diytube.com

I'm working on a preamp, and planning to use the 6BW4.

http://www.radiomuseum.org/tubes/tube_6bw4.html

In the schematic diagram, the first cap says 40uF. That is fine. The resistor after says 100 ohm max. Is that accurate? What about the PI filter resistors after that? Do they have to be small values also? Can the caps after the first filter section be bigger than 40uF?

Thanks!

Blair

Don't use larger than a 40uF cap immediately after the rectifier. 20uF is fine. In subsequent stages, the value of the cap can be higher than 40uF.

Blair wrote:I'm working on a preamp, and planning to use the 6BW4.

http://www.radiomuseum.org/tubes/tube_6bw4.html

In the schematic diagram, the first cap says 40uF. That is fine. The resistor after says 100 ohm max. Is that accurate? What about the PI filter resistors after that? Do they have to be small values also? Can the caps after the first filter section be bigger than 40uF?

Thanks!

Blair

I would say yep 40uf max, the 100 is 100ma max. The 6BW4 is like an inbetween of the EZ80 and EZ81. But the 6BW4 will drop more voltage than a EZ81, rectifying the same current. I like to use a EZ81 over a 6BW4, but a 6BW4 is cheaper. JJ makes a EZ81 for $11, but I don't know if I would buy it.

Thank you!

What about that first resistor? Can it be a bigger value than 100 ohms?

OK. Got it.

Yep, they do drop lots of voltage. I'm thinking about adding another one for FWB rectification.

Oh you can't add another for a bridge, because the cathodes are connected. A 6BW4 will work great in a preamp. You can wire the socket to accept a 6BW4, EZ80 or EZ81. To cut down on noise I'd use a big choke (8-10H) after the first capacitor. Gary has many for under $8.
http://oldradioparts.com/2a23afl.txt

That's an option. I didn't even think about the single cathode:)

I'm considering using SS rectification to get a higher B+, then feed that into one side of the 6BW4 into a cap like a resistive load.

That should work OK, and then I don't have to plug the hole:)

You could go with a tube/diode hybrid FWB - simpler, fewer components, less current draw, etc. than building an all tube FWB.

Fig. 3 below:

Now, that's spiffy. Does it still get the voltage drop across the tube rectifier?

Thanks!

The rectifier tube is bypassed by SS diodes. There is less of a voltage drop from a bypassed hybrid, than from an un-bypassed hybrid. The tube rectifier merely sets the "tone" for the amp/pre-amp; SS diodes are doing the actual rectification.

Here is the wayback's copy of the original webpage. It gives some info on how to construct pure tube and hybrid FWBs.

http://web.archive.org/web/200508280545 ... ifier.html