[GreenKeys] 6AU4 SS replacement?

[John Lawson]

On Sat, 10 Mar 2007, Eugene Hertz wrote:

> Hi All,

>
> While trying to set the FSK shift on my transmitter last night, it 
> coughed up some smoke. I pretty sure the smoke was from one of the 6AU4 
> rectifier tubes used in a 600V supply (for 6550 finals).

    Ummm - probably not. Tubes do not let out magic smoke, except in the 
rarest of circumstances. I would think that the smoke came from some poor 
component (perhaps a resistor) that got stressed due to the tube shorting.

  Time to get the transmitter on the bench, upside down, and check for 
evidence of BarBeQue.... ;}



>
> I am thinking about replacing the rectifiers with solid state. You may 
> ask why? Simple answer is there are 3 rectifiers in the transmitter 
> consuming a total of 37W of filament power and heat.  For those 
> concerned, I always run the transmitter through a variac and can adjust 
> it down to lower the B+.  Also, as far as inrush through the power 
> transformer, I think I'm ok because the power transformer does not 
> directly feed filter caps, it first goes through some mighty chokes. So 
> I think the choke will moderate the current filling up those capacitors.
>

    Recall that solid state rectifier replacements (and I run the 866 SS 
replacments in my Valiant) have a much lower voltage-drop across them, 
hence the output voltages (and the stress on the old components) is 
greater.

    If the transmitter (what make/model, BTW??) was designed to run on 
'nominal' 110 VAC, and you install SS rectifier replacements, this might 
be a good idea.  Be aware, though, that the filaments must be lit 
optimally to produce the longest life and best operation - too high and 
they burn out much faster, too low and the emission suffers, the internal 
currents go up, and the tube dies from starvation of the cathode.

   In the case of my (1958) Valiant - I run it on my normal line, 118-120 
VAC most of the time - for the last year, everything has been fine. Knock 
on bakelite....


> In any case, the real question is, does anyone have any experience with 
> SS replacements for the 6AU4? My google research has only located one 
> which can be found here:  http://tinyurl.com/2crs5e

   Gene, the 6AU4 is a 'damper diode' for color tv use - it will handle 
4,500 volts at 1,300 mA (1.3A).... for 10 microseconds.  The overall DC 
plate current is 210 mA, and the average dissipation is 6.5 watts - these 
are 'maximum' figures above which you'll make even more smoke.  Also, 
there are restrictions of 900 volts negative from cathode to heater - 100 
V postive.

    In this case - if a suitable "plug in" SS replacement can't be found, 
I'd be tempted to 'sacrifice' the tube, break it all out of it's base, 
then mount a single SS diode in the base to the appropriate pins. Mind how 
the filaments are connected to be sure that eliminating the 6AU4 from the 
filament supplies will not disurb anything.

>
> This is the only one I've located. Anyone ever use this brand? Anyone 
> know of any other product? Seems like embedded in this is a (or more 
> than one?) 1N1262. I wonder if they do anything else like series 
> resistance, good heatsinking or anything fancier. Otherwise, why 
> couldn't I just wire in a 1N1262? (maybe 2 in parallel for more forward 
> current?).


   See above.  Be careful when paralelling diodes - a bit of work needs to 
be done to make sure they share the current equally.



>
> Transmitter also has a 5U4 which I will probably replace with the weber copper cap.
>

   You would get better info if you'd give us the specifics of what 
transmitter you're using - also do you have the service manual or 
schematics for it? Thst's going to be essential fairly soon, IMHO.


    Finally - please be very careful and cautious when working on HV 
circuits, and transmitters and boatanchor radios in general.   Recal that, 
if the 'bleeder' resistors across the HV filter caps have become open, the 
caps can retain a substantial charge for days - been there, done that, 
have more than one little scar to show for it...   ;}


   Cheers

John
KB6SCO
DM09fg