Build / mod, understanding a Magnatone PI circuit

[Leif H]

Im looking at the magnatone 213, tone master 214, and trying to figure out the phase inverter. I've read that this circuit uses 1/2 of a 12AX7 for the phase inverter, however looking at it, is the PI split in half, with the vibrato in the middle?

I'm wanting to try the circuit without the vibrato,and need help with the PI. This is of course based on my assumption that the maggie uses a simple cathodyne PI circuit.this being cant begin to think what to do after the tone stack, and before the output section.

Here it is in it's entirety:

magnatone_213.pdf

This is what I think should be removed:



I'm going from the tone stack into the cathode of the PI, then out from cathode and plate to the coupling caps going into the output section.

Is this going to cook, or rock?

Any comments or corrections would be appreciated.

[R.G.]

Im looking at the magnatone 213, tone master 214, and trying to figure out the phase inverter. I've read that this circuit uses 1/2 of a 12AX7 for the phase inverter, however looking at it, is the PI split in half, with the vibrato in the middle?

No. The phase inverter is entirely in the 12AX7 half that is connected to the output tubes. It's a simple split-load phase inverter with equal plate and cathode resistors.
To remove the phase shift section, you need to connect the plate of the preceding tube (270K plate resistor) to the 470pF capacitor leading into the grid of the phase inverter half-tube. No connection needs made to the cathode of the preceding tube. Delete the lower red line entirely.

The vibrato/phase shift circuit is its own phase inverter, but its outputs are recombined into the 470pF grid capacitor for the PI tube.

You may need to tinker with the value of that cap to get the low frequency response you want. 470pF seems awfully small; but then the grid is connected to its bias (cathode resistor) through what looks like a 10M resistor, so maybe it's OK.

[Leif H]

No. The phase inverter is entirely in the 12AX7 half that is connected to the output tubes. It's a simple split-load phase inverter with equal plate and cathode resistors.
To remove the phase shift section, you need to connect the plate of the preceding tube (270K plate resistor) to the 470pF capacitor leading into the grid of the phase inverter half-tube. No connection needs made to the cathode of the preceding tube. Delete the lower red line entirely.

The vibrato/phase shift circuit is its own phase inverter, but its outputs are recombined into the 470pF grid capacitor for the PI tube.

You may need to tinker with the value of that cap to get the low frequency response you want. 470pF seems awfully small; but then the grid is connected to its bias (cathode resistor) through what looks like a 10M resistor, so maybe it's OK.

R.G. thanks,

to make sure I understand this, the plate, before the plate resistor,that is connected to the B+ Right?
and jump to the 470pf cap like this?

[R.G.]

Yep.

[Leif H]

Yep.

Awesome, thanks!

[loudthud]

There seems to be a ground missing from the phase shifter circuit. There is no way for the cathode current to get to ground.

[Leif H]

Sorry loudthud, I didn't fix my redline drawing before I posted it.

This is my plan.

[echuta13]

You're gonna have an extra triode floating around in the soup. Methinks you should do something with it. :-)

[Leif H]

You're gonna have an extra triode floating around in the soup. Methinks you should do something with it. :-)

I don't want to do another gain stage, and the chassis wont allow for another chanel, we have input, volume and tone only, no pilot light, on/off switch is on the tone pot. I'm using the Filmosound chassis I have lying around here. this is a quick and dirty amp.