Will a 400 volt filter cap for the preamp filtering in the board be high enough voltage?

Discussion in 'Building the Classics' started by Impetus, Oct 26, 2021.

  1. Impetus

    Impetus Member

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    Hey, is 400 volts high enough for the preamp filtering on the board in a 50 watt marshall? My other filter caps are 500 volts but I'm wanting to try a different value and all I have on hand is 400 volts. What's the "minimum" voltage requirement for that particular filter cap? I current have a dual axial 33+33(64) at 500 volts but I'm wanting to lower the value and use two 16uF 400 volt caps tied together instead
     
  2. neikeel

    neikeel Well-Known Member

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    Yes. You can go as low as 350v rating in the 50w preamp can but 500v mains and 450 screens/pi are sensible.
     
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  3. Impetus

    Impetus Member

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    Thank you! I was curious because on plate voltage on this one is around 465 but I didnt know if that would effect it. And my mains and pi/screens are at least that voltage if not all 500v. I've been having people tell me that lowering the caps in a 50 watter would be terrible but I think it sounds and feels so much better. I'm using 32 mains, 16 screens, 16 PI and about to swap the 64 preamp to either 32 or 16 and see how that goes. I also like mains at about 48uF and sometimes take the PI to 32 but i think this sounds great and the lower filtering seems to help combat some of the less than desirable 4x12 traits when you are standing directly in the speakers path. Not a bit of ghosting at any volume level. Sounds and feels so good (very forgiving too when you play)
     
  4. printer

    printer Active Member

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    Put a voltmeter on the node and turn on the amp. You will see the voltages involved. For testing with keeping an eye on the amp it would be fine. Long term I do not think it a good idea to have the preamp ratings less than the output stages. Before the tubes warm up the diodes rectify the voltage and with no current draw the voltage dropping resistors do not limit the voltage as normal. When the caps first charge up you will get a current to chatge them but once they are at voltage the resistors are not developing voltage across them. If someone turns on the amp without the output tubes drawing their load the preamp voltages will go higher than normal.

    Under normal conditions it may be fine, depends how bulletproof you want the amp. I want my voltage rating to be able to cover the voltage which would occur with no tubes in the amp.
     
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  5. Pete Farrington

    Pete Farrington Well-Known Member

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    Yes, the idle voltage on parts is only part of the picture, certainly not the be all and end all.
     
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  6. Impetus

    Impetus Member

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    Ok. I'm going tit ake your advice and try to see if I like it and then I'll order an appropriate voltage rating. Thanks so much! Oh, btw..unrelated but do you know what the difference is between the old Allen bradley carbon comps with a silver vs gold multiplier on the resistor? Did they just change from silver to gold one day?
     
  7. Pete Farrington

    Pete Farrington Well-Known Member

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    That band probably isn't a multiplier, silver = 10% tolerance, gold = 5%
     
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  8. mAx___

    mAx___ Well-Known Member

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    Silver and Gold are the tolerances. 10% and 5% if I'm not mistaken.
     
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  9. Impetus

    Impetus Member

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    Again thanks! I just had a guy that used to own a RadioShack sell me hundreds of a bunch of old Allen Bradley's for just 20 bucks so I took them. Most of them are 1 to 2 watts though but I've always used carbon film but I'm gonna give these a go just to see what I think. The leads on some are kind of thick though so I'll have to get creative when putting them on the board but should be easy enough. I figured 20 bucks for that and a days worth of "fun" just to try them out would be worth it. To settle curiosity anyways!
     
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  10. Impetus

    Impetus Member

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    Right, on thanks so much!
     
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  11. Impetus

    Impetus Member

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    I just got home and I cant seem to find the other cap for the preamp filtering I was going to use so I'm just going to have to order one anyways. This amp used to be a 1987 model but ive converted it to 1986 spec and the preamp filtering is a dual axial 33+33(64) with the 10k resistor between the two 33uFs in the dual cap and another 10k resistor AFTER both those caps too along with the one between them. All I have at the house to just try and see if I prefer lower filtering is a single 33 and a single 20uF so I'm going to solder their negative leads together and then attach their positive leads in their locations with the 10k still between them. Basically, my question is will using 2 different values throw anything off having that 10k between them or would it still work as a 53uF like how my dual 33 works as a 64uF cap? I realize this is a very small difference but I'd like to see what about 15 less uF would do there. The 64uF cap actually reads around 70uF. Also does anyone sell a dual 8uF cap for this position..I cant find one on the internet but I found a dual 16 I'm going to buy but would also like a dual 8uF to try in that position...

    Sorry for such a long winded question
     
  12. neikeel

    neikeel Well-Known Member

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    Two different caps are no problem
    I would not go lower than 16 in any element
     
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  13. Impetus

    Impetus Member

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    Thank you! Do you mean wou
    Thank you! You mean you wouldnt go lower than a dual 8uF for 16uF total? Because after trying the dual 16 then the only other one I plan to try is a dual 8 for 16uF total..
     
  14. neikeel

    neikeel Well-Known Member

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    Why use two 8uF caps in parallel if you have an appropriately rated 16uF.
    By putting a dropping resistor between the two it is no longer considered a single capacitor.
    Personally (other than on an original vintage JTM45 which has a single 16uF) I would stick with 32uF/10k/32uF in preamp. I would not go too low on the mains (your rectifier, ss vs valve will decide that) 50uF is minimum, and the size/rating of your choke. What affects the feel (at least of a Marshall 4-holer) is the screens filter, again 16uF is as low as I would go and from your B+ quote 500v would be sensible. The pi can be 32 or 50uF - you probably will not notice much of a difference.
    Lowering filtering is a compromise between a soft feel to the guitar/amp vs spongy low end and ghosting.
    Maybe we should just let you experiment for yourself, although I did this a while ago and just sharing my experience/opinions
     
  15. Impetus

    Impetus Member

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    Hey, I was wanting a dual 8 because the way it's all wired up is with a dual axial 32+32 so I dont have to remove anything or do anything extra like removing the 10k resistor etc. Also, I've played with the filtering besides preamp quite a bit and the only reason I keep going so low is for the more forgiving spongy low end. I have an old fender choke and some vintage OT that I'm not entirely sure what it is. But I'm not getting any ghosting at all with the low filtering and it's only that way because I think it "feels" best. I said earlier that sometimes I bump the mains from 32 to 48uF and that i raise the PI every now amd then too like you mentioned. But this is the way I think the amp feels best.

    Also the amp is actually putting out around 35ish watts clean. So I dont think the its a 50 watt transformer and I always go back and forth between jtm and 1986 and I just dig this filtering...the amp is SS rectified btw.

    But yeah, I've just been wanting to use the dual axial like the 8+8 because that's just the format that's there already. Theres the 10k resistor between the two caps and then another 10k resistor right after the two caps but that's from when I built this to be a 1987 model and I'll be honest, I'm not really sure what BOTH of the 10k resistors are for other than the 1987 schematic showed them there so I put them there. I dont think the bass model amps have 2 10k resistors there so maybe I should remove?


    Oh, I just remembered this. I also went with low filtering because the guitar kept sounding closer to hendrix at fillmore east the lower I went lol (to me at least) and when using my 4x12. I realize he wasn't using a 1986 amp and with there being so many variables its hard to say if it was even the jtm45/100 or whatever else but I digress. I just know the amp was at least sounding closer to the recording which is the type of thing I'm after with what I currently have anyway
     
    Last edited: Oct 27, 2021
  16. neikeel

    neikeel Well-Known Member

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    The 10k droppers are there to regulate the voltages to the preamp. The bigger the value of the resistors the lower the B+ voltage down stream. If you want a browner, lower headroom amp then keep the resistors if you want clarity etc you can remove one of the 10k or temporarily clip a bus wire across one of them - not both.
     
  17. Impetus

    Impetus Member

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    Heres a picture and yeah the second 10k that's bridged to the first 10k resistor/filter cap is going to v2 preamp tube and connecting to a 100k resistor there. Which one should I remove for path of least resistance? I thought I would need the 10k resistor BEFORE the 2 caps if I was only using one of the 10k? And for some reason I was thinking that if using a dual capacitor instead of a single cap that the 10k would have to go between them...but that's not correct? I think that's usually the way I've seen schematics so I just figured that for a single cap in bass spec amps the 10k goes before it and for a dual cap the 10k goes between the two caps. And then I've also got a schematic like the one I used to build this which was a 1987 with how they are now with one after the caps too


    I am thinking I should take out the second 10k completely and just put a jumper lead where it was and move the blue wire connected to it to the first connection of the first 10k where the cap and resistor are soldered together? Leaving the first 10k between the caps?

    https://imgur.com/a/wWdU5re
     
    Last edited: Oct 27, 2021
  18. neikeel

    neikeel Well-Known Member

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    No I mean bridge one of the 10k between PI and screens (next to NFB) leave that one as is for now.
     
  19. Impetus

    Impetus Member

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    Ak ok. The ones going to the choke? I should have a single 1 watt 10k I can pop in there to span the whole length and just take out both of the shorter 10k's leaving the middle post untouched...that works? Thanks! I dont really have any other questions if that's correct but thank you so much for the time you've spent helping me!
     

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