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Thread: (Slow) Build log for Vox JMI-era AC15 OA-031 circuit

  1. #36
    Don't forget the joker g1's Avatar
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    Quote Originally Posted by dchang0 View Post
    I haven't figured out how to get all the wires at that point onto the cap can's terminal.
    Maybe one of those 2 into 1 spade lugs would work for this?

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    They make such a thing? Awesome!

    Unfortunately, I don't know what keywords to use when searching Digikey/Mouser. So far, typing in "spade lug" pulls up single, crimp-type male spade connectors, no splitters or adapters. Any suggestions on keywords?

    I suppose in the end, I could solder a 1/2" length of thick bus wire into the slot in the terminal and then solder the other wires to the bus wire.

    Alternately, I could cut some of the heat shrink off the leg of the 22.1K resistor and solder the wires to that, but it seems flimsy and easy to break. Would make changing that resistor out later on tough, too.

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  3. #38
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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  4. #39
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    Awesomeness! Thanks very much! The chair adapter will be perfect.

    For anyone reading this thread, I took the keywords from the links g1 provided and searched Digikey, who calls the class of similar items "Terminals - Adapters"

    http://www.digikey.com/product-searc...apters/1442878

    I'll measure the blade widths and pick the right one(s). I'll move on to the aluminum chassis while waiting on these parts to arrive.

    EDIT: Blade width on the F&T 16uF+16uF 450V cap can is 0.165in. That's pretty small; I had to cheat and get a terminal adapter meant for 0.187in. quick detach blades:

    http://www.digikey.com/scripts/DkSea...48647369071451

    I am hoping the curled-in parts grab the F&T's terminal tab, which is split like a forked-tongue. It is possible they will fit into the middle, empty slot and not grab at all. If that occurs, I guess I'll crush the adapter so it grabs the forked-tongue.

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    Last edited by dchang0; 12-05-2015 at 04:19 AM.

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    Added B+ center-tap fuse to ground. Ground is the center lug on the same terminal strip used for the heater wires.

    No further soldering on the steel chassis until the diodes and terminal adapter arrive.

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    Last edited by dchang0; 12-05-2015 at 04:23 AM.

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    Had to drill one more hole in the aluminum chassis that already looks a bit like Swiss cheese. This hole is for the 1in. P-clamp that holds the F&T cap can.

    Due to the large size of this cap can and the way the P-clamp sticks out pretty far to one side AND the fact that the power transformer is going to sit flush against the other side of the chassis, there were relatively few places this hole could be drilled. I lucked out and found a spot between the legs of the power transformer's bell cover.

    Another way to position it would have been to flip the P-clamp around so that the legs point to the left and drill a hole near the V2 socket. This would have moved the cap can about 8mm to the left and 8mm down with this particular P-clamp. I chose the current location because the hole is in a relatively out-of-the-way spot if it turns out that I have to move the P-clamp somewhere else, AND I was hoping to use it as the safety ground bolt-down to the chassis, but I've decided to use the nut trapped under the fuse block for the safety ground. That screw contacts the power transformer's bell cover very firmly, so it would serve as a better conductor to even the lower, steel chassis. The trapped nut makes it more likely that the safety ground won't come loose, a lucky accident.

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    NOTE: I got this P-clamp from Home Depot but do not like it. The hole that is punched in it is very, very large, so large that an 8-32 pan head almost goes through it. I would rather have bought an Essentra Components aluminum 1" P-clamp, but Digikey and Mouser do not stock these in single units. Essentra does ship free samples, so that is one way to get this clamp. In the end, it was a good thing that I didn't get the Essentra one because I needed the clamp to bend upwards to clear the nut for the screw securing the PT bell cover's left foot to the aluminum chassis (see it under the P-clamp in the photo). Home Depot's cheap clamp bends easily.

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    Last edited by dchang0; 12-05-2015 at 05:43 AM.

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    Upper chassis now has heater wires and pilot lamp wired.

    The sockets I bought aren't clocked the same way as the sockets JMI used, so the layout sort of worked against me. Oh well.

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  8. #43
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    Finished the interconnects on the upper tagstrip board (nearer the controls). Yeesh--so tedious. The little cross-wires between the tags are so numerous. The photos don't look like much was done, but it took a long time. This would be the downside to using tagstrip boards instead of eyelet boards or turret boards. I imagine sluckey's turret board layout is a lot easier to finish.

    I will test the board out with a multimeter and match it to the schematic before starting on the lower board.

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  9. #44
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    Happy New Year, guys!

    I finally got around to working on the build again, this time finishing up the steel chassis with the parts I ordered.

    First of all, g1's quick disconnect adapter idea was brilliant, but alas, due to the usual size and shape of the F&T capacitor's solder lugs, the smallest adapter I could find still didn't fit, and forcing it to fit would have been too much work. So I went ahead and just soldered all five leads to the single lug. You can see the quick disconnect adapter in the first photo.

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    Over on the PT side, I added the diodes and finished wiring up the rectifier. I went with a pair of single diodes instead of a pair of double diodes since I didn't want to drill a hole for another 3-lug terminal strip.

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    The heater wires and the virtual center tap aren't done yet--those will wait till the joining of the two chassis together. For the most part, the steel chassis is complete minus the wires that run to the aluminum chassis.

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  10. #45
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    All intra-board wiring for the tagstrip boards is done. Just need to be double-checked, and then they can be bolted to the aluminum chassis.

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    Last edited by dchang0; 01-11-2016 at 11:14 AM.

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    The tagstrip boards tested A-OK and were installed in the aluminum chassis.

    Working on the interconnecting wires along the top edge of the upper tagstrip board (closest to the pots). Note the Brilliance switch is replaced by a bass-cut shelving control from figure 10.7b on page 233 of Merlin's book on preamp design. I have no idea whether this will sound good or not; I just didn't want to go with 6-discrete-positions switching. I also am not sure whether I got the clockwise/counterclockwise direction correct--it should be easy to switch out either way.

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    Last edited by dchang0; 01-14-2016 at 07:24 AM.

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    Installing the bus wire through the input jacks. I had to bend the bus wire towards the chassis behind where the chassis screw goes through the cage nut so that it would not short and create a ground loop.

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    Wired up some of the pots.

    Note that the Vib/Trem pots are the Ted Weber mods:

    1) The 500K Audio trim pot (VR4) that normally sits on the lower tagstrip board is now an actual pot on the console. This pot has a push-pull on-off switch (SW3) for the Vib/Trem effect. Pull will be Vib/Trem off, push will be on.

    2) The 1M Audio trim pot with the 220K resistor sticking out from it will replace the Vib/Trem Speed 3-way switch (SW1).

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  14. #49
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    On a friend's 60s JMI AC50, I found that the cause of hum was a 0V loop created by the 0V bus wire along the controls and jacks, as the 0V return from later stages (eg tone controls) interferes with that of earlier stages.
    Separate 0V returns for each stage, back to a pre-amp star point on the pre-amp chassis, as per Merlin's multiple star ground arrangement The Valve Wizard resolved the hum.
    Just something to bear in mind in case you find it's a problem.

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  15. #50
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    I wondered about that, while examining the ground scheme on the vintage layout. Thanks for the heads-up.

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  16. #51
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    First Power Up Comments

    dchang0,

    Have you formulated a plan for how you will do first power up and testing of this project? Your work is meticulous but the finished amp is going to be difficult to troubleshoot because of the compact space and the lack of wiring service loops. From experience I know the pain of undoing connections to troubleshoot. A lot of high quality electronic equipment has been hand built like your project so I'm not suggesting that your approach is bad. In fact, when everything is correct, you will have a beautiful creation and the potential for lower noise and increased stability.

    There are often questions on MEF regarding the recommended sequence for powering up a new build for the first time. An incremental first power up approach can be very useful for a first article build like yours. Are you familiar with first power up procedures? When you are in control of the complete, from scratch, build you donít even need to wait until the wiring is complete to start checking out live circuitry. For example:

    1. Complete the power supply wiring and verify the current draw (No fuse blowing) and open circuit voltages are correct. Note that there are sub-steps to this such as first test without the rectifier tube installed. This is also an opportunity to do some burn in to check for infant failures, excessive heating etc.
    2. Complete the power amp and confidence test the operation. Verify that the hum level is acceptably low. If there is too much hum then trouble shoot the problem before continuing. No need to complicate things by having a pre amp connected to a hummy PA.


    You donít need 100% final dressed wiring to do the preliminary tests. Some connections can be temporarily solder tacked in place or connected with clip leads.

    I hope this unsolicited input is useful to you. Even if all your wiring turns out to be perfect you will be sequentially verifying the integrity of the individual components. Itís no fun to find out at the end of a long build process that something like the power transformer was bad as originally received. It happens.

    Cheers,
    Tom

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    Hi, Tom--

    All constructive input and criticism is welcome--not just for myself but for other readers who may stumble across this log looking for tips on building an AC15.

    I haven't thought too much about the first-power-up process. I do have an incremental-power-up procedure saved somewhere from the other times I built amps, but I haven't looked them up yet for this build. Whatever this procedure is, it was the one recommended by amp-builders at the time several years ago. Maybe a newer, better write-up is available now.

    Obviously from my "final dressed wiring," I'm not guarding against failure. The most I'm doing is triple-checking my work as I go (to one layout and two different schematics), but this does not guard against, as you pointed out, a bad PT.

    A few questions: (If it's easier to point me to some other thread than answer here, please do so.)

    1) What is a "wiring service loop," exactly?

    2) Do you have a recommended first-power-on procedure written up somewhere? If it is the same as what I had written down, great. If it's different or better than what I had, even better!

    3) If it turns out I did receive a bad PT or OT, how do I prove to the seller/mfg. that it was bad as received and not that I blew it up (so I can ask for a free replacement under warranty, however long or short that warranty might be)?

    Thanks for the tips. I will follow your advice in points 1. and 2. that you posted and perform some testing sooner rather than later.

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    Last edited by dchang0; 01-14-2016 at 05:38 PM.

  18. #53
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by dchang0 View Post
    ...What is a "wiring service loop," exactly? .
    Simply enough slack that you can remove and replace components without the need to completely replace a wiring harness. Sometimes utilized by leaving long leads on items like transformers until the exact placement/orientation is verified. Then they can be shortened up or stowed in a bundle. A related consideration is making sure that parts are not trapped behind other parts. At lease not too many occurrences. Donít want to be like some old cars that required removal of the starter to get to the last spark plug.


    Quote Originally Posted by dchang0 View Post
    ...Do you have a recommended first-power-on procedure written up somewhere? If it is the same as what I had written down, great. If it's different or better than what I had, even better! .
    I donít because I work alone so the list is in my head.


    Quote Originally Posted by dchang0 View Post
    ...If it turns out I did receive a bad PT or OT, how do I prove to the seller/mfg. that it was bad as received and not that I blew it up (so I can ask for a free replacement under warranty, however long or short that warranty might be)? .
    Donít worry about things until they happen. That was just an example of something that could happen. A reputable manufacturer will make it right if the time frame is reasonable. Thatís why, especially for a build that will take a long time, itís advantageous to confidence test major parts sooner rather than later.

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  19. #54
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    Cool--thanks!

    I did take a while between ordering the transformers and actually starting the build, but it is certainly still under a year. Better get my butt in gear and finish this amp!

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    One idea I got from m-e-f, and R.G. Keen in particular, is to form the electrolytic PS caps before the signal tubes are installed. A 100k resistor temporarily soldered between the rectifier and first filter cap allows the caps to see full voltage while limiting current to the oxide-forming level. Lets you catch leaky caps before they blow up in an amp, too: http://music-electronics-forum.com/t36367/#post344622

    Whether this is necessary with modern caps is debatable, but it feels like "good practice" and I enjoy rituals. I'm thinking about building a small PS jig to occasionally (re)form the caps I have in the drawer.

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    Last edited by ThermionicScott; 01-14-2016 at 07:37 PM. Reason: tried to make it a little clearer

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    Man, this place is a treasure-trove of knowledge--I had no idea that caps, modern or not, could be "formed." Thanks for the tip!

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    By the way. I suggested the incremental power up, starting even before the amp is finished, just because it makes sense to check out one thing at a time so that any potential problems are isolated to one section of the circuitry at a time. It makes sense even with a simple amp like a Fender Champ.

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    Thanks, Tom.

    I'm dividing up the work into stuff that is easy to change after the two chassis are joined and stuff that is difficult to change after they are joined. Some connections will always suck to access. For instance, getting at the upper edge of the top tagstrip board (the edge that is just under the pots) is never going to be easy once the lower edge of that same tagstrip board is soldered to the tubes.

    Put another way, the only way to build this with the intention of testing everything incrementally would have been to assemble the circuit outside of the final two chassis, using temporary fixtures, etc. Now, I'm just going to have to proceed with the fact that I have painted myself into a corner in some parts of the build.

    Anyway, I've wired up the top edge of the top tagstrip board (fairly easy--there are only a handful of connections) as well as most of V5 and all of V6 on the bottom edge of that same tagstrip board. Photos are attached.

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  24. #59
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    That looks very tidy!
    Just to note that I've encountered ground loop problems in old Voxes that seemed to be caused by the shared buss used by the input sockets and all pot 0V connections.
    Hopefully it will be fine, but if not, following Merlin's guidance on the topic can work out very well The Valve Wizard
    The multiple star arrangement described in 15.8 probably being most applicable.

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    Dang--I wondered about the grounding scheme but did nothing about it... Well, I'm prepared to cut the bus wire to break the jacks off of the pots. Thanks for the tip!

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    It's probably more of an issue for models with the higher gain / additional cascaded stages required for tone stacks (top boost in Vox speak).

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    Okay, thanks--I'll probably skate by on this grounding scheme then. No plans to add any gain stages nor Top Boost--ever.

    A little more progress: all but one pin on V1.

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    Last edited by dchang0; 04-07-2016 at 04:12 AM.

  28. #63
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    I suggest to put more of a loop on the EF86 yellow plate wire, to keep plenty of separation between the heater and signal wires (especially early, high impedance circuits), eg 1/8" / 3mm, as if the wire insulation was another 1mm thicker.
    Bearing in mind that the impedance of the pentode's plate circuit will be pretty much equal to its plate resistor, 220k.

    For the 12AX7s, rather than running the red heater wire around the outside of the socket body, consider Merlin's method of taking them across the centre of the socket body (best to remove the socket's centre terminal, where fitted); see The Valve Wizard but yours would be flipped around 180 degrees, so that the red wires would run through the space between terminals 1 and 9.

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    Last edited by pdf64; 04-07-2016 at 09:38 AM.

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    Thanks for the tips--I can definitely move the EF86 yellow plate wire. The red heater wires might be highly impractical to move now.

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    Last edited by dchang0; 04-07-2016 at 07:42 PM.

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    Fixed the EF86 yellow plate wire.

    Also finished soldering V2 and V7. That means that all the valves on the upper chassis have all their pins connected to their destinations except the two shielded wires running from the inputs.

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  31. #66
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    Finished wiring the input jacks to the valves. I followed the OA-031 schematic rather than the commonly-seen alternative on the TurretBoard.org layout. As I understand it per this discussion, http://vintageamps.com/plexiboard/vi...p?f=2&t=108211, the original OA-031 wiring results in lower noise. I also saw that BLC 15s use the original OA-031 wiring.

    I couldn't tell for sure whether J1 or J2 is supposed to go to the right or left from the many gut shots I found of vintage JMI AC15s (all of those I found use the alternative wiring documented by TurrentBoard.org's layout), so I just followed BLC 15's choice of J1 (marked 2) on the left and J2 (marked 1) on the right.

    In other words, I didn't find any gut shots of JMI-era AC15s that had true-to-the-schematic OA-031 wiring.

    The Vib/Trem Speed control is a pot, not a rotary switch, and the idea is taken directly from Ted Weber's 6V15 kit that never made it to market. It might be wired backwards (clockwise or counter-clockwise)--we'll find out which way is correct after the amp is finished.

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    Last edited by dchang0; 07-05-2016 at 05:40 PM.

  32. #67
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    Finished the tremolo push-pull pot and the cut pot (that was miswired in all previous photos). The tremolo push-pull pot is inspired by Ted Weber's 6V15 but doesn't exactly follow his schematic. Mine follows the original OA-031 schematic exactly, just combining the Depth trim pot and the rotary switch into one control. It MIGHT be wired backwards (clockwise/counter-clockwise) in the pot and also in the push-pull switch. These will be corrected if needed after the amp is finished.

    I also added the "floating" 0.1uF caps that connect the two tagstrip boards, C29 and C31.

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    Last edited by dchang0; 07-10-2016 at 09:44 AM.

  33. #68
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    I finally got around to double-checking everything that is currently soldered on the upper chassis to the OA-031 schematic. The plan is to join the two chassis together next, and I wanted to minimize tearing stuff apart in case there were some errors.

    One thing I discovered was that Turret's full-color layout and matching schematic has the Cut pot wired in backwards in that C11 is connected to C7 when it should be connected to C9. That means I originally had the Cut pot wired in correctly, flipped it around to match Turret's layout, and now flipped it back.

    Now the wiring matches the factory OA-031 schematic. The Cut pot may still be in backwards in terms of clockwise vs. counter-clockwise, but at least the fix will be easy--just move the jumper between the center leg from left to right.

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    Last edited by dchang0; 09-13-2016 at 04:02 AM.

  34. #69
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    Fastened the two chassis together and discovered that the gap between the feet of the power transformer and the upper chassis is quite large--large enough to warp the upper chassis when the screws are tightened (and pull the control panel out of alignment). I had to insert some 1/16" lock washers to fill the gap.

    Added the missing orange wire between R31 and R56.

    Mounted and wired in C4 (half of the cap can).

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  35. #70
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    Added the C8 to R17 wire.
    Added the C20 to R57 wire.
    Added the C41 to R16 wire.
    Added a temporary wire to sub as the Trem footswitch ground between R54 and R60.
    Wired output transformer to the 3-position terminal block and ground spade (wish it were a 4-position terminal block to make it easier to attach the speaker to ground).

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