[R-390] 6080 in place of 6802 - RMS ???

Thu, 10 Jan 2002 00:23:30 -0500

Dave, guys ....

As JFK used to say at his entertaining press conferences .. "Let me say this
about that."  Of course, I'm no JFK, but ..

Notwithstanding all the theoretcal controversy (I'm not into that stuff -
too political for me) ...

In the most recent issue of HSN -- which isn't all that recent anymore due
to a lack of author-contributors (hint) -- the 6082 situation is dealt
with -- BTW , it's 6082, not 6802 as the subject line would indicate.

The approach runs along the lines of Dave's #3, and I think it's more
elegant than wiring an additional filament transformer to run 6080's.
Here's the gist of the article:

I acquired an R-391, which is electronically the same as the non-A, the
autotuner is elecro-mechanical and not involved.  Turned out to have a
solid-stated P/S and audio amp.  The audio module, from which normally
"hang" the 6082's upside down so as to better cook the works, was almost
completely gutted -- only one tube left in the thing.

In place of the 6082's was a piece of blue perfboard with a fairly simple
circuit on it.  I'm no engineer, so I enlisted the assistance of Dr. Jerry
Johnson to figure it out -- basically reverse engineer the thing.  It then
became his remote assistant (a la Igor) in preparing the article (in
addition to being editor of HSN).  So, it was as if the NIH Frankenstein
monster was already alive, and the good doctor set out to determine why.
Not a cookbook article.

Basically, the circuit consisted of a Darlington pair with a pair of zener
diodes.  Now, this thing had apparently been put together some years ago and
the exact same transistors are probably not easy to find, but I don't think
there was anything all that special about them.  There were no markings on
the tiny zeners.

Some of the list members might remember that I called for '390/'391 owners
to help out with an experiment -- to use a variac to replicate low/hi AC
power levels (about 95 up to 125) and track the B+ which is supposed to be
regulated at a nominal 180 volts.  What we found is that the solid state
circuit performed equivalently to the stock modules.  (There were one or two
Motorola non-A's that were better -- dead on 180 no matter what, but the
Collins's were similar with about a +/- 1 volt variation from 95 to 125 vac
supplied.

We wanted to also check on the effects of loading -- mostly that would be
audio power consumption -- but couldn't really do that as this modified '391
also has one of those ECG inline flat pack audio amp modules, so it's not
drawing any B+.

Now both Dave and Dr. J. expressed some concern over how to mount the
zener(s) and/or transistors to dissipate heat, but this radio runs stone
cold in that area.  Not only that, but the tab mount transistor isn't even
sunk to anything -- just floating up from the perfboard.  It looks like (to
this homebrew forensic specialist) that the tab mount transistor was
originally mounted to one side of the shield/baffle as there's a hole with
residue of heat sink grease there.

I probably should point out that this rx is running on diodes, not 26Z5W's
and has a resistor where the ballast tube used to do it's lava lamp
impersonation. ;-)

This was published a few months ago with the hopes that someone would
replicate the circuit with new or at least more identifyable parts.  Haven't
heard anything yet.

The advantage with this approach is a great reduction in heat vs. the 6082's
which tend to cook the audio module.  I don't understand why the circuit
doesn't throw much heat -- but if your design requires it, you can heat sink
the zener or transistor(s) right on the side panel.  However, it will
probably be necessary to use mica insulators and teflon feed-throughs with
heat sink grease as the tab on a tab mount or body of a TO-3 would be hot.

Dr. J. was at first surprised that it worked as well as it does.  The parts
needed, including the perfboard, would fit in your shirtpocket with room
left over for a deck of cards and a cell phone.

Will somebody please take a look at the article and whip up a new circuit?
It's possible to build it on a piece of board that will fit in where the
6082's go and hook right up with an octal plug into one of the 6082 sockets,
so it can be done reversably.

Also -- I need author-volunteers.  So, c'mon now .. hey ....

Barry

----- Original Message -----
From: "David Wise" <[email protected]>
To: <[email protected]>
Sent: Wednesday, January 09, 2002 11:01 PM
Subject: RE: [R-390] 6080 in place of 6802 - RMS ???

> Sorry for the big quote
> after my post; I fat-fingered this
> little laptop keyboard and sent
> by accident.
>
> If you want to run 6080s, I see three
> ways to handle the heaters.
>
> 1. Transformer.  Just a 6.3 or 12.6V
> filament transformer on 120.  If there's
> room, JUST DO IT.  It wins on simplicity,
> cost, effectiveness, RF noise, and heat.
> IMO it's the only way to go, but I don't
> know the realities of the R-390.
> Somebody give me one and I'll try it :-)
>
> If a transformer won't fit, or
> you [lunatic :-) ] insist on a diode:
>
> 2. Diode + resistor.  My off-the-cuff
> numbers here are wrong, but they
> make a reasonable starting point for
> cut-and-try.  I said that the tubes
> get 8.8Vrms each, so you need
> to reduce it to 6.3, that's 5V .
> The tubes pull 2.5A, oof! so I'd
> try a 2-ohm 20W resistor and see
> how it goes.  This solution generates
> more heat than the 6082s.
>
> 3. Diode + Zener diode.  The zener
> reduces the peak voltage to one
> that results in the right power.
> I haven't calculated the required
> zener voltage, and it's actually
> somewhat involved if you want an
> exact answer (integration of trig
> functions over less than 90 degrees).
> Much easier to cut and
> try until you get the right brightness.
> To measure the power you'd need a
> true wattmeter because the waveform
> is weird.  Start with a 7- or 8V
> 20W zener.  It needs to be heat-sinked.
> This solution also generates
> more heat than the 6082s.
>
> Regards,
> Dave Wise
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