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Spare (Blues Jr) Parts Build

Discussion in 'Shock Brother's DIY Amps' started by mcabe, Jun 16, 2015.

  1. mcabe

    mcabe Tele-Meister

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    After completely rebuilding my Blues Jr., I have the old cabinet, speaker, power transformer, and output transformer lying around. Since these tend to be the most expensive parts of an amp, I started to look around for low watt push-pull amps that I could build. I ran across the Dragonfly on the AX84 site that looked like a good option. Here is the schematic courtesy of Doug Hammond.

    The Dragonfly calls for a Hammond 269JX power transformer which is rated for 500VCT at 69ma. The Blues Jr transformer is 240V at 125ma with no center tap. So the first change is that I'll need to use a bridge rectifier instead of a two phase rectifier. Unless I am totally off, I think the B+ will be pretty comparable.

    The second issue is that the output transformer for the Blues Jr. has a much lower impedance (~5000 ohms) than what the Dragonfly calls for. But using both ECC99s, I think I should be able to power an 8 ohm speaker just fine.

    So what do you think, does this project have a shot, or am I missing something?
     

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    Last edited: Jun 17, 2015
  2. mcabe

    mcabe Tele-Meister

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    Here is a layout that I put together for a 5x13x2 chassis. I plan to mount it hanging from the back of the cabinet like the stock blues junior. This layout follows the schematic with the following modifications:
    1. uses a bridge rectifier;
    2. has elevated heater voltage;
    3. uses a 20 volt lamp powered by the unused winding on the blues jr. transformer;
    4. changes the location of the standby switch to before the bridge rectifier;
    5. does not have an option of using only 1 ECC99 for 2 watt operation; and
    6. adds a pre phase inverter master volume.

    So how does this layout look? Any suggestions for improvement?
     

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    Last edited: Jun 17, 2015
  3. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    B+ will be significantly higher- best to plan on ~350B+ so you may need to do some additional voltage drop. The OT really needs to be ~22k for the 12au7. If you wanted to run the stock OT, check into a 6n1p output section. May be a good fit here.
     
  4. mcabe

    mcabe Tele-Meister

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    Thanks Dan! Unfortunately the dragonfly schematic doesn't list voltages, but I've seen one person report that he had 340V B+ on his dragonfly. From what I've read over at valvewizard, the output voltage from a bridge rectifier or a two phase rectifier is 1.4xVrms. B+ for the Blues Jr. should be around 340V (~240x1.4) like you said.

    The Hammond 269JX is 500VCT, so shouldn't that be close? The voltage from the center tap to secondary should be half of the total voltage, so B+ would be 350V (~250x1.4). I think earlier dragonfly designs were based on the firefly which runs at a much lower voltage.

    The dragonfly uses ECC99 output tubes which are beefier than the 12au7. I also have a 16 ohm speaker lying around, so hopefully the Blues Jr. output transformer will sound OK in this set up.
     
  5. mcabe

    mcabe Tele-Meister

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    Here are the spare parts and new chassis that (if all goes right) will be transformed into a working amp in the near future. I found an old 1970s era drill press on craigslist that I am hoping to pick up this weekend. Should make drilling the chassis and staking the turrets much easier. Othewise, I have all the components on hand and am ready to start building.
     

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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Here's the schematic I used for my Firefly build, it shows voltages:

    [​IMG]

    Notice it shows a 207v non center tapped power transformer using a bridge rectifier and also a 190-0-190v (380v) center tapped PT with a conventional two diode full wave rectifier.

    [​IMG]
     
  7. mcabe

    mcabe Tele-Meister

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    Cool build Rob! The dragonfly uses a different power transformer (Hammond 269JX). It's the ECC99 version of the firefly.
     
  8. mcabe

    mcabe Tele-Meister

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    Here's the new (old) drill press that I was able to grab off of craigslist. It's a Rockwell 11 inch press. I think it's from the 1970s based on the yellow protective guard, but the seller had no idea when he got it. I think it had been sitting in his basement for years with very little use. Should be more than adequate for everything I'll need. Seems much more solid that the inexpensive new drill presses you can get at homedepot.

    First order of business was building an anvil and swaging tool to stake turrets out of some 5/16 inch hex bolts. Tubedepot has some great videos and instructions, BTW! Hopefully, tonight I can start drilling the chassis and turret board.
     

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  9. mcabe

    mcabe Tele-Meister

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    I have been using DIY Layout Creator for this project, and it has been really useful. Other than planning layouts, I have also used it to create drill plans for the chassis and turret board. It is easy to delete all the components and wires, and then export a layout with only the potentiometers, turrets, and eyelets to a pdf. Here is a picture of the turret board drill plan.

    DIYLC will also create a bill of materials. I still somehow managed to make 3 separate trips to homedepot and far too many mouser orders, but it's a nice feature nonetheless.

    I am sure that there are other tools that can do a lot more, but I appreciate the simplicity of DIYLC.
     

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  10. mcabe

    mcabe Tele-Meister

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    And here is a picture of the chassis drill plan and drilled chassis. I cut out the power transformer hole and IEC socket using a jigsaw and file. The holes are pretty rough and a little crooked, but everything fits. Not too bad for a first build, although any tips for future builds would be appreciated!
     

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  11. Commodore 64

    Commodore 64 Friend of Leo's

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    Cool little project you have here. I need to repurpose my harbor freight drill press for turret staking. I need to build the anvil/swage setup too.

    I've drilled a lot of holes in chassi, and I always end up using my hand drill with step bit as opposed to the drill press.

    I have a nibbling tool, and my square holes are always ragged. I've used Hammond steel and aluminum boxes for chassis, but given my druthers, these days, I use a PPwatt pre-punched chassis whenever possible (whether I'm using a ppwatt PCB or not).

    http://www.tubeface.com/chassis

    The jr. chassis has pilot holes, not hole cutouts so it's a bit more versatile for eyelet/turret builds.
     
  12. mcabe

    mcabe Tele-Meister

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    The Universal Chassis has the tube sockets right in the middle of the amp which might not work with some layouts. The Jr. chassis looks pretty good though.

    After this project I am only going to work in large chassis though. Everything is more difficult working in tight spaces. Last night I found out that the speaker output jack won't fit. The turret board is 3 inches wide and the input and speaker jacks are more than an inch long, which is a problem for my 5 inch wide chassis! Current plan is to run the speaker wire into the amp and internally solder it to the output transformer leads. Luckily this is a combo, and I don't absolutely need an output jack.

    In the picture below, the speaker wire will enter through the rubber grommet between the tube sockets and the IEC. I mounted a standoff in front of the grommet to attach the speaker wire and output transformer leads. Planning on using heat shrink tubing to help hold everything in place. Think any additional strain relief is needed?

    All the hardware is attached, and the turrets are staked. Hopefully there won't be any more surprises. At least I know everything will (barely) fit now.
     

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  13. mcabe

    mcabe Tele-Meister

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    I am having second thoughts about wiring the speaker directly to the output transformer. Sounds like if I forget to plugin the speaker, the resulting open circuit could cause major damage. I have some 5 watt 250 ohm wire wound resistors. Would running one of those across the output transformer secondary be enough of a safety feature, or should I find a way to fit a shorting jack?

    Here is the current progress. Turret board is done except for the section that generates elevated heater voltage. Next step is to wire the potentiometers and heaters and mount the chassis in the amp.
     

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  14. mcabe

    mcabe Tele-Meister

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    Made some good progress on the amp this week. The chassis is about 3 inches smaller than the Blues Jr. cabinet, so I screwed a couple of pieces of 2x4 to the sides, so it fits snugly. Problem is the the screw heads stick out enough that the turret board no longer fits! Time to break out the file, and make a couple notches in the board.

    The push-pull pot and the standby switch are pretty large controls, that are also giving me some trouble with fitting the turret board. Stopped by radio shack this morning and got a smaller standby switch and a mini-toggle switch for the boost, so I can replace those controls with something smaller.

    Lessons learned:
    1. Always get a bigger chassis than you think you need.
    2. Install all the hardware needed to fit the chassis into the cab/head before you do anything else.

    Anyway, here is the current progress. Tube sockets and transformer leads are all dressed and wired. Decided to leave out the speaker jack and wired a 250 ohm resistor between hot and ground to protect the output transformer. All that remains is to wire up the pots, and install the turret board.
     

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    Last edited: Jun 30, 2015
  15. Commodore 64

    Commodore 64 Friend of Leo's

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  16. mcabe

    mcabe Tele-Meister

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    Funny you should mention the standby switch. I was reading up on the misunderstood control yesterday. I was thinking I could possibly move the power switch over, omit the standby switch and put a fuse in the extra hole. Unfortunately the fuse does not quite fit, and I don't want to have an empty hole in the front panel.

    One thought is to turn the standby switch into a mute switch. The master volume is close by so it would be easy to ground the signal with the same switch. Not much harder to do a proper stanby. So standby or mute? Which is better?
     
    Last edited: Jul 1, 2015
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Be careful running signal lines close to the AC power wires and power switch. I ended up having to relocate my 120v power light on the Deluxe Micro because it was next to the input jack (Duh!) and injected a lot of 60Hz hash noise. Adding a tiny SPST ON/OFF mini-switch might be a better idea for a mute switch.
     
  18. mcabe

    mcabe Tele-Meister

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    That's a good point. The signal wire to the mute switch would be like a little antenna for all that AC noise. There are no other wires or controls between the master volume and standby/potential mute switch. The AC power would probably be about an inch away from the signal wire. How close was your signal wire to the 120 volt lamp when it gave you trouble?
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    About 1/2 inch from signal terminal to AC terminal (again, Duh! on my part).
     
  20. mcabe

    mcabe Tele-Meister

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    I don't know about you, but I learn the most when I make mistakes. The signal at the master volume should be less susceptible to interference, since it is already amplified. I could probably wire the switch as a mute without problem, but you definitely have me leaning towards standby.
     
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