This is a project I started some 5 years ago. It is the first tube-amp I built (and the only one so far), but I made an error in the calculations back then which led to absolutely nasty sound. No lows, oscillation and unproportional hi-end. Need me to say why it was sitting on the shelf ever since?
Yesterday I redid the calculations, now that I think I have a better understanding of how tubes want to be treated – and this time around I was successful. The frequency plot needs yet to be done, but from listening I like it very much so far. The tubes are also no longer the original ones, it was originally equipped with two EL83 from TESLA and two EF860. The EF860 did an okay job as a preamp but was very microphonic – that is, picking up vibrations from the casing – whereas the EL83 simply can’t handle much anode dissipation power, about half that of the EL84. Output power was limited to just above 2 Wpk.
Some basic facts:
- JJ EL 84 power pentode as output tube, single ended configuration
- ORION EC 86 single-triode as preamp tube
- 275V anode operating voltage
- HSGM (german company) output transformers for EL84 SE
- Total amplification preamp ~ 10-fold (optimized for consumer audio devices)
- Global feedback including output transformers
- Peak output power ~ 4 W
- Beech wood & brushed aluminum casing
- And the blue tape on the transformers is there to creep off anyone (me) thinking of accidentally sucking power off the transformer rails with his finger or other body parts while ogling it in the dark. This is not to become a permanent aesthetic feature, but there are no transformer caps available for this type. I will build something out of epoxide PCB, I guess.
I will put my schematic, the bode plot and exact part types up here in an extra post as soon as I get my scanner to work. Until then, I advise you to visit the following sources which helped me a lot and are always an inspiration for me. Dig through the news section, there are some GREAT projects.
- Jogis Roehrenbude (german webseite)
- Pentode SE amp using 5B/110M tubes (german reader’s project on Jogis Roehrenbude, my schematic has become very similar. Great explanation of the necessary calculations)
Seems like my biggest mistake in the past was not to account for the voltage drop over the output transformer, induced by the bias current. *facepalm* I totally forgot that.