FM3 rectifier tube

I am interested in replacing the rectifier in my Dynaco FM3. The V9 is given originally as a 6V4/EZ80. I was wondering if I could use a 6CA4/EZ81 instead. As I understand the pinouts are the same but that the EZ81 is rated for 150 mA while the EZ80 is rated for 90 mA, and the EZ81 draws more heater current (1 A vs. 0.6 A). The reason I ask is that I can get a new JJ EZ81 for a very reasonable price and I would give that tube a try if it will work. Thanks for any information you can give me. Bill

eickmewg wrote:I am interested in replacing the rectifier in my Dynaco FM3.  The V9 is given originally as a 6V4/EZ80.  I was wondering if I could use a 6CA4/EZ81 instead.  As I understand the pinouts are the same but that the EZ81 is rated for 150 mA while the EZ80 is rated for 90 mA, and the EZ81 draws more heater current (1 A vs. 0.6 A).  The reason I ask is that I can get a new JJ EZ81 for a very reasonable price and I would give that tube a try if it will work.  Thanks for any information you can give me.  Bill

Why not use 2 1n4007 instead ? They could be soldered on the tube-socket.

ez81 could do, but it will load the transformer with 3w extra, it might be deadly for it.

Go for 1n4007 or ez80

In looking at the schematic, it looks like the red transformer inputs go to pins 1 and 7.  The output to the 40 mFd can capacitor lug comes off pin 3.  There is a lead off pin 5 to ground (lug 3 of the 5-lug terminal strip) and finally there is a lead to lug 2 of the terminal strip from pin 4.  Where would the diodes go, does it involve moving the red transformer leads now on pins 1 and 7?  If the tube is removed, what else would need to be done in rewiring the pins?  I've got plenty of appropriate diodes having done the diode modification on my ST-120.  Would I lose any "soft start" capability with the diode change?  Maybe in the end I should just get another NOS EZ80?  Nonetheless, thanks for the suggestion.

The diodes should be "pointing" towards the 40mFd cap. ( the end with the band )
thus one diode from 1 to 3 and another from 7 to 3 .

There is no rewiring needed for this, just remove the tube.

You will be softstarted as the tubes will come online gradually ( the caps will be loaded fast but they
will cope with this)

Peterh, many thanks. I believe I understand this option now. See if I have it: 1) remove rectifier tube, 2) solder one diode from pin 1 to pin 3 with the band towards pin 3, 3) solder one diode from pin 7 to pin 3 in a similar fashion, 4) you're done! Looks like it would be very easy to convert back to a tube by simply clipping out the diodes. Do I "have" it? Thanks again. I've really enjoyed playing around with this little tuner. I think it works and sound great and is pretty cool to look at too.

Yes, you have it.
The only thing is that you might need a resistor in series ( 100 ohm wire wound) to reduce the
surge at power on, but that could be done if it blows fuses at power on.

And the whole thing could be built in a D9 connector ( if you can find one) that plops into the
tubesocket.

If you want a true plug 'n pray solution, Weber makes a copper cap to fit ...

WZ80 replaces the EC80 ... same spec.
WCA4 replaces the EC81, but the voltage drop might be a concern ...

http://www.webervst.com/ccap.html

PS ... Weber copper caps are already slow start ...

Thanks, Peterh and sKiZo. I was wondering if there was a Weber Copper cap that might be appropriate. I think that the WZ80 might just be the way to go. I am using a WZ68 in my ST-120 and that has worked out well.

sKiZo wrote:If you want a true plug 'n pray solution, Weber makes a copper cap to fit ...

WZ80 replaces the EC80 ... same spec.
WCA4 replaces the EC81, but the voltage drop might be a concern ...

http://www.webervst.com/ccap.html

PS ... Weber copper caps are already slow start ...

One learns every day ! Thanks for the tip!