Cathode biased Blues Junior new project

[acheld]

I'm planning a build around a Hoffman Blues Junior, though with a cathode biased output section.   This project was brought on by having some BJ iron hanging around unused (a Mojo780 PT and an AllenAmps TO20B), and I want to learn more about cathode biasing.   

And, was intrigued by Fender's switching to cathode biasing for the BJ IV; as most of you probably know, Fender had traditionally used a very hot fixed biased output section which caused frequent (EL84) tube failure in the BJ series.

I have been unable to find a verified schematic for the BJ IV.  There is an unofficial schematic over at:  https://www.tdpri.com/threads/fender-blues-junior-iv-schematic.1104154/ -- for convenience sake, I have attached this PDF.  I don't know anyone with a BJ IV that I can inspect to confirm this schematic, so beware.

My proposed schematic is attached (BJ cathode biased).    I've taken Hoffman's BJ Project (a very solid amp itself) and adapted it to cathode bias.

I have two main questions:   What is the expected value of the cathode resistors?   The TDPRI schematic calls for 270R.   One of the EL84 tube sheets I looked at suggested this value, but virtually every other cathode biased EL84 amp I've looked at uses far lower values, and thus hotter biasing.

What is an appropriate range for the cathode bypass caps?   Again, I've seen multiple values in various amps, up to as high as 100uF.  I'm not sure what to expect from various values.

You bet I will be substituting various values in and out for both Rk and Ck!  All part of the fun . . .

[sluckey]

270Ω is fine for separate cathode resistors, but why use separate resistors? There are plenty of successful EL84 amps that use a shared 130Ω or 150Ω. (I prefer the 150Ω which would translate to 300Ω for separate resistors.) I've seen the bypass cap anywhere between 50µF and 500µF.

Look at my AC-15 layout. I used shared 130Ω/50µF. I also used two 1Ω current sense resistors to be able to easily check individual tube bias.

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

     http://sluckeyamps.com/VAC15/big_guts.jpg

[acheld]

Thank you.   

I spec'd separate resistors/caps only because I had the parts on hand, but I'll rethink that and take a look at your AC15.

Of course if I combine the caps/resistors, I'll have some room for the 1R current sensing resistors.   Ok, I'm liking the idea!

[sluckey]

I spec'd separate resistors/caps only because I had the parts on hand

Very good reasoning. You don't need any 1Ω resistors when you have separate cathode resistors.

[tubeswell]

so you’ve got a spare triode? (what would you use it for)?

[pdf64]

An advantage of having a dedicated cathode bias resistor (and bypass cap) for each valve cathode is that valves settle to broadly similar idle currents, it might be seen that by the magic of self bias, they match themselves.
Whereas sharing a cathode bias resistor seems to force a hotter running valve to run even hotter, a colder idling valve to idle even colder.
And if one valve dies or its screen grid resistor fails, the remaining operational valve will idle super hot, as shared cathode bias is predicted on all valves being functional.

[tubeswell]

... cathode bias is predicted on all valves being functional.

Predictive text's a beeatch :-)

[acheld]

so you’ve got a spare triode? (what would you use it for)?

The Hoffman BJ uses V2 as paralleled triode as the recovery stage following the tone stack. 

Brings to mind a question:  Fender BJ's use only V2a - the b section is not used - in their amps.   Why wouldn't we use the extra triode as paralleled in V1, and then use V2a to supply the tone stack, with V2b recovering? 

The Matchless Spitfire (https://el34world.com/charts/Schematics/files/Matchless/Matchless_spitfire.pdf) parallels V1, which then drives a simple tone stack without a recovery stage prior to its LTP.  Maybe a paralleled V1 in a BJ would be cool.

  I have fallen down the rabbit hole once again.

 

[thetragichero]

allow me to throw another idea at you: i used the unused triode in a blues junior to put a cathode follower prior to the tone stack. maybe not very fender, but then el84s aren't very fender either (save for a fairly rare brown tremolux) so why not marshall it up even more?

[tubeswell]

allow me to throw another idea at you: i used the unused triode in a blues junior to put a cathode follower prior to the tone stack. maybe not very fender, but then el84s aren't very fender either (save for a fairly rare brown tremolux) so why not marshall it up even more?

I too was going to suggest this. (And it is an old ‘Fender’ trick - 59 Tweed Bassman)

[shooter]

predictive or not, being able to tweak each separately does add flavor to the mix.
I've "unbalanced" the bypass caps, and the R's, there are changes even I could hear


the biggest problems I ran into was predicting the outcome when hammering the inputs, sometimes you got a great OD, sometimes you got bad death metal, never took it far enough to figure out the whys though.  the 84 is so easy to drive, you'll spend most of your tweak time in the preamp.  I suggest adding extra use turrets between gain stages for voltage dividers and coupling filtering

[mresistor]

... cathode bias is predicted on all valves being functional.

Predictive text's a beeatch :-)

did you mean "predicated" ?

[tubeswell]

... cathode bias is predicted on all valves being functional.

Predictive text's a beeatch :-)

did you mean "predicated" ?

I think so

[pdf64]

did you mean "predicated" ?

Yes