Main Reservoir Capacitor charging and discharging route

[Gary_S]

Can anyone point out to me the route through which the main cap is charging and discharging?

It's obviously getting charged from the PT and building up a voltage but then my thinking is how is it discharging? through what route?

I thought it was going back through the diodes in the rectifier to the CT  of the PT which is at ground potential the same as the other side of the cap.

Most of the tech stuff i've been reading shows the cap discharging through a load resistor but there's no Rl anywhere near this cap. C23 it is right next to the PT.

[HotBluePlates]

Most of the tech stuff i've been reading shows the cap discharging through a load resistor ...

Equivalent circuit.

It's easier to draw a resistor than a mass of circuitry. If there is a voltage across a resistor, then there is a current through it, which depends on the amount of resistance  (ohm's law). So any load could be represented by a resistor which draws the same current from the power supply.

It's obviously getting charged from the PT and building up a voltage but then my thinking is how is it discharging? through what route?

What is the main cap connected to?

+ end of Main Filter cap -> OT center tap -> output tube plates -> through tube -> Ground -> - end of Main Filter cap

There is also the parallel path of the choke and screens, as well as the portion of the supply which feeds the preamp. But output tube plates draw the majority of the current.

So let's say you have a 4kΩ plate-to-plate OT. The power supply cap is connected to the center-tap, so you might say you have 2 parallel paths through output tubes to ground. You could say each half of the OT primary is 2k, and those halves are in parallel. While you'd think that's like the supply seeing a 1kΩ load resistor, in fact when one tube is conducting more the other is conducting less, so until one of the tubes is completely cut off it looks to the cap like a 2kΩ load.

So for calculation with a 2-output tube Marshall, you might assume you could replace the output section with a 2kΩ resistor (or 1.7kΩ if your OT is 3.4kΩ plate-to-plate).

[Gary_S]

Equivalent circuit.

Ah! i get it now. I was taking the diagrams in the power supplies module in NEETS too literally and i'm saying "where's the resistor for the cap discharge path!"

What is the main cap connected to?

+ end of Main Filter cap -> OT center tap -> output tube plates -> through tube -> Ground -> - end of Main Filter cap

Thanks HBP that clarifies it for me. Man reading your advice is like a cool college course for pointing us relative newbs in the right direction.

[TIMBO]

Hi guys, Timely topic, The standard silver jubilee has a similar PSU, so I have added a 220k resistor as a bleed to discharge caps as this seems to be good practice.

On my build when the standby is turned off the sound will decrease as excepted, but when the power switch is turned off it gets a small rush of power and the sound comes back for a second or two.

So whats going on there. Thanks

[alerich]

When I shut off my amps I switch the power off and leave the standby on. The power tubes are still hot and are still conducting and they discharge the power supply pronto. Faster than a using a discharge probe.