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Blackvibe 6V6 layout - grounding thoughts

Discussion in 'Shock Brother's DIY Amps' started by BigDaddy23, Sep 13, 2020.

  1. BigDaddy23

    BigDaddy23 Tele-Holic

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    Hey all - I'm in the planning process for a build of Rob's 6V6 Blackvibe design. Given all the recent talk about single star grounding, I thought I'd do a bit of a faff around in DIYLC for the first time to try to incorporate some of those single bus/input jack ground thoughts. Thanks for the layout and schematic, @robrob .....bloody fabulous for DIYLC rookies like me!

    I plan to add the Presence and a Frondelli Master Volume. I'll use dual core coax on the MV (one going board > pot and a second one for pot > grids) and ground those shields at the pot end. I also have a switched 1meg pot so thought that might work as a push/pull bright switch on the vol/gain control.

    Have a look at what I have done with this layout and please comment regarding the grounding. Note I have thrown in a nominal AB763 bias circuit but haven't settled on values yet given the 50v bias secondary. Thanks for your thoughts.

    Blackvibe Grounding.jpg
     

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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That looks great. The only thing I would do is move the HV center tap to the first filter cap negative eyelet for a little more isolation from the bus.
     
  3. fasteddie42

    fasteddie42 Tele-Afflicted

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    man..


    you ought to feel blessed that you ask a question about the blackvibe and none other than the man himself robrob answers first :lol:
     
  4. andrewRneumann

    andrewRneumann Tele-Holic

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    That is sweet man. Are you building the board from scratch or ordering something?

    I will follow along. The first thing that came to me eye was where to ground the presence control. My suggestion:

    E6DC8E7D-BBCD-4F41-BC98-321F6990DBAE.jpeg

    Have you considered flying the ground bus off the board? This might give you some more options in achieving a true “distributed star” grounding scheme.
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    Another suggestion is to ground reference the OT secondary as close as possible to LTP ground. Most amps work fine without this, so it’s up to you as to how far you want to take the concept. The connection can be made next door on the bus wire, it doesn’t have to be literally on the eyelet/turret.

    26160B46-4891-4885-BC31-A60D1CFD8563.jpeg
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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  7. Nickfl

    Nickfl Friend of Leo's

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    Instead of hanging the two 33K input grid stoppers off the input jacks, it would be a better practice to hang them directly off pin 2 of the tube socket. Either way they are hanging and it is much better to have them at the socket than at the jacks. It may seem like having one long wire from the jacks to the socket is preferable to having two, but the benefit of moving the grid stoppers outweighs the benefit of reducing the amount of wire. That one long wire between the grid stoppers and socket will act like an antenna, the two long wires between the jacks and the grid stoppers won't.
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Since you are running a PT with lower voltage, you do not need the two caps in series for your first filter.
    You can save your shekels.;)
     
  9. BigDaddy23

    BigDaddy23 Tele-Holic

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    Wow - thanks for all the great responses fellas!

    I'll implement these changes and drop an updated layout into the thread. I'll put up a separate build thread once I have finalised the nuances of the layout.

    The other are of contention is where to ground the x2 single core coax shields that run from the master volume pot to the power tubes. By my reckoning, I have these options: The case of the dual gang pot (easy) or the bus bar itself. Thoughts on that? I want to use coax there due to the long run back to the power tube sockets. I could even use a third, dual core coax from the grid caps to the MV pot. By the looks of things, that will be pretty close to the pot so 2 individual, standard hookup wire may be fine.

    With respect to specific questions/responses:

    Awesome - will do this.

    Thanks for these! With my other builds, I've always used a flying bus bar so will be doing the same here. I will build my own board too. I was thinking about the Presence ground when I was adding it in...you have confirmed my thoughts. The other suggestion on the OT secondary/ground is excellent. I want to apply these grounding principles as practically as possible.

    Thanks mate - I guess this is something I can easily test. I'll connect it to the first cap as per Rob's suggestion in the first instance. If no good, I can provide a dedicated ground tab on the chassis and see what difference that makes.

    My PT is running the 330-0330 spec as per Rob's design (Deluxe Reverb). The only change I made to his design was to flip the second cap in series so the negative was closer to the bus and the grounding itself. I've got the caps in stock, so no issue there. TBH, I was considering using a JJ cap can with the 40-20-20-20 sections (ala Princeton Reverb) to reduce the board real estate. From a $ perspective, that's more expensive as compared to individual caps.......then there's the grounding principle I'm trying to evaluate. :)

    Thanks! I'll definitely hang them off the tube socket with a 3 lug terminal strip and some dual core coax. Do you think it would be better to ground that coax at the tube socket or the input jack end?
     
  10. BigDaddy23

    BigDaddy23 Tele-Holic

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    Righto - I've added in the changes as suggested.

    A couple of other things to ponder:
    1. Where best to ground the Frondelli MV coax shields. I figure the back of the pot is fine?
    2. Should the star ground at the input be a dedicated bolt or is the jack fine with a star washer?
    Thoughts?

    I'll most likely relocate the switches over near the rectifier as I've got a large Hammond black steel chassis and plan to use an IEC socket for the mains. The fuse can go into the tray (blackface champ style). That should allow me to route the Standby switch wiring and B+ feed of the GZ34 together to the left of the power tubes. The eyelet sharing the choke and standby switch wires can also move left between the A filter caps. I'd like to keep the noisy stuff over between the GZ34 and the power tubes if that helps things. Perhaps that's a bit OCD :D??

    Blackvibe Grounding v2.jpg
     

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  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    That ground is not critical. The back of the pot is fine.
    A bolt is the preferred method as jacks loosen sometimes. The jacks are often used though.
     
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  12. Nickfl

    Nickfl Friend of Leo's

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    A three lug terminal strip to mount the grid stoppers is fine but unnecessary. I posted some pictures in the thread a couple weeks ago showing where I measured half Watt and one watt resistors compared to an equal length of 600 volt wire and the resistors mass is negligibly different than the wire itself so you don't have to worry about supporting them, they don't put any more stress on the joint than plain wire, probably less than something heavy like coax. That being said there's no reason not to mount them if it makes you feel better.

    As to which end of the shield is grounded, I think that's very much angels dancing on the head of a pin stuff, especially since shielded input cable is probably unnecessary in a relatively low gain design like this anyway, especially with the grid stoppers mounted right on the socket, but it's not going to hurt anything to use it if you already got some coax lying around. Just make sure you ground one end and one end only, I would go with the end by the input jack as that is convenient.

    If you use an IEC socket with a built-in fuse, as many of them have, you can simplify things (and cut cost) by leaving out the fuse holder altogether.

    Also, a standby switch is neither necessary nor a particularly good idea for a couple of reasons (see the valve wizard site for reasoning). I'd just eliminate it entirely for simplicity, or you could put in a mute switch if you like the functionality. That might be good use for the pull switch on your volume pot actually.
     
    Last edited: Sep 14, 2020
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You have the HV center tap connected between the first two series filter caps. It needs to be connected to the left A Filter negative terminal.

    You don't need to run the bias to the circuit board. Take it direct to the master volume.
     
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  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    Excellent! I'll implement these changes.

    @Nickfl - I was looking at the fused IEC sockets and thought about incorporating one. The only thing that kept me from deviating was the fact I had standard IEC sockets and fuse holders on hand. To be honest, I think it is a great idea to use the all in one option.....one less hole to drill!
     
  15. BigDaddy23

    BigDaddy23 Tele-Holic

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    Righto - updated layout. I've added in the fused IEC socket, shifted the B+1 eyelet left and removed the standby switch. I've also substituted shielded wire between the volume pot and V1b.

    Thoughts? Now to start looking at bias circuit values - in particular, the 470R and 37K total provided by the pot and resistor to ground. I have that 100uF/100V cap on hand so will use that for sure.

    Blackvibe Grounding v3.jpg
     

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  16. andrewRneumann

    andrewRneumann Tele-Holic

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    Looking good. For those interested more in the Blencowe grounding method and what "distributed star" means, I created this drawing. I'm not saying this is the better way. As you can see, the grounds are grouped together in local stars related to their respective supply filter caps. The individual ground leads coming in from volume/tone controls, cathodes resistors, etc. are all grouped physically as close together as possible. Naturally this will create clutter compared to a bus bar where we connect a ground at the nearest point on the bar, regardless of where its local star would be.

    A couple other notes. Blencowe does not ground anything at the B+1 supply negative terminal. That is reserved for the HT C.T. (or full bridge rectifier return). The bias supply gets grounded at the power amp star which is associated with the screen supply cap. Any post-P.I. master volume also gets grounded at the power amp local star.

    This is all from the Blencowe pdf/Ch. 15 of DTPFGAB.

    Blackvibe Grounding v3 (2).jpg
    Hope this helps, not as a "thou shalt", but as an idea for what your options are. I'm a Merlinite, but I have seen and read enough to know there are many ways to do this.
     
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  17. BigDaddy23

    BigDaddy23 Tele-Holic

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    @andrewRneumann ^ this is super. I've actually got Merlin's book but have struggled to understand this star concept properly despite the nice pictures! The penny has dropped with your explanation.....thanks heaps!

    With respect to the master volume ground point; I assume that applies to those types that send unwanted signal to ground?
     
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  18. andrewRneumann

    andrewRneumann Tele-Holic

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    Yes, the way I understand it, any circuit that connects to ground after the P.I. would ground as close as possible to the power amp (screen supply filter) local star.
     
  19. BigDaddy23

    BigDaddy23 Tele-Holic

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    Here's an up to date version incorporating the star grounds.

    I've drilled and populated the board with eyelets to this layout. I've used x2 turrets where the bus bar connects to the board (ie left most A Filter cap and at the D filter cap. All other points are eyelets. That will get the bus up in the air a bit without the need for extra bends in the bar which should be more sturdy and neater from a lead dress point of view. I may look to put a third turret in about half way along the bar (near the red Presence wire) for extra support only.

    Blackvibe Grounding v4.jpg
     

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  20. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Here is a real world example of what Andrew is talking about:

    IMG_20200726_190507866.jpg
     
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