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Bassman Micro Build Thread (w/ Pictures)

Discussion in 'Shock Brother's DIY Amps' started by jchabalk, Nov 21, 2020.

  1. jchabalk

    jchabalk Tele-Holic

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    For the Phase Inverter Master Volume in this circuit - is there any reason to use a push/pull POT to completely remove it from the circuit when not in use, or is a standard 1M Audio pot a good choice here. It's pretty much the easiest thing to swap out. I have mine mounted on the rear of the chassis so i just need to fly a couple of wires over the circuit board. I tested it out with some alligator clips a few days and and it seems to work fine.

    I'm thinking - and i don't know why i didn't do this out of the gate - of adding an impedance selector switch to the rear here. Anyone have any advice on where i should:
    • add a selector switch (which means drilling another hole)
    • reclaim the line-out jack and wire it as a second impedance speaker output
      • I'm leaning this way as i hate drilling a hole in completed amps if i can avoid it and i don't think i'll use the line out.

    [​IMG]
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If you go the dual speaker jack--dual impedance route you will have to disconnect the main speaker jack shunt between the ground and switch (shunt) terminals shown on the layout above. You will lose the protection the shunt offers for powering the amp without a speaker attached.

    If there is adequate space between the line out and speaker jacks you could put the impedance switch in the line out hole. That would be my suggestion.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I have used the circuit displayed on the Hot Rod Deluxe schematic for dual impedance. The circuit maintains the shunt yet allows for two impedance choices.
     
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  4. jchabalk

    jchabalk Tele-Holic

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    I've kind of painted myself into a corner (i mean - not really but my control plate will need to be "updated" with the brother p-touch which is kind of a drag :( )

    Reusing the line out is the most appealing option from one perspective as i don't think i'll really use it. I almost didn't put it in there to begin with but i went ahead with it anyway. I'll check out the HRD schematic that @Lowerleftcoast mentioned.

    Otherwise i can add the switch next to the line out. It'd be cleaner to have the speaker output right next to the switch etc...

    I ran the amp through a 16ohm 2x12 cab (from the 8 ohm tap i've got wired). It sounded fine but was much quieter than i expected. I rewired the terminal block from the 8 ohm/22500 set (wires 2,4) to the 15 ohm/24000 (wires 4,6) and it was significantly louder. Not that i'm looking for volume here but i don't want to run the amp inefficiently either. I assume this is the best way to feed a 16 ohm output with this OT.

    IMG_3435.jpeg
     
    Last edited: Dec 9, 2020
  5. jtcnj

    jtcnj Tele-Holic

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    Very nice build!
    I just got started a again on a JCM800 micro 1w that I started in ....... April.

    I was going to weld nuts to the back of my chassis mounting flanges, which can be iffy.
    Saw your cage nuts and just ordered some - thanks.

    On an earlier build, I drilled faceplate and front panel holes together - but found everything stayed together better with small C clamps instead of tape. Never did finish the face plate but the holes are in the right place.

    I did not do this for this build so i have to clamp up and drill the plexi faceplate from inside the chassis Gently and hope for not a lot of blow out on the holes.

    ** wish I would have thought of drilling just pilot holes as was mentioned"**

    I may do this in 4 layers:
    1. through the chassis holes into a
    2. space layer to help center the bit
    3. through the faceplate
    4. through an outer layer giving the faceplate support and let the outer layer take the edge blowouts.
     
    Last edited: Dec 10, 2020
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  6. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    What thickness acrylic did you use? I’ve tried going that route but ran out of thread space on components....
     
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  7. jchabalk

    jchabalk Tele-Holic

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    The acrylic is 1/16" thick and the chassis is about the same, so it's 1/8" in total - which is thick.

    What i found was that the normal switchcraft jacks fit and the alpha pots that i had also fit, but in both cases you wouldn't even get a full turn of the nut. I swapped out the pots to CTS pots and i dug up the "long" version of switchcraft jacks that i had from a previous project. Those components fit great, including with lock washers. The normal micro switches fit perfectly with no modification needed, in fact they could handle an even thicker material.

    I got my plates made by Sandy at Precision Design I didn't check the material thickness with her beforehand. In the future i'd do exactly the same thing but i'd have the right pots and jacks on hand from the get go.
     
    Last edited: Dec 10, 2020
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  8. jchabalk

    jchabalk Tele-Holic

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    I think this sounds like a good plan, i probably printed out 8-10 different mock-ups - a lot of that was to figure out good placement of components - part of which was ensuring i had enough space around components. i iterated on that a few times. When it came time to actually drill i held off until i had the control plates in hand so i could verify that they actually lined up with my paper templates and then i wanted to confirm exactly where the control plate placement would be so i didn't leave a lip on the top of bottom of the chassis unnecessarily.

    The control plates came with full sized holes cut but i waited on painting them until i had the chassis holes cut - basically i left the painting until the very end.

    Once i had the placement correct i taped down my paper template and drilled 1/8" holes as pilots for each chassis hole and then drilled them to the proper diameter. The holes placement would have been a little more crisp if if they were done by a machine but it all ended up working out perfectly. I cleaned up the edges with a deburring tool, and to be honest even if you marred the edge of the plastic a bit it would be under the finish washer and nut.

    I used a step bit to enlarge the holes to the right size, the step bit i have has something close to 1/8" thickness per step so i don't think i could drill through the chassis and plastic as the same time without marring the plastic.

    To summarize though - doing this is something i've stayed away from in the past because i figured it'd be a mess, not only the control plates but drilling the chassis, and it turned out great - and that was using a hand-drill. I won't hesitate at all to do this for future builds.
     
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  9. DrewB

    DrewB Tele-Afflicted

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  10. jchabalk

    jchabalk Tele-Holic

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    I think i found an answer to this in a previous thread. I had a thought earlier this morning that maybe i don't need to get a full size impedance selector switch. I'm only used them a couple of times and when i do i've always bought one of the large-ish switches sold for that purpose.

    I'm wondering if i can use a On/On microswitch for 2 options (in this case 8 and 16ohm). The switch rating is [email protected] or [email protected] and the solder lugs will fit 20 gauge - which is what i'm using on the OT secondary already.

    I've never thought to use one of these for this purpose before but it seems like it'd work fine. Anyone done this?

    [​IMG]

    UPDATE: here the other thread i'd found - from robrob too - seems like i'm probably good.
     
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  11. jchabalk

    jchabalk Tele-Holic

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    Of course i ran out of wire - well not really, i've probably got 100 yards of wire but i ran out of the stuff i was using on this build, so instead of using cut ends of mismatched stuff i ordered some more and a couple of others bits i needed - including the JJ ECC802s that rob recommends in his write-up on this circuit.

    On thing i noticed as i've been playing the amp is that the tone controls aren't too interactive. The treble seems to work pretty well but the bass doesn't do too much. I tried a few things this afternoon and i think it's probably just due to my mid pot being a 100K Audio pot but rather a 100k linear (i couldn't find a 100k audio taper pot with enough thread for my chassis). The operating range of the pot appears to be something like 0 -> 3 or 4, but even with it rolled all the way off the bass control is pretty minimal. I'll play with it some more this evening.

    It still sounds great. Rolling the mid pot up to dial the tone stack out also sounds great and ups the gain a good bit. Overall being able to play with overdrive at home (the only place i play, at night, is not something i've ever really been able to do.

    I also ordered a head cab for it yesterday, it'll be a few weeks but i think it'll look great. I'll post pics once it arrives.

    Here's my impedance switch, it almost looks like it was meant to be right there. I just need to do some low-key work with the P-Touch and wire it up when i get the wire in. I probably could have put it even with 8 ohm marking but it gets pretty right back there. If i were to do another build i know what adjustments i'd make on the control plates.


    IMG_3465.jpeg
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You really do need a 100k audio or log pot for a high value Mid control. The "normal" range of the mid resistance is around 6.8k and it will be difficult to set that value. One option is to put a 6.8k "Mid Min" resistor between the 100k mid pot and its ground so when you dial the mid pot full down you will get the standard 6.8k that is used in AB763 amps with no Mid control.
     
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  13. jchabalk

    jchabalk Tele-Holic

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    That's a good idea, i bet i have one too.

    I just went looking around and found that amplifiedparts sells the 100k audio CTS pots, i'll figure out a couple of other small parts i need and get the right part.

    It's always like this with the builds, i go to 2 different places to get everything and end up having 1 or 2 parts that neither of them carry :rolleyes:

    Although at this point my parts bin has a lot of parts in it and more and more i have on hand the thing i need to fix a problem - just not in this case.
     
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  14. jchabalk

    jchabalk Tele-Holic

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    ok parts are ordered. i'd expect them all to arrive over the holidays sometime.
     
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  15. jchabalk

    jchabalk Tele-Holic

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    Since this amp works great and i'm just sitting here playing while i wait for some wire to arrive i was wondering about 60 cycle / AC hum. This build is pretty quiet, but there's still a little there (when i'm not playing and it's turned up pretty loud).

    One difference in my build as opposed to robrob's layout is that i made an artificial center tap for the heaters using 100 ohm resistors connected to the lamp assembly and grounded to a lug i drilled in the chassis in that location.

    Rob has the 100ohm resistors connected to the filaments on V4 (the power tube) and grounded to the cathode of the power tube which flows through the 820ohm cathode resistor.

    This afternoon i lifted my current ground lug and connected it to the same point on the cathode resistor rob uses in the layout using a wire with alligator clips. The hum was mostly the same as it was using the original location.

    I'm wondering if i might see a notable difference by actually relocating the resistors over to the filament locations on the tubes, or perhaps trying a different location.

    FWIW i plugged an instrument cable into the amp and moved the tip around inside the chassis (a few inches above the components). It's definitely loud over by the Mains (light, fuse, switch), and then from the rectifier over to the filter caps, but then at the filter caps and on through the rest of the amp it's pretty quiet.
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Do you have the chassis in a cab? Most amps are noisier when sitting open on the bench due to RFI.

    If in a cab do you have the chassis opening covered with metal foil? As in foil attached to the wood cover? That can help with RFI too.

    Have you tried chopsticking the leads around with the volume up?

    More troubleshooting:
    https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Hum_and_Buzz
     
    Last edited: Dec 18, 2020
  17. jchabalk

    jchabalk Tele-Holic

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    Right now it's sitting on my desk just on a plywood plank i put together until i get the cabinet. There's definitely more hum up there, and i've been testing with it upside down so the guts are exposed.

    I'm not worried about the hum that's there, it's pretty low. i'm just trying to figure out if there's anything i can optimize. I've used the 100ohm virtual center tap on a number of builds and i've always attached them to a chassis lug. This was the first time i'd become aware of putting them on the actual tube socket and attaching to the cathode resistor.

    Does that provide a potential benefit over attaching directly to a chassis ground? I was reading around last night and found a few threads claiming they went from very a very low hum to absolutely 0. I've been trying to better understand ground loops and where they get introduced. The Bassman Micro layout seems very well thought out in that regard so i was mostly curious about the placement of the 100ohm resistors in that context.

    (update: forgot to mention i put copper tape down on the plank to cover the open area of the amp, the chassis ears tighten up to it).
     
    Last edited: Dec 18, 2020
  18. jchabalk

    jchabalk Tele-Holic

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    I swapped in the 100k audio pot for the mid control this afternoon - it's amazing the difference having the right part makes ;)

    I'm going to play with it a little and i might add the 6.8k mid-min resistor later on. I checked in my meter before installing the pot and it ~6.8k shows up around 2-3 on the dial.

    I have a question on wiring up the impedance switch, mostly about what to do with the negative feedback resistor. Should i:
    - attach the negative feedback resistor directly to the speaker jack tip (so it is used for both the 8 ohm and 16 ohm selections, or
    - wire it directly to the switch lug with the 8 ohm tap - leaving the 16 ohm tap with no negative feedback?
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Wire it to the 8 ohm tap. It will provide the correct amount of NFB even when the 16 ohm tap is used.
     
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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    This clear article from Merlin tells you all about heaters and heater hum. For your question, see especially the section on 'elevation.'
     
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