Introduction
It is common knowledge among the guitarists of the world that when playing Electric, the nicest sounding amplifier is one using vacuum tubes (or valves as we say in the IRL & UK). This is due to a number of reasons, such as the introduction of musically pleasant harmonic distortions of varying degrees, and the ability to overdrive the amplifier without it sounding terrible. In fact it can sound brilliant when overdriven! This is all in comparison to the more modern solid-state/Transistor amplifier, which is cheaper to build but has none of these nice qualities...
Well where can these precious tube amplifiers be found, other than in a Music shop with a large price tag on them? Vacuum tubes were used in all sorts of Electrical devices up until the maturing of semiconductors (transistors, microchips, etc!) which then took over their role. Old radios and televisions pre 1970s are the best place to look. All of these had some form of low power audio amplifier to drive the loudspeaker, ranging from a fraction of a Watt in power for very small radios, up to maybe 10 Watts for larger radios and TVs. So as for finding them, most valve radios are collectors items so you won't find them in your local Recycle Centre/Trash heap. But as for the TVs, these things are big, heavy, and generally not considered pretty on the mantlepiece :-). So this is where this project started!...
DISCLAIMER: READ FIRST!
For those of you new to building with vacuum tubes, or new to electronics in general, PLEASE be fully aware of the sheer danger of messing with Electronics such as this. TVs are one of THE WORST appliances to work on, never mind to chop them up and experiment with! Therefore I take no responsibly whatsoever to any amount of electric shock you might be unlucky to experience if you undertake a project like this. It is simply up to your own sensibility to stay safe, ALWAYS unplug it from the mains AND CHECK voltages every time you work on it. Capacitors can stay charged for days, and you WILL find that out the hard way if you're not careful! Anyway, enough of the Pepp-Talk, Let's explain this ridiculous project!!!
Trash Refresh!
I picked up this old Ferguson model 3547 Television from the Recycle Centre one day, knowing that it was a vacuum tube model (one look through the back grille will tell you ;-)). Upon getting it home I opened it up and saw no tubes missing, and no signs of burning or other electrical disasters inside. I decided to cautiously plug it in to see if it actually worked. And hey presto! After a minute of warm-up it came to life, with a lovely static blasting from the speaker :-). Sure enough though, after a few minutes I saw smoke and quickly unplugged it. The only section I was interested in was the audio amp, which consists of a single PCL86 tube, and handful of resistors and caps, and a little output transformer. This was, luckily for me, all situated at one corner of the PCB, meaning that I could carefully cut it away from the rest once I knew what was needed in the circuit.
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| Inside the Ferguson Telly :-) |
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| Cutting away the Audio Amp from the corner of the main board |
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| Audio Amp PCB with large smoothing cap fitted, from original PCB |
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Amp undergoing testing with two transformers;
one for step-down (heaters), and one for step-up (HT) |
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| Wooden panel fitted within CRT opening, ready for a 12-inch driver |
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| Everything fitted, including a Yamaha 12-inch driver from an Organ :-) |
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Now for some testing!...
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And now for the messy schematic!
Modifications, barely...
Apologies for the messy drawing! Most of the schematic for the amplifier remains unchanged from the Ferguson design except for a few things; The first thing I did was to move the volume control from before the triode over to sit between triode and pentode. This will now allow the triode to be overdriven with a high input signal while still being able to reduce the final volume :-). I also removed the negative feedback (NFB) path as this will deduce the overall gain and is there to make the amp more accurate and clean, something we DON'T want! There are many early guitar amps which don't use NFB such as the Vox AC30 and the Fender Champ. I then looked for ways to increase the gain further, as with a standard Telecaster I was only managing to slightly overdrive the amp at full-tilt. I then spotted that there was no bypass cap on the R88. So after adding a 25uF cap there I did get a little bit more gain.
To give some tonal control I took the now redundant Brightness pot (500K), added a 3nF cap to ground and it then became a Treble Cut control. I experimented with the Contrast pot (1K2) in the cathode side of the triode but it yielded little change, as I expected, so I left it out in the final steps.
The Power Supply
Nearly all of these vacuum-tube TV sets used a rather nasty method of getting their supply voltages. They simply rectified the incoming mains (230VAC) to get about 300VDC, then used this to run all circuits through a series of resistors. The heater supply for all valves was also taken from this rail through resistors and all tube filaments were connected in series. In this way the stingy little capitalists managed to build these TVs with no mains transformer! Shocking Stuff eh! :-). So the biggest addition to this whole project was to provide a safe HT supply, and a low voltage for the heaters of the PCL86. With series heater elements you need to limit the current to 300mA, therefore ending up with a voltage of 10-13V across the heaters of this PCL86. I achieved this using a big wire-wound 10R resistor in series with the low voltage output of the first mains trafo (14VAC). If the PCL86 had been dead then I could have replaced it with an ECL86. The E means that the heater is voltage-fed and not series/current fed. then I would have needed to provide a supply of 6.3VAC (or DC).
The Speaker driver
At this point you might be expecting me to install a Greenback or a Bulldog... Jee that would put this project WAY over budget! I had a bunch of drivers on hand from scrapped organs so gave them a go. My first candidate was a 12-inch driver from a Yamaha Electone Organ. It turned out to be quite unfulfilling with a nasty top end, and no body at all with a very stiff suspension. No surprise really! The next candidate to try was a driver from an early 70s Solina Organ. This one had a much more relaxed suspension, meaning (hopefully) higher sensitivity, and it looked to be of higher quality. Massive improvement! I now had more volume, more body in the mids, and still was at zero cost :-). This one actually has a twin cone for improved HF response, but it isn't too harsh. I did try to connect tho original eleptical TV speaker across the main driver through a cap but it just killed the volume, and was messing up the impedance on the output trafo...
Anyway! Enough of my rambling! Go out and find an old vacuum tube TV and tear it apart! I'm delighted with mine :D
Fair play man interesting read. Must have a look out for one of those tv's!
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