Little Bear T10 Chassis Upgrade

Discussion in 'Tube Audio' started by neevo, Sep 17, 2016.

  1. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    I've been on the lookout for a chassis to mount my Little Bear T10 phono preamp in so its a little more aesthetically pleasing and matches the quality of my Kenwood KA-7100. Whilst trawling the auction site the other day I came across this:

    [​IMG]

    In addition to mounting the T10 in the chassis, I'm also taking the opportunity to do some "mods" and keen top post them up here to ensure I am doing it right/safely.

    I am replacing the on/off switch with an illuminated LED round push button. I've used these before and they look quality and I have managed to get a 240v switch with a 12v LED. In order to run the 12v LED I have purchased a LED ballast and plan on mounting that near the transformer to run the 12v for the LED. In order to isolate the power from any audio signal I have purchased some Alu mesh so I can run all the wires in between the chassis and this mesh. I assume that should be enough to try and stop any stray signals getting into the audio path?

    I wanted to have the tubes out the top of the chassis as per the original plastic box, however this was going to be difficult as I needed to mount the PCB on the bottom of the chassis (to create clearance for the power button on the front), which meant the tubes would not really protrude out the top. The fix? I have bought some tube sockets and plan on mounting the tubes directly on top of the chassis and have them completely visible. The plan was to desolder the original sockets and run wires up to the new ones.

    Questions:

    1. What gauge wire should I use to solder in to the PCB to run up to the tube sockets?
    2. Should I twist any of these wires together? (I have 3x 9 pin sockets and 1x 7 pin socket - rectifier)
    3. What options do I have to be able to disconnect the sockets from the PCB so I can take the lid off?

    I have taken the T10 out of the audio setup for the moment so I can start on drilling holes and work on getting it all together. Appreciate anyones help/knowledge to guide me along the way.
     
  2. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    This is the rough end plan:

    [​IMG]
     
  3. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Step 1 was to mark the base plate for the screw holes to hold the PCB. I used the base acrylic sheet to mark the holes:

    [​IMG]

    Drilled and lightly chamfered the holes:

    [​IMG]

    PCB legs attached using the old black bolts from the T10:

    [​IMG]

    Test fit showed I would have to remove a few parts to make it fit. Not an issue as all these parts were always going to be removed:

    [​IMG]

    The acorn nuts worked perfectly to attach the PCB:

    [​IMG]

    Next up I bolted up the feet using the air slots that are in the (now, as it used to be the top) base:

    [​IMG]

    Looking at the bolts I had to check clearance to the PCB (although on reflection I think I may reverse them and have the nut in the rubber foot):

    [​IMG]

    All good either way:

    [​IMG]

    Next up I removed the power plug, power switch and RCA's:

    [​IMG]

    Test fit:

    [​IMG]

    I started drilling the hole on the front for the power switch but ran out of drill sizes, so I have a 16mm on order.
     
  4. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    So my 16mm drill bit came in and I managed to get a bit more done. Essentially I have all the power stuff done now and I am waiting on some shielded component video cables to come in before I can also finish off the audio cables.

    I managed to work out all the placement of the items on the bottom cover. This included a hole for the transformer bolt, plus I moved the feet and drilled new holes for them too. All holes drilled and chamfered:

    [​IMG]

    [​IMG]

    PCB back in for about the 60th time:

    [​IMG]

    Pulled the PCB back out and soldered in the wires for the power and also the on/off switch:

    [​IMG]

    I put the wires in from the bottom to provide as much clearance as possible for the audio cables above the PCB. This meant I had stubs of wire poking out of the PCB, so I dropped blobs of hot glue on each of them as a safety measure:

    [​IMG]

    After a bit of fiddling I got it all wired up. I used some connector blocks hot glued to the side of the case so I can remove the PCB:

    [​IMG]

    Test of the final look:

    [​IMG]

    I also managed to cut the holes for the IEC connector and RCA's for the input. I will drill the output RCA's when they come in:

    [​IMG]

    All that's left to do is:

    1. Mount and wire up the LED switch transformer
    2. Wire up the audio cables from PCB to RCA's
    3. Connect the earth to the PCB
    4. Mount the Alu mesh for shielding audio cables and power lines/transformer from PCB
    5. Drill holes in lid for tube sockets
    6. Work a solution to wire up the sockets from the PCB

    I finished tonight with a test. Button works fine and tubes were glowing softly as per normal. Hope to have it finished off soon.
     
  5. ferninando

    ferninando Lunatic Member

    Messages:
    12,456
    Location:
    San Jose Ca.
    are you sure you have enuf heat sink?
     
  6. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Needs a bit more you think ;)
     
  7. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    I have been reading up on preamp hum, as my T10 has always had hum at high volume levels (12 o'clock or higher). Its not been a major issue as I never listen to the amp that loud (it would be exceptionally loud), however I am keen to remove all hum where possible.

    I've tested various solutions but none have worked so far, then I read up somewhere that mentioned earth grounds from TT on the L&R RCA's should not be connected prior to the power amp. Is that true?

    The T10 uses a common earth rail for the RCA's which you can toggle on /off to the mains earth if required, however they are always connected at the pre amp.

    Is this could be a source of hum, I was thinking of putting a DPDT switch in to the chassis for the earths on each RCA from the TT where I could switch them from the original setup (where they are both fed through the common earth on the PCB) or I can switch them to connect directly to the separate RCA earths that go to the power amp (and therefore would be separated).

    Thoughts?
     
  8. ferninando

    ferninando Lunatic Member

    Messages:
    12,456
    Location:
    San Jose Ca.
    you mite consider this.

    The shield for RCA inputs on preamps and poweramps, should be connected to ground
    at the RCA end only and not elsewhere internaly to the circuit. Just connect the signnal wire.
    Thus keeping ground at the chassis only preventing ground loops which can cause hum.
    Not sure if I expplained that clearly.
     
  9. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    That sounds like what I'm building in as a switchable option.

    Position 1: both shields are joined together on the PCB (common earth)
    Position 2: both shields are kept separate and sent direct to the amp and do not touch the PCB earth

    That way I should have 4 earthing options as I also have the option of earthing the PCB or not too. Hopefully with those 4 options there should be 1 where ground loops are eliminated.
     
  10. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Finished up my audio cable shields today. I wanted to solder them together as its a bit more permanent, however I quickly discovered you can't solder Alu :p

    So instead I used my hot glue gun and sandwiched glue in between each joint. After a lot of fiddling I had it complete:

    [​IMG]

    I need to work out a solution to attach it to the chassis still, but overall I'm very happy with the fit:

    [​IMG]

    Wondering if I should look to ground the cage.
     
  11. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Latest updates, its moving quite slowly as I wait for parts to slowly arrive. My shielded RCA cable came in so I soldered that in to the PCB:

    [​IMG]

    Turns out the cable is not that flexible so I am going to have to review my Alu shield a bit (might just remove the end pieces and have it run the length of the chassis.

    I also finished off the back panel. The previous mini switches I used were pure junk (I couldn't really get continuity across some of the pins), so I scrapped them for much larger versions but they are significantly better:

    [​IMG]

    I used battery cell jumpers to link the left and right pins so now I have the option of passing the shield through to the amp (and not linking the L&R shields at all) or passing it on to the PCB where the shields are joined together (where I also then have the option of earthing the PCB or not):

    [​IMG]

    Since this photo I have also scrapped the fuse on the PCB (as one of the switches was fouling it), this has been relocated to the back panel too, so that nearly all functions are available on the outside.

    I have also soldered in an LED driver for the 12V power switch. This takes power from the switch itself so the driver is not on all of the time. Yet to test fire and see if it works... we'll see:

    [​IMG]

    I am waiting on some new PCB stays and bolts before I mount the PCB for the final time and get to completing the soldering.

    Then last night I worked my solution to extend the tube sockets. I wanted a removeable option as the new tube sockets will be mounted on the top panel and therefore if I soldered in the wires, I wouldn't be able to take the top panel off. The idea here was to buy some tube extensions and solder in some wires which I can then solder to the sockets on the top panel and would allow me to separate the top from the PCB:

    [​IMG]

    In the vice for soldering:

    [​IMG]

    Test fit:

    [​IMG]

    Questions:

    1. I used 13x0.12mm stranded wire for joining the 2 sockets. Not sure what current these wires will see, so is there any risk here?
    2. Is there going to be issues with these wires being close to each other (introduced artefacts in the audio signal)?
     
  12. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Reading a thread online it looks like the wire is capable of 1.95A of current. Anyone know what the feeds on the tubes are on this amp for both the main ones and the rectifier? I might need to grab some other wire if this isn't going to cut it.

    Also cruising google images I came across this setup of using different tubes in different holders (off a headphone tube amp forum), so maybe the principle is ok and shouldn't introduce noise. Just need to check current carrying capacity of the wires.
     

    Attached Files:

    Last edited: Oct 18, 2016
  13. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

    Messages:
    1,269
    I'm confused about that LED. What's that white box in the 4th picture of post #11? An LED only needs a resistor to limit the current and a diode if you're running it off AC. If the white box is something designed for LED household lighting it's totally unnecessary and a potential source of noise.
    The aluminium mesh is not a bad idea but it's not really going to tackle the major issue which is the proximity of the mains transformer to the PCB. Get the transformer as far away from the audio circuit as possible. Put it in a separate box if possible. If that isn't an option you need to experiment with its location within the chassis to minimise hum.
    Twisting wires is done to minimise the radiation from the wires due to the current flowing down them. The idea is this: a current flowing through a wire creates a magnetic field around it. Current flowing in the opposite direction would create an equal but opposite field. Two wires with equal but opposite current flowing through them would cancel each other's magnetic fields if the wires were in exactly the same place. Obviously it isn't possible for two wires to be in exactly the same space but twisting them together gets as close as possible. So if you have two wires carrying the same current to and from a load - such as wires to a valve heater - they should be twisted together to cancel the noise they radiate. That applies to any two wires, so the wires to/from the primary winding of the mains transformer should be twisted together, so should the wires to/from the secondary.
    The idea of the valve socket extenders is a bad one. I recommend you don't do it.
     
  14. GChief

    GChief AK Subscriber Subscriber

    Messages:
    659
    Location:
    Eastern NC
    Nice job! I have been thinking of doing something with mine after reading some of the "fixes" and you have just motivated me, thank you sir!! I work mostly on Marine Gas Turbine control upgrades and repairs for the US Navy so I am thinking of using an old skool JB enclosure kind of like the one attached, the proper size of course, and the shielded armored cable we use because of the RF these system generate. I figure it works in turbine and generator controls it should kill any RF from a tiny little transformer.

    IMG_2825.JPG
     
  15. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    @rothwellaudio the LED in the illuminated switch runs off 12v so the driver was to step down the voltage for that. Is there a simpler way of doing it?

    Also take your feedback about the tube sockets on board. How would you connect tube sockets on the lid to the PCB?
     
  16. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

    Messages:
    1,269
    Yes, there's a much simpler way to illuminate the LED - just use the AC for the valve heaters. You just need a current limiting resistor (about 2k) and a diode in series with the LED to protect it from reverse voltage when the AC goes "the wrong way".
    I wouldn't have the tube sockets on the lid unless I was building it that way from the start. You could put the PCB on tall stand-offs so the tubes poke through holes in the lid but some of the capacitors on the board are a bit big for that. I can't see the details of the board but it may be possible to put those caps on the other side of the board if the holes are through-hole-plated.
     
    Lavane likes this.
  17. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    6,153
    Location:
    northern cal.
    Since there are slots on the bottom why not use a unibit and bore some 1" holes In the top in a nice patern wnd cover the holes with screen or perf. Steel under the top? rothwellaudio has a nice idea but its easier to measure for tube holes with a blank pcb as a template. And mount the taller parts like caps under the pcp making sure of orientation. Ive tried the running wires from a board to sockets mounted on top and had hum issues to deal with from all those wires.
     
  18. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Thanks for the advice. Looks like I'm back to the drawing board a little then. Change of plan will look to:

    1. Remove the LED transformer and use the heater voltage feed (15v) to power the LED in the main switch with a 2k resistor and diode in series
    2. Desolder the large caps and flip them underneath the PCB so I can lift the whole PCB up and have the tubes poking out the top vs mounting to the lid.

    Much more work and redesigning to do.
     
  19. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    Ok. So it took to the amp this evening to fix the issues raised above. First up was thinking of a way of raising the tubes up, the Bakelite extensions I bought and soldered wires to are perfect in fit and lift, but the ones I've got are ruined now as they have solder in them. So I've ordered 3 more.

    The 7 pin socket was not as easy to find a solution for so I decided to lift the pre existing socket. First I desoldered it from the board, then cut some TIG welding rod and soldered it to the socket legs:

    [​IMG]

    Some heat shrink and soldered it to the board:

    [​IMG]

    I used the old PCB upper stays to lift the PCB as high as possible:

    [​IMG]

    Next up I cut the holes in the top and test fit the tubes to see how they would look:

    [​IMG]

    Plenty out the top and will get a nice glow from these as the heaters are all pretty much above the lid.
     
    Last edited: Oct 21, 2016
  20. neevo

    neevo Well-Known Member

    Messages:
    723
    Location:
    Sydney, Australia
    I've also removed the LED driver and need to work out what parts to buy to drop the 15V AC feed for the heaters to 12VDC.

    Anyone guide me on that?
     

Share This Page