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First build...now the questions.

Discussion in 'Shock Brother's DIY Amps' started by alias23k, Aug 7, 2009.

  1. alias23k

    alias23k Tele-Meister

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    A few months ago i decided to do my first tube amp build, read up (actually still reading up and there will probably be lots more reading) and what started to be a 5e3 build has become a scratch build from an electromatic tube chassis. This chassis is pretty much a bogen db-110 chassis which in all actuality is still a 5e3 build of sorts but with the different cap and resistor values, suspecting it to be a dirtier cousin. Once i learn the affects of caps and resistors i'll let you know if my suspicions are correct.

    Anyways, turns out the PT was bad (maybe...the rectifier burned bright red so i shut it off and proclaimed the PT bad) and ended up buying a philco PT with a 800vct and a choke to hopefully drop the B+ to 370v, followed by two 180uf/450v caps in series. I'll run the light bulb test at my first warm up to see where i need to be with my currents at the tubes.

    To my question (actually questions but isn't it always), the power rail calls 3 power resistors, 2 i can find, the third calls for a 18k/3w resistor. I've read that you should always replace resistors with the same values but with it be on the power rail and also different voltages, will this still be a factor. I know what my variables are with caps so i know i'm good (but maybe with altered tone) there if i need to sub any of those out.

    1) Can i sub the 18k/3w with a 10k/5w or 2w resistor on the power rail?

    2) Can i sub a 330ohm/7w cathode resister (power tube) with a 300ohm/7w?

    3) This question is about filter caps on the power rail. it's asking for three 20uf/450v and one 16uf/500v filter caps. Can i sub with four 22uf/500v caps?

    The basis for these questions are typical, will i burn anything if i sub these out and how will it affect the amps characteristics?

    Please remeber i'm a newbie when it comes to all this jargon. to give you an example i learned what the hell was the B+ maybe 2 weeks ago even after a couple of months of researching tube builds. 90% of my parts have arrived and i'm almost to the point to start soldering. i'm still drawing up my layout but i have the schematic which i will attach (though you may have to save to see the small print). you can see where i added the new components so let me know if i'm wrong anywhere. (don't pay attention to the tube pin voltages in the bottom left, they haven't been altered yet).

    And of course, thanks in advance for the replies.

    PS. A lot that i know now i learned from the guy that i bought the philco PT from. and no he's not a local guy, i bought it off of ebay ,gobble75 is his username, and has helped me immensly and i feel i need to give him a good plug.

    [​IMG]
     
  2. alias23k

    alias23k Tele-Meister

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    ok, so i just read up that the power resistors aid in discharging the filter caps, so i know now that the have little affect on tone. but not knowing the math, anyone else know the discharge time for safe handling of the circuit? will the light bulb test and a voltmeter be enough. i'm thinking i just increasing discharge time through the filter cpas by decreasing the value to 10k as opposed to 18k.
     
  3. alias23k

    alias23k Tele-Meister

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    Cathode resistor - large values reduce the gain, smaller values increase the gain.

    Guess i shoulda read up before i posted this.

    Is it bad when you have a conversation with yourself online?
     
  4. gayler

    gayler Tele-Meister

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    No, But make sure no one can hear you if you answer yourself!:D
     
  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Sometimes I have conversations with myself online, in the supermarket, or at the mall.

    The one place I try to avoid talking to myself is at the bar...
     
  6. celeste

    celeste Friend of Leo's

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    1. looking st the schematic, the only 18k/3w I see is a dropping resistor. If you reduce it to 10k, that is almost in half, you increase the voltage to all the tubes it feeds.

    You can increase wattage of a resistor with out worrying, if it is the same kind, the higher wattage will be quieter for a given current flow.

    As for the resistors discharging the caps, yes they do that, but their primary function is to set a minimum current flow through the choke. Since you are relieving on the choke to keep the voltage down by using a choke input filter instead of a cap input, you have to keep current flowing in the choke to get it to act like a choke. Otherwise voltage will rise from .9AC to almost 1.4AC

    2. Yes, but it changes the bias for the tubes, they run at a more negative bias, so less current, closer to class B and a bit more headroom before power tube overdrive sets in.

    3. Yes , 20uf 500v will be find in each position.
     
  7. alias23k

    alias23k Tele-Meister

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    Guess i'll have to rethink and reorder my #'s 1 and 2.

    I'm checking my voltages and currents to make sure i'm good with that. do i need to consider the 10% voltage drop after warm-up when considering this?
     
  8. celeste

    celeste Friend of Leo's

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    the bleeders should take care of that
     
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