Trainwreck-y Cathode-Biased and with a 5879

[Ledzepp007]

Can it be done convincingly?

I hacked together some stuff and made an "idea" schematic.

[printer2]

How much gain do you want?

[Ledzepp007]

How much gain do you want?

Is this a rhetorical question lol? Actually I would be aiming for about the same amount of gain as the normal TW circuit. Haven't adjusted the 5879 for that just yet. But looking for something along the lines of mu of 100 for the 5879, not EF86 levels.

My half-cocked idea is to put the 5879 in place of an AX7 not for more gain, but for different tone.

[tubenit]

I think that would work.  My guess is that when you get it built that you might need to lower the initial cathode cap values of 25uf to something more like 5uf.

With respect, Tubenit

[tubenit]

This is a Trainwreckish idea that I drew up a few yrs ago.  Never have built it.  I think a version could be done with 6V6's also.

[HotBluePlates]

Can it be done convincingly?

The Trainwreck style OT from Magnetic Components is 6.6kΩ.  You have a B+ of ~400vdc.  Why were you considering cathode bias?  What was your goal for choosing that over fixed bias, which would be more typical for 6V6's at that supply voltage?

[Ledzepp007]

I prefer cathode biased amps in general. There shouldn't be a reduction in amp stability, just efficiency, correct?

[MakerDP]

Nothing wrong with cathode biased power if that's what you prefer. But something to consider... if you use a "large" cathode bypass cap then the sound of the power amp will become more like fixed-bias.

[Ledzepp007]

Nothing wrong with cathode biased power if that's what you prefer. But something to consider... if you use a "large" cathode bypass cap then the sound of the power amp will become more like fixed-bias.

Maybe 100uf to keep things from getting too flabby? 220uf?

[MakerDP]

As Merlin told me on another site:

No bypass cap = Stiff, maximum headroom.
Smallish bypass cap = Squishy, minimim headroom.
Large bypass cap = Stiff (probably), minimum headroom.

He goes into it a little bit on his website in the PP Poweramp article.

By "large" we are talking 2200uF range. By "small" we are talking 25 to 100uF.

Some people put like a 50uF and a 2200uF on a switch so they can go back and forth at-will.

[HotBluePlates]

I prefer cathode biased amps in general. There shouldn't be a reduction in amp stability, just efficiency, correct?

It's fine to use, and shouldn't impact efficiency much.

At 400vdc and a 6.6kΩ OT, you'll likely need a cathode resistor a good bit over 250Ω for optimum bias.  Tubenit can tell you the values he typically uses (usually between 300-400Ω?).

Folks here know better, but at a different site there was a lot of discussion to explain why cathode bias does not equal "more compression".  I was just making sure you weren't thinking along the same lines, while still using a supply voltage and OT that's otherwise suitable for 6L6.

[tubenit]

I mostly use 330ohm or 360ohm with cathode biased 6L6/5881 amps.   I have used 270ohm to 430ohm range with 6V6's.

With respect, Tubenit