Another installment in my “take a cheap ebay-kit and tweak it” series – a tube headphone amplifier :D. I do what I normally do with these – I prefer to buy a kit instead of a finished board (even if the price difference is very small) and then replace any parts that look “questionable” or should be upgraded.
The circuit in question is a White Cathode Follower (WCF) and the tubes needed are (equivalents of) ECC88 (input) and ECC99 (output). I haven’t actually checked if the circuit is properly calculated as a WCF, but it’s probably close enough for me… :) The main attraction of this kit was that that it was simple, cheap (25 USD shipped excl. the Alps pot and the tubes) and that it needed voltages that I already had a suitable spare transformer for (200VAC and 6.3VAC). I have reused most of the components that came with the kit, diodes, resistors, screw terminals etc. The tube sockets I replaced, but the ones that came with the kit were quite good as well. For 25 USD including postage anything other than the PCB I almost consider a bonus, really :)
One thing I have replaced though is all the electrolytic capacitors. They looked fine on the outside, but when I checked them against the manufacturers datasheet I found problems on a couple of them – the case size wasn’t listed for that particular capacitance/voltage combination!. This is a major warning sign that either means that they are a special-order item bought cheaply as surplus by the kit manufacturer (possible) or that they are of questionable origin, i.e. counterfeit, relabeled used caps or similar (which is sadly also definitely possible).
Since we’re dealing is a high-voltage amp I did not want to take any chances and so I purchased new electrolytics from Mouser which should be completely safe. I also chose different values (for better bass with low-impedance phones) and higher temp. rating (for increased reliability).
Soundwise this definitely sounds like a tube amp :) The bigger output caps gives decent bass even with mid-impedance (60-150 ohms) phones and moving to higher impedance phones seems to improve the sound further. I have no imminent plans to case this yet, but as a cheap and cheerful build I am quite happy :D