[R-390] Audio Capacitance

[Barry]

Mike,

C604 and C605 are the ones I was jumpering with 0.033's.  I replaced C609
with a 10uF too.  I need to try this again, this time with the calibrator
signal and maybe watching an output meter.  I may see some increase at the
low frequencies that way, but if I can't really hear the difference, then it
won't matter.

One thing I wasn't doing during the experiment was to listen through the 600
to 8 ohm transformer.  That makes quite a difference too.  I'll try to hook
that up in the test this time.

I'm currently in the process of making a jumper cable to allow me to power
the PTO (and the RF deck for that matter) away from the radio and onto the
bench.  I don't want to take any more chances on shorting anything else out
while doing my PTO linearity work.  BTW, this one looks pretty bad
linearity-wise.  I plotted the output in Excel and it looks pretty pitiful.
Hopefully I can replace the capacitors someone mentioned and improve this
thing right off the bat.  I'm worried, though, that the "curve" looks like a
sawtooth pattern in places.  Maybe someone else has already tried
"correcting" the stack.  Dunno...

Barry - N4BUQ

----- Original Message ----- 
From: "Michael Murphy" <mjmurphy45 at comcast.net>
To: <Flowertime01 at wmconnect.com>; <r-390 at mailman.qth.net>
Sent: Tuesday, April 19, 2005 7:17 PM
Subject: Re: [R-390] Audio Capacitance


> 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