Cathode bias for Express?

[dynaman]

Anyone try it? Results? Thinking of a pair of 6V6s in output for this. Prolly tube rectified too. Anyone see any probs?

[M Fowler]

I can only hear a better sounding amp to my ears going cathode bias.

[Richie]

hmm i would think just the opposite. different, but maybe not better?

[dartanion]

I've done it and depending on what value of bypass cap on the power tubes, it will be too squishy with a low value (below ~400uF, or sound pretty much the same with a very high value (~1000uF or greater). With 6V6s, I have found that around 1500uF works quite well and EL34s around 2000uF keeps it nice and tight.

[RJ Guitars]

I was thinking I read about somebody's work (here on the forum) where they had this as a switchable option. Some clips would be cool if anybody has a sonic comparison.

If we believe that cathode biasing reduces the ultimate voltage potential between the cathode and the plate (I don't think anybody will argue with that??)... then I would also be curious if the output level changes and the clean to mean crossover point changes as well? This might be too subtle to hear but with the Express' reputation for needing everything just right, you might expect something to be different. Could that be part of the technical explanation for the sonic differences that have been mentioned in the previous posts?

rj

[dartanion]

There is no point in using a tube rectifier, but cathode biasing does work and doesn't change this too much. Output is a little lower, hardly noticeable at all.

[dynaman]

I was simply going to use different rectifiers to manipulate the B+ voltages.

[jjman]

I'm using cathode bias JJ 6v6 but with lower voltages from using the lower voltage taps on toneslut’s tranny. Plate=346V @ 40ma, cathode=20V for idle of 13watts each. Max output with no clipping is 9watts. Plenty loud but never painful. Had to reduce the grid leaks to reduce crossover distortion when cranked.

[skeezbo]

Has anyone tried the power reduction switch that lifts the bypass caps off the individual bias resistors on a cathode bias Express? I have always wondered if you could still get the power tube distortion that the Express is famous for at a lower volume with this kind of setup.

Skeezbo

[rfgordon]

I did a cathode biased Express, and it was profoundly loud and aggressive.

[M Fowler]

Rich I find that the cathode bias amps are smoother, what are your thoughts on that?

[rfgordon]

I can't really say if the amps was smoother than a fixed bias Express, as it's the only one I made. It sang like crazy, though--definitely a great lead amp.

I built it that way because I used vintage transformers from an old PA amp that was cathode bias.

[JamesHealey]

I'm building a 6V6 Cathode Biased Express at the moment..

I'll report back with clips and pics and what not in the thread i posted.

[M Fowler]

Sounds good JamesHealey, we like reports and opinions makes for good conversation.

[rooster]

I did this to my TS tranny equipped Express, no rectifier. I would suggest anyone doing this take a look at a Matchless Chieftan schematic. Here you will see a 270 ohm cathode resistor paralleled to a 250uf cap per EL34 with 415 plate volts. This, the Matchless amp, is a HOT amp. I changed the resistors to 300 ohm and even this is HOT so consider at least 320 ohms or more for an average EL34. IF you can buy cold ones, you are done of course. (dartanian? where did those cap values come from? Is there a production gtr amp out here somewhere that is using a bypass cap higher than 250uf?).

Any, the amp needed gain reduction bigtime in the preamp and slightly in the PI for it to actually work as intended. Mark, you are on it, it actually sounds really good. Fat, and throaty and not flubby. Too, the volume is more like the Rocket now, which translates to 'not so loud'. Changed the volume pot to 500K. Its kinda like a Rocket overall now basically, but different because of the power tubes. Yes, I changed out the Presence control for a Cut control.

But its a balancing act. If all you do is cathode bias the back end, it will be mush as Richie describes.

I have it out with a guy tonite but I will get it back and do some sound clips. To me, it is totally playable/gigable where the stock Express was just too much. And, too, the 'Normster's Reverb Rocket', swapped to EL34s that rawnster is making is also on the same path, but with lower plate voltage than mine.

Also, FWIW, I was told by someone here when I posted about my experiment and asked who else has done this, that 'many people had done this, etc..' so it was nothing new. ??? Now it seems *new*, Funny how that goes.