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Micro Deluxe, first build.....issues....

Discussion in 'Shock Brother's DIY Amps' started by tdoty, Dec 24, 2017.

  1. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    So, I started this quite a while ago.....then it got put on the back burner because we were moving. A year later, I pulled it out and started troubleshooting.

    The amp is based on Rob Robinette's schematics and layout. I left out the master volume and the tone stack because I just wanted to make it work. I subbed a Hammond 6K88VG power transformer from Allied.

    Specs on the transformer are: 500VCT @40mA and 6.3VCT@2A. Since the Micro Deluxe draws 17ma of B+, I figured the transformer would be an ok sub. It has a solid-state rectifier. It was just easier to start with. I do, however, have more tube sockets and an EZ81 that I forgot about.

    With a GT 12AX7 (from my Hot Rod Deluxe) in V1 and a used (vintage) Conn 12AU7 in V2 the voltages are as follows:

    PT Output: 280-0-280
    B+1: 326
    B+2: 292
    V1A Plate (pin 1): 250 (!)
    V1A Cathode (pin 3): 0.74
    V1B Plate (pin 6): 189.6
    V1B Cathode (pin 8): 1.383
    V2 Plates: 326
    V2 Cathodes: 303 (!)

    To me, V1B is the only one that looks close to in the ballpark.

    My electrical experience is industrial stuff (480 3-phase, etc.). I've done some solid state electronics. The closest I've come to building a guitar amp before now was working in an electronics factory winding transformers for Crate and Ampeg amps. We wound Fender stuff too, back when there was a Fender amp factory in Hoopeston, IL. After that closed, of course, the CBS and Fender business went away. Champ cabinets were quite plentiful in our area in the late 80s and early 90s though....but I digress.....a lot. Oh, and I modded an old organ amp into non-functionality. I'll get back to that one, eventually.

    Any ideas?

    I've attached a pic of the board, but the only camera I have handy is my GoPro, so it's pretty fuzzy and it looks pretty weird. It's just kinda prototyped....the lead dress is almost non-existant except the heater wires....no chassis, none of that...wanted to see if I could make a working tube amp first. If I can get it working, then I'll put it in a proper chassis. If not, I'll start over - I bought more than enough parts to build another couple of amps, aside from transformers and tubes.

     

  2. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    34
    510
    Oct 8, 2016
    Melbourne, Australia
    Without a chassis, how have you setup your grounding scheme for PT centre taps, cathodes, inputs etc?

    Does it make any noise?
     
    Last edited: Dec 24, 2017

  3. sds1

    sds1 Tele-Holic

    Age:
    41
    589
    May 4, 2017
    Orlando, FL, USA
    Cool thread, I'm bound to learn something as well.
     

  4. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    Just a very faint hum.....sounds like 60hz hum to me.

    Everything I can think of is grounded. I didn't have the case of the volume pot grounded, but I've taken care of that.
     

  5. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    34
    510
    Oct 8, 2016
    Melbourne, Australia
    Have you got more detailed photos of the back side of the board, and how grounds etc are hooked up?
     

  6. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    There are just 3 jumpers on the back of the board. All of the grounds are tied in with the copper buss wire where the alligator clip is located in the upper right of the pic. All of the grounds are tied to here and/or go directly back to the ground terminal on the plug.

    Apparently I had V1A and V1B wired backwards (wire the layout shows going to pin 1 went to pin 6, pin 3 to pin 8, etc.). I removed the wires and sorted them so they matched the schematic/layout. Apparently, my lead dress was better than I thought; moving the wires changed their routing and I got a little extra hum. I'll sort that later. I guess I thought it wouldn't matter whether V1A was actually A.

    However, I can now hear guitar sounds from the output! Very, very quiet (yes, the volume is up) and extremely distorted, but it makes noise!

    The V2 cathode voltage is still way out of whack, but the others seem to be in line , according to my latest measurements.

    B+1: 312
    B+2: 269
    V1A Plate (pin 1): 172.7
    V1A Cathode (pin 3) 1.3
    V1B Plate (pin 6): 202.6
    V1B Cathode (pin 8): 1.8
    V2 Plates: 319.5
    V2 Cathodes: 295(!)

    Could V2 be bad? I have a Chinese 12AU7 and JJ ECC82, but I figured I would use the Conn since I didn't buy that one new, it came out of an old radio. If I destroyed the tube, I wouldn't be out anything.

    IMG_0739.JPG
    A peek at the back side of the board. Not real clear, but there isn't much there.

    IMG_0740.JPG
    Close up of the ground buss - which the red alligator clip is attached to. All grounds lead back to the ground lug on the plug and it measures 0.2 ohms from the buss to the lug on the plug, as measured with my trusty Fluke 111. The T5-1000 isn't precise enough for these voltage measurements, so I don't have Cat III 1000V protection, just Cat III 600. The T-5 has trouble registering 1.3VDC.
     
    Last edited: Dec 25, 2017

  7. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    And it looks like the ground buss is at issue! Total duh moment, I forgot the ground wire for the V2 cathode. Oops! I ran a wire from the ground side of the cathode resistor to the ground wire for the PT.

    Now, with the new Chinese 12AU7 in V2, I have noise..............BUT...............it only sounds like a proper guitar amp for a few seconds after measuring voltage from V2 pin 2 to ground. Otherwise, it is extremely weak and fuzzy. If I hit the meter leads to V2 pin 2 to ground, it comes to life, then gradually fades out again. Capacitor?

    Any idea what I'm missing? I guess I should go back over everything wire by wire, after finding the missing cathode ground and the V1A/B swap combined with the fact that I soldered it together a year ago and then just let it sit. I wouldn't think the absent Master Volume would make that much difference, but I'll wire that in the next time I get a chance to play with it - which probably isn't going to be on Christmas Day.

    Edit: The more I think about it, the more I think the Master Volume is just a contolled leak to ground from V2, so it may be just what's needed.
     
    Last edited: Dec 25, 2017

  8. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    34
    510
    Oct 8, 2016
    Melbourne, Australia
    Is V2 cathode cap the correct polarisation?
    Sounds like the issue is around ground on V2 though.
     

  9. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    It works! It's a bit early on Christmas morning to plug it into my 1x12 extension cabinet and give it a proper go, but it does work! Right now it's wired to a pair of 6-1/2" car speakers (wired in series for 8 ohms). I woke up to pee and managed to find an hour before everyone else in the house woke up, and started going over the circuit again.

    Lessons learned from the troubleshooting stage:

    1) Go back and check the layout and schematic from square 1! The cathode resistor for V2 had a ground jumper over to the B+1 filter cap.....but that terminal had no connection to ground. Thusly, the cathode was not dropping voltage - which explains the 300 volt reading on the cathode.

    2) Don't start modding the circuit until you really know what is going on! By jumpering out the Master Volume, V2 had no grid leak. Wiring in the Master cured that. The resistance through my meter was acting as the grid leak, momentarily. I probably could have jumpered in a 1 meg resistor from the V1B coupling cap or the V1B grid stopper to ground to provide the leak as well.

    3) Tube electronics are apparently more robust and durable than solid-state. I made a few mistakes here and nothing seems to have been damaged....no magic smoke.

    The 6K88VG power transformer was under $30 from Allied Electronics, where I bought most of the other parts. The "tube-specific" parts came from Tube Depot. I couldn't tell you what the total outlay for the amp was, as I bought lots of spares, parts for future mods, and parts for unrelated projects all in the same shot.

    The raw material for the eyelet board was originally an insulator sheet in a power supply. The power supply died, and I thought that fiberglass sheet might come in handy at some point. I think I was right. The eyelets came from Hobby Lobby.

    I managed to make my own eyelet board from scratch and wire it up, and make it work. That makes me feel good! Now I can start thinking about going back through my 2xEL84 organ amp and getting that 18-Watt circuit up and working!

    I've been programming, troubleshooting, wiring, and repairing industrial robots and automation for 20+ years, but this is my first successful tube project! I'm a little excited!

    To Rob Robinette: Thanks for the great design! I may have taken a little liberty with the power transformer, but, by following your layout and schematic, I have built my first tube amplifier! I plan to play it for a while (after getting it in a chassis and sorting the heater noise I induced into the circuit) and then (maybe) start exploring your mods. I only had a few minutes of playing time this morning, but I'm digging the sound!
     

  10. arcticbreaze

    arcticbreaze Tele-Meister

    121
    Aug 2, 2014
    Oamaru NZ
    congrats on a working amp,
    sorry i was a bit slow ,could have helped with the gridleek i have had that before.

    now the bad news, if you thought cocaine was addictive.....
    expect to here about amps 4 and 5 soon lol.
     
    tdoty likes this.

  11. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    Already eyeballing #2.....and did some mods to #1 to make it a little more humbucker friendly, namely coupling and bypass cap changes.

    Had an odd thing last night though.....no glow at all from V1. Still seemed to sound ok. Checked the heater voltages and the heater voltage on V1 was odd. Pulled the tube and it measures 35 ohms from pin 4 to pin 9. The other 12A*7 tubes I have measure more like 11 or 12. Swapped out the Fender tube for an old Silvertone 12AX7 and it lights up fine, works well, and still sounds good. Just struck me as odd.

    Now I have to look into a new cabinet and a speaker since I'm leaning toward a 10". I hooked it up to my 1x12 cab (based on the AX84 plans) and it's a little on the modern side for this amp, I do believe. I might hook it up to the speaker in my Hot Rod Deluxe, since the extension cab was for the Hot Rod to tame it a bit. The AX84 cab has some serious bottom and is a little dark with the Tweedy Deluxe Micro. Maybe I'll do some swapping of the wires.

    All in all, it was a great learning experience, with more to come!
     

  12. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    I'm kicking around the idea of adding another gain stage to my Deluxe Micro.

    Does this look like a reasonable option:

    gainstage.JPG

    Sorry for the quality, it was done old school.

    Is this correct for adding 1/2 of a 12AX7 as a cascaded gain stage? Would it be better to use another 12AU7 instead, to bring the gain down......at least to start with? I thought I had a 12AY7 and a 12AT7 kicking around somewhere, but I haven't found that box of stuff yet.

    I don't know that the Micro Deluxe needs another gain stage, I just feel like experimenting.

    Tim D.
     

  13. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    I redrew it in more traditional tube schematic style, and it seems to match the gain stage info I found on Aiken Amps web site. With any luck, I'll wire it up today and see what I get. It's all set up on a breadboard and just needs 4 wires hooked into the circuit.

    I am a little concerned that I'm going to need another filter cap to filter the ripple and create B+3. I have the parts.....

    new_gainstage.JPG
     
    Last edited: Jan 2, 2018

  14. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    Well, it worked. Sounds like crap, but it works. Back to the books.

    Back to the original schematic too.
     

  15. dan40

    dan40 Tele-Holic

    856
    Aug 19, 2015
    Richmond Va
    Congratulations on building a "micro"! I have been wanting to do one of these just to try it out. You mentioned above that all of your ground wires ran back to the safety ground from the power cord. Normally you would want to keep the preamp grounds (or ground buss) near the input, power amp grounds on the opposite end of the chassis and the safety earth from the power cord on it's own dedicated lug (preferably attached with a machine screw, nut and lock washer for a solid connection).

    Are you able to post any soundclips? I'd love to hear how one of these sound!
     

  16. dan40

    dan40 Tele-Holic

    856
    Aug 19, 2015
    Richmond Va
    With the added stage, you may need to set up a voltage divider to dump a little gain so it doesn't overload the next stage. You could wire it up as a "gain" knob with potentiometer. It would also help to lower the coupling cap value in the new stage...try something between .0022 and .022uf.
     
    robrob likes this.

  17. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    Thanks! It's pretty satisfying!

    About the grounds: all of that will make a difference once it's all in a chassis; right now it's kinda laying on a table (dangerous stuff). In the end, all of the grounds go to the same place; at least that's my understanding as an industrial electrician. The power section is not connected to the preamp ground buss (directly), but the preamp and power amp eventually connect back to the earth ground (as if it were attached to the chassis.

    Sound clips? The way I "play"? I might have to think on that one a bit.

    When I built the amp, I went with a few of the mods Rob suggests. I changed the cathode resistors on V1 to be equal, as I didn't particularly care for the "asymmetrical clipping". I went with smaller value coupling and cathode bypass caps on V1A (.1uf coupling and 1uf bypass) and have since gone to smaller values on both to better deal with my humbucker equipped Dot. Otherwise, it's got pretty much the sound I expected. I'm playing around with some mods for now until I get things together to build another amp. It's the sound I expected.....just not sure it's the sound I want.

    This is fun stuff!
     

  18. tdoty

    tdoty Tele-Meister

    155
    Oct 17, 2011
    Opelika, AL
    Missed the second post.

    Yes, an added gain stage is going to take a bit more research. For now, I've returned it to "stock".

    Coupling cap was reduced to .0022uf because I just happened to have a Mallory 150 in that value handy. I don't know why my schematic says .1uf. Maybe that was just what I had in mind before I built it?
     
    dan40 likes this.

  19. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I've been extremely busy with non-tube amp related stuff so I just saw this thread. Congrats on getting it up and running. Your prototype is open to a lot of environmental noise so putting it in a chassis will most likely quiet it down some and then you can also tidy up your lead dress for a little more improvement. Like dan40 said, you need to add a voltage divider to an extra gain stage to dump extra gain. It's actually best to reduce the gain of the stage before the extra gain stage too. Several lower gain stages will typically overdrive nicer than a couple of stages running at max gain. Turning one of the gain stages into an unbypassed cold clipper is a classic trick for high gain preamps.
     
    dan40 likes this.

  20. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    54
    Nov 18, 2010
    Vancouver
    What’s your goal with the added gain stage? Can you get there with a different set of tubes? I’m looking at the Micro amps and wondering if a 12BH7 might be a welcome addition.
     

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