[R-390] Re: R-390 Voltage Regulator

[Barry Hauser]

Hi Drew

I acquired an R-391 (same electronics) a few years ago that turned up with a 
solid state VR setup.  In addition, the audio circuit was also subbed out 
with one of those amplifier modules -- only one tube left on the audio deck. 
I posted to the list about it and we did some comparisons of regulation --  
B+ at various AC line voltage levels, from about 95 to 130 vac.  The results 
were fairly close to those of others with stock 6082's, etc.  The mod had 
apparently been done some years ago -- I'd say maybe 15-20 years based on 
the components.  Basically, as I recall, it was a dual Darlington pair with 
a couple of zeners and a few other components.  I'm not well up on theory, 
so worked with Dr. Gerald Johnson at the time who analyzed the circuit and 
wrote it up in Hollow State Newsletter.

It 's possible to build the same or similar VR in a reversable way, on a 
small PC or perfboard cut to fit right into the shielded area occupied by 
the pair of 6082's.  The board can be mounted onto a single (or two) octal 
tube or plug basis so it's simply "plug 'n play".  There was some concern 
about where to locate the heatsink  -- inside or out of the chassis --  
however, the mod in my '391 doesn't have much in the way of that. 
 Another -- availability of high voltage zeners, but there are workarounds. 
Actually, I think mine has two 90 volt zeners to get to 180 - not sure.

Anyway, one of the list members did just that and built a plug-in board. 
His results were similar and was in the process of writing a follow-up 
article for HSN last we communicated.  We will hopefully be finishing that 
one day.

While 6082's can be a bit pricey, they're not up to the level of 3TF7's 
(yet) and tend to last a fairly long time.  The main purpose of all this is 
to substantially reduce the heat generated that wafts up from their low, 
upside-down position.  Of course, it's a good idea to place a muffin fan on 
that side of the '390/'391 to draw the heat out through the existing 
"portholes".

Barry

>
> On a soiled-state replacement for the series pass tubes in the R-390 non-A 
> Roger L. Ruszkowski  wrote (not in the order I quote):
>
>>Third good thing is a zener reference and series pass type transistor 
>>regulator will work fine.
>
> Someone used that approach and published the circuit and results in, IIRC, 
> Electric Radio some years back.
>
>>We will have to do the math on an R390 B+ line for the real current and 
>>the real voltage that is
> dropped.
>
> IIRC, current drain is about 200mA or so.   The input voltage to the 
> regulator has a high ripple content because of the Non-A's puny filter 
> capacitor.  The ripple is not a problem; regulating action removes it. 
> Ripple peaks reach about 300 volt, IIRC.  Finding Regulator power 
> dissipation calls for calculus.
>
>>A 2N3055 will do a 100 watts.
>
> Only sorta'.  That would be when perfectly thermally coupled to a heatsink 
> whose temperature was 25 deg. C.   Any reasonable heatsink would get 
> hotter than that.   Two or more  pass devices operated in parallel would 
> be preferable.  Devices in parallel need some emitter ballast resistance 
> to share current equally.  I would venture a guess at somewhat less than 
> 10 ohms for each device.
>
> The 2N3055 hasn't sufficiently high VCEO to run at voltages encountered in 
> a 'non-A.  For easy availability and overkill on voltage rating one could 
> use a TV horizontal output transistor, though somewhat low on hfe.
>
>>Rather than play with the 47 ohm resistors under the chassis, solder a 
>>wire into the socket pins for >each 6082 cathode (a total of four wires) 
>>and each plate (another four wires). <snip>
>>This will draw the current through all 4 of the 47 ohm resistors.
>>This also keeps the current spread over all the tube socket pins. Rather 
>>than
>>pass the full current through any one socket pin, keep the power spread 
>>out.
>>Your new plug in wiring may take the load, but consider all the original 
>>wiring
>>under the deck. Its so much easier to just build a mod that will work and 
>>play
>>well with a little prior planing than to rewire a burned chassis.
>
> A single wire would handle 200 mA easily.   For simple plug-and-play, 
> though, 4 wires would effectively place all of the 47-ohm resistors in 
> parallel and thereby reduce the output impedance and improve voltage 
> regulation.   Just one wire to a cathode pin could be used if its 
> accompanying 47-ohm resistor were jumpered out under the chassis (so much 
> for plug-and-play, but you were planning to tear down the radio for 
> refurbishment anyhow.)
>
>>The next operation is to build up a suitable reference voltage. The 5651 
>>is
>>only 85 volts. The solid state regulator wants a reference .6 volts above 
>>the
>>desired B+ voltage. My choice would be for a couple 5 watt zener diodes in
>>series with a proper filter cap across the zener diodes and a voltage 
>>dropping
>>resistor between the high power supply voltage and the junction of the 
>>regulating
>>transistor base and zener reference.
>
> Power zeners can be tough to get and are harder to mount than lower 
> powered devices.  An NPN with zener collector to base, a couple K resistor 
> base to ground , emitter grounded and collector used as the power zener 
> cathode substitute would work.
>
> The reference would be more free of ripple if a diode (1N4007 suitable) 
> were inserted in series with the reference dropping resistor.  That would 
> keep the reference's filter cap from discharging back through the dropping 
> resistor during ripple valleys.
>
>>Depending on heat sink preference, and the regulating transistor, 
>>everything can be mounted on the >relay octal sockets and plugged in.
> [in place of the 6082's]
>
> A heat sink to fit in that space would need fan cooling.  A small fan 
> could be mounted atop the plug-in and powered off the filament pins.  A 
> small DC computer-type fan (with rectifier and dropping resistor) would 
> serve.
>
> I would prefer Roger's "put the heatsinks outside the radio with a long 
> cable" approach.  With heat thusly relocated, evah' li'l bit he'ps.
>
> Instead of bipolar pass devices, power MOSFETs could be used. The IRF820 
> (available from Mouser) would suffice.
>
> That approach is used in a high voltage regulator in a tube tester 
> project.  Go to members.aol.com/sbench101.  Click on RAT Tube Tester.
>
> Same considerations about sharing power dissipation between several 
> devices apply.  Use a few ohms ballast resistance in series with each 
> MOSFET's source.  Connect 1k (exact value not critical) in series with 
> each gate terminal to quell oscillation tendencies; locate the resistor 
> right at the gate terminal with short leads.
>
> The high gate impedance (zero DC current) makes a reference using low 
> power zeners practical.
> Stack low-power zeners to, say, 200 volts for a reference.  Bypass with 
> .1uF poly for zener noise.   Regulator input goes to 1N4007 then to 10 uF 
> 250v electrolytic then to 47k (2 mA zener current; must be above the zener 
> knee), 47k to zener stack.   Strap a few hundred K pot across the zener 
> stack and connect wiper to resistor'ed  gate terminals.  Pot will adjust 
> output voltage.  Selected fixed resistors could be used instead.
>
> For a voltage reference more in keeping with the theme use a few neon 
> bulbs in series to replace the zeners.  The parallel capacitance would 
> have to be kept low to prevent relaxation oscillations.  NE-2's would be 
> run at about half a milliamp.  Pretty to look at.
>
> Another interesting approach can be found in the National Semiconductor 
> Linear Data book.  That circuit uses the LM317 sort of cascoded with 
> series pass transistors to stand off the high voltage.
>
> That's mine 2 cents worth. Thanks go to Roger for bringing up this 
> interesting topic.  Any more ideas to share?
>
> Drew
>
>
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