had just been acquired, and had been in a dusty environment - it was filthy. So my first job was to give it a real good clean, with a soft brush and vacuum cleaner, and then take a good look inside. I could see that someone had already tried to restore it, as most of the capacitors had been replaced already.

When I powered it up, I first checked the power rails, which were good. However, I could not get a signal through it.

I found several schematics online, but none were completely accurate, and I had nothing to tell me the location of components. So I spent quite some hours just tracing the circuit through and noting what was where. Working on an valve amp demands caution. The presence of 400V DC means you don’t go poking around in there carelessly! Making measurements has to be done with great care, and after powering off it is important to make sure the power rails have discharged before touching anything. Mistakes can be a shocking experience!

So, in a case like this, taking time to get to know the unit with power off makes sense.

investigating the Dynacord "Eminent" amp

While figuring the amp out, I found a few problems; a wire had fallen off one of the front panel controls, and a damaged PCB track. Powering up again - hey presto! I could at least get a sound. However, it was quite noisy - some mains hum.

Some wire straps on the back of the old PCB

Now I did all of the mods. Most of the sockets were falling apart, so, apart from the unused mic inputs, I replaced them. I gave the two instrument inputs slightly different impedances, one 33k and one 1M ohm, to offer some tonal variety with electric guitar. I took the preamp output via a small audio output transformer so that to avoid earth loops. Some speaker terminals were also falling apart, but there were enough good ones that could be used, so they were also rewired.

Interior of the amp after some rewiring

With the amp now more or less working, I set up the output bias. There were also some adjustments to reduce mains hum by balancing the valve heater currents.

The heaters in these things take quite some current - the two output valves take 3A between them. The current in the heaters is 50Hz AC, and it has to pass quite near to sensitive low level signals, so the way they are wired is quite critical. I decided to rewire them, as I knew I could make them neater and shorter - thus reducing pickup.

the Dynacord "Eminent" is powered up and working a treat!

Finally, I improved the ground wiring. I didn’t trust the 50 year old PCB to give much of a ground plane, so I added a number of straps to ensure that the earth reference was solid. I also rewired the earth wire from the mains transformer to the smoothing caps, rerouting and shortening it.

Ready to go back to its loving owner

With this done, mains hum became almost non-existent. A play test revealed a sweet, warm sound. The two direct outputs have a different character - the preamp output sounds warm and sweet, like a classic 1950s jazz amp; the direct out from the speaker side is grittier.

This was a satisfying project. The restoration took some time but was really worth the effort.