Cathode bias for Express?

[M Fowler]

Rooster,

I had run across a schematic where they were using 370R/15w for the EL34 cathode bias. I bought that resistor but decided that the norm is 200R-250R so that is where I stayed at. Actually 200R/25w with 220uf/64v cap.

I favor taking the prescence out and going with cut control as well.

I use my Ceriatone Express as my testing bed and I have done many changes with that amp. Next I plan to change the cap stack to two can caps. JJ 50/50 and 40/20/20/20. I am thinking about going with a 15H choke instead of the resistor to see how that works as well.

[ampdoc1]

Why not have both SE and PP?

[rooster]

Wow, ampdoc1, that is a very clever build in so many ways! Beautiful work indeed. That 12DW7 tube and the pentode are just well thought out. Very cool!

I have to say, and I guess you would know, but to me, once you get the Cut control on board, and you reduce the gain of the PI, the amp doesn't really sound the *same* when you come back with the negative bias. Eh, but you may have an exception here, I dunno. Comments?

Mark, yeah, its hard to tell what all the various tubes can produce, you may have the perfect bias for that set. I am using the J/J KT77 and these are typically - overall - in the HOT range. Eurotubes told me that when a customer calls for tubes for his Chieftan, they reach for the 'coldest' numbers they can find in this tube, and still they wish they had colder options.

At 300ohms X 2, I don't have cherry plates but I can't help but wonder how the amp would behave if things were a bit colder. (BTW, this is exactly 25 VDC at the cathode.) But it sounds good, definitely more usable amp for me, as much as I love the Express.

Oh also, about he KT77s, where I like them alot, they do not survive a combo too well I have learned. They develope a 'shake' pretty quick. Good in the head cab though.

[jaysg]

I did .... and it was profoundly loud and aggressive.

[Structo]

I can't remember where I saw it but I think it was at a 18 watt forum where the guys were using 1000uF cathode bias caps.
I was a bit surprised but somebody said it worked well.

[paulster]

I can't remember where I saw it but I think it was at a 18 watt forum where the guys were using 1000uF cathode bias caps.

That's the first place I saw the idea of using very large cathode bias caps.

To some degree it gives you fixed bias since the cap can hold enough charge to resist a voltage change on the cathode.

Probably closest to combination bias in real terms (using fixed bias at the grid and a cathode bias resistor).

[JamesHealey]

I've just finished my 6V6 cathode biased wreck..

It's Liverpool Tonestack and Phase Inverter

3n3 + 100k on the RC filter for less gain than true express, but more than liverpool yet retaining the Express Voice.

No Bright Caps

Presence Control, Liverpool style.

220uF Cathode Bias

18W Marshall OT

220k Grid Stopper on V2a (3rd Stage)

And it sings like a mofo... really like it, pics + clips soon.

[jjman]

I can't remember where I saw it but I think it was at a 18 watt forum where the guys were using 1000uF cathode bias caps.

That's the first place I saw the idea of using very large cathode bias caps.

To some degree it gives you fixed bias since the cap can hold enough charge to resist a voltage change on the cathode.

Probably closest to combination bias in real terms (using fixed bias at the grid and a cathode bias resistor).

Some hifi guys use diode (strings) instead of cathode cap/resistor. No impedance to the ac signal in it but sets a fixed +DC voltage (at the cathode.)

[Ken Moon]

This amp is an Express a1a preamp mated to a non-NFB, cathode-biased power amp (similar to a tweed Fender or 18watt Lite), but with a low-ripple, less saggy PSU. B+1 is about 410V. I think about 6 or 7 folks have built these now.

http://moonguitaramps.com/images/talon_schematic.png

In my experimenting, just switching an Express to cathode bias sounds nice at lower volumes, but not very good at higher volumes (past 1 o'clock or so on Volume knob). (A stiffer PSU can mitigate this effect.)

One of the Express versions in the big pdf file has switched fixed/cathode bias - that's the version I used to try it out - it's an easy mod if you already have fixed bias.


Removing the NFB, but keeping the fixed bias, yields a more punchy clean sound at low volume settings, without the mushing or smearing of notes at higher volume that I experienced with cathode bias.

Removing the NFB is easy too - it wouldn't be hard to put both options on switches so you can try every combination

Cheers,

Ken Moon