Rickenbacker M11 schematic

[Diverted]

A friend of mine gave me an old amp of his to fix, but it was missing some parts and apparently there is no version of this exact circuit online ... there are several incarnations of the M11, but none that I have found that show the one I have, with a 12AX7 preamp and 12AU7 phase inverter.

So before I start digging into it, I decided to draw out a schematic based on what I think should be going on here. All that were missing were a few coupling caps and connections between those caps, and the instrument inputs, to the board, pots, etc.

I would like to have this to make sure everything is legit, and to put a good schematic of this amp online. Apparently a lot of people have been looking for this schematic for some time, and it doesn't seem to be out there. I would really appreciate any extra eyes going over this so I can correct it if it needs it.

Thanks!

[sluckey]

I think there should be a cap between V2A plate and V2B grid.

V2A cathode resistor 3.8K is not a standard value resistor. Maybe 3.3K, or 3.6K, or 3.9K? I'd recheck.

[Diverted]

I thought there should be something there too, based on looking at other phase inverter schematics. But the socket leads (1 to 7)are bent over and soldered together, so no cap originally.

You know, that 3.8 is a typo. Thanks. 3.9K is what it should be.

Thanks! See any glaring errors otherwise?

[PRR]

I think there should be a cap between V2A plate and V2B grid....

It's clearly a mistake. However it seems to work OK? Two resistors are not needed, but what the heck.

[Diverted]

I was reading up on different types of cathodyne phase inverters last night. I admit I don’t understand them very well, but apparently it’s not a mistake. This is a DC-coupled cathodyne PI as opposed to one that has a cap between plate and grid, which is the more common AC coupled version.

I always thought keeping DC out of the grid was crucial but apparently it’s done. The plate in V2A has to have quite low voltage for them to work... it makes sense. When I first drew it out I was surprised to see a 470k plate resistor there but now I can see why.

[Diverted]

Quick question: the 5M preamp grid resistors to ground are both way high, in the neighborhood of 10M ohms. I have no 5Ms on stock, but do have 3.3Ms. Close enough? Or no?

[sluckey]

In the case of grid leak bias, 10M may actually be better than 5M.

[Diverted]

Here's the final version of the Rickenbacker M11 schematic, with voltages added.
Put it all back together today and it's a great-sounding amp so far.
If the detail isn't good enough on the photos, the originals can be found here:

https://imgur.com/a/GYBFJ9U

Thanks,

Ted

[PRR]

....This is a DC-coupled cathodyne PI as opposed to one that has a cap between plate and grid, which is the more common AC coupled version.....

Yes; but if that is what they wanted they could have saved two resistors. See Fisher SA-100, for just one of many-many examples.
https://positive-feedback.com/jeffsplace/wp-content/uploads/2018/06/fisher-tube-circuit.jpg

We more often see cap-coupled in g-amps. The cathodyne gets kinda screwy when OVER-driven. More than a hi-fi guy would ever do. The cap-coupled *may* de-bias itself in a more musical way.

[Diverted]

The overdrive on this amp is great, starts breaking up nicely in the mic input around 6.