Note I have no affiliation with Elekit other than being an admirer of what they do for DIY audio hobbyists.
Elekit’s kits seem to hit attractive price points for what’s included (tubes, transformers, components, and a chassis) and the quality of the documentation. VK Music (Canadian importer of Elekit) just announced a new 300B amplifier on diyaudio.com here. Although details are sparse, we can maybe glean some ideas from the specs and the previous incarnation (Elekit TU-8300).
Specs for TU-8600:
• Tube Set : 12AU7 X 2 + 12AX7 X 1 + 300B X 2
• Now compatible with low to high impedance headphones
• Frequency response (-3dB) : 10Hz – 80kHz
• Max. output (THD 10%) : 8.3W + 8.3W (Input voltage : 250mV r.m.s)
• Residual noise : 42uV rms (IHF-A)
• Power consumption : 80W when no signal; 80W at max. output
The previous TU-8300 used MOSFET regulation for a B+ of 375V and because the new amp is rated for the same power, I think we can assume the new version operates at about the same voltage with a similar bias (-60V). Rather than two stages of 12AT7 on the input, the new design uses two 12AU7 and one 12AX7 triode per channel. So how are these arranged?
Let’s reverse engineer the numbers for an educated guess. We get full output of 8W from a -60V biased tube with an input of 250mVrms. If the 250mVrms (0.7V peak to peak) fully drives the output tube’s -60V bias (120V peak to peak), we have a gain of about 160x. That could be two successive stages of 12AU7, but then what is the 12AX7 doing? We appear to be using one half of the 12AX7 dual triode per channel. It would be an odd choice for a buffer stage, but it’s a plentiful tube and would make sense in that regard.
The other likely explanation is that we have some feedback at work and the 12AX7s are used for voltage gain. Maybe this is a grounded cathode 12AX7 into a 12AU7 SRPP. In terms of driving the Miller Capacitance of a 300B, this seems like a plausible arrangement. Gain would be in the 400-500x neighborhood, but feedback would knock this back down to the 160x overall and lower the output impedance.
Lastly, this amp includes a headphone output. Maybe the extra triodes are employed in some kind of follower specifically for the headphone section. A 12AU7 white cathode follower seems like a potential candidate. Whatever it is, I’ll be anxiously awaiting the manual and schematics to see what Mr. Fujita has come up with!
If you’re headed to the LA Audio Show (June 2nd-4th), the amp will be on display in the VK Music booth.