[R-390] R-390A IF and AGC

[Bob Camp]

Hi

Oddly enough there are two ways to do a tube diode and the equivalent=20
in solid state. A triode grid will work as a diode and it often does a=20=

better job than a straight vacuum tube diode. The main advantage is=20
that it's "forward voltage" is a bit lower so it makes a more efficient=20=

detector.  The same thing turns out to be true with the base emitter=20
junction of a transistor. It makes a lower leakage diode than a normal=20=

silicon diode. You can also get low barrier hot carrier diodes in solid=20=

state.

An AM detector has a bunch of characteristics. The first is the ratio=20
of RF power in versus the audio power out. Solid state diodes are lower=20=

barrier but also lower impedance. That makes things a little hard to=20
compare directly. The next characteristic is signal to noise. Here=20
lower impedance is generally better. I suspect the combination comes=20
out in favor of the solid state diode. Assuming the IF strip has enough=20=

gain and that signal to noise is set before the detector neither of=20
these things probably are a big deal in a properly designed real radio.=20=

If you swap parts back and forth in a radio without redesigning it I=20
would bet the difference might be more noticeable.

What probably is important is the linearity of the diode. Most grid=20
based tube detectors have a noticeable non-linear component to them.=20
The solid state diode may be a bit more linear but it's a toss up..=20
Neither one of them is as linear as some other ways to do the rf to AM=20=

conversion process. What is for sure is that each will have a different=20=

distortion characteristic. Most people like the tube distortion better=20=

than the solid state distortion .....

No matter how you look at it the fast attack / slow decay that the=20
common AGC mods add to the radio work better than the stock AGC ....

	Enjoy!

		Bob Camp
		KB8TQ




On May 6, 2004, at 7:51 PM, D. ball wrote:

> Hello all.
>
> I have been having great success by using the IF section of a SSB CB=20=

> to demodulate the RF coming from the 390A=92s IF out. I cannot say how=20=

> much fun it was to do this and watch it work so well. I can also feed=20=

> the audio back into the 390 by using the diode out. Now that I can=20
> hear SSB much better than the product detector mod someone installed I=20=

> am considering reversing the mod to stock.
>
> Question 1: Does anyone think a tube diode makes for better AM audio=20=

> than a diode?
>
> The other is the ACG mods.
>
> Question 2: Would the stock AGC work better when the radio is used as=20=

> a down converter rather than the typical AGC mods found in these=20
> radios? The CB has it=92s own AGC in the IF stage.
>
>
> On another note I removed a scary mod in the audio section. Someone=20
> had used a line transformer as a plate transformer!!!!.. Did this guy=20=

> know anything about voltage ratings?
> I removed that mod quickly and lucky there was no damage due to the=20
> mod. The old caps were replaced with new ones for safty and all works=20=

> well. I bought a 500 to 4 ohm transformer and installed it in the=20
> speaker. No need to mount a transformer on the back of the radio.
>
> thanks
> Joe
> KE1MB
>
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