Using Paddyboard Construction
And a low cost step down transformer as an output transformer.
This project started out to be a design using 12AX7’s and EL34’s, but when I went scratching through the junk box I could not find any EL34’s so decided to use what I had. With just a couple of modifications the design will work with either the 6DQ6 or the EL34.
Here is the circuit and the values for the EL34, using the 6DQ6 the cathode resistor has to be changed from 120 ohm to 330 ohm and the screen resistor raised to from 100 ohm to 22k ohm. The 13.5 volts at the cathode will be 28 volts with 330 ohm resistor to correctly bias the 6DQ6.The only other change is the transformer tap used for the speaker.
The paddyboard (manhattan) construction is achieved by using in this case a piece of single sided circuit board and cutting small pieces of board and sticking them down with super glue in the places you require a connection or join, this example is pretty rough as it started out as a prototype with a single channel and had an extra channel added then an extra stage pre amp was added because the overall gain with only the two tubes was too low with all the feedback use. Better use of the available real estate will be achieved with the final version.
The output transformer.. I was under the impression that if you did not use a transformer specifically designed for this application the distortion particularly at low frequencies would be unacceptable. I have now tried several step down transformers and thy all seem to perform very well. The transformers shown in the photo above are M2155 from Altronics. I am using the 12.5 volt tap for an 8 ohm speaker, this gives a plate load of 2944 ohms close enough to the 3K ohm required for the 6DQ6, if the circuit was using EL34’s then the 15 volt tap would give a plate load of 2048 ohms close enough to the 2Kohm required for the EL34. Using a Lindos Audio test set the frequency response at normal listening level was -1.35 dB down at 30 Hz -0.61 db at 63 Hz the within 0.1 db from 250 Hz to 31 Khz. Distortion was 0.15 % at 1Khz rising to about 1% at 30 Hz. Listening to my favorite music through a pair of Celestion speakers is very pleasant. Using the 240 Volt winding for the plate here is how the impedances of the transformer work out.
The 100 ufd 350 volt Capacitors in the above photo were capacitors recovered from an old defunct switch mode power supply and are super glued to the board to hold them in place. ( they are actually 400 volt rated) It’s getting harder to find a cheap source of high voltage caps, however I think the input capacitor on switch mode supplies will give an endless stream of good high voltage filter caps. The power supply. High Voltage transformers and filter chokes are also hard to come by cheaply, however there are ways around these problems with easily obtainable transformers at Jaycar and Altronics.
With two 240 to 15 0 15 transformers connected back to back you will get an isolated 240 volts and when rectified will give close to 330 volts if you want a little more grunt for the EL34’s drop one of the right hand side taps down to the 12 volt tap and you will end up with 260 – 270 volts.This will give 30 volts out of the first transformer into a 27 volt tap on the second theoretically this should give a rectified voltage of 377 volts.
The 4 amps of 6.3 volts for the filaments is the hardest challenge at the local electronics outlet, but I have found the easiest way around this problem is to get any low voltage torroidal transformer that is of the appropriate VA rating in this case 25 VA I would go for 30VA. Before you start measure the output voltage then wind off some of the secondary turns (say ten) then measure the voltage again it will have dropped, from this you can calculate the number of turns per volt. Multiply this number by 6.3 and you now know how many turns of wire you will need to wind on to give you your 6.3 volts. Its lucky that the primary or 240 volt winding is always on the bottom. The winding will need to be made using about 16 gauge enameled copper wire.
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