Amp measurements (updated)

Here go the measurements. I use RMAA 5.5 free to do these.

All measurements were taken off an 5R6 load resistor. The maximum output power reached at 1 kHz sine into 5.6 ohms was 2.49 Weff resp. 4.97 Wpk, which is right on the money. Soft-clipping occurs right above that. I am not yet satisfied with the rise above 20 kHz, which I suspect to originate from a missing bypass cap in parallel to the feedback resistor. The cap short-circuits the feedback resistor at high frequencies to prevent oscillation (which currently occurs at 30-50 kHz when the volume isn’t turned all the way to the max).

The 50 Hz spike, on the contrary, is lower than I expected as I didn’t bother placing the output transformers away from the power transformer or putting them at an angle to prevent the magnetic fields from interfering. Another of those times when function follows form. Shielding in the wiring compartment could be improved a bit, though. Dunno if I should really photograph this mess.

UPDATE: And here comes a screen of the schematic. No need to do it in EAGLE as there is no PCB, the whole amp is air-wired. Enjoy.

The usual word of warning: This circuit operates at voltage levels of >= 275 Vdc at relatively low supply impedance, so please be aware that you build this circuit at your own risk, and proceed with caution! I am not responsible for any damage/injury that might occur related to this circuit.

As I already mentioned, there are lots of parallels to the linked amps from the last post. A few annotations concerning the schematic:

  • C5 is still missing in my case, estimate by trial and error.
  • Some parts dissipate a noteable amount of power, as marked – choose for at least 2W of dissipation to be on the safe side. You might want to oversize a bit here, to minimize aging of the parts due to thermal stress.
  • The coupling caps C3,C4 should be designed to handle voltages > 300 Vdc at least.
  • R9 is the loudspeaker, of course ;-)

Another piece of advice I learned during testing: If the connected audio source device is somewhat valuable, it is wise to install a suppressor diode directly across the input in such a way that is shorts high potential at the input to ground. In case any high voltage potential gets through the capacitor in form of a spike (like it did through a damaged socket in my case), this will at least save your soundcard a lot of undue stress.

EL84/EC86 Tube amp

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.

  • 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.

See also: Second post, containing schematics & measurements…