No Decoupling on Negative Rail?

[phase]

While freshening up an SAE 2300 recently, I noticed that most of the SAE range of amps were produced without any electrolytic caps on the neg rails. These are 100-220uf caps, as typically used there, and which are in place on the positive rails.
Is there a reasonable explanation for this? Saving pennies would seem like a possibility, given that the rest of the caps were pretty low grade.

Any guesses?

 

[vzaichenko]

Hi Phase,

I believe, the way of thinking of SAE engineers was the following.

CR1 (51V Zener diode) sets the lowered +25.5V and -25.5V rails for the input stage. C2 decoupling electrolytic (100uF 50V) is placed on the positive side of the Zener (cathode). The negative side of that Zener (anode) is "rigidly tied" to the positive one - consider the Zener as a level shifter.
So, as soon as the positive side of the Zener is decoupled to ground with C2, its negative side accurately follows its positive side with the voltage level difference of 51V, making C2 working well for both the positive and the negative sides of +/-25.5V rails.

Cheers,
Valery

 

[phase]

Thanks you for that explanation, I can see that the diode is directly tied to both rails.

 

[llwhtt]

The older SAE amplifiers had the capacitor floating in parallel with the Zener. Early 2400s were like that and the 2400L had the capacitor tied to ground. I think all of the later models like your 2300 had the capacitor tied to ground. I'm working on a 2400 now and it has an early drive board and a later board, contemplating whether I should update the early board to the later board method.

Craig