18 watt build help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tfletchii, May 8, 2015.

  1. tfletchii

    tfletchii Member

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    New build I"m working on and would love some help.

    I'm blowing a fuse on power up so it has to be either the diodes (ss rectified) or the power transformer so I'm guessing I have the power transformer wired up wrong. I'm using the Mojotone 18 watt power tranformer and I'm using the 18 watt lite iib (baby will) PCB . no tubes installed. I have it wired like this...

    0v (black) run to the neutral on the ac in.
    120v (black/white) to hot on ac in
    the two 290v (red) run to T1 and T2 on board
    the two 6.3v (green) to T5 and T6 on board
    the red/yellow to ground
    the green/yellow to ground
    then I have the 6v 2amp (white), 0v (yellow), 5v 2amp (yellow/white) and the 240v (black/red) all tied off connected to nothing.

    here is a link to the power transformers wiring layout.
    http://www.mojotone.com/transformers/BRITISH-18W-PT.pdf

    18 watt lite iib (babywill) PCB
    http://www.guitaramplifierpcbs.com/images/version 2_3.jpg
     
  2. Blue Strat

    Blue Strat Member

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    The photo of the board doesn't help a lot. A schematic with T# markings would be more useful.

    I would disconnect EVERY secondary wire from board, or wherever it's going, and try again. If the fuse stops blowing, the transformer and primary wiring are ok. Then start connecting the secondary wires one by one, testing after each new connection. When the fuse blows again you'll know where to dig deeper.
     
  3. tfletchii

    tfletchii Member

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    Thanks I'll do that soon. T5 & T6 go to the tube heaters. T1 & T2 go to the diodes (solid state rectifer)
     
  4. tfletchii

    tfletchii Member

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    Removed the secondaries and the fuse isn't blowing. Any ideas on what I'm doing wrong?
     
  5. tfletchii

    tfletchii Member

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    reattached the (green) 6.3v to T5&T6 and the fuse didn't blow so it has to be the way I've wired up the (red) 290v to T1&T2. but now I'm stuck.
     
  6. Blue Strat

    Blue Strat Member

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    Those would connect to the diode rectifiers, right? If so, are they oriented correctly? Are the filter caps oriented correctly? Etc.

    Post a photo of the schematic and the stuffed board.
     
  7. tfletchii

    tfletchii Member

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    best I can do on the schematic I've had to adapt the solid state rectifier. guitar amplifierpcb.com doesnt have a schematic for this build. Frustrating.

    Schematic:
    http://mhuss.com/18watt/schematics/18wattLite2b.gif

    I'll post pictures of my board in a second. thanks again
     
  8. tfletchii

    tfletchii Member

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  9. tfletchii

    tfletchii Member

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  10. Avatar Tech

    Avatar Tech Member

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    Your PT is set up for a 2 phase rectifier, which means that it has a center tap that you would normally ground, like when using a valve rectifier. That pcb looks like it's set up for a bridge rectifier. Usually with a bridge rectifier, the HT winding doesn't even have a center tap. I think you could just lift the center tap for that winding. Try and have someone else confirm before attempting because I'm not 100% sure.

    Colin
     
  11. Xlch721

    Xlch721 Member

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    What version is you Baby Will board? I have the Valve JR conversion PDF and all of the scratch build docs for your PCB. Just PM an email addy and I will send them to you. That or go over to the 18 watt forums and ask there. The Watkins family of forums is where those originated from great bunch of folks over there.
     
    Last edited: May 8, 2015
  12. tfletchii

    tfletchii Member

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    THanks xlch721. That might help. I'm with avatar tech in that it's got something to do with the way the transformer is tapped and my misunderstanding of it.
    I have the baby will 2.3.1 and id love any documents that you have because they don't have a schematic or wiring layout on their web page
     
  13. xtian

    xtian Member

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    Build yourself a light bulb limiter so you stop blowing fuses and putting stress on the PT.
     
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  14. Xlch721

    Xlch721 Member

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    Here you go
    http://www.guitaramplifierpcbs.com/liteiib.html

    You should be able to find all the info you need here. Last time I checked the site it was down but it looks like Terry's Wife has everything back up and running now.
    Terry was tragically killed in a motorcycle accident shortly after all of his PCBs were released but his wife kept things going to honor his memory. It was a sad day for all of us that had the pleasure of knowing him. He was a good man very helpful always went out of his way to help his customers both online and on the telephone.

    Make sure you post some clips when you get it all done. I have been rocking one of these builds for years now its a great amp for very little investment. Built mine with an EZ81 Tube rectifier and its a beast but the current drummer weve been jamming with hits like John Bonham and drowns my 18 watt and a DRRI out so I had to step up to something bigger to play over his hard hitting. I almost have my new amp completed and then I am gonna drown his butt out with a 50 watt JMP built to 68 lead specs. LOL The whole band had to go out and buy new gear to play with this guy but he is one og the best drummers I have ever played with so it was worth it.
     
  15. Xlch721

    Xlch721 Member

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    Make sure you download all of the files for both the conversion and the scratch builds that way you have everything. Between both sets of docs it should be a really easy build and thats what Terry's goal was with the PCBs. Some of the conversion file will be useless for a scratch build but the rest will walk you right through it.
     
  16. Xlch721

    Xlch721 Member

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    I have to agree its the next best thing when you dont have a variac handy to use and the light bulb will blow before the fuse or Tranny will go out. Heck i built mine with cheap parts from Wal Mart in the middle of the night so I could fire up my first build. It was the same amp your building now. LMBO
     
  17. tfletchii

    tfletchii Member

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    It is a very easy build. I think if I can figure out how to use this transformer everything should fire up fine from there. I have those documents but there isn't a schematic or a wiring layout for the baby will there is for the superlite. Again, I don't think a layout will help because it will be for a different transformer. I'm going to talk to the folks at mojotone (who made the transformer) and see if they can help me out.

    Thanks for all the help so far everyone. Its been nice
     
  18. Avatar Tech

    Avatar Tech Member

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    You really need to figure out which type of rectifier that pcb is using. I suspect it's a bridge rectifier because those pcb's were made to convert the Valve JR. The Valve JR appears to have used a non center-tapped transformer (requiring a bridge rectifier). Using this type of rectifier with a grounded center tap will blow fuses. You could lift the CT but your voltage will be higher than you want it most likely.

    Colin
     
  19. Xlch721

    Xlch721 Member

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    The docs tell you how to use both types of rectifiers I built mine with a tube and an overspec tranny so i had to add a couple turret strips and 3 6.3 volt zener diodes to drop the B+ about 20 volts.
     
  20. Xlch721

    Xlch721 Member

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    I have the schematic and the layout for a scratch build on my Mac if you pm me your email addy I will gladly send them to you. I checked the site and I see now that those docs are not on the site. Sorry about that I figured they were there but now I see that they are not posted there. I will have to lookup your power tranny to see what the layout for it is

    OK the 2 red wires are your HV windings 290v the red with yellow strip is your center tap if used it should go to ground these go to the diode rectifier
    The 2 green are for your heaters 6.3 volts the green with yellow is your center tap these go to the pcb if you need to use a Center tap the striped wire should go to ground
    The yellow wires are for running a tube rectifier with the striped wire being the center tap. if your not running a tube rec you wont use these just cut the stripped wire off and cover them with heatshrink tubing
    For 120v Mains you use the Black and the Black with white stripe
    240v mains you would use the Black and the Black with Red stripe
    I can tell you right know that with that power tranny your B+ is going to most likely be to high so your gonna have to to do the zener diodes on turret strips to get your B+ inline the schematic for the scratch build and the conversion used either the Valve Jr or the MPS tranny that specs at 275 v on the HV windings . Your Tranny is like the Classic tone model I used 290v so your B+ is probably going to be 20 to 30 volts to high and your tubes will redplate. If I remember right this build uses an artificial CT but is easily modded to use the CT on the Tranny by installing a Jumper wire and not using a resistor. Your best bet is to go to the 18 Watt Forums and ask around about your build.Thats what I did when I had the the high B+ issues .
     
    Last edited: May 8, 2015

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