Has anyone done a fixed bias Phase Inverter?

[leadfootdriver]

I seen one on a Boogie Studio 22 amp schematic.

http://www.webphix.com/schematic%20h...ogie_22cal.pdf

It seems like it would deliver a firm drive to the power section, but I've never played one.

[jpfamps]

I've used fixed bias long tail pair phase splitters.

The main reason I did this was to dc couple from the previous stage to eliminate the coupling cap into the PS.

I use a fixed bias cathode follower to set the bias voltage on the LTP.

The main aim of this approach was to avoid blocking distortion.

Can't really comment on how it sounds compared with a self bias LTP.

[leadfootdriver]

That's neat. There's a ton of things I need to learn.

I need a real platform so I can try this stuff!

[Prattacaster]

How about using an LED to fix bias instead of messing with a bias tap and supply. Just a single component and you're there. A red LED would set the bias at -2.1v.

[mooreamps]

I'm not seeing how a led would work like that...

-g

[kg]

a forward biased LED would be a lot easier to use. just forward bias it and put it between cathode and ground!

BUT, part of what makes a schmitt/LTP splitter work is a shared common mode impedance in the cathode circuit.

put another way, a properly designed and working LTP will have minimal AC voltage at the cathode.

[redelephant]

That's not fixed biased. Just b/c it uses a negative rail doesn't mean it's fixed bias. I use a negative rail like this for my LTPs as well. That way I can DC couple the inputs, and often get to use a larger resistor in the tail.
If your asking if using a neg rail to place the tail is a good idea, I'd say yes. It certainly works well for me. The only caveat is the extra complexity in the power supply.

[kg]

good catch redelephant. i misread the schematic and thought the F node was AC grounded through a cap, but it isn't.

very odd to place the shared Rk on the "power supply" portion of the schematic!

[leadfootdriver]

It looked like fixed bias to me 'Shows how much I know...

[loudthud]

That's not fixed biased. Just b/c it uses a negative rail doesn't mean it's fixed bias. I use a negative rail like this for my LTPs as well. That way I can DC couple the inputs, and often get to use a larger resistor in the tail.
If your asking if using a neg rail to place the tail is a good idea, I'd say yes. It certainly works well for me. The only caveat is the extra complexity in the power supply.

Is it "fixed" bias or is it "cathode" bias? It's really a combination IMHO. Is the Paul C mod "fixed" bias? It biases a concertina or split load phase inverter by setting the grid voltage at some DC voltage above ground, the cathode current is set by a resistor between ground and the cathode. So the grid to cathode voltage is set primarily by the tube's characteristics but the cathode current is primarily set by a fixed voltage and a resistor.

I've used this type bias on an LTPI so the inputs would be at ground. This eliminated a capacitor in the feedback loop which included a resonance control. What I didn't like about it was when the amp was on standby, you get significant grid current unless you also shut off the minus rail. My minus rail was the -60V output tube bias supply and the tail resistor was 33K.

[kg]

in confusing cases like this, i always find it helpful to go back to the degeneration at DC requirement/definition for self bias...

if raising the cathode current through the tube makes bias voltage more negative, it's pretty much cathode biased.