[R-390] AF LF Frequency, Distortion, & Power Mods

[Dallas Lankford]

Recently I did Chuck Rippel's change of C604 and C605 to 0.022 or =
greater for improved LF response and found they worked very well =
provided you don't increase them too much, which may cause motorboating =
at higher LOCAL gain settings.  I also tried the conversion of XV603 for =
a 6AQ5 as a triode as well as associated changes.  It worked poorly, =
with increased distortion and power output was not increased.  So I =
restored XV603 for the stock 6AK6 and paralleled 1200 ohms across R614, =
which increased the gain but also increased distortion, and 330K & 200K =
paralleled across R612 for increased negative feedback to reduce the =
(excessive) gain and reduce distortion.  The net effect was an increase =
in 6AK6 max power output from 0.90W to 1.0W into 8 ohms real (through an =
impedance matching transformer) and a reduction in distortion (compared =
to unmodified).  With  Graham Maynard's 6AQ5 mod, which cuts off pin 7 =
of the 6AQ5 and then replaces the 6AK6 with it, lower distortion is =
maintained while max power output is 1.4W RMS into 8 ohms real.  If you =
want more power, you should probably use the LINE out to a hi fi.

My mods require no removal of parts, only paralleling resistors across =
existing resistors.  If you don't like the mod, you can easily remove =
it.  There is a picture with description on the Yahoo R-390A reflector =
FILES section. 

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