Raising Capacitance of First Main Reservoir Node?

[FourT6and2]

I'm considering raising the two main reservoir caps to 100uF/100uF in a modded 1959 or 2203 circuit. The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering. But I want to try raising this to 100uF total. Depending on the 100W Marshall schematic I look at, I see the first main reservoir cap listed as either 50uF total or 100uF total. I'm not sure if the schematics are flat out wrong, for an outlier amp, or possibly a mistake carried over from a 50W amp—where the author copied the 100uF node from the 50W over to the 100W without thinking.

1. Has anybody seen STOCK 100W Marshalls (1959 or 2203) with 100uF filtering at the first node after the rectifier instead of 50uF?

2. Will upping this to 100uF risk stressing the PT too much due to inrush current?

3. What about the time constant of this larger capacitance? It will certainly take longer to fully charge the caps compared to 50uF/50uF.

[cdemike]

Not sure about the answer to number 1, but I think if your HT fuse is spec’d appropriately your PT should be safe. To know the time constant, I think we’d minimally need to know what specific capacitor(s) you plan to use (or their ESR) and the HT secondary resistance of your PT. That’d be a simplistic estimation since it wouldn’t account for the tubes’ current draw or current flowing to the screens, PI, and preamp nodes in parallel with charging the reservoir cap.

If you have the parts on hand to either do the mod or to temporarily simulate it with capacitors in parallel to the existing reservoir capacitor(s), I’d opt to just try it out since the math is both of limited use in telling you how the amp will sound/feel and very complicated if you want a high degree of accuracy for the time constant.

[Helmholtz]

See corresponding thread on the marshallforum.
I experimented with increasing the reservoir cap value of my '68 1959. Didn't like it.

[FourT6and2]

Not sure about the answer to number 1, but I think if your HT fuse is spec’d appropriately your PT should be safe. To know the time constant, I think we’d minimally need to know what specific capacitor(s) you plan to use (or their ESR) and the HT secondary resistance of your PT. That’d be a simplistic estimation since it wouldn’t account for the tubes’ current draw or current flowing to the screens, PI, and preamp nodes in parallel with charging the reservoir cap.

If you have the parts on hand to either do the mod or to temporarily simulate it with capacitors in parallel to the existing reservoir capacitor(s), I’d opt to just try it out since the math is both of limited use in telling you how the amp will sound/feel and very complicated if you want a high degree of accuracy for the time constant.

I have 100uF/100uF caps. I can also simply parallel another 50uF across the existing caps to get 100uF total. I'm not too worried about how it will sounds/feel, as that's simply a "try it and see" sort of thing. I'm more concerned about how the PT will handle it over time.

[jabguit]

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering.

how does one series/parallel 2 caps?

[Colossal]

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering.

how does one series/parallel 2 caps?

They are two 50uF + 50uF caps in the same envelope (F&T, ARS,etc).

[Roe]

I'm considering raising the two main reservoir caps to 100uF/100uF in a modded 1959 or 2203 circuit. The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering. But I want to try raising this to 100uF total. Depending on the 100W Marshall schematic I look at, I see the first main reservoir cap listed as either 50uF total or 100uF total. I'm not sure if the schematics are flat out wrong, for an outlier amp, or possibly a mistake carried over from a 50W amp—where the author copied the 100uF node from the 50W over to the 100W without thinking.

1. Has anybody seen STOCK 100W Marshalls (1959 or 2203) with 100uF filtering at the first node after the rectifier instead of 50uF?

2. Will upping this to 100uF risk stressing the PT too much due to inrush current?

1 yes, there are some reported cases with 200uf caps on mains and screens.

2 not really

[Roe]

See corresponding thread on the marshallforum.
I experimented with increasing the reservoir cap value of my '68 1959. Didn't like it.

It may work for metal, but not for blues. Also, it sounds better on low-voltage amps than on high-voltage ones normally

[FourT6and2]

See corresponding thread on the marshallforum.
I experimented with increasing the reservoir cap value of my '68 1959. Didn't like it.

it sounds better on low-voltage amps than on high-voltage ones normally

Interesting. My personal amp is a '70 Super Trem modded for 5 gain stages. Vp is 510v.

[FourT6and2]

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering.

how does one series/parallel 2 caps?

Two dual-section radial caps. The two sections of each cap are paralleled with themselves, then each cap run in series.

50uF + 50uF paralleled = 100uF
100uF in series with 100uF = 50uF

Marshall originally did this because they were cheap. High-voltage 50uF caps were expensive and large. So they used underrated dual-section caps run in series-parallel to up-rate the voltage handling instead of using 100uF caps or a single 50uF cap rated for the appropriate voltage. Then this just carried over throughout the years until the late '70s or early '80s when they moved to a more modern arrangement.

[jabguit]

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering.

how does one series/parallel 2 caps?

They are two 50uF + 50uF caps in the same envelope (F&T, ARS,etc).

well then - that's a horse of a different color....

[Roe]

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering.

how does one series/parallel 2 caps?

Two dual-section radial caps. The two sections of each cap are paralleled with themselves, then each cap run in series.

50uF + 50uF paralleled = 100uF
100uF in series with 100uF = 50uF

Marshall originally did this because they were cheap. High-voltage 50uF caps were expensive and large. So they used underrated dual-section caps run in series-parallel to up-rate the voltage handling instead of using 100uF caps or a single 50uF cap rated for the appropriate voltage. Then this just carried over throughout the years until the late '70s or early '80s when they moved to a more modern arrangement.

these 50+50uf radials are used in series from 1969 to 1985 roughly. But the voltage rating of the caps increased from 350v to 500v.