[avahifi]

Do you have an old Dyna ST-70 or an early version of our Super 70, Ultimate 70, or Ultavalve amplifier that uses a 5AR4 rectifier tube?

Attached is a simple circuit that will protect and extend the service life of your 5AR4 rectifier tube.

Check your 5AR4 tube socket and if these protection diodes are not already installed, we strongly recommend that you do this little upgrade.

Use two 1N4007 diodes.

Frank

[ArthurDent]

Thank You for the tip Frank. Will check the Ultra next time I have the chance.

JD

[Scroof Neachy]

So now we have the diodes doing the rectifying and not the 5AR4. All it does now is drop voltage. Looks cool though.

[avahifi]

Actually, the 5AR4 still provide a slow voltage ramp up at turn on which protects the output tubes and protects the Diodes too.

Frank

[rlee8394]

Then short pins 4 and 6 together and the two diodes will do all of the rectification while the tube becomes a paralleled forward biased diode with increased current capacity and acts solely as a slow turn-on device which is desirable. 

-Ron

[avahifi]

You don’t want to short the 5AR4 out of its series connection.

This will raise the rectified raw voltage by 50V to near 500V DC and risk failure of power supply capacitors, output tubes, and other parts.

Not a good idea.

Leave protect diodes connections as shown on drawing above.

Frank

[DaveC113]

New 5AR4s fail often due to arc-over on start-up but NOS has not been an issue in the ~12+ years I've been using my current amp and the several years I had a preamp that used one.

So yes, you may need diodes with new tubes but maybe not with NOS, then you can avoid SS rectification if you wish. IMO, this is the result of issues with new tube production and shouldn't be necessary.

[rlee8394]

"You don’t want to short the 5AR4 out of its series connection."

Not shorting out the series connection. Just paralleling the plates. Still acts as a series diode.

-Ron

[Scroof Neachy]

You’ll cut the voltage drop in half won’t you? If the 5AR4 drops 20 volts let’s say, it will be 10 now, I think. May be a good thing.