Fixed vs. Cathode bias

[Denyle_Guitars]

I'm in the process of tweaking the Bruno mod'ed bassman that I bought a few weeks ago and while I was inside adding a master volume (because it's so GD loud) I noticed it was fixed bias with no adjustment, other than a resistor change. The amp is bright and punchy.

Will cathode bias make the amp sound more compressed/slower?

It has a 3 prong cord so I could use the ground switch for cathode/fixed bias. I'll probably add a bias adjustment either way. So, is it worth the effort?

I could also use it for pentode/triode switching.

[Old Tele man]

...Yes, cathode-bias will tend to be more "compressed" when pushed versus fixed-bias at the same level. The reason is that with cathode-bias, the "average" bias voltage tends to change and "lag" behind when played hard...whereas, with fixed-bias it's always the same, idle or full power (unless the PT is being "saturated" by too much current load).

[jetlag]

When I tried this on a BFPR, I noticed a little difference, but not a lot. Then I omitted the cathode bypass cap and noticed a huge difference. Just what I was looking for. I also recommend a switch for regular (stock) fixed bias operation with optional cathode bias. I use the fixed bias mode 75%, but the CB sure comes in handy for small rooms.

[Blue Strat]

Originally posted by jetlag
When I tried this on a BFPR, I noticed a little difference, but not a lot. Then I omitted the cathode bypass cap and noticed a huge difference. Just what I was looking for. I also recommend a switch for regular (stock) fixed bias operation with optional cathode bias. I use the fixed bias mode 75%, but the CB sure comes in handy for small rooms.

Omitting the bypass caps allows for ever earlier breakup especially on the low end.

How effective cathode bias is with the bypass cap depends on how hot the tubes are running at idle (ie, how small the value of the cathode resistor is).

[jimmisixstrings]

Hi,
On this subject:

I have a '68 Fender SR that has had tweaks over the years, mostly by Jeff Bober (Budda-PAT) and it has served me well. My issue now is that the amp is too much for most venues I'm in and I'm finally sick to death of how akward it is to carry around. I've been thinking about buying a new cabinet and going for 2x10", most likely 1 alnico and 1 ceramic from Ted Weber. That's been a plan in mind for some time now, BUT my local amp tech in PA (Rick Coberly) has planted the seed for cathode biasing my Super and has implied that it will be most pleasing to me when done, whether or not I do the cab/speaker changes.

Ok, there's my life story. My questions are what to expect? I've been told that cathode biasing is a "tweed-era" tone. I think it's supposed to lower the output, which is great as long as it's not too low. I want to be able to really open it up and let it breathe. Will the amp need to be basically rebuilt if this change is done? Will it never sound the same as is did before? How easy is it to make it a choice, switchable or something along those lines?

Help,
jimmi

[jimmisixstrings]

bump for a little help...

I'm also starting to think about doing the cathode biasing and a 3x10 all alnico speaker compliment ala Bandmaster-ish vibe.

Please, any and all opinions are welcome,
jimmi

[trdlasvegas]

Jimmi,
I have a customer who's 68 SR I've been modifying for years. He's in search of that "Holy Grail of Tone" that only he can identify but it sounds very similar to what your looking for.

Currently we have it Cathode Biased with a 5UB rectifier tube in it. I also changed the Ground switch to a Pentode/Triode switch to give him a "half power" setting.

He swears by the Cathode biasing, and I'm still waiting for his verdict on the rectifier.

I think the next trip in were planing on either KT-66's or the GT 6L6GE's. Going to be a pain to rebias for these with those Cathode Bias resistors but that's what he likes so far.

-Tony