Tubes and Semiconductors

Tubes and Semiconductors – the great controversy.

So you finally decided to take the plunge and build or get yourself a tube (radio valve: UK) and discovered to your amazement all the joy that you had been missing over the last 1000 years.  You may be kidding yourself of course.

70W SE 300 Tubes
70W SE-300 Tubes – Courtesy: By Scscalal / Wiki

Radio valves have their place. As a designer and kit builder of many RF and audio amplifiers over the years thermionic emission devices were never as mysterious as semiconductors. Their inner workings were almost tangible. They were easily modified and to get brute strength of 50W or more took a different anode voltage and a more powerful output transformer. And this, oh boy, came at a price. Solid state amplifiers hold more of a mystery because they are prone to thermal issues, biasing can be critical for optimum performance and of course, they burn loudspeakers if directly coupled.

A well engineered solid state amplifier will however last much longer than a tube amplifier. Radio valves lose emission, become hissy and because of the high voltage, capacitors do some strange things like blowing up when switching on. Don’t get me wrong, thermionic tubes have their place – we all love them.  Right now however the marketers love them more than audio enthusiasts.

When I started out in electronics in the 1970s the two transistors which were cutting edge, the 2N3055 and 2N2955 could dissipate 100W into a 4 Ohm load. At one eighth the price of a 35W EL34 audio amplifier. They sounded different and although Elektor magazine took great pains to explain the differences to us learners we weren’t really interested. Youngsters like power, not quality.

Tubes used in audio amplifiers are actually remarkable in that they require very few components to get working but they do have one drawback – the high voltage anode and screen supply rails. This can be a problem if you are over enthusiastic and impatient.  The thermionic devices may be resilient but not once the anode starts glowing a bright rosy red, normally an imminent sign of ominous things to come. A 3kW transmitter I worked on many years ago would go into standby 15 seconds after switching to HT because the forced air cooling system had failed. This just shows how quickly these radio valves could overheat. Radio valves do like cooling so don’t think about enclosing them. Of course modern times enthusiasts like to display their wares out in the open so this should not be a problem. I think the mere fact that those glowing heaters look sexy gives us an added dimension to sound quality. And just possibly herein lies the problem.

Power, power and more power

MOSFET audio amplifiers also use very few components, they are after all voltage devices like tubes. A FET amplifier I built many years back  knocked the socks off any radio valve device I had either heard or built.  Although the amplifier was designed as a 100W into 8 Ohm module the device actually was way under-rated and could deliver in excess of 150W.  Another amplifier which I was very impressed with and I believe still very popular was the Maplin 150W version.  You can get these from Yebo Electronics in Cape Town – at under 50U$ this is pretty cheap.

On an international level many highly rated design engineers of audio equipment are antsy Mosfet amplifiers and from a design standpoint some of their arguments are valid and hold water. BJT topologies have also seen some changes through the years and although from an engineering standpoint much of this is now well established with most audio circuits nowadays using NPN and PNP output stages. The very simple circuits as in the NAD3020 have proven to be extremely reliable and whilst we can argue that simplicity is best this is only really true for the under 50W audio circuits, the beasts of power often having more sophisticated safety circuits than the actual voltage and current amplifiers in the audio chain. Safety circuitry used in the biasing of  tubes are often less sophisticated and of course, this bears testament to the resilience of these devices. Directly coupled transistorised amplifiers more often than not suffer catastrophic failure, sometimes even just when switching on.

BJTs and Mosfets

In the author’s opinion BJTs and Mosfet audio amplifiers do make the best audio amplifiers and with the ultra efficient class D stages driving high powers into very low impedance loads it is doubted that the versatile tube will be able to play catch up except through snake oil marketing ploys.  They cost less, are way more efficient and even in cheaper entry level equipment they sound better than a poorly designed valve amplifier.  Good valve amplifiers cost upwards of $2000.00 which puts the semiconductor version into upmarket for the same price. What is upmarket anyway?  Much of this may just be snob value. It’s a known fact that mass produced audio circuits always carry a cheap-arsed supply. Anyone with a modicum of interest in electronics will be able to change this for a few dollars. This will never be the case for tube amplifiers.

In the 60s the top bands used equipment which possibly only topped 500W at full power combined whereas with class D amplifiers it’s not unheard of to have a band with over 100kW of audio amplification equipment. But loudness does not equal quality and this is where a line needs to be drawn. Most tube enthusiasts are happy with their 10W per channel setup and possibly this is where the distinction comes in. One could not go to a head stomping party with a 6W per channel audio setup but in fact 6W per channel is more than adequate for most home systems if quality listening is the desired result. Move over for home theater systems.

Hellfire and Brimstone – move over Motorhead

Although the older venues for live music was often the local church or village hall and were the pre driver to our modern death and destruction sound systems many retired or now deceased musicians blamed headphones for causing deafness.  What about the multi kW sound systems used in motor vehicles these days?

Not too long from now we will see the result of this, just as headphones definitely cause deafness when used irresponsibly (or when drunk or smoked up). A well designed tube amplifier may not cause listening fatigue but often the critic is not reflecting the entire truth. Program source material is very important. Sound engineers have complained about CD labels applying too much compression in their recordings and this is often the case, especially in modern pop and rap culture.

Home theater systems may very well be applying all the very worst habits into the audio whilst the best into the video effects.  In many cases a simple stereo (or 2.1) setup has much better quality but in order to keep with the Jones’s we crank the power through our 7.2 DSP setup. Whilst it’s nice to have all the bells and whistles possibly this is just the problem – the musos and philes of this world have had enough and want to go back to what music is all about. Hearing the fingers on the fretboard, the cymbals and the true tenor of the voice.  Your vinyl collection may have just that and this may be just where tube amplifiers perform their magic.