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Mojo 5F11 Kit - Rookie Build

Discussion in 'Shock Brother's DIY Amps' started by GTG_Gopher, Feb 25, 2021.

  1. GTG_Gopher

    GTG_Gopher Tele-Meister

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    Bought a Mojotone 5F11 kit about 16 months ago. Got off to a solid start, but then life started to throw a couple nasty curve balls my way. So the amp has been a semi permanent part of my workbench. The life happens stuff has been clearing up, so time to really dive into it and get it finished.

    While this is my rookie build, I've had some expererience with electronics and a very healthy respect for high voltages. I've also had plenty of time to pour over Rob Robinette's excellent material on amps.

    I've made a few small mods from the Mojotone plans.
    1) I put the fuse and switch both on the hot lead from the wall
    2) I added backup diodes to the rectifier socket
    3) I moved the resistors for the heater's artificial center tap to 1st 6V6 socket from the pilot light socket.
    4) I flubbed up drilling the hole on the eyelet board for the mounting bolt on one end. I wasn't super thrilled with that set up to begin with, so i took the opportunity to change it up a bit. I got some nylon stand offs to go between the backer board and the eyelet board. That way I can leave the backerboard mounted to the chassis while being able to remove the eyelet board a little more easily.

    I've been using the manual from the Mojotone 5E3 kit, and the Stew Mack 5E3 kits as a guide for assembling mine, for the parts they have in common.

    So I'm at the point where I need the second set of eyes on some things.

    The eyelet board, front and back jumpers

    Board Front Left.jpg Board Front Right.jpg Back of Board 2.jpg

    I have a couple jumpers that I need to rework, either wrong length, or in one case the cloth insulation came off.
    Also, while soldering, I nicked one of the filter caps. Is that something that will be a problem? Nicked filter cap.jpg

    On the power tubes, pins 1 & 8 on each tube needed to be grounded to the chasis. I had already used the small grounding lugs on the power transformer bolts, so I took the larger 3 position lug and used that to create a mini ground bus bar for those grounds. is that something that's going to cause issues?
    Socket Wiring.jpg
    And switch, fuse, and pilot light wiring. Power Wiring.jpg

    So, hoping everything is looking ok, but being the rookie builder, wouldn't be shocked (no pun intended, ok a little pun intended) to have some additional rework to do.

    Thanks in advance for any comments and suggestions!
     
  2. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Looks like it may work. Please take what I am about to tell you in the kindest most helpful way possible. I know you are learning and I mean no offense.
    The v2 cathode resistor is not soldered to the bypass capacitor. It is also better to solder the resistor and the capacitor to the eyelet so it will be easier in the future to replace the capacitor. When using radial parts like the orange drop capacitors you must not break the epoxy where the case meets the lead. That is the hermetic seal for the part and it keeps moisture out. Yours are all broken.
    I see long exposed lengths of wire where the transformer leads meet the rectifier tube socket. I would shorten them, a 1/16” is permissible. Hold the wire with tweezers before the insulation to prevent melting the insulation.
    I would also get paraffin sealing wax and seal the parts board before installing it. That will prevent moisture from absorbing into the fish paper. Also grounding pin 1 on the 6V6gt sockets is unnecessary unless you plan to run metal case 6V6 tubes. Other wise save pin 1 as a mounting point for grid stopper resistors. I wish you luck on the rest of your build take your time and have fun.
     
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  3. Jazzerstang

    Jazzerstang Tele-Afflicted

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    I made one of those about a month ago BD84F3B1-2857-4801-B759-8DFF32653E15.jpeg 946AA382-D6AB-4196-BFEF-8CDC202DEB58.jpeg
     
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  4. The Ballzz

    The Ballzz Tele-Afflicted

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    Nice start! Many do it and even more might argue with me about it, but I would NEVER use the transformer bolts for grounding, for a multitude of picayune reasons. Instead I drill a hole with a bolt for the lug and on a steel chassis, I use a big honking iron or soldering gun to solder those lugs to the chassis!

    Also, not a real fan of your soldering, especially evidenced on the bottom side of the board, although I'll sheepishly admit to coming close to that on some occasions! Look especially at those green under board wires that are touching multiple connections and only insulated by fabric. Not the way I would want to leave it!

    And yeah, although it's likely OK, I'd probably replace that "nicked" cap!

    Not trying to offend, just bustin' your cojones enough to encourage you to more closely inspect, think about and critique your own work. It's never fun going back in after the fact to correct something you knew fairly well might bite you in the wazoo! You know what its supposed to look like and should put a bit more thought in getting it to be a bit closer to that way!
    Keep Us Posted!
    Gene
     
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  5. JuneauMike

    JuneauMike Poster Extraordinaire

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    I see a lot of what could be cold solders, just like my first 5F11 build.

    I did mine paint-by-numbers not really understanding the circuit at all and I used one of those cheap Weber solder wands that was not really up to the job. It worked sorta, but I wasn't that happy with the kit. So I spent a ton of time at @robrob's site, I built a 5F1 from scratch and scrounged for everything I could find on the Vibrolux. Then I completely rebuilt my 5F11 from scratch, putting some more thought into all the components. Got a proper soldering station, and got good at desoldering and prep work (remember, heat = speed). It is the amp I am most proud of now.

    A couple years too late for me, but you're in luck because one of Uncle Doug's best videos was a real 5F11 that he serviced. Everything I needed to know about that amp, and it was all free!





     
    Last edited: Feb 26, 2021
  6. The Ballzz

    The Ballzz Tele-Afflicted

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    Uncle Doug absolutely rocks the universe. Sadly, I think his last couple/few videos show him to be winding down a bit! Maybe he's getting bored with shooting all the details, or maybe he's just getting on in his years. His instruction and information sharing style has always been amazing and even entertaining, with a wonderful sense of humor! I'll be certainly sad to see the day he shuts down! Might not be a bad time to start archiving some of his best and legendary stuff?

    His most recent Morley Oil Can Echo vid was kinda lame. He skipped over a lot of operational details that could have been quite informative. Of course, the device design was kinda lame, right from the get go! Interesting, but not really very useful for much! On the other hand, the 1940s DeArmond tremolo from a few months back was pretty slick! I'm one of the first to notice when folks start going down hill, 'cause I'm gettin' pretty close to hittin' that brick wall myself. I do so love using guitars and amps to Screech & Squawk! It's pretty much been the greatest love of my life!
    Sorry For The Ramblin'
    Gene
     
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  7. GTG_Gopher

    GTG_Gopher Tele-Meister

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    No offense taken here, after almost 20 years with my ex, this is child's play lol!

    @J. Bonkosky This is the feedback i was looking for... No one likes to hear that their baby is ugly, but rather find out from you guys than from physics and chemistry being the teacher when I power it up.

    The leads for the cathode resistors were not quite long enough to make it from eyelet to eyelet, so thus the wrap around the bypass cap leads. Not soldering that one is a D'oh.

    I'm guessing those orange drop caps will need to be swapped out now?

    The gap on the PT wires to the rectifier are now on the rework list.

    What are peoples' experiences with drilling the chassis from Mojotone? I thought about doing that, but i thought there was a note from Mojotone saying not to do that, could mess with the chrome plating. Agree that mounting the ground lugs with the PT bolts is not the best design.

    I have a decent soldering station, but looking at my photos on my big screen I see that the joints are kind of ugly. I have a fine point tip on my iron, would a chisel tip be more appropriate for this job?

    The silver lining in this is that I get some more time to break in the speaker, have it mounted in the cabinet and can use as an extension speaker for my little monoprice amp!

    Thanks again guys for the feedback!

    Peter
     
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  8. mrfitz98

    mrfitz98 Tele-Meister Silver Supporter

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    I drilled a hole about 1.25" from the hole for the pot (towards the front of the chassis) for my filter and CT grounds, and I drilled a hole on the end of the chassis neat the PT for the mains ground. Neither one of those places had chrome and with a sharp bit there wasn't any trouble.
     
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  9. OldPup

    OldPup Tele-Meister Ad Free Member

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    If you don't mind, I have a few questions.
    1) I can guess, but what tips you off that the seals are broken?
    2) I assume the amp could be fired up with that epoxy seal is broken and sound fine at first. If that is true, how long would it take the caps to start leaking DC, roughly?
    3) If the problem is ignored (which I am NOT suggesting!) what symptoms would the amp display? I'm interested particularly in how it would sound different, but I'm also interested in what other problems might typically result.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I like your attitude. :) If you do replace your OD caps, the good news is nice but inexpensive Mallories or equivalent will fit waaaay better and make it much easier to read values. If you use that type, let us know and we can talk about the simple way to test for the open foil — or splurge on the pretty Sozo or Jupiter type where it’s marked.

    Yes to chisel tip.

    A neat trick for the output cathode grounds is to make them using 1% 1ohm resistors — 1/2W size fit well — so you can measure cathode voltage in mV across the resistor and realize that if I = V/R, I = V when R is 1, so mV = mA. Makes bias calculation *super* easy fast and safe. I ground each one separately (ignore the yellow tip jack for a moment)

    upload_2021-2-26_9-15-10.jpeg

    but you could replace your bare wires with resistors going to your shared tag strip. Do make sure the leads don’t pass too close to the other socket lugs.
     
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  11. GTG_Gopher

    GTG_Gopher Tele-Meister

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    Ok, this makes sense. Looked again at my chassis, and the side where the pots and input jacks go is a separate piece from the rest of the chassis and appears to be spot welded on.

    And as I think more about how the chassis sits in the cabinet, the better this way looks. it seems like a lot of weight hanging off the PT Bolts when it's sitting there sideways, and I'm not a mechanical engineer, but I think you would want to avoid anything getting between the wall of the chassis and the nuts holding the PT to it.

    Curious, do people ever put a thread locker (Locktite) on the PT nuts?
     
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  12. GTG_Gopher

    GTG_Gopher Tele-Meister

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    I ordered up a new filter cap to replace the one that got damaged. I also ordered a set of Mallories to replace all the Orange Drops that I cracked open. That should also make it a lot easier to check values like @King Fan said. I think it was just a hair over $20 so a relatively cheap lesson that will double as a slight upgrade.

    I really like that idea of using the 1ohm resistors for the cathode grounds. All for making things safer where I can.

    Since it's going to be a few days before the new caps get here, I think I'm going to use the old ones and practice up on my soldering skills here.

    Stay tuned!
     
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  13. JuneauMike

    JuneauMike Poster Extraordinaire

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    deleted
     
    Last edited: Feb 26, 2021
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yes. Even if you wisely avoid grounding to PT bolts. Even on separate ground bolts, it's smart to use a star washer under *and* keps nut over *plus* thread locker. The mains/household green ground is even more important (and should be even more separate, mechanically and electrically.) So the idea by @mrfitz98 is ideal. Here's an example of the sidewall safety ground; you can barely see the blue loctite but it's there.

    upload_2021-2-26_11-8-50.jpeg
     
  15. pugnax

    pugnax Tele-Meister

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    Why not ground the power side to the transformer bolt? That's the way it's done in most of the examples I've seen from the Hoffman site and elsewhere.
     
  16. The Ballzz

    The Ballzz Tele-Afflicted

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    If it were me and I'd made enough of a mess already of the board, components, etc, I might go ahead and continue soldering practice (after getting a different tip or two) on that board, while waiting for a new eyelet board, or even better yet, go with a turret board! I very much dislike tag/eyelet boards.
    But That's Just Me,
    Gene
     
  17. JuneauMike

    JuneauMike Poster Extraordinaire

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    I love turret boards too. I just think they look cool and are easier to make multiple connections to. The trick that I always heard about Fender's eyelet boards is that the gals soldered the backside of them instead of the front. Made the solder joints on the front look beefy and uniform. FWIW.

    You should get yourself a solder sucker and some desoldering wick (my personal favorite) if you don't already have it. And practice desoldering too. It's pretty amazing how handy that skill is in amp building. You could wick away some of those cold solders and then burn in the rest with a really hot chisel tip and you'd be golden.

    Have you fired it up yet?
     
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  18. JuneauMike

    JuneauMike Poster Extraordinaire

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    I never have, but I should. The PT can vibrate, not like a Harley Davidson, but there is stuff going on in there at some frequency or another. Locktite and star washers would be cheap insurance.
     
  19. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Hey Old pup!
    1) the epoxy should look as if it flows into the lead. Like a good solder joint. These have an abrupt plateau because they are broken.
    2/3) Yes you can start the amp with the seals that way. The caps may be fine for years it depends on the environments the amp is exposed to. Lots of moisture in the air (like Seattle) they may not last long. Then you start to get interstage DC in your signal. Crackle, dull lifeless tone, weird distortion, etc...things you most definitely don’t want.
     
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  20. GTG_Gopher

    GTG_Gopher Tele-Meister

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    Over the summer I picked up a Weller WES51 soldering station. It came with a pretty fine point tip. I think that's where I was running into trouble, it worked ok on socket pins, but couldn't heat up an eyelet fast enough to get a good connection. my solder is 60/40 Rosin Core. I have desoldering wick, and a solder sucker

    I can see why at the Fender factory they soldered the back of the boards, thought about doing that myself as I was assembling things. Would have saved that filter cap.

    Definetly have not fired the amp up yet! Still need to assemble my light bulb current limiter and wire up the filaments once the board is squared away.
     
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