[R-390] Off Topic - Transistor R390A sounding good.

[Bob Camp]

Hi

One of the more interesting mysteries of the world is the enormous 
number of transistor part numbers out there compared to the number of 
tubes. If you look at a tube manual from the R390 era you will find 
roughly 1/100 the number of tubes compared to the number of transistors 
and diodes in an equivalent solid state book these days.

We don't worry to much about a particular brand of tubes in the 390 but 
the audio guys go a bit nuts on specific manufacturers of "classic" 
tubes. As far as I can see the R390 approach (don't sweat it) works 
just fine except when you have a tube that's just plain bad. I have 
never seen a problem that always happens with a specific brand of tube 
in a 390.

If you look in your 390 there are a bunch of fundamentally different 
tubes in it. You are not going to swap a dual triode for a pentode and 
have much success. Even if you stick to triodes they come in a wide 
variety of bias and gain setups.

Bipolar transistors are different. You have two basic flavors NPN and 
PNP. Provided you don't run to much power, current or voltage do their 
thing ok. At a given current level the gain of most of them is very 
similar in a normal circuit.

If you already have a bag full of diodes or transistors, try plugging 
one in and see what happens. It just may work and save you a major 
hassle.

The Lankford SSB mod on the 390A is an very good example of this. There 
are probably thousands of diode part numbers that will work just fine 
in the circuit. They are easier to find by body style than by part 
number. If the diode looks like it will work it probably will.

So if transistors are more like each other than tubes - why so many 
transistor types?

Marketing rules ....


	Take Care!

		Bob Camp
		KB8TQ



On Jan 1, 2005, at 7:29 AM, Charles B wrote:

> Hi all,
> My R-390A is sounding good lately.
>
>
> I don't know where to go for this info, so I am checking here?  I need 
> to replace a transistor.  I have gone to the
> electronics store in West Palm Beach and they can't find it?
> I checked with a source here on the web and it's not listed
> in their catalog.  Anyway, if anyone could take a look
> at the following number and let me know if they can
> cross reference it, I'd appreciate it?  You can
> get me off group at my email address of:
>
> ka4prf at us-it.net
>
> The transistor's number is     MPS 6530
> I think the case has a large "E" on it?  The
> resistor is an NPN type. O18 case
> Thanks
>
> Chuck
>
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