Rob Robinette Blackvibe Twin/separate chassis questions!

Discussion in 'Amp Tech Center' started by Diverted, Jan 23, 2020.

  1. Diverted

    Diverted Tele-Meister

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    Hi all,

    Sorry this is a little bit long but please bear with me.

    A few months ago I wanted to see if I could build a small, relatively light single channel/no tremolo version of a Fender Twin AB763, with all its power. I used my own layout (just deleted the stuff I didn't want) and built it on a Princeton chassis with a very light single 12" neodymium 200W speaker). I was able to squeeze everything in and it all works but it is noisy; I wasn't happy with it and so I've decided to try again and could use help.

    Because of the small chassis I didn't have room on top to put the filter caps in a doghouse, and I had to put the output transformer all the way to the right and snake the wires a good distance on top of the chassis to make room for a speaker. The four 6L6s barely fit next to the power transformer .. overall, it was noisy and I couldn't get the hissy, white noisy floor to go away, despite trying a bunch of different electrolytic layouts. Plus, with all the transformers on the chassis, it was seriously top heavy.

    So I'm going to try again and could use some advice. This time I'm planning to use Rob Robinette's single channel/no tremolo AB763 "Blackvibe" layout found here:

    https://robrobinette.com/RR763_BLACKVIBE.htm#AB763_Single_Channel_With_Reverb_But_Tremolo_Deleted

    My idea now is to put the power transformer on a small chassis at lower right of the cabinet. The power switch will be on that chassis but everything else (faraday shield, HV secondary and center tap, and heater wires) are going to go to an umbilical on a six-pin octal socket. I'm planning to snake that from the right side of the lower chassis to the top left of the upper chassis, where I'm hoping to put the choke and output transformer. I'm going to use a doghouse on top of the chassis this time and I'm going to layout the four 6L6s in a square pattern, as opposed to the four in a line pattern, to keep everything together.

    There are six wires in the 18g unshielded umbilical I have:
    1. One connects ground of power supply to ground on top chassis.
    2. Two heater wires
    3. HV-0-HV

    I want to put the two big transformers on opposite sides of the chassis for balance; speaker would go roughly in the middle.

    I have a few questions though:

    1. Will snaking that AC from one side to another cause issues? I can't find shielded six conductor cable in a heavy enough gauge to supply the heaters.
    I was going to run the umbilical straight up from power transformer at right and tuck it right underneath the faceplate, then head to the other side from there, to keep it as far away from the reverb tank as possible.
    2. The reverb tank will have to be a short 9" spring to fit next to the power transformer on the bottom. Should I move the reverb jacks to keep them away from the power transformer? And if so, should I run shielded wire to the jacks inside the chassis, since they won't be in their standard position at back right?

    Thanks for hanging in there on this long post. So many questions and unknowns. I'm committed to building a quiet amp and could use any advice on keeping it quiet as I can get. Thanks!
    IMG_2032.jpg
     
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  2. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    I don't think I would run the ac by the preamp section. Not so good to have the PT near the reverb tank either.

    There are shielded 3 wire power cords, or maybe Faraday cage the cables.

    If you put the PT on the left, maybe you could get one of those vertical reverb tanks mounted on right side.

    Of course all of the heavy bits would be on the left so to make it balanced you may want to get an ElectroVoice SRO speaker. The gravitational force is strong with that one.
     
  3. schmee

    schmee Poster Extraordinaire Silver Supporter

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    You may be better to put the OT away from the tubes instead of the power transformer. Mount it on the speaker like some old amps?

    BTW, I have done a big output transformer on the opposite end of the Princeton chassis, long wire, with no noise issues, so I doubt that was your issue.
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    ^I like these suggestions. I'd just offset the carry handle to compensate for all the iron being on one side.

    I don't think you can get "quiet" from your current layout.
     
  5. tele_savales

    tele_savales Tele-Meister

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    I like the concept of a small, lightweight 4x6L6 amp.:D
     
  6. tele_savales

    tele_savales Tele-Meister

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    I had a hell of a time getting the reverb to stop squealing and sounding like general trash on my build of Rob's same design, until it finally dawned on me to get the reverb tank OFF the bench three inches away from the idling amp/transformers, which cleared the problem immediately. So I'd be concerned about having the tank and the OT in the bottom of the cab.
     
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  7. Diverted

    Diverted Tele-Meister

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    Well I've finished the circuit/layout work.

    I am putting the transformer chassis at far left, bottom of cabinet. Output transformer is roughly at left center of top cabinet.
    The power chassis contains the power cord, fuse, tranny and artificial heater center tap resistors. The earth ground from power line is grounded to a transformer lug and in turn brought over to one of the pins on the socket. That one terminates at another ground point in the amp. Nothing else is on that ground. The faraday shield grounds at another transformer lug and nothing else is on it.

    Other sockets are HV secondary, HV secondary, HV center tap, AC heater supply, AC heater supply (with 100 ohm resistors running from each to the HV center tap lug, which in turn goes to the main power ground in the upper chassis, which has first two stages of filtering and bias ground), and power line hot and return from switch, which runs into the fuse.

    Next, on to the cabinet.

    I haven't had any issues with the reverb squealing, and the tank has been very close to the transformer on my bench. It did hum a bit initially but I put in shielded wire to and from the pot, and the hum's pretty much gone. I'm very happy with this build actually. It's very stable biased at about 60-65 percent depending on the tube. It's dead quiet at idle and runs very smoothly, sounds great and boy is it loud with its single 12" Eminence EM12N speaker. It'll never move as much air as a twin so won't have that presence, but this was built to be as close as we could get to a Twin configuration in a Princeton-sized chassis and cabinet. I think it's going to rock. It'll weigh just under 40 pounds.
    Amp's the one furthest over to the left in the group pic. I know, no doghouse yet. Going to bend one later this week.

    IMG_2165.jpg 2020-01-26 18-51.jpg
     
  8. Diverted

    Diverted Tele-Meister

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    And thanks to Rob Robinette for the layout! I had tried a single channel no trem AB763 before but didn’t account for a few things he did. Huge help!
     
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  9. AndyPanda

    AndyPanda Tele-Holic

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    I don't know if this would matter on your setup (since only the power transformer is separate) but in my old amp that had untwisted heater wires going thru an umbilical between power amp and preamp (and lots of noise), I used a separate length of good quality microphone cable to run the heater voltage between the amp and preamp (tight twist surrounded by braided shield grounded on only one end) and now that amp is very quiet.

    If I were putting the power transformer separate, I think I'd also put rectifier and filter caps on the PT chassis so filtered B+ is going thru the umbilical.
     
    Last edited: Jan 30, 2020
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  10. Diverted

    Diverted Tele-Meister

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    I initially was thinking about putting the choke and first filter cap down there, but for a couple of reasons I didn't do it.
    I didn't want to snake that B+ inside the umbilical next to those heater and power wires, and I wanted to have the switch on the faceplate, not the bottom. I was thinking I could do it if I could shield the power and heaters, but the amp really needs 18g heaters at least for the 6L6s. I don't have any shielded wire of that gauge on hand so I thought I might as well not do it. It all ended up working out this way. The socket carries everything I need up top and there's no hum coming off it so I'm very happy.

    One other concession is I had to use an 8ab3c1b tank which is 9", not 16" like most, to fit in the bottom of the cab next to the power chassis. Didn't know how it would sound but it's a three-spring and I really can't tell the difference between it and normal sized reverb springs. I'm pretty psyched!
     
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  11. Diverted

    Diverted Tele-Meister

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    61E60182-311D-4800-9D48-480FBF4764A8.jpeg 84C911F7-6BEF-4A7C-9660-4781518C716A.jpeg Put the amp together for a dry run. It all fits! A slight bit of white noise at idle, but no big deal. It weighs 41 pounds.
    Some more odds and ends to do on the amp and power supply, and I have to Tolex it, but the end is in sight.
     
    Last edited: Feb 2, 2020
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  12. Corvus

    Corvus Tele-Meister

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    One thing about amps with the power transformer separate is safety in case of a short so I guess you would need to run a ground wire from the grounded chassis to the tranny in some wy?
     
  13. Diverted

    Diverted Tele-Meister

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    Yes the earth ground is run up through the umbilical.
     
  14. tele_savales

    tele_savales Tele-Meister

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    Man, that PT is burly!
     
  15. Diverted

    Diverted Tele-Meister

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    It sure is. It’s almost half the weight of the amp. HV secondary@450ma!
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congrats on getting it up and running. It's an interesting build, that's for sure.
     
  17. Diverted

    Diverted Tele-Meister

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    Thanks! Your site was a big help. As I said I tried this before I knew about your Blackvibe stuff and got it mostly right, but not totally. I appreciate it.
    Other than a hissy/white noise floor that's coming from V3 and I can't for the life of me knock out (audible hiss with volume at 0) I'm very happy with it!
    If I can't root it out then, oh well. The amp is a surprise gift for a friend of mine who likes to play loud, will use it live and probably won't record with it far as I know. So the hiss should not be an issue for him and if it is, too bad! It's free :p
    Seriously, I would love to knock it out but having very little luck so far. Will keep at it.
     
    Last edited: Feb 3, 2020
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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    A bad tube can cause excessive hiss. Have you tried a different V3? Switching the input resistor and first stage grid stoppers to metal film resistors can also help with amp hiss.
     
  19. Diverted

    Diverted Tele-Meister

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    Thanks Rob.
    Yes I tried a bunch of different tubes in V3, the only ones that knocked it down a bit were 5751s and 12ay7s.
    The amp’s built with metal films so those should be good.
    It’s really not too bad, and like I said soon as you hit a note it’s buried underneath sweet blackvibe sound.
     
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