[R-390] R392 help

[Charles Steinmetz]

Dave wrote:

> Finally, the final amplifier (26A7) is boiling hot;
> from all I can find that is the way it is expected to
> work (more contact bias)!
>
> I am highly inclined to do something with the audio.
> I have thought about a couple of things:

Generally speaking, the first AF amp works OK as it is -- the real 
problem is the 26A7 output amp.  Here are two possible solutions:

(1) "Super simple" mod (re-biases the 26A7 stage).  This works great if 
all you want is the stock power output (~200mW) with much lower 
distortion and tube power dissipation:

<http://www.ko4bb.com/getsimple/index.php?id=download&file=08_Stuff_Not_Sorted/8_Sept_28_2014_Uploads/R392_26A7_super_simple_audio_mod.pdf>

(2) Power MOSFET replacement for 26A7:

<http://www.ko4bb.com/getsimple/index.php?id=download&file=03_App_Notes_-_Proceedings/R392_26A7_MOSFET_replacement_Johnson_annotated_updated.pdf>

Finally, for reference purposes [only -- do NOT build one of these...], 
here is documentation on the factory "transistorized amplifier module":

<http://www.ko4bb.com/getsimple/index.php?id=download&file=08_Stuff_Not_Sorted/8_Sept_28_2014_Uploads/R392_transistor_audio_module_schematic.pdf>

The modification to accommodate the transistorized amplifier module is 
the addition of an 8.2M resistor from J613-1 (the AF section output) 
back to J613-16 (the AF section input).

Best regards,

Charles