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

[dchang0]

The small parts finally arrived.

I drilled the slider board for the speaker and footswitch jacks and used chrome jackplates to hold the jacks. The footswitch jack is wired to be ON when nothing is plugged in.

The local hardware store was out of matte black paint, so I left the slider board holes unpainted.



I finally got to test the footswitch. It shuts off the tremolo as expected, leaving a very tiny bit of vibrato.

[dchang0]

I replaced the unshielded input jumpers with shielded cable, even though it made very little difference. The unshielded green jumpers didn't pick up any significant noise in the first place.

I also added the 330K bleeder resistor as Kevin advised. It was easy--it goes from the positive terminal of C40 to the nearby chassis ground tab. I wish I had a 1W resistor on hand, but 1/2W is double the expected 0.27W. It does not get hot when it does its job. I will probably replace it later with a Dale RN70-series 470K 3/4W resistor rated for 500V just to be safe. The 330K resistor is rated for 350V, too close to the measured 341V for comfort.

All that is left is to order and add the pull-down resistors to the Brilliance control, and then I will install the reproduction copper faceplate. Currently, the bass-shelving control is installed and the 6-position control has been removed.

I removed the two cage nuts up on the top edge of the chassis because as it turns out, the holes are in the wrong place relative to the pre-punched holes in the back panel. At least according to this photo at the Vox Showroom, the square holes are not used anyway.

http://www.voxshowroom.com/ct/amp/uk...15_chassis.jpg

Instead, they use two 10-32 clip nuts. When I eventually order the two brass oval-head 10-32 screws and the matching finish cup washers, I'll drill the holes and add the clip nuts. That may never happen, same as with painting the slider board holes black.

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The amp is basically complete at this point.

[dchang0]

The ordered resistors arrived.

It is a good thing I replaced the 330K Dale RN65 resistor rated at 1/2W and 350V with a 470K Dale RN70 resistor rated at 3/4W and 500V. When I measured the actual voltage it turned out to be 361VDC rather than the prior measured 341VDC, over the rating of the RN65 resistor. It probably would have died over time.

Also included are close-up photos of the sluckey 6-position, 2-pole rotary Brilliance switch mod (reversed from his drawing so that further clockwise = less bass). I am including these photos now before I add the pull-down resistors because the assembly will be complex and cramped. Sorry for the blurry macro photos--no way around the autofocus choosing the wrong subject.

There is a bare semi-circular bus wire going around 6+1 posts of the switch. That's all of the positions of the "blue" pole plus one position of the "white" pole (the most counter-clockwise white position). That one white position is of course the fully-shorted position that passes all the signal through. The other five "white" positions are the various capacitor values sluckey recommended, arranged in order.

[dchang0]

I added the 10M pull-down resistors. The pop is gone from the positions with capacitors, but interestingly, the one position that still has a loud pop is position 1, with no capacitor and just a straight short across (and one 10M pull down under it).

Why would this be? The 10M pull-down on all six of the positions of the first "blue" pole, plus the first position of the second "white" pole, should act as a pull-down for the 0.01uF coupling cap C3, right? Or do I have to reduce the value from 10M to something lower to shunt the DC voltage to ground faster?

Is this caused by the effective resistance mismatch between position 1 (shorting, no capacitor at all), which has a 10M resistor that does not have another 10M resistor following after it as with the other 5 positions with capacitors?

Would one way to work around this be replacing the short on position 1 with some suitably wide coupling capacitor that effectively passes through most or all of the guitar signal, along with another 10M pull-down on the tail end of that capacitor? If so, what would this value be?

EDIT: I removed the 10M from position 1 (all the positions, really), trying to emulate the schematics in the Mr. Black article on pops and pull-down resistors, and the pop is still present. It might even be worse, but that is hard to tell. Next, I will try 2M resistors.

EDIT 2: Stepping down from 10M to 2M definitely made a big difference, and the pop is barely there. Still there, though. I discovered something else--it does not seem to be necessary to have the 2M resistors on the tail ends of the other capacitors at positions 2 through 6 if there is a 2M resistor on position 1 (and thus 2 through 6).

I considered taking the 2M down to a 1M to get rid of the pop, but this will probably affect the volume control too much.

Overall, it is looking better and better to just keep the Merlin bass-shelving cut which has none of these pops or any affect on the volume pot.

If I had to do this all over again, I would follow the original OA/031 Brilliance switch precisely (short out an always-connected 250pF cap or not), thus not dealing with the pop.

[LuciMan]

@dchang0:

Nice work you've done!

I guess you've seen my builds I've done in 2014 & 2015: Surrounded by clones I have used turret's fine layouts too.

Cheers

[dchang0]

Thanks, LuciMan! Yes, I researched this build extensively before starting my own, and your build thread was a huge help. I would have thanked you sooner over at vintageamps.com, but I didn't have an account until just yesterday, LOL. There were a couple of other build threads like the one over at Charlie Rose or something like that that were also a huge help.

BTW, did you run into this problem when doing your builds? Does the choke block access to the tube shield of the one 9-pin tube socket behind it (on the upper chassis)? I refer to this photo (choke has been moved 3/8in away from its mounted position and is still blocking the tube shield as shown):

[LuciMan]

T...BTW, did you run into this problem when doing your builds? Does the choke block access to the tube shield of the one 9-pin tube socket behind it (on the upper chassis)? I refer to this photo (choke has been moved 3/8in away from its mounted position and is still blocking the tube shield as shown):

[ATTACH=CONFIG]41191[/ATTACH][ATTACH=CONFIG]41192[/ATTACH]

Yes - it does here too.
If I have to change the tube, I just remove one screw of the choke. I wonder what they were thinking in those days by placing the choke like this.

[dchang0]

Thanks, LuciMan for confirming that--so I am not the only one!

I am searching for confirmation with owners of the original 1960s JMI AC-15s to make sure they really did have this problem. It might be possible that with different sizes of choke (skinnier ones) and shorter tube shields, they do not have this problem. And if they were able to get around it, then we might be able to find the same parts and get around it too.

I do have another idea that may be easier to change the V5 tube shield and tube:

1) Remove back cover and slide out slider board no more than half way. Discharge capacitors using bleeder probe and observe all safety procedures.

2) Remove upper transformer screws (x4) and nuts and set aside. (Circled in red in attached photo.)

3) Loosen lower chassis screws (x3) without removing them completely. (Red arrows in attached photo.)

4) Tilt back upper chassis without removing it. Be careful not to sever any wires.

5) Replace V5 tube shield and tube.

6) Tighten lower chassis screws, starting with center one and using the tilted chassis to keep the unreachable Keps nut engaged while turning the screw.

7) Replace upper transformer screws and nuts, making sure grounding tabs are tightly secured.

8) Slide chassis back into amp and replace back cover.



The advantage of this method is that you do not have to remove both chassis from the slider board to remove and replace the choke screw and nut. If done right, it can be done in the field without removing the slider board with chassis from the amp!

[dchang0]

Finally got the small parts needed for the last two screws (top edge of the top backboard). I needed two small "u-nuts" or "clip nuts" and 3/4in 10-32 brass oval-head slotted machine screws with #10 brass finishing washers.

The holes in the backboard were pre-drilled, so I just punched through to the chassis, but that wasn't wise. The hole on the right was higher than the one on the left, so I had to egg-out the right-hand hole. Luckily, the clip nut covers the ugly hole completely.

I installed the copper faceplate finally, and then I noticed an error: I had swapped Ch I and Ch II's input jacks. An easy fix (move the shielded wires).

Now the amp is done!

[loudthud]

This thread is almost exactly one year old. It would drive me crazy to have to wait so long!

[dchang0]

Work kept getting in the way. I remember there was one night I sat down to work on this amp and only got to solder in one whole wire (two ends!) before I had to quit for the night.

Then there's LuciMan who cranked out multiple JMI-era clones in the span of a few weeks... Amazing!

Ah well, at least I finished it. I've got a buddy who has started 3 tube amp kits and not finished a single one (job+wife+kid=no time).