Mallory main filter caps - 650uF / 350V

[StanHavars]

Hi everyone,


I have a question about the main filter capacitors in guitar amplifiers. At first, I thought it was just a coincidence, but after digging deeper, I kept coming across the same type of caps in every amp that I liked.


I’m talking about modded Marshalls with large main filter caps—Mallory caps, supposedly 650µF 350V wired in series, which results in 325µF at 700V filtering.


The first time I noticed these was in a Mark Cameron–modded Marshall JMP from 1979 located in Australia. After that, I saw the same setup in Ossie’s (Three Monkeys Amps) Jose-modded Marshall JMP. I think that in a discussion with Dave Friedman, they mentioned that the amp was modded by Lee Jackson and that he may have installed those caps.


That’s not all—Doug Aldrich’s 1978–79 Marshall JMP, which was modded by John Suhr, also has similar caps.


Now, everyone who hears about that amount of filtering flips out, but there must be something about this technique that contributes to the way those amps sound. In the Cameron “Old Bitch” amp, there are also filter caps in a similar range (without going into exact values).


I’m curious—has anyone here experimented with such large filtering values, and how did it go? Is it worth trying? And please don't say the amp is dead sounding 'cus I really have no f..'n idea what the hell should this mean.


I understand that this will stress the transformers and rectifiers when switching the amp on, but I’m still interested. I’m also wondering how this kind of filtering would affect the overall sound of the amp or it will be mainly feel thing?

 

[griff10672]

 

[baron55]

Hi everyone,


I have a question about the main filter capacitors in guitar amplifiers. At first, I thought it was just a coincidence, but after digging deeper, I kept coming across the same type of caps in every amp that I liked.


I’m talking about modded Marshalls with large main filter caps—Mallory caps, supposedly 650µF 350V wired in series, which results in 325µF at 700V filtering.


The first time I noticed these was in a Mark Cameron–modded Marshall JMP from 1979 located in Australia. After that, I saw the same setup in Ossie’s (Three Monkeys Amps) Jose-modded Marshall JMP. I think that in a discussion with Dave Friedman, they mentioned that the amp was modded by Lee Jackson and that he may have installed those caps.


That’s not all—Doug Aldrich’s 1978–79 Marshall JMP, which was modded by John Suhr, also has similar caps.


Now, everyone who hears about that amount of filtering flips out, but there must be something about this technique that contributes to the way those amps sound. In the Cameron “Old Bitch” amp, there are also filter caps in a similar range (without going into exact values).


I’m curious—has anyone here experimented with such large filtering values, and how did it go? Is it worth trying? And please don't say the amp is dead sounding 'cus I really have no f..'n idea what the hell should this mean.


I understand that this will stress the transformers and rectifiers when switching the amp on, but I’m still interested. I’m also wondering how this kind of filtering would affect the overall sound of the amp or it will be mainly feel thing?

Larger filter caps will tighten up the bottom end and make the amp "faster" at higher output levels. Think of a filter cap as a large air tank connected to an air compressor. The larger the tank the more air it can deliver at a specific moment before the pressure drops. So, a large filter cap can deliver more power before the voltage drops. But a larger filter cap does need to be re-supplied, so initial powerup it pulls more inrush current.

As for "stressing" the power transformers, fake news. Older rectified amps had a limit on how bike the capacitors were due the tube rectifier limits.

All modern voiced (High Gain) tube amps made today have large capacitor values in the power supply

 

[StanHavars]

Larger filter caps will tighten up the bottom end and make the amp "faster" at higher output levels. Think of a filter cap as a large air tank connected to an air compressor. The larger the tank the more air it can deliver at a specific moment before the pressure drops. So, a large filter cap can deliver more power before the voltage drops. But a larger filter cap does need to be re-supplied, so initial powerup it pulls more inrush current.

As for "stressing" the power transformers, fake news. Older rectified amps had a limit on how bike the capacitors were due the tube rectifier limits.

All modern voiced (High Gain) tube amps made today have large capacitor values in the power supply

So basically when the amp is powered on I should wait like a 5 min for the caps to charge up and it should be fine?

I have heard a lot of times Doug Aldrich complaining that the transformer of his amp just kept blowing off so he brought it to J. Suhr and he moded the amp.

Should I consider balancing resistors for the caps and should i consider changing the screen resistors?

Maybe solid-state rectification or the stock will be just fine?

BTW I'm thinking to try this on SLP reissue from the 90's.

 

[baron55]

You don't need 5 min to charge filter caps, they charge instantly. That's why on new amps, techs tend to power the amp with a variac with low voltage and work up. This was common with older design capacitors back in the 1950s - 1980's. Modern caps can take it no prob.

Marshal amps used crappy low grade and under specked parts thus the history of failures.

Most musicians especially famous ones have zero clue about how amps work. (they spend more time playing) They just repeat and many times incorrectly about what they heard or interpret.

When putting filter caps in series you should use a resistor voltage divider to as you term balance the voltage to them.

Screen resistors limit current to the screens, since in guitar amps screens are run almost double what they were designed to.So as the screens pull current the large resistor causes the voltage to sag. Typically EL34 based amps start at 1K @ 5 watts, and 6L6 amps start at 470R @ 5 watts. Not a set in stone values, but typically what you see. Marshall has gone as high as 2.2K in some amps.