Of Modules and Tubes
You just realized that you keep some hidden treasures in your locker? Or you have spotted one of those little gems on eBay and now feel magically attracted? Those inconspicuous metal boxes with a lever mechanism on the front and a single multipoint connector on the back... But: How are they actually connected? And: Do they still work?
Using the example of classic studio amplifiers in cassette form with those cryptic designations such as V72, V74a or V76, I would like to show you what you should keep in mind when reactivating these or other old tube amps for your studio.
From the outside, at least, you can't tell from these plain cassettes that under the hood they are top-quality tube units that were benchmark amplifiers in broadcasting studios until the 1980s. These modules are instantly recognized by their grey (RAL 7030) front panel, labelled with the model type, serial number, manufacturer's logo and featuring the typical high gloss nickel-plated quick release lever, with which these cassettes could be exchanged in seconds without tools and on the fly during operation.
Inside such a cassette, however, there is a complete, multi-stage tube amplifier with input and output transformer, selected and individually numbered components, expensive long-life tubes and an integrated power supply.
Most of these devices were developed in the 50s and 60s by the central technology department of the NWDR (North-Western German Broadcasting Institute), and they were designed for operation in the whole ARD (General German Broadcasting Institute) region with 220 volts, as an export to other countries was not intended. The advantage of having an internal power supply is obvious: Stand-alone operation. But especially in this regard, some important conditions have to be considered.
Mains Voltage Then and Now
I would like to start with an aspect which is particularly important for the operation of old tube technology today: The mains voltage supply. Until 1987, the mains voltage in large parts of Europe, including Germany, Austria and Switzerland, was 220V with a tolerance of ±10%. The nominal voltage of 230 V which is valid in Europe today was defined as the standard voltage in the international standard IEC 60038 of 1983. From 1987 onwards, the voltage was changed in several stages to 230V. The tolerance limits initially ranged between +6% and -10%. From 2009 onwards, the mains voltage of 230V may fluctuate by ±10%. Thus, the nominal voltage range extends from 207 V to 253 V! These new tolerance limits can result in an early end for quite a lot of vintage tubes.
Preventing Premature Death
When studying company publications, notifications, technical books and data tables of the famed tube manufacturers (Telefunken, Valvo, Siemens, etc.), it becomes noticeable that special attention must always be paid to the power supply for the cathode heating system. Especially here, according to the manufacturers in unison, voltage fluctuations in both directions are literally fatal for a tube.
For the so-called long-life tubes with particularly narrow tolerance data, the allowed voltage fluctuation of ±5% is even lower. This means that for the heating of a classic E-tube with a nominal heating voltage of 6.3 volts, the limits granting the longest possible lifetime lie in between 6.0 and 6.6 volts. If such an amplifier is operated on 230 volts mains today, the resulting supply voltages are just too high.
Overheating causes the cathode layer to be destroyed by evaporation, while underheating results in a depletion of effective Barium, caused by a lack of additional supply from the oxide layer due to insufficient heating. In the worst case, overheating can lead to burning out the filament.
Over the years, it has shown that heating at the lower tolerance limit causes the least damage to a tube. Other sensitive components in the se devices are the mains rectifier (usually the classic Selenium type) for the anode voltage and the filter capacitors inside the amplifier.
These parts react sensitively to the new (too high) mains voltage. It is particularly critical if the capacitors have lost capacity due to long storage and the insulation resistance has decreased. A replacement of these parts usually works wonders.
A solution for the standard-compliant operation of the entire amplifier with today's mains conditions is an autotransformer (transformer with a winding for 230 volts and a tap at 220 volts), since many of the classic studio modules have a power consumption of only 8 - 12 watts. Thus, several units can be operated with little effort. Such an autotransformer reduces the mains voltage to approx. 220 volts +-10% with relatively low loss and good mains stability. Users from countries with other mains voltages (e.g. 120 volts) need to use a transformer which can convert their specific mains into the 220 volts needed here.
Still, when a module is put into first operation this way, the temperatures of the individual components within the device should be carefully observed: The tubes need to get warm – the other components must not!
Danger is imminent when the electrolytic capacitors get hot. If they are heated up by internal creeping currents, they can explode as a result. Your device then operates in "single-shot mode". You should also make sure that only the cathode glows inside the tubes, but nothing else. If the grids glow, or even the anode turns cherry red, then the voltages at the electrodes are not correct. This is usually caused by leaking capacitors. Switching off quickly usually saves the situation. Tubes do not die as fast as semiconductors, and: You can see the problem coming.
Maybe you have enough spare tubes in your drawer? If so: Are these "NOS" or used? If they are just used but not used up, this can actually have advantages over NOS. NOS is very popular with tube enthusiasts, because it means "New Old Stock". These tubes are very old, well stored, mostly in sealed original packaging, the glass bulb still has the white lettering, there is an unfilled warranty certificate and the tubes have never been in action – and this can be a big problem: Even the best Barium cathode coating slowly comes off the nickel tube when no anode current is flowing. In other words: The tubes can become unusable in their original packaging over time.
It was not without reason that the radio stations and the German Federal Postal Service (where tube amplifiers were used for communications over thousands of kilometers) had instructions to put storage tubes into operation for a few hours every six months to prevent the cathode layer from coming off.
If your precious module is in silent run mode after inspection, control and revision, you can assume that nothing serious will happen in the foreseeable future. The quality of these devices and the components is just too good, even after half a century and more.
And if you are successfully sending your productions through your venerable, restored classic amplifier, don't be disappointed if a change in sound is audible in (beautiful!) nuances: These good old tube devices do not produce tons of obvious coloration like some might expect. However, the old technology is – properly restored and maintained – as good as 50+ years ago.
Next time, we will talk about reactivating tube microphones and their power supplies – something for precision mechanics with steady hands in silk gloves. Among other things, I present the rescue of a venerable U47. It was the tube and not the capsule that made the microphone sizzle... I had three new and one used VF14 at my disposal. And now guess which tube was chosen for providing the silent run mode...
Important: Anyone who opens tube devices and works on them alone or operates opened appliances unattended is acting negligently. You are dealing with life-threatening voltages! Never work alone! Only proceed if you know what you are doing! Show a colleague where the power must be interrupted in case of an emergency!
This article is presented with kind permission of its original publisher, the amazing Studio Magazin, enriching Pro Audio since 1978! The author, Uli Apel, is an incredibly versatile and experienced engineer as well as one of the most qualified experts in vintage broadcasting and audio technology around these days.