AB568

[Stevem]

Can someone explain to the moron that I am at times how it is that on these part cathode biased AB568 Fenders That how it is that the 5uf output tube bypass cap that spans between both pin 8 on these amps works with no reference to ground?

[martin manning]

The AC voltages on the cathodes are 180 degrees out of phase, but shorted together by the cap so the net result is zero, same as bypassing. I suspect that this arrangement would also eliminate the rise in cathode voltage (and bias excursion) due to the cap charging up, since the average voltage across it with time is zero.

[Stevem]

Does that not assume possibly wrongly that the output from the PI is truly 180 out of phase?

Maybe that's why these amps sounded so poor?

[Tony Bones]

It's a weird setup. The basic intention is to make the amp tolerant of mismatched output tubes.

The separate cathode resistors provide some 'autobias' for each tube, thus balancing the DC current to some degree.

The cap between cathodes forces some improvement in the AC balance. You can think of it as making the separate cathode resistors look like a single shared resistor as far as AC is concerned.

[Stevem]

It's really kind of ironic in that here Fender/ CBS in 68 and 69 are stepping on the bottom rung of the class A output stage Ladder if you will and it's a financial disaster, and just 1 year or so later Randle Smith does the simil class thing with running 2 tubes out of 4 in full class A and he can't make enough of the amps fast enough to sell em!

[martin manning]

Cathode bias does not equal Class A. This circuit is part cathode bias and part fixed bias, but still push-pull Class AB. I'd be inclined to bypass the cathode resistors in the conventional manner, or convert it back to fixed bias. Except for that unusual feature it's a typical Fender. Maybe it just didn't sound very good, or maybe people were looking for something new.

[Stevem]

Yup, I am quite aware that a Cathode biased output stage is no guarantee of it being class A.

CBS was just looking to get finished amps out the door faster, hence the reworked wiring layout that lead to oscillation issues, that lead to suppression disc caps on the reverb return and output tubes.

[Stevem]

I never dug in and looked before at a bunch of these two output 1968 Fender amps , but now that I have it seems CBS / Fender where without a doubt experimenting and sell those experiments at the same time!

Some of these amps have only one Cathode resistor and one cap and some have 2 resistors and 2 caps .
It seems like the Bandmaster was the only amp that got one Cathode resistor and one cap, all the other two output tube amps got 2&2.