Yesterdays little play with the TDA2003 amp had me trying different speakers on the end of it simply because at such low wattage output the sensitivity of the speaker is all important. Being the enclosure was quite limiting I couldn’t use the 12″ which actually sounded the best and had to make do with a 5″ vifa I had set aside for an eventual set of monitors.
But I’m now warmed up and upon visiting DIYstompboxes I came accross a thread on sticking a 12AX7 tube before the amp and this isn’t a bad idea as theres an old trick where you can use two power transformers back to back to get the power requirements you need to power a tube with both 12V for the filaments and anywhere over 250VDC for the plates.
The first transformer is set up normally. 240 in and 12 out and you run the filaments for the tube on that. Also you’d throw in a diode bridge and get 12-15VDC off that to drive the TDA2003. Then you grab another 240/12 transformer and throw the 12V you already have into the 12V winding of the next transformer and it gets brought back up to the 240VAC on the other end. This is bridge rectified and smoothed with 400V caps because 240VAC rms becomes about 340VDC. This is more than enough to drive the plates on a 12AX7 tube and it may be prudent to knock it down a bit as the max plate voltage for a 12AX7 is something like 300VDC.
Going into 15-18V windings, instead of 12 will cut it down a bit as it’s the ratios that count here and a 240/12 has a ratio of 20/1 whereas a 240/15 has a ratio of 16/1. 12 x 16 = 192, 192 x 1.414 = 270 and thats a bit closer to be suitable for the plates of a 12AX7 tube.
The 1.414 is about changing rms to a peak value as rms means root mean square and is the AC waveform averaged out but because we are charging caps after a set of diodes then those caps will charge to the peak voltage available in the AC waveform.
Incidentally for those of you that might not know. We measure the wall voltage here as 240VAC and what that means is that we have both positive and negative sides to the waveform and the 240 means that we have peaks running from 240 x 1.414 which is 340 to the peaks so when one is zapped the waveform going into your body goes from 340 positive to 340 negative which adds up to 680VAC +/- accross the complete waveform. So the next time someone says that 240V zaps are quite a shock you can say it was actually closer to 700 that nearly found the heart muscle.
Next thing you do is measure your resistance from hand to hand or from hand to foot and you’ll be able to ascertain how much current you’ll actually have passing through your body at the speed of light. My resistance from hand to hand is about 2 megs so 700 / 2,000,000 = .00035 which is just about half one thousandth of an amp. No wonder they just surprise me. The lower your resistance the higher the current will be and this is why some people get terribly deadly shocks while others merely go “f#ck!” ( a few other things count like perspiration etc, and that counts alot as the salty water is quite a good conductor and so much of the current travels through the perspiration of a sweated person, paths of least resistance and all that.)
But back to amps and I’m going outside to build a little amp using a glass bottle (containing thoriated tungsten), some iron and copper and the odd bit of doped silicon. Hopefully it’ll be louder than all my recent attempts… which still aren’t loud enough to cut above the noise floor at Handmade.