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Old 18th January 2013, 02:03 AM   #1
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Default Stripped down HK A-300

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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Old 18th January 2013, 11:04 AM   #2
DF96 is offline DF96  England
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Voltage gain of about 15 means that 0.7V in is 10.5V out. Across 8 ohms that is 14W, which sounds quite plausible for a pair of 6V6.

I am puzzled why the designer decided to deliberately unbalance the phase splitter. Maybe he likes a bit of extra 2nd to 'enrich' the sound?
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Old 18th January 2013, 12:20 PM   #3
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Probably,
also with the tone control section in place, (not in my schem), even at the supposedly "flat position" the FR of the whole amp, shows a behaviour similar to a hard wired , non detachable, BSC filter.
Sure this design would sound very "nice" with some fullrange speakers out there
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Old 18th January 2013, 03:01 PM   #4
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Quote:
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.
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Old 18th January 2013, 09:09 PM   #5
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Quote:
Originally Posted by Eli Duttman View Post
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...
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Old 18th January 2013, 09:35 PM   #6
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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.
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Old 18th January 2013, 10:58 PM   #7
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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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Old 18th January 2013, 11:09 PM   #8
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Originally Posted by Eli Duttman View Post
FYI, AA5 radios totally lack power trafos. They are incredibly dangerous.
Of course I mean these 50C5 amps powered trough an isolation transformer 120 to 120, or 230 to 120 , not directly from the wall socket...
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