Metropoulos Forum

Filter caps on 100W Super Lead...

What are the functions of each of these:

1.) the group of 4 filter caps (that seem to be related to the power tubes?)
2.) the filter cap under the board near the preamp section
3.) the remaining filter cap

I have been reading about how lowering the filtering can affect the tone and either tighten up or loosen up the feel of the amp - but i am not clear on which caps do this - a little reader's digest version explanation of each of the above would help me out a lot. Thanks.

You basically have 5 filter sections. The 2 cans in the grouping of 4 closest to the power tranny are the mains filter section, in the case of 100W amps it is known as a voltage doubler. Each filter can has 2 caps in it which share a common ground terminal. The mains filter section uses 2 of these cans (4 caps) only to double up on the voltage rating. 4 50/50 cans wired in parallel then in series with each other still gives a total capacitance of 50. The mains filter section is fed the DC/B+ directly from the rectifier diodes. After the mains a wire carries the B+ to a fuse for safety. The OT "taps" the B+ from here. Then the power supply chain goes to the choke. The other end of the choke goes to the next filter section- the screens.

The two cans in the grouping of 4 closest to the board (with the resistors wired on the terminals) is for the power tube screens. It supplies the DC voltage to the power tubes. This will have the largest influence on the amps tone of all the filter caps. Usually a wire will connect this filter to the board were a turret is used as a junction point for the choke "output" and another wire that goes to pin6 of every power tube. Theres also 2W resistors connected to this turrret. The 2W resistor(s) lead to the next filter section- V3.

At this point the value of the filters effects the tone minimaly. I guess because the demand from the preamp and PI isnt nearly as large. The voltage rating doesnt need to be nearly as large either so you can use one side of a can. Those 2W on board resistor(s) (atached to the screens on the opposite side of the board) have a wire jumping from the turret their connected to to the V3 plate resistors (100k/82k). A wire also leads from this junction to the filter can all the way in the corner behind the PT. Its the filter for V3. That filter is usually 16 or 50u. I just use one side of a 16/16 can or both sides for 32 since my amps are more JTM100s then 1959s. A wire then jumps from the V3 plate resistors to the first of 2 10k2W resistors near the preamp.

Attached to either end of the far 2W resistor is either side of the filter can under the board. It supplies the voltage for the preamp sections. A wire goes directly from the first resistors turret to V2pin6. A jumper goes from the far eend of the last resistor to the plate resistors of V1 (100k/100k).

Thats more or less the entire power supply.

Excellent info. Thanks!

That really gets me curious about what exactly the filter is doing to the power/voltage/current (exposing my ignorance now) and why it works but that is probably more of an electronics 101 kind of thing - so, I'll stick to my original question.

Ok - so, would it make sense then to try different values for the 2 cans closest to the board that supply the power to the power tube screens? Can these values be changed independently of changing the values for the mains filter section?

You can change the two cans for the screens independently of the other cans. You could raise the values if you have ghosting issues, or lower the values for a looser feel (although I might try lowering the two cans for the main filtering to loosen things up first, and keep an extra current reserve for the screens with bigger caps.)

The filter cans are there for a kind of twin reason. First off they help to filter the voltage supply. Even after the rectifier diodes the voltage isnt totally smooth DC. So the caps and choke help to smooth it out. But they also are like a storage bin. After each resistor drops the voltage in a node, the cap is there to store that voltage and supply it smoothly. Its different from resistors in line with the B+ on the tube screens and plates which arent decoupling 2 filter sections but dropping the voltage in realtime as it goes to the tube.

In this schematic, does each 100u cap represent 1 can with the 2 50u caps inside it wired in parallel or am i a confused donkey?

No you got it right. Actually it doesnt necessarily represent anything. You could use 4 25u caps in parallel or one big 100u cap. Its just giving you the capacitance but the common way to achieve that 100u is by paralleling 2 50u's since its convenient to do in a 50/50 can. Maybe it makes a difference how many cqaps you actually have and a big 100 or 2 50/50's etc. sound different. But in theory the only thing that matters is the total capacitance.

For the screens supply in that schem you could use 1 1000v 50u can if you wanted. It would work the same though I dont know if it would effect the tone in any way or not. But the mains filter needs to be the way it is because its a voltage doubler configuration and as you can see in the schem the PT center tap goes to the junction of the 2 series 100uF capacitances. Thats the way a VD works.

what is the lowest voltage rated cap could be used for Screens, and mains? why not 600v? is there voltage spikes higher than that for a 50w?

awangotango wrote: ↑

Sat Oct 28, 2023 12:49 am

what is the lowest voltage rated cap could be used for Screens, and mains? why not 600v? is there voltage spikes higher than that for a 50w?

To be on the safe side I'd say 500v for 100w amps and 450v for 50w amps.

thanks. to that end. If one has dual 50uf /500v daly cans...the only time marshall ever used both sections for screens or mains (in a 50w) is to run them 'parallel'? (treating the two sections as separate), and use them for different purposes (one section for PI, the other for screens ). Marshall never did or would have used them in 'series'? like you see in a superlead where the two sections are jumped by the + terminals together, therefore doubling the voltage? as this apparently is not needed (450V). But if one did use both sections for one purpose, say screens, then the two jumped cans are now effectively a 1000v can

with capacitance (50uf+50uf = 25uf actual


and if one did this, what value resistor would be needed across those two + terminals? 10k/1W?

It's impossible to hook up a single Marshall style two condenser filter can in series since these caps only have three solder lugs (two positive and one negative). For the mains filtering on 50w solid state rectified amps Marshall ran one filter can in parallel so you'd end up with either 48uF/450v, 64uF/450v, 100uF/450v, or 100uF/500v depending on which era of amp your viewing.