Little Bear T10 Chassis Upgrade

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

  1. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Thanks @rothwellaudio appreciate the encouragement. I would love to have a go at point to point in the future as I have found the process of building this chassis great fun.
     
    Last edited: Nov 5, 2016
  2. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Finished off the chassis today as my aviation plugs came in for the transformer umbilical.

    Mounted on the back:

    [​IMG]

    [​IMG]

    I still need to wrap the cables but it's good.

    So I plugged it all in and I have a new noise through the speakers. A regular tick, tick, tick... about 3 per second.

    Haven't diagnosed by pulling things apart yet but wondering if anyone knows why?
     
  3. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

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    That kind of noise doesn't sound like the sort of thing that analogue electronics generates easily. My guess is that it's noise picked up from something near by. Is there any other equipment in close proximity?
     
  4. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Let me check when I have some time at home. Trying to work out what's different as the setup is essentially exactly the same, however I did replace the power board from a 4 way to a 6 way as I needed extra. Will change that back out for the old one and see what happens.
     
  5. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    If the problem isn't motor boating I don't know what it is. Sounds like a distant boat chugging along a river.

    Tested a few things tonight to see how I could affect it:

    1. Changed to old power strip. No change
    2. Removed phono input. Chugging sound completely disappeared but loud hum was introduced
    3. Touching the chassis made the chugging quieten down but only by about 25%. Couldn't get it to disappear.
    4. Tried disconnecting 0V rail earth. No change

    If I can get this chugging sound fixed the amp will be awesome as the noise floor on it now is uber quiet. Unfortunately the chugging is at levels that is audible when the vol is cranked up so I'll have to track down where it's coming from.
     
  6. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Trying to think about what I changed since the last setup to see if something could be causing the issue.

    1. Umbilical attached to separate the transformer (doubt its the cause as I was running the cables there is test before anyway)
    2. Extended the tube sockets (not by much though as last time I did that with rectifier only it made no diff)
    3. Had a broken pad on a main power feed to the rectifier tube (tube wasn't lighting up). Replaced with wire, long bit, fairly thick and tried to copy the trace on the board. Could this cause an issue? Read something about impedence on wires to tubes causing motor boating.
     
  7. Lavane

    Lavane AK Subscriber Subscriber

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    northern cal.
    You used mig wire to extend the sockets? It may be copper coated but it may also be resistive? Maybe like using a metal coat hanger as wiring.
     
  8. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    I used solid copper wire for the main tubes and the TIG wire for the rectifier. Might look to remove that TIG wire and find some thicker copper wire instead.
     
  9. Lavane

    Lavane AK Subscriber Subscriber

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    I worked in a welding shop for 9 years and we used a lot, of core and 35 wire. I would think it would be resistive to some point since you want it to melt and weld. I could be wrong though.
     
  10. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    I'm keen to remove it as a possibility so I've purchased some solid 1.6mm copper wire and will replace the TIG rod I used. Will also allow me to replace the fix for the broken pad by extending the leg slightly vs having to use a wire.
     
  11. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    My transformer chassis came in the other day, so I set about mounting that up and soldering up the wires.

    Mounted:

    [​IMG]

    Chassis finished:

    [​IMG]

    Pity about the motorboating noise in the audio signal as otherwise I would be mostly finished (except mounting the tube guards).

    Nearly there though.
     
    Last edited: Nov 12, 2016
  12. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

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    ?? Shouldn't that multipole socket be on the back? A power-on switch and LED on the front would look a lot nicer.

    On second thoughts, I was the one who said power supplies should be ugly and kept on the floor, wasn't I?
     
  13. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Hahahaha. Correct @rothwellaudio as this will be hidden underneath my turntable cabinet out of sight. Power switch has been kept on the main chassis (so I thought I would ugly it up by putting connector on the front).

    The only things that are a concern with this idea are:

    1) the chassis is not earthed, so I've run an extra wire down the umbilical to connect each multi pole connector and effectively linking both chassis in case anything goes wrong with the transformer chassis
    2) if the umbilical is not connected and the amp is switched on, then 2 pins exposed on the back will be 240v, I tried to find a connector that had the male part as the chassis connector but no luck. It's something to be aware of and luckily the connectors lock in very well so chances of falling off are essentially zero.
     
  14. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    @rothwellaudio do you think my motorboating issue could be caused by the TIG rod I used to extend the 7 pin socket as lavane suggested? I've ordered some copper wire to replace it anyway.
     
  15. rothwellaudio

    rothwellaudio Forums Sponsor Sponsor

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    Doesn't seem likely to me. You described the noise as a ticking sound - I don't think motorboating makes that noise. Anyway, the problem may be caused by the power supply voltage being modulated. You could look at it with a 'scope if you have one. If not, the frequency might be low enough for you to be able to see something with a voltmeter. A digital meter might show it but an old-fashioned analogue meter will probably show it better.
     
  16. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Ticking isn't the best description in hindsight. It sounds like I low sound at a very consistent frequency. Like a little boat far away.

    Goes up and down with the volume on the amp. Also linked to the gain on the PCB (gain down, sound goes down). I can make the sound get quieter if I put my hands on the chassis. Sound completely goes away if I unplug turntable but then I get a big hum.
     
  17. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    I finish my umbilical cord for the transformer today. Wrapped it in braided shrink wrap and annoyingly forgot to thread the earth wire through it, so I'll need to fix that up too:

    [​IMG]

    So essentially the preamp is complete mechanically. All that is left to do is drop in the tube extensions when they turn up and thread the lid for the tube guards (waiting on some bolts).

    Pretty happy with the end result and it's been a fun project.

    On to the sound issues I'm having. Well I have a development there too. I have some new speakers that I'm getting set up and today I was giving them a go when I noticed the same sound through the speakers. Odd as I didn't have the preamp installed. So here's what I'm assuming:

    1. The previous test where I disconnected the TT from the preamp made the sound go away, but I got hum, so it would appear the sound is originating from the TT
    2. The sound appears to be worse with the preamp installed which I assume is because it's adding gain to the signal (I have to run less volume with the preamp installed for the same SPL)

    So I'm on the hunt for working out how to solve the sound from the TT now.
     
    Last edited: Nov 20, 2016
  18. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    I redid my interconnects too this evening. Changing out the ends for something more aesthetic:

    [​IMG]

    I also noticed on the cable that it has a direction indicated. Didn't know cable was directional!

    Either way I thought I would completely strip the system down tonight and see if I could isolate where the noise was coming from.

    Well good news and bad.

    Good news: I found where the sound was coming from. I turns out my EOP on the circuit is causing the "motorboating" sound. Fixed temporarily by shifting power points.

    Bad news: background hum is there. Not too bad that I can't live with it, however on quiet passages on when a record has finished I can hear it and so would love to get the noise floor lower still (wouldn't be surprised if it was still the interconnects).

    Something to work on.

    Noise or not, I'm loving listening to records through the preamp again, it does wonders for vinyl and my new speakers are sounding better and better each day :)

    [​IMG]
     
    Lavane likes this.
  19. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Found this on the web. Is this a suitable test technique to try and determine where the hum is coming from (specifically the part where you short the phono input on the preamp)?

    Blair G.Forum Resident
    Found this on the net, from the KAB site:

    Item three is where I'm focusing. Do their suggestions for testing make sense and is it safe?

    The Stereo
    Switch the stereo to CD or AUX, and with nothing playing, turn the volume up to your normal listening position. If there is no hum now, then we can eliminate the stereo. If there is a hum, powersupply service is probably indicated.

    The Phono Preamp
    This will require one accessory, a shorting plug. You need to get a couple of standard RCA plugs from Radio Shack for instance, and you need to short the center pin to the outside ground. Now, in place of the turntable, plug the shorting plugs into the phono input. Now, set the stereo to phono and turn the volume up to your normal listening position. If there is no hum now, then we can eliminate the phono preampor phono stage. If there is a hum, phono stage service is indicated.

    The Turntable Wiring
    This will require one accessory. a pair of alligator clips. You can also get these at RS. You need to clip together, I.E. short out the left and right cartridge pins. Do this on the back of the cartridge. You needn't remove the cartridge connections. Just connect one pair of clips between the red and green pins and another between the white and blue pins. Now, set the stereo to phono and turn the volume up to your normal listening position. If the hum is gone now, then we can eliminate the turntable wiring. If there is a hum, something is amiss with the turntable wiring. either a bad connection. or perhaps someone has changed the factory wires for some "fancy" wires that do not give sufficient shielding.


    The Phono Cartridge
    If you've come this far, then the problem must be the phono cartridge.
    Most cartridges use hi permeability steel shells to protect the coils from electrical fields that can cause hum. However, not all companies use this system. As such , there are some cartridge brands that are sensitive to external electric fields and will hum. The only solution you have is to replace the offending cartridge. Or, if you love the sound and want to keep it, you will have to play with the location of the turntable and try to minimize hum. Sources of hum fields are power transformers in equipment, wiring in the walls, certain turntable drive motors. Experiment by listening to the hum while you move the tonearm through its arc(cued up!!) and see if you can find a null location that will give you the best results.
     
  20. neevo

    neevo Well-Known Member

    Messages:
    696
    Location:
    Sydney, Australia
    Used a couple of spare RCA connectors and shorted the outer and inner connectors as per the instructions above.

    The result was the same hum level as with the TT connected, so I've determined the hum is coming from the preamp.

    I need a plan to try and make the amp better. Currently thinking:

    1. Still replace the TIG wire for copper (just to rule it out).
    2. Extend my Alu mesh around the ends to try and get more coverage for the signal wires
    3. Try making some covers for the tube socket extensions
    4. Play around with the wire routing for the power feeds and power switch to see if that makes a difference.
     

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