People have asked us how far can they push their SE5 amplifier, power-wise.

 As it turns out, quite a long way.

 By connecting a full-wave voltage-doubler circuit to the AC output from the power supply, it's possible to achieve much higher HT voltages. Using an HT voltage of 360V, just below the maximum DC voltage rating of the capacitors (400V), and KT88 tubes, we were able to get about 8.5 Watts RMS sine-wave output @ 5% distortion - 10 Watts when pushed.

The power supply has jumpers that select output taps on the transformer. Make sure you don't exceed 400V when tubes are cold, use ultra-fast 1kV diodes (UF4007 are suitable), and 400V high-frequency pulse-rated capacitors for the voltage doubler (1uF polypropylene are suitable).

We used 50V zener-diodes in series with R10 and R10B (the 10k resistors in the HT line to the input stage) to cut the voltage down to less than 300V for the input tubes.

The 220R 5W cathode resistors on the output tubes may need to be increased a little to keep the anode current less than about 90mA, necessary to avoid OPT core saturation (100mA rated). The input sensitivity will be about 1.5V RMS with the standard 12AT7EH input tubes, much less if you use 12AX7's with appropriate parts changes. 

So, is it worth it - probably not. All the increased heat and component stress will give you about another 3.5 Watts which is only a 2.3dB increase. You may notice a very small difference but it's certainly not going to be earth-shattering! See our dB tutorial page.