Tube Temptation

"I have heard the music of men and believed it was the music of angels......... In the blindness of my idolatry, I danced fearful pagan Rites with Nijinsky, I took the A-Train, I heard Salieri's pulse quicken, I heard Puccini's stop"

Today brethren, dark clouds are a'loomin' on the horizon. For I am aware that I have already failed in my attempts to dissuade some of you from endeavouring to construct your own valve power amplifiers (see this column September '92). Now I hear that there are amongst you good hum-fearing people some who have strayed so far into the swamp of confusion and idolatry that you are considering constructing your own pre-amplifiers.

Once again I say to you brothers and sisters. Don't do it. Valves are the serpents of hi-fi - luring we innocent hi-fi victims with their warm, cherry-red charms only to dash us to pieces with their lethal high voltages, troublesome base connections, cost and tintinnus-like microphony. Steal yourself from this temptation. Join AA (Analogues Anonymous). Sign the tube pledge. Once you let this awful vacuum into your life, you will be changed.

I have to tell you, I myself fell. And only near bankruptcy raised me up. I have known earthly rapture. I have sat in the listening-seat of iniquity. I have heard the music of men and believed it was the music of angels. I was ensnared. I thought I had glimpsed heaven but the heaven I had glimpsed was the heaven of a bottle - a line of bottles glowing red with a sheen of blue. And those bottles carried me away from the world I had known to a realm of appalling dissipation. In the blindness of my idolatry, I danced fearful pagan Rites with Nijinsky, I took the A-Train, I heard Salieri's pulse quicken, I heard Puccini's stop. A curtain was not so much lifted as rent in twain and I became blind to the pleas of multinational advertising, blind to the heroism of efficacy and mass-production. Blinded to that most profound truths brethren; that things always get better.

I want you always to remember chil'n that in our lives two forces are wrestling for us; valves and semiconductors. For a time it looked like old valves was a'winning. Then it seemed as if transistors was a'getting the upper hand. To and fro they fought. But history has scattered valves in the imagination of our hearts. History has put valves from their seats, and exalted them of low degree. Solid-State has filled the hungry with good things........

But this Rich it has sent empty away!

So, Fig. 1 shows a design for a vinyl disc pre-amplifier that I designed and which ran in my system for some years during the apogee of my idolatry.

I hope it will be of interest to those considering construction because it is a slightly unusual design. It incorporates no overall negative feedback, passive RIAA equalisation and employs a cascode input stage.

In transistor equipment, the problem with passive equalisation is the risk of overloading the first (necessarily high-gain) stage due to high level treble signals. With valves this does not present a problem because of the enormous headroom when using a power-supply of several hundred volts.

The usual choice for a high-gain valve stage is a pentode but these valve generate more shot noise than triodes because of the action of the cathode current as it splits between the anode and screen. Instead I used a cascode circuit. Like so many other valve circuits this has its origins in radio. Its characteristics are such that the total stage noise is substantially that of triode V1a. But the gain is roughly the product of the anode load of V1b and the working mutual conductance of V1a. In other words it works like a pentode but with lower noise!

The RIAA equalisation is shown in a dotted box. This is to disassociate myself from this section of the circuit. If you have your own ideas about RIAA eq. then you can substitute your own solution for mine!

On a general note, I disagree with those who say there is no place for valves in low level circuitry. Well designed valve circuitry can give superlative results. Hum can sometimes be a problem. In this design I left nothing to chance and powered the valve heaters from a d.c. regulated power supply. The h.t. was also shunt-stabilised using cold-cathode glow-discharge tubes which look great! The power supplies were built on a separate chassis. The more common ECC83 valve would be suitable as the first stage cascode valve except that the ECC82 is more robust in construction and therefore less microphonic. I have found, from bitter experience, that there is no alternative but to select low-noise valves individually for the first stage valve.

I must stress that, like my September column, this is not a constructional article. My intention is to both warn and tempt. My own ideas and mistakes are presented as grist to that most enjoyable of design phases: The Armchair Phase. And to Hell with the warnings - welcome to thermionic Heaven!


© Richard Brice 1998