Dynacord Eminent Restoration and Modifications

This is a nice old German made valve amp 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. There were also effects send and return connections.

Dynacord Eminent Tube Amp

The owner wanted to use this for recording guitar. So, we decided on a few mods:

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 idly - it’s just too dangerous. Every time you make an observation or measurement, it is best to turn off the power, discharge the unit, attach test equipment, and then power up again - so that you avoid putting hands inside it with the power on. It takes longer but it is much safer. So, in a case like this, taking time to get to know the unit with power off makes sense.

Adjusting bias on Dynacord Eminent Tube Amp

In the procss, 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.

Dynacord Eminent Tube Amp

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.

Dynacord Eminent Tube Amp

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.

Dynacord Eminent Tube Amp

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.

Dynacord Eminent Tube Amp

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 the first time I had worked on valves for quite some years, and I must say, I liked it. I hope there will be more in future.