• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Stripped down HK A-300

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Hi,
I read about this one at "Fun with Tubes", Max Robinson's web site.
Just for the sake of curiosity I wanted to try it on LTSpice, so, here is it:
Sadly I miss the nice feature of using four 12AX7s heaters to cathode biasing the finals, but the rest of the circuit still seems of interest..
On paper it promises 12W out, at pretty low distortion,
Thing that bothers me is that it goes into clipping with just 0.7 Vrms at the input, even with the heavy NFB applied
Suggestions for some fine tunning ?
(Included the LTSpice file ready to run, just delete the *.doc extension)
 

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Sadly I miss the nice feature of using four 12AX7s heaters to cathode biasing the finals

The "Cheap Charlie" method of DC heating 'X7 heaters is HORRIBLE. Separate RC bias networks for each PP pair make for easier and less costly matching. The thought of the 'X7 heater current being modulating by the audio signal is frightening. In this day and age, it is neither difficult nor expensive to build a proper regulated DC supply for 'X7 heaters.
 
The "Cheap Charlie" method of DC heating 'X7 heaters is HORRIBLE.

Even if it isn't technically perfect, I think this method is still neat ingenious,
I am naturally fascinated by these tricks of wit that achieve that same element or circuit meets two or three functions at the same time. Similar case for the "All American Five" where the one and only transformer feeds the series heaters string, and provides also the same 115V AC for the B+ PSU...
 
FYI, AA5 radios totally lack power trafos. :( :( They are incredibly dangerous. The famous Atwater-Kent firm went out of business in preference to starting AA5 production.

While some of the tricks are "cute" and definitely pump profits up, they are VERY poor engineering practice and may be outright illegal under current laws.
 
Well, as said by DF96, if You provide a perfectly balanced inverter outputs (by making R2 equal to 24K, instead of the original 20K) , You can observe , at 1 watt out, that THD diminishes from 0.03% to 0.01%, but then, the FFT analysis clearly shows that residual 3dh and 5fth score up respect to 2nd and 4th, where in the previous situation, the FFT behaviour looked very "single endish" ..
 
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