Blackface Twin Reverb Build (my first from scratch!)

Discussion in 'Shock Brother's DIY Amps' started by JKube, May 24, 2017.

  1. sds1

    sds1 Tele-Afflicted

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    Thanks! I guess then my question becomes -- shouldn't there be a second ground (to chassis) from the first 2 caps in the dog house? Looks like you followed Mojo's wire color exactly, so I see A/B/C/D wrapped inside the white preamp ground, but not that other ground wire. What am I missing?
     
  2. JKube

    JKube TDPRI Member

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    That is the black wire coming from the doghouse over to the power transformer lug (rear interior) where the PT and power tube cathodes are grounded (from the two large caps). You could use the front lug, but all the advice I got said to share a single ground between those three things.

    The second ground from the cap board comes out with the B+D behind the cap board near the channel 2 controls. Look for the black and yellow wire comi g through a grommet. The white shield performs no grounding of components at all. Just shields the power wires.
     
  3. JKube

    JKube TDPRI Member

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    Edited above. Make more sense?
     
  4. sds1

    sds1 Tele-Afflicted

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    Yes I think I got it now. Allow me to summarize and tell me if I got it right?

    1) The power amp ground from cap pan is a black wire coming from the left-side grommet and is ground to chassis lug near PT
    2) The preamp ground from cap pan is also black coming from the right-side grommet and is connected to the preamp ground bus
    3) The white curly wire is connected to the preamp bus and nothing; it is a shield only

    Appreciate it, I was looking at this pretty wrong to start out but hopefully I got it now. :)
     
  5. JKube

    JKube TDPRI Member

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    That is exactly right
     
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  6. sds1

    sds1 Tele-Afflicted

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    Thanks! So helpful.

    So last question, it sounds like you'd definitely recommend the high voltage shield, it was only to whether it was grounded on both ends or not that you'd argue was perhaps inconsequential?
     
  7. JKube

    JKube TDPRI Member

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    It was so easy to do, why not do it?I would definitely recommend doing it. I could not comment on the long term difference in one end/both ends grounded, but the science seems to support one end. I never ran without the shield, so I don't know how much difference it made. As far as grounds, by securing it to a lug on the transformer end, you're creating a preamp bus reference to ground very near the power bus/star ground point. Most say that's a no-no. Maybe it's kool aid, maybe not. The goal is to ground your preamp bus as far from the power bus as possible, which is the input jack. Connecting one end of the shield to the bus and the other to chassis at or near the PT would negate that.

    I took a long thick phillips screwdriver, coiled the wire very tightly around it, then butted the tip of the screwdriver against the end of the 4 wires exiting the cap doghouse and slid the coil over. Ground one end to the preamp bus, cover the other, and you're golden.
     
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  8. sds1

    sds1 Tele-Afflicted

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    Great, thanks! Agreed, it's no extra effort whatsoever -- just can't say I had ever seen this done before. But I'm new and only paying close attention to some things for the first time. Really appreciate the guidance, I'll certainly be following it.
     
  9. JKube

    JKube TDPRI Member

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    All good man, I hope it works like a charm for you. Mine is quiet as a church mouse, I think in large part to a very focused effort on the heater wiring. If you need an assist, feel free to send me a message. I'll try and send you whatever pics/videos I have that might help.
     
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  10. sds1

    sds1 Tele-Afflicted

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    Had some questions about your grounding again, this time with respect to the pots and jacks. Here is what I think I see:

    1) Your pots cases are connected together via bus wire making a single connection to ground per channel
    2) Your input jacks are grounded together with pots per channel? And the jacks are all isolated from chassis?
    3) The single grounding point for all that + all ground from preamp is that big lug on the chassis

    Not sure about all that, thanks for any clarification you can provide. Thanks!
     
  11. JKube

    JKube TDPRI Member

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    Questions answered above.
     
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  12. sds1

    sds1 Tele-Afflicted

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    Thanks! I think understand your wiring now. I think I am missing some concepts with respect to grounding in general. I think I now understand that the lug is in this case is equivalent to the first input in terms of a ground. But why connect lug to jack with bus wire, is it a more reliable connection than the chassis? Also I think I picked up somewhere that all but the first jack are isolated. Are they chassis grounded to eliminate noise?
     
  13. JKube

    JKube TDPRI Member

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    That lug, for all intents and purposes, could have been isolated by nylon washer/screw. The wire connection is the "primary" ground to the bus, although basic physics would dictate that it is negated by the lug. Worst case, it brings the point of ground forward to the lug, which is still before the preamp circuit, so I figure I'm safe from getting anything caught in a loop. The input jack had a direct reading to ground on the third contact. I didn't do anything special to isolate the jack, so I assumed it was default-grounded. I could have isolated the lug and used the input jack as the sole ground point, but I didn't think it would really matter (and it didn't). I'd check the unused contact on your input jack set. It might (should) be a chassis ground.
     
  14. JKube

    JKube TDPRI Member

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    Also, this is what worked in my build. Your results may vary, and plenty of dudes on here make my skill and knowledge seem toddler. Anyone else care to weigh in on preamp grounding, input jacks, etc?
     
  15. sds1

    sds1 Tele-Afflicted

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    This is great help, thanks very much. The knowledge provided here is backed by a successful build, advice worth heeding I would say! I have much to learn, but what I did know or thought I knew so far was in line with the grounding scheme you've followed here. I'm doing a first build with no kit, so down to the nitty gritty on things like buying input jacks -- which do I buy? Forcing me to look at every little detail so I don't get to the end with a handful of issues.

    I am still a little unsure about shorting jacks vs non-shorting and when to use either. I think I got something stuck in my head a while ago about this and it's holding me back now.
     
  16. JKube

    JKube TDPRI Member

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    Ladies and gentlemen. Sorry to ressurrect this thread (or am I?), but I wanted to follow up. I didn't mean to go radio silent, just finished up a cross-country military PCS. The amp traveled well. It might have had a better seat than my children. IMG_0195.JPG IMG_0197.JPG

    It made it to Missouri and I got it all hooked up again. It sounds like Christmas in 1965 all over again. I'm slacking on the sound clips, but once I get the interface plugged up I'll get some basics posted.

    90 day report: The full JJ's glass set I got has been nothing but problems. Not that I'd blast them overall, I've generally had really good luck with JJ's, but the 12AX7 set I got had a single microphonic right off the bat. I had a spare, so that one went in. I was having reverb noise issues, so I swapped the reverb driver tube. Problem solved. After the trip, i had some faint static, almost like a very quiet AM radio station in the background. It was there on both channels, but only volume-affected by Ch 2. Even with nothing plugged and all volumes down. Still faintly there. Ended up being the V2 12AX7 inserting noise. Swapped with my final spare tube and she's dead quiet and sounds like the amp of my dreams. Maybe just a bad run, but on the third of 5, I might make a call.

    Other than those issues, and some noise from cheap RCA cables on the reverb tank, trouble free so far. Sound clips to come....
     
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  17. Twin Brother

    Twin Brother TDPRI Member

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    Hi,
    I'm drooling over the pics and insights. In fact I am building my own amp. I bought the kit from Mojo tones. My request is.. I wish someone will update the Mojotone circuit to what is right or based on the Fender Circuit since I saw some corrections that you mentioned because their circuit was modified for more gain.
     
  18. Twin Brother

    Twin Brother TDPRI Member

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    This is the mojotone circuit and the Fender circuit AB763_Twin_Reverb_Schematic  June 2019.jpg
     

    Attached Files:

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  19. robrob

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    [​IMG]
     
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  20. Jmorrie

    Jmorrie NEW MEMBER!

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    Such a great thread. Look forward to referencing this on my own build!
     
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