Rectifier tube question

[thumbs]

Earlier this year I converted my Weber Proluxe (6L6's in fixed bias) to cathode bias. What would be the best rectifier tube to use with this configuration?

[tubeswell]

Cathode bias will typically result in a lower plate voltage and less power than fixed bias all other things being 'equal' (i.e. assuming the same PT, rectifier, filter, tubes and OT reflected load) (at least this is my observation from having tried this at home in the past).

So if you want to stiffen it up a bit more, then maybe go with a rectifier that drops less voltage than the one you've got (if that's possible - and making allowance for the requisite current draw on the rectifier winding), or vice versa if you want more sag.

[thumbs]

Thanks tubeswell. I've been using a GZ34 with the 6L6's, but was looking at a schematic for the old cathode biased Fender Pro and wondering if a 5U4 might be more correct.

[tubeswell]

I'm guessing those, 5c5, 5d5, 5e5 etc had a PT about 325-0-325 or 330-0-330 to get 395 B+ with a 5U4G?

I have found the sovtek 5U4G (coke bottle) is a good variant of this tube. I have some of the specs for it someplace. It supplies about 250mA from memory (i.e.; slightly more than the 225mA of a typical 5U4G, but slightly less than the 275mA of a 5U4GB)

[pdf64]

Cathode biased outputs tend to sound sweeter close to class A. This reduces the difference in current draw between no signal and full signal.
The cooler such a cathode biased stage is biassed, the more the cathode voltage will rise at high signal levels, which will force the stage further toward class B. Such that when overdriven, the crossover distortion makes it sound thin and weedy.
Best to bias it such that the cathode voltage doesn't rise much more that say 25% from no signal to full power.
And it's much safer to bias close to class A if the B+ isn't too high, so a 5U4 might be a better option. Peter.

[thumbs]

Thank you for the replies. The amp is sounding pretty good, but I'm always looking for something a little better. I think I'll give the 5U4 a shot.