Fun With Radio Valves

Long before the internet, young boys would experiment with getting the best out of a costly valve.

6V6, KT66, KT88, 6L6, 6K6, EL34

A classic series of power output valves loved by guitarists and audio enthusiasts alike. The problem was they all shared the same International Octal valve base. Even worse, they were all inter-changeable. You could chop and change as much as you liked.

Our knowledge was based on Clarkson Effect, the idea that if a specification for one valve had higher figures than the other it had to be better. Much the same way the Top Gear presenter will insist that a 402 horse-power car is better than a 400 horse-power car.

The carnage was shocking. Swapping a 6V6 with a KT88 would triple the anode current saturating the output transformer giving the impression of much more bass until it burned out. You might be saved by the cathode resistor acting like a fuse, but the heavy load on the mains transformer meant the days of that bit of kit were numbered.

I remember sitting in a pretentious restaurant where they cooked food at your table. While our Chef Du Jour explained he was sealing in the flavour, all I was reminded of was hot transformer smell.

A DJ friend of ours swapped 6V6s for EL34s in the hope of getting more power. At the local village fete, his amplifier burst into flames while playing The Wombles. Many saw this as a blessing.

EL84, 6BW6

There was a time when these were seen as miniature valves. Being small did not stop them from being abused in the same way as their grown-up chums above. They tended to be used in domestic sets with nice wooden polished cabinets. Rich wax polish and over-heating valves increased the fire risk to the extent that you were not asked to fix Granny's wireless ever again.


An icon, a wonderful transmitter output valve good to 60MHz, they were destroyed in their thousands by the build-your-own-kit brigade. The thinking seemed to be that if the book said you needed 600 volts on the anode, then 1000 volts can only be better. We have seen 807s run so hot, the glass envelope melted.

Another thing you could do was under-run them. The 10 watt limit on Top Band was nothing to an 807, so no thought would be given to how it was set up. Bad wiring would make the valve take off, radiating parasitic oscillations up to its 60MHz limit and beyond on the rich range of harmonics. One free-running 807 could take out all the televisions in the village.

These oscillations would steadily build up to the point when the cathode resistors exploded. Many years later in the Lowe Electronics Workshop, the whip-like crack of failing resistors heralded the death of another pair of 6146s. I always thought the muted round of applause was a bit over the top.

ECL80, ECL82, ECL86

A triode pentode combination, a complete audio amplifier in one valve. As you went up the ECL Series you got more gain and more power. Jeremy Clarkson would be so proud.

The redesigning of the record-player amplifier circuit you stole from The Radio Constructor culminated in the rare and expensive ECL86. No attempt was made to update the input wiring so the extra gain got you more hum and noise. Earth loops happened to other hi-fi enthusiasts, not us. We knew everything.

We had a sneaking respect for the ECL80. It had a common cathode for the triode and pentode, each section needing different grid bias voltages. This lead to all sorts of split cathode resistor arrangements, a rich area for experimentation and damage.

We remember with some affection a TRF design using an ECL80 and Denco plug-in coils. The triode was a regenerative detector and the pentode just about drove a loudspeaker. As AM gave way to single-sideband on the amateur bands, the detector was driven hard into oscillation to act as a BFO to resolve these strange new messages. As a thirteen-year-old Radio Engineers, we thought RF decoupling was a capacitor and resistor too many, causing the pentode to power-amplify the oscillations causing interference to ham stations six miles away. At some point in The Radio One Roadshow, the output transformer burned out.

EF50, SP41

We mention these as so many were available for a few pennies as war surplus in the Sixties. While everyone was swinging, we used them in the pre-amplifier stages of our so-called hi-fi amplifiers.

The hiss sounded like a distant waterfall. Add to this the fact that all the other components had been salvaged from equipment already fifty years old, the hiss took on pops and crackles of a thunderstorm slightly more distant than the waterfall.

Those who used the SP Series valves looked down on us. They used the Mazda Octal valve base, different enough to International Octal that you had to use Mazda valves. Forced brand-loyalty long before Apple came along.