Cathode to Fixed bias

[tubenit]

I've been pondering experimenting and changing one of my cathode biased amps to fixed bias to simply hear the difference in tone (if any?).

I am not sure what bias topology and values I would need for this since there is not a dedicated bias tap already?

The schematic shows a 5V4 but IF I am using a GZ34 with around 390v on the plates of 5881's,  would this current bias structure work or would I
change the values ?

With respect, Tubenit

[sluckey]

The only thing you may need to change is that 220K resistor (call it bias range resistor). The rest of the circuit is rock solid. However, I like a wider range on the bias pot so I use a 50K pot. I also have several small 50K locking pots on hand so I'm sure that has an influence on using that value too.

If I had to buy a pot I'd get a 50K.

[2deaf]

Change the 220K resistor to 100K, the 25K pot to 50K (as per sluckey), and the 47K resistor to 15K.  Should be able to bias anything from a 6V6 to a 6550.

[tubenit]

Excellent!  THANKS, guys!

With respect, Tubenit

[jjasilli]

Alternate Point of View.  Small changes in pot rotation make for large changes in bias voltage; whereby, large value pots are hard to control.  Solution, a small value pot, around 10K; or a multi-turn pot for larger pot values. 


To find fixed bias voltage, measure the cathode voltage while cathode biased.  Shoot for that voltage with fixed bias.  One way to do that is to set your bias pot at 50% rotation. Temporarily, use another pot in place of the fixed resistor under the bias pot to get your desired voltage.  Play your guitar & experiment with hotter & colder fixed bias to see what you like best.  Then measure the resistance of the "under" pot and replace it with a fixed resistor. 

FWIW: I added SW-able cathode bias to my fixed bias EL84 Traynor Bassmate.  To my dismay I can't hear a difference.  Haven't tried this on other amps.

[tubeswell]

Look at the Fender Princeton Reverb bias supply and copy that (but make it adjustable). Try something like 100k input resistor-diode-10k pot+15k tail with 100V caps

[shooter]

To my dismay

If the amp has NFB, disconnect it for cathode bias n put all the gain n vols close to 10, you should get closer to metal