[R-390] Audio Capacitance

[Michael Murphy]

Barry,

Beefing up those caps is good practice. But, it is a little like standing on
your sprinker hose with both feet and taking one foot off.

Do the Rippell - C604 and C605 to 0.033 uF and a 10 UF cap for C609. This
should get you somewhere near 100 - 200 Hz for your -1dB point on the low
end and your high end should be fine. Perhaps too fine. My top end was
peaking above normal. Removing or reducing the value of C612 (68 pF) will
flatten the high end. In any case you should be going out above 10KHz to
the -1dB point. This should get you to 300mW at under 3% distortion. 1 Watt
or so is about the maximum I could get out of the stock 600 Ohm iron for 11%
distortion with this mod.

If you should try to bypass R614, the cathode resistor, with a 100 uF
electrolytic in order to increase gain, the positive feedback at R615 will
cause trouble, producing a novel circuit - more suited to a code practice
oscillator. The positive feedback produced by R615, the 56 Ohm job, is yet
another mystery circuit of the R390A. I have elected to short this little
bugger out.

If you are willing to do a simple rewire to replace the 6AK5 with a 6AQ5,
lower R614 to 270 Ohms or so and install a small all-american 5 type output
transformer, you can easily get to 1 watt at less than 1% distortion and
obtain 30 Hz to 20 kHz bandwidth. With a better transformers and more
fooling with the circuit, 2 - 3 Watts is possible.

Warning - Playing with this circuit is addictive, buy another audio deck.

Mike Murphy WB2UID


----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, April 19, 2005 6:28 PM
Subject: Re: [R-390] Audio Capacitance


> Fellows,
>
> Yes I did say go from .01 to .1 or .3
>
> A jump to .03 from .01 just will not give you enough to hear the
difference.
> As some of the other post pointed out the transformers and other things
are
> still
> effecting the changes
>
> Roger KC6TRU
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