1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Turning a Champ into a Champ Micro

Discussion in 'Shock Brother's DIY Amps' started by Goudacris, Jan 24, 2021.

  1. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Hey all,

    I've been working on transforming my first project (5f1) into a Champ Micro (https://robrobinette.com/Champ_Micro.htm).

    The first build was so poorly wired that I wanted to pull it apart for a rewire, and while reading up to try and be sure I did a better job, I became interested in the Micro, mostly for volume purposes.

    The rub: I'm determined to build in the 5f1 chassis, and the 270AX I bought does not drop in (I was under the impression it would, but now I'm not sure how I figured that...)

    Anyway, I'm very new to the this, so I may need you to talk down to me.

    Currently I'm wondering: Can I use a 290AX for this build? If I do, would I use the red/wht 275v HT and the green 6.3VCT with the green/yellow CT?

    More info: I plan on using the 5y3 rectifier, and 2 12AX7s while I wait for my 12AU7 in the mail. My OT is a 1750A, and I didn't plan on wiring the boost or NF switches or the headphone jack. And the 470K resistor that's on the board, I was planning to just stick on a hookup and heat shrink tube to cover it.

    I'd love feedback! Thanks!

    The future: When buying supplementary parts for this build, I also bought most of what I need for a BF PR that I'm planning for next. The 290AX would be great for that, so if anyone has a suggestion for mounting this 270AX in the 5f1 chassis with some kind of adapter, or just having it be ugly mounted with new holes and a big gap, I'm all for it. It's going to be in a cab behind a panel anyway... But the cleaner the better! Thanks!


    290AX spec from hammond:
    [​IMG]

    [​IMG]
     
    guitarmikey likes this.
  2. dan40

    dan40 Friend of Leo's

    Posts:
    2,149
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    You could use the two red secondary leads on the 290AX for a higher B+ voltage if you like but I would probably go with the red/wht secondary wires for a lower B+ voltage in the Micro amp. Ground the red/yel centertap to the chassis or directly to the main filter cap's negative lead. The second method helps reduce hum a bit more than grounding it directly to the chassis. For the filament circuit, you would connect the two green wires to your pilot light and then on to your power tube. The grn/yel centertap will ground to the chassis.

    You mentioned that the 270AX does not fit the chassis. Is the mounting bolt spacing too large or too small for your chassis?
     
  3. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Thanks for the reply!

    The 270 would need a smaller hole. 290AX screw hole distance is 2.5"x2" vs 1.75"x1.5" for the 270AX.

    Glad to know the 290AX can work. About the 5y3 rectifier, on Rob's site he says:
    "If your power transformer does have 5V available and you'd rather use a standard, full size 8 pin rectifier tube like a 5Y3GT then just wire the 8 pin socket with the HT to pins 4 and 6, 5V heater lines to 2 and 8 and the B+ power line to pin 8."

    Does that mean I have to use the 290AX? I don't believe the 270AX has 5v heaters?
     
  4. dan40

    dan40 Friend of Leo's

    Posts:
    2,149
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    If you want to use a tube rectifier then yes, you will need the 290ax. If ss rectification is fine, then the 270ax will work.
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,302
    Joined:
    Dec 29, 2012
    Location:
    United States
    Without a 5v secondary you could use the EZ81. It's ok to use common 6.3v on all the tubes including the EZ81. The 275-0-275 + EZ81 should work out fine for the Champ Micro.
     
  6. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Hey Rob, thanks for the reply.
    I decided on the 5y3 because I happen to already have one, and it's looking like the best option since I'll be using the 290AX because of it's footprint.

    I'll post some pictures of my progress later this evening.
     
    robrob likes this.
  7. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Do the diodes on 1+2 and 6+7 need a different home as well if I'm using the 5y3 with red/white HT on 4 and 6 and the 5v on 2 and 8?
     
    Last edited: Jan 25, 2021
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,302
    Joined:
    Dec 29, 2012
    Location:
    United States
    Wire your 5Y3 like this:
    [​IMG]
     
  9. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Troubleshooting time!

    I don't have my voltage readings handy, but they appeared to be within somewhat normal ranges. I can certainly post them later if you think they'll be helpful.

    I'm getting what I believe is an oscillation noise when the Master is up passed 9. The frequency of the oscillation changes when turning the other volume knob.

    Here's a video:

    A couple things: I did not include any of the switches or jacks for mods, but the only small component I deleted was the 10k headphone resistor, which I replaces with a jumper.

    Pictures of the build:
    [​IMG] [​IMG] [​IMG] [​IMG] [​IMG] [​IMG]
     
  10. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Thanks, this was super helpful!

    I'm adding voltages, also wanted to add that I've been chop-sticking around and I think it's coming from the v1A cathode bypass area. Touching around those leads causes change in sound when in oscillation. I did use a 50v cap here, because that's what my 5f1 scheme called for, but I do have some 25v's I could try.

    I also wanted to call attention to the shrink wrapped wire on the back: this is my 470k v2 grid stopper which I didn't have room for. I thought running it under the board like this would be cleanest, but I wondered if my scrappy solution was actually not good.


    B+1 398
    B+2 313
    v1
    1 208
    3 1.5
    6 202
    8 1.45
    V2
    1&6 397
    3+8 7.28
     
  11. Bitsleftover

    Bitsleftover Tele-Holic

    Posts:
    515
    Joined:
    Oct 17, 2016
    Location:
    The North of England
    Hi,
    Sorry to hear you’re having problems.
    I went through a similar process a while back and it can be soul destroying, but you will solve it eventually.
    Not that i claim to be an expert at all.... But it looks to me as though you’ve created quite a complete ground loop right around you input jack where the signal is weakest. Maybe get rid of that white wire. (Coloured in red) and connect your V1 bypass cap directly to the ground turret for your cathode resistor (Coloured green)
    Does it make that noise when you have a guitar plugged in? I’d check everything around the input jack wiring next. 5AF484E5-E629-4CAB-9434-C04824B82E7D.jpeg
     
  12. Bitsleftover

    Bitsleftover Tele-Holic

    Posts:
    515
    Joined:
    Oct 17, 2016
    Location:
    The North of England
    Actually, shouldn’t that 1meg resistor ion the input jack go from tip to ground? Not to the switched connection?
    I think you have 1 meg between input and switched ground instead of grounding out your input signal.
    That could be why you are getting the noise with no guitar plugged in. I don’t have a switched Jack in front of me to compare with your photo so i could be wrong.
     

    Attached Files:

  13. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Looking at Rob's layout makes me think you're dead on with the resistor needing to go T to G. I'll try that and report back!

    I actually had no signal when I finished my 5f1, so at least we were makin' noise!
     
  14. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Update: Moved the resistor on the input jack from T-S to T-G, cleaned up as much dress as I could without major de-soldering. Not sure what's next. I suppose I could try disconnecting that white wire, like you mentioned @Bitsleftover. Is the reason Rob has it laid out the way he does because having them separated allows for the switch to only engage or disengage the cap? I suppose I could try taking the cap out of the circuit all together. I have tried laying one on it in parallel using alligator clips, but all that did was drop the frequency of the oscillation.

    I'd really love to understand the problem once we find the cause. It seems like a distinct and consistent problem. Master passes 9: loud oscillation, volume and chopsticking v1a bypass are the only things that impact sound when oscillating, oscillating happens both with guitar signal and without.
     
  15. Bitsleftover

    Bitsleftover Tele-Holic

    Posts:
    515
    Joined:
    Oct 17, 2016
    Location:
    The North of England
    D9CAB09C-CDA3-4AC5-8845-5C1C2789E46D.jpeg
    Yep. Exactly That. Without the switch it may as well be connected directly across the cathode resistor.

    Jumping the gun a little, but I had a similar problem, only high pitch squeal with volume above 6 or 7 on my bassman micro build. I convinced myself it was the first gain stage and cathode bypass cap by chopsticking around but in my case it turned out to be a heater supply centre tap issue. I elevated it even further (see robrobs amp mods section) and it solved it completely. That might be a bit extreme just now but certainly check your centre tap or artificial centre tap wiring and reflow your solder joints throughout.

    I’d also try and route that filthy unfiltered AC from your fuse to your power switch on the back of your MV way away from the signal wires.

    Good luck. Don’t get disheartened. You’ll solve it eventually.
     
    Last edited: Jan 29, 2021
  16. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Great notes, I'll get on those tonight! The lower wire is a bit of a problem... Unfortunately that black lead from the PT to MV is pretty taught, and the other (black to white) is even worse. Any advice on lengthening that? Is it worth trying to solder and shrink an extension to it to get some more give? If there a best practice on extending clipped leads? I didn't forsee reusing this PT when I put it in my 5f1 initially, so I clipped everything way too short to be comfortable in the repurpose...
     
  17. Bitsleftover

    Bitsleftover Tele-Holic

    Posts:
    515
    Joined:
    Oct 17, 2016
    Location:
    The North of England
    The black one probably won’t make much difference as it it. The bit I would focus on would be where the white one from your fuse comes close to the signal wire from your Master Volume. (Circled in blue)
    Id just pull it away with a chopstick and see what happens. (Somewhere away from everything like marked 98D6D692-A5D5-4189-B0E5-279295D64688.jpeg in red)
    Hopefully someone with more experience will jump in with some more suggestions over the weekend.
     
  18. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Groovy, I'd love to see pics inside your Bassman, especially of the elevated CT. I think I understand it from the mod page, but seeing it would be cool.
     
  19. Bitsleftover

    Bitsleftover Tele-Holic

    Posts:
    515
    Joined:
    Oct 17, 2016
    Location:
    The North of England
    I had a look through my pics but cant find any. If get chance Ill take the back panel off and take a snap.
     
  20. Goudacris

    Goudacris TDPRI Member

    Age:
    30
    Posts:
    13
    Joined:
    Nov 28, 2020
    Location:
    21701
    Moved the AC wire, reflowed solder on the HT center tap to (red/yel), removed redundant bypass ground. Starting to feel tempted to pull the board out. At this point I'm worried I might have accidentally flowed solder through a turret and on to the chassis. Would probably try the elevated tap before pulling the board. @robrob I'd love your thoughts on the noise I've got going on.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.