Dear Romy, thank you so very much for mentioning our amplifier to your excellent forum. Indeed, during the last 5-6 years there are a lot of manufacturers “from non traditional hi-fi manufacturing countries”. In particular, in poor old Greece we are more than 50-60 very small companies building some interesting audio products even with the extremely awkward local financial situation which is leading to extinction a lot of us.
Anyway, as the description on our site on second paragraph says, the third 6C33 located at the back, is used as the pass element of the regulator (controlled by solid state circuitry) providing 220V with 100uV of random noise & Hum at full load with very low and uniform, across the audio band, output impedance.
On the other hand, 7721 was chosen amongst many other driver tubes tested including 6E6P-DR and 6E5 too. To tell you the truth I really liked 6E6 but the gain was lower than needed and after 1 year on the test amp it started to degrade in sound and characteristics. Also 7721 (specified for 10000hours service life which is good for a commercial design) seem to my ears marginally more analytical than 6E6, though 6E6 had marginally better articulated bass. 7721 is not a small tube at all and at 170V(at anode) and 14mA (triode strapped) provides very low distortion even at full power. In any case the circuit can easily accommodate 6E6 with very minor internal adjustments (though no soldering is needed).
The quoted 15W of output power is restricted only by the distortion characteristics of 6C33, if you do not mind higher distortion (say 7-8%) the output power can reach easily 20 or more watts. Moreover, because of the adjustable regulator, someone may increase voltage, from 220VDC specified, to 230 or more and play with the operating point, but the sound is not that correct to my ears, I really prefer 220V @ 210-220mA but my taste is just my taste.
You are correct though that frequency response is not at full power and you can be assured that I am not at all evil minded as I am not stating that the response is at full power. The frequency response stated is measured at 8W@8Ohm done just for ease of measurement as the 8V mark on my millivoltmeter happens to be right above the 0db mark. For your information, I just measured the actual full power bandwidth at 15W and it is -3dB from 9Hz to 122KHz which is not bad and not very much different from what it is stated at our site and very respectable for a zero feedback SET amplifier. This good bandwidth is the product of good old transformer calculation and construction execution using a total of 10 primaries and 5 secondaries carefully layered. The core is of type E120 with 8cm of laminations providing a total of 10H gapped for 250mA.
On the other hand, we got no distributor in the US yet, as we launched our products in Munich Hi-End Show (together with Tune Audio Anima speakers) in May and had no contacts or interest from any US distributor.
Sorry for the long post,
Remaining at your disposal for any questions you may have