Can someone help me understand this ckt?

I have posted a diagram to:
alt.binaries.schematics.electronic
refer to: Meesa Boogie Bias Ckt

I guess I can't see the way current flows in this ckt.

HOW DOES CURRENT FLOW IN THIS?

My goal is to be able to adjust the bias using a trimpot.
Should i put a 50k pot in place of the 33k?
Would changing the value of the 170K effect the cathode supply voltage, it
must remain at 2 volts.

Why is the cathode supply voltage lower than the grid bias? They both have
the same source and the cathode supply has a resistor (15K) HALF the value
of the grid bias (33k) resistor.

I looked at this so long now I can't SEE it!

Any help appreciated

Vin Collins

"J. Vincent Collins"

Can someone help me understand this ckt?

I have posted a diagram to:
alt.binaries.schematics.electronic
refer to: Meesa Boogie Bias Ckt

I guess I can't see the way current flows in this ckt.

HOW DOES CURRENT FLOW IN THIS?


** The current passing via the diode charging the filter cap s more than
that trying to discharge it via the cathode circuit. When first switched on
the voltage on the 220uF, 63 volt cap is way higher than -19 volts - it
settles down to -19 when the PI tube warms up.



My goal is to be able to adjust the bias using a trimpot.
Should i put a 50k pot in place of the 33k?

** No.


Would changing the value of the 170K effect the cathode supply voltage, it
must remain at 2 volts.


** The value is 100 K in my copy of the circuit.


Why is the cathode supply voltage.....


** It aint a supply of current.

Current flows into the circuit from the PI cathodes.




............ Phil

Hi Phil,

** The value is 100 K in my copy of the circuit.

This is a Studio 22+



** The current passing via the diode charging the filter cap s more than
that trying to discharge it via the cathode circuit. When first switched
on
the voltage on the 220uF, 63 volt cap is way higher than -19 volts - it
settles down to -19 when the PI tube warms up.

OK, so the current is fed back to the bias supply from the cathodes of the
PI.

Why?

What do the 2.2meg grid leak resistors effect?


My goal is to be able to adjust the bias using a trimpot.
Should i put a 50k pot in place of the 33k?

** No.


Then how to arrange bias swing to accommodate all tubes?

Vin,
Lost on this one for sure.




"Phil Allison" wrote in message
u...

"J. Vincent Collins"

Can someone help me understand this ckt?

I have posted a diagram to:
alt.binaries.schematics.electronic
refer to: Meesa Boogie Bias Ckt

I guess I can't see the way current flows in this ckt.

HOW DOES CURRENT FLOW IN THIS?


** The current passing via the diode charging the filter cap s more than
that trying to discharge it via the cathode circuit. When first switched
on
the voltage on the 220uF, 63 volt cap is way higher than -19 volts - it
settles down to -19 when the PI tube warms up.



My goal is to be able to adjust the bias using a trimpot.
Should i put a 50k pot in place of the 33k?

** No.


Would changing the value of the 170K effect the cathode supply voltage,
it
must remain at 2 volts.


** The value is 100 K in my copy of the circuit.


Why is the cathode supply voltage.....


** It aint a supply of current.

Current flows into the circuit from the PI cathodes.




........... Phil

"J. Vincent Collins"


Phil A wrote:

** The current passing via the diode charging the filter cap s more
than
that trying to discharge it via the cathode circuit. When first
switched
on the voltage on the 220uF, 63 volt cap is way higher than -19 volts - it
settles down to -19 when the PI tube warms up.

OK, so the current is fed back to the bias supply from the cathodes of the
PI.

Why?


** The negative supply is used to provide a current source for the PI
cathodes - it helps to make the PI more linear.


What do the 2.2meg grid leak resistors effect?


** With the 220 K ohm feeders - a voltage divider.

Then how to arrange bias swing to accommodate all tubes?




** The best solution is to add a second bias supply ( extra diode,
resistor cap and trim pot ) independent of the PI tube and adjust that.




........... Phil