Have been focused on the push pull western amps like 86, 92 or 46. Been looking into the 91a with “R” and we are still going strong learning.
The push pulls have choke and capacitor filtering after the sublime 274a/b. I was surprised to learn that the 91a is restitor capacitor. Also the 91a/b hosts 25uf electrolytic capacitors that in western electric terms compared to the we1086 is high.
If any audio16 readers could help understanding the reasons here it will be appreciated.
[…] Thanks to “JF” in Switzerland the mystery from Read here […]
Your point is relevant. There is no choke at all!!!
Seems to me, the resistances (50KOhm) in parallel with the capacitors, stand as bleeders: They insure the voltage on the capacitor in normal operation and mainly, they are intended for safety when the power is switch off:
The high voltage stored in the capacitor has path to ground when the amplifier is turned off within a time constant RC
In this case, I am wondering how WE insure that he capacitor is not leaky through these bleeders in normal operation?
I would expect a switch to open those resistors in normal operation.
Still I’m wondering why no choke filter is used in the 91A:
1- Does it mean that the variation di/dt is low enough with respect to the capacitor tangent alpha and value being used (capacitance and supply current)?
2- Does the cut off frequency introduced by the overall RC is enough to cleanup spike on DC supply?
Hi, you can read in later post that there is in fact a choke. The choke is the field could of the speaker in the 500a system,