SUPER COOL OLD GEAR: RADIOMETER COPENHAGEN 100db AMPLIFIER

This beauty came from a Canadian Government auction, through one of my favourite surplus warehouses, ESI. Made in Denmark, this is a laboratory grade vacuum tube amplifier. It runs 5 EF40 pentodes, an EZ40 rectifier, an 85A1 voltage regulator, and a UL41 output pentode. The tubes all date from early 1952, so it's safe to say this unit was crafted some time shortly after that. Massive power transformer and thoroughly elegant layout design. 10 MegOhms input impedance and 500 Ohms output impedance, both unbalanced. The single control is a rotary switch that sets Amplification in 10db steps. First thing I thought when I saw it was: gotta try this as a BASS DI!

Panel closeup 1

Panel closeup 2

Badge closeup

It needed a new pilot light assembly and jewel, and its signal capacitors required replacement: they were the type that look like stale chocolates. Exact values of excellent reliability mylar film and foil Philips Mustard axial capacitors were employed. A general spruce-up and some new rubber feet, and it was time to check it out.

chassis

wiring

Look at the beautiful construction details like little ID tags affixed to the tube socket mount screws.

It came with a cloth 2-conductor power cord that's still supple and mates neatly with the inlet port, so I decided to leave that be as it works well. I set the voltage for 127... now, here's the counterintuitive thing about voltage settings with old gear: you want to be kind to it, and not give it too much voltage, and you see a setting marked 110V and you think that'll be great... but NO. STOP. If you apply today's modern 120-125V to a transformer set to transform 110V, the resulting voltage on the components will be almost 10% HIGHER that it should be. The correct approach is to choose the setting that is EQUAL TO OR THE NEXT NOTCH ABOVE what's coming out of your wall. In this case, set to 127V, if I apply 120V, the voltage on the components will be just a hair LOWER than design-spec, which is definitely preferable to HIGHER for gear that is over 60 years old.

The input and output connectors are instrumentation-type affairs with separate banana plugs for grounds. I thought about replacing the jacks with 1/4" phono jacks, but decided for now it would be better to leave the unit unmolested, attach 1/4" plugs to the ends of the cables, and build a breakout box. Using a McGohan MC-1 microphone transformer (like Altec/Peerless 4722), I could now test the unit with direct instrument input and low impedance microphones. I added an output attenuator to control the signal it kicks out, and as it turns out that was a good idea, because this thing has MONDO GAIN. With a bass guitar plugged in, it will distort (satisfyingly) with the rotary control set anywhere above 30dB. I have yet to hunt down replacement tubes - or even test the ones in the unit - but aside from a little hiss at higher settings, it's quite a joy to use.


Check out this sound sample. Everything you hear was recorded using the Radiometer Amplifier, a Strymon BlueSky, an old Shure SM58, the Hammond S6, some hand percussion, and a BOSS RC-3. Transferred to a Nagra IV-STC at 7.5ips dub-style, whatever high frequency hiss you hear is a combination of the Radiometer (multiple passes in the loop) and tape hiss.



Click here to go back to the main page.