Magnatone MP3 verb PP driver

[Willabe]

A good friend of mine has a Maggie MP3, someone changed the reverb circuit over to a SE Fender type. He's ordered a Hammond universal PP OT with all the different primary/secondary connections to put in.

How does this PP reverb driver work?

It's listed on the schemo as; 12AU7 Mixer, Phase Inverter, Reverb Power Amp, V4. 

On the schemo attached below, V4's 12AU7 grid/pin 2 is fed from the trem channel and grid/pin 7 is fed from the normal channel. (Schemo has both triode's labeled as V4B.)

Then V4's plate pins, 1 and 6 feed the 2 opposite primary ends of a CT PP transformer, schemo says, T1=25K to 8 ohm. T1's 8 ohm secondary feeds the verb tank.

So how does it get the other half of the signal wave when signal only goes into 1 or the other triode?

1. Is it self inverting because of V4's common K R?

2. And why did they bias up the tube with dcv on the 2 grids? Looks like 2- 5.5M, 1 feeds each grid, taken from B+ node D.

3. Reverb recovery triode, 12AX7, V5A, has a grounded grid, with the verb tanks output fed the V5A's K. Why?

4. Is it safe to try a 12AT7 for v4?

https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_mp3_5.jpg


Thanks guys,  Willabe 

[sluckey]

How does this PP reverb driver work?

V4 is a differential input/differential output operational amplifier (op-amp). Notice that both channels have reverb?

So how does it get the other half of the signal wave when signal only goes into 1 or the other triode?
1. Is it self inverting because of V4's common K R?

The common cathode resistor is the key. The signal on one grid is amplified and inverted at the plate but also appears in phase on the cathode. Since the cathodes are tied together this in phase signal is also amplified by the other triode and appears on the plate but it is NOT inverted. Remember, the input signal can be applied to the grid or the cathode since the tube bias operates on the signal BETWEEN the grid and cathode. Doesn't matter which is grounded. So now you have the two out of phase signals on the plates to drive the PP transformer.

2. And why did they bias up the tube with dcv on the 2 grids? Looks like 2- 5.5M, 1 feeds each grid, taken from B+ node D.

Designer's choice. There are several ways to bias a tube. Fixed bias is a good method and probably simpler in this case due to the shared cathode resistor. Another example of a fixed bias circuit is the 6G15 Reverb Unit cathode follower.

3. Reverb recovery triode, 12AX7, V5A, has a grounded grid, with the verb tanks output fed the V5A's K. Why?

The cathode is probably a closer impedance match with the tank than the grid. Remember, you can input the signal to the cathode just like the grid. You get the same thing on the plate except there is no inversion.

You can try a 12AT7. Should work, but why? Ain't got no 12AU7s?

[thetragichero]

if you don't have any 12au7 i could send you a handful of organ pulls if you were to pay shipping. got em coming out of my ears

[Willabe]

How does this PP reverb driver work?

V4 is a differential input/differential output operational amplifier (op-amp). Notice that both channels have reverb?

So how does it get the other half of the signal wave when signal only goes into 1 or the other triode?
1. Is it self inverting because of V4's common K R?

The common cathode resistor is the key. The signal on one grid is amplified and inverted at the plate but also appears in phase on the cathode. Since the cathodes are tied together this in phase signal is also amplified by the other triode and appears on the plate but it is NOT inverted. Remember, the input signal can be applied to the grid or the cathode since the tube bias operates on the signal BETWEEN the grid and cathode. Doesn't matter which is grounded. So now you have the two out of phase signals on the plates to drive the PP transformer.

2. And why did they bias up the tube with dcv on the 2 grids? Looks like 2- 5.5M, 1 feeds each grid, taken from B+ node D.

Designer's choice. There are several ways to bias a tube. Fixed bias is a good method and probably simpler in this case due to the shared cathode resistor. Another example of a fixed bias circuit is the 6G15 Reverb Unit cathode follower.

3. Reverb recovery triode, 12AX7, V5A, has a grounded grid, with the verb tanks output fed the V5A's K. Why?

The cathode is probably a closer impedance match with the tank than the grid. Remember, you can input the signal to the cathode just like the grid. You get the same thing on the plate except there is no inversion.

You can try a 12AT7. Should work, but why? Ain't got no 12AU7s?

Thank you Sluckey. Much clearer now. 

And I did think of the Fender stand alone verb when I saw the bias on V4a,b.

My friend has plenty of 12AU7's, just asked so he could have an option to try.

[Willabe]

if you don't have any 12au7 i could send you a handful of organ pulls if you were to pay shipping. got em coming out of my ears.

Thank you thetragichero, very generous of you, but I have plenty of 12AU7's, and so does my friend.

Willabe

[Willabe]

My friend got the new Hammond CT PP verb transformer in and the verb is now working. 

But he was having a problem with when he turned up the verb, the amps output volume would drop the more he turned up the verb. He found a couple wires that were only connected by a single strand, re-did those and now the amp works great, and the trem is now working too.

Thanks guys,  Willabe