Grounded Cathode

[LastManOnEarth]

I'm restoring a Rickenbacker M11, and the first thing that jumps out at me is that the preamp (in my case, 12AU7) uses a grounded cathode rather than a cathod bias resister. I understand how this works, but it's different than the canonical cathode biased preamp on every other circuit I've seen.

Is there any advantage or disadvantage of using a grounded cathode versus cathode biasing? On the one hand, you'd be getting gain and headroom (similar to fixed bias) since it isn't dropping some of the plate voltage across the cathode resistor. The downside would perhaps be that the bias current flows through the input side of the signal path, which would introduce more noise?

Any thoughts?

LMoE

[MWJB]

There shouldn't necessarily be more noise, I've built & modded amps that have "grid leak" or "contact bias" (like the Rickenbacker) and cathode biased channels side by side and have never noticed additional noise.

Some people say that grid leak is looser, squashier, softer, rounder. A bit less gain (distortion) than cathode bias. You actually end up with less plate voltage than cathode bias, all other factors being equal. It's the way that most guitar amps were configured up to the mid fifties. It can't always handle big signals, e.g. from tube screamers etc.

A 12AU7 has considerably less gain than the 6SC7 that the schematic shows. Perhaps a 12AY7 may be a better match?

[LastManOnEarth]

Thanks for the reply.

What's odd is that the circuit of this amp is different than the one published. By all appearances, its is the original wiring, tubes, everything (except the speaker), including the 12AU7 and 12AX7 tube covers labeled as such. There are a few wiring differences, particularly the phase inverter.

As for the "squashiness", it would seem that the grid bias voltage is going to fluctuate with the signal, since there's nothing equivalent to the cap in parallel with the cathode bias resistor, and you can't cap the grid bias resistor.

I'll probably get it running with the existing circuit first and then perhaps mod it with a switch to toggle between grounded cathode and cathode biased, for S&Gs.

LMoE

[Enzo]

Look up grid leak bias and study it. With the grounded cathode, electrons collect on the grid, giving it a negative charge. THAT is where the bias comes from, not the signal. Yes, at any given instant the signal adds to the bias (or subtracts as the case may be), but that happens in conventional common cathode stages as well. In the case of a cathode resistor, the plate current flowing through it makes the voltage across it vary. The plate current does not flow through the grid, so this variation doesn't happen. The grid resistor is typically very high in value, yours has 4.7 meg grid resistors. And those .05 coupling caps to the input jacks block any DC, so the grid can climb to a nice negative volt and be happy. The signal will move about that reference level, but bias will remain there across that resistor.

I wouldn't worry about losing plate voltage to the cathode, after all, most cathodes in gain stages in our amps have maybe 1 or 2 volts on the cathode, not much to subtract from the couple hundred on the plate.

Bias current is extremely small, as in microamps.

As to the amp being different from the schematic? Welcome to the world of amps. Gibson made a career of not matching amp to drawing. At some point in time, they decided to shift the circuit over to more modern tubes, so I don't doubt it is original.

And if you got the drawing like the one on Schematic Heaven, watch closely. For example, I see the lower 6V6 has two 240k grid resistors side by side. That doesn't seem likely.

[mbruffey]

I have an M-11 with the 12ax7/12au7 circuit, too. I inherited it from my dad. Back in the 70's he hadn't played it for a long time. One day we turned it on and played a while (15 minutes). Then it made a great honking noise. We put it away. I took it to Savage Audio in about 2005-6. They fixed a couple things in it and it sounded good. They put a Sovtek 12au7 in that slot (marked 12au7 for slot V2).

Funny thing though. Being curious, I tried ALL KINDS of other 12au7's in V2 as marked, including the original 12au7 which was still fine, but mainly I got a distorted signal from low to high volumes. Any two 12ax7's side-by-side in V1 and V2 sounded fine together, however. My final solution was to reverse the position of the 12ax7 and the 12au7 so that the 12au7 is in V1 and the 12ax7 in V2--contrary to the markings on the chassis. Sounds perfect that way. Lots of clean at low volumes!

In addition to what Savage had done, I replaced a couple more caps, the cap can itself, and a few resistors that showed out of spec. None of this, however, helped the distortion problem--only reversing the 12ax7 and the 12au7--hmmmmmmmmm . . . .

It's still a mystery to me why the amp does not work properly with the 12ax7 and 12au7 positioned as marked on chassis. I am certain it originally did.

I never could make heads or tails of the 6SC7 schematic in relation to this amp. This is an entirely different critter. If ANYONE out there actually has a schematic for the 12ax7/12au7 model, I would LOVE to have it!!

Mark