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

[dchang0]

You guys can ignore this thread for a while. I'll post progress on my Vox JMI-era AC15 OA-031 circuit build incrementally as work progresses, so it will be a long while before there is anything interesting to see in here. The end goal is a mostly-period-correct repro of a 1963-64 fawn 1x12 combo with copper faceplate, 12" Celestial Alnico Blue speaker, ClassicTone transformers and choke, and NCM licensed replacement cabinet. Current total cost of all parts is just under $1500. The most expensive single item is the ~$500 combo cabinet.

To start, here are photos of tagstrip boards made in England from paxolin. They are made and sold on eBay by Steve in the UK (expensive, as this is the only source out there) and started out as 42-lug (for AC30s). I cut them down to 36-lug with a Japanese-style fine-tooth pull saw (better fine motor control with a pull saw) and started populating them with Dale 1/2W mil spec resistors and CDE WMF-series 400V foil-and-polyester-film capacitors as well as CDE 500V silver mica capacitors and a handful of Sprague Atom electrolytic capacitors.

A couple of capacitor values were not available for sale in the CDE WMF-series, so I had to use CDE's Mallory 150 series instead or step up to 630V WMF-series (white colored instead of yellow). I don't think these substitutions are in the signal path, but I didn't check closely.







The lower tagstrip board (further away from the controls) is raised by 3/8" standoffs (installed). The upper board will eventually get 3/16" standoffs when they arrive. I'll put the remaining components on that board after installing the standoffs.

[dchang0]

The NCM 1x12 repro combo cabinet includes a slider board that hasn't been drilled.

I had to assemble the empty chassis halves with faceplate and fit it into the cabinet to determine where the holes will be drilled.

It ended up being off-center by about 8mm to the left (when facing the back of the amp) in order to get the faceplate to be centered in the cutout at the top.

Photo shows the lower chassis half just before drilling (you can't see the pencil lines on the slider board).

The two holes in the rear cover that attach to the top chassis half need to be moved to the left too (not shown).

FYI, the chassis halves and faceplate are made and sold by the same Steve from the UK on ebay that made the paxolin tagstrip boards. When shipping, ask him to pack it extra carefully. The one I ordered got damaged in shipping (one corner of the soft aluminum chassis half bent inwards). I'll have to figure out how to flatten/re-bend the dent out.



Next photo is of the Classictone transformers and choke installed in the lower chassis half, along with rubber grommets.



I originally planned to install the transformers flush against the chassis, but due to the copper shield curving a bit too far away from the power transformer's windings, I would either have to widen the rectangular hole in the chassis or raise the transformer off of the chassis to get it to fit. I decided to raise it a tiny bit using the plastic bushings that came with the bell cover.



The copper shield on the output transformer fit perfectly flush, but I raised it too to match the power transformer.

[pdf64]

The NCM 1x12 repro combo cabinet includes a slider board that hasn't been drilled.

I had to assemble the empty chassis halves with faceplate and fit it into the cabinet to determine where the holes will be drilled.

It ended up being off-center by about 8mm to the left (when facing the back of the amp) in order to get the faceplate to be centered in the cutout at the top.
...
The copper shield on the output transformer fit flush, but I raised it too to match the power transformer.

I built a head cab for my 1964 JMI AC30, and that 'off centre' thing surprised me, nearly caused a problem.


I've raised its Woden PT up from the chassis slightly with some washers, as the windings were pressing down on the slider board. The tape over the windings had worn through, and I was worried that the winding insulation would be next. It looks like the heaters are the outermost windings, and the rectifier heater will be at full VHT. Unlikely to cause an actual problem in use, but may be an unexpected hazard when on the bench.

[dchang0]

Thanks! So it seems I'm not the only one who has had to deal with these fitment issues... Good to know! Luckily, the ClassicTone transformer windings are not so "fat" as to strike the slider board as in your case.

Next up: slowly installing the fixtures like capacitor clamps, terminal lug strips, barrier strips, and fuse block onto the chassis halves. Sadly, I didn't order enough 4-40 Keps nuts, so I can't finish the install of all the tube sockets nor the 3 standoffs on the upper tagstrip board. That'll hold up the build for a week while I wait for parts to arrive.



The capacitor clamp on the steel chassis didn't align with the holes. Turns out the chassis is out of spec--one of the mounting screw holes is very far from the center of the largest hole. You can see from the photo that it "pulls" the capacitor off-center. I had to use a Dremel with tungsten carbide carving bit to egg out the hole just to get the 6-32 screws through.



I finished installing the hardware components to the steel chassis (anything bolted down directly to the chassis). No soldering yet.

FYI, the 16uF+16uF 450V capacitor can is an F&T, and the 32uF 450V electrolytic is a CDE. Both were chosen mainly because they match the correct clamp sizes. Finding these physically-large caps is difficult.

[dchang0]

I ran into a problem with the fuse block placement:

I tried to use an existing #6 hole in the aluminum chassis for the fuse block--only had to drill a small hole for the alignment pin (pictured).

Turns out this was a mistake. The existing #6 hole caused the fuse block's mounting screw/nut to intersect with the power transformer. The proper way to handle this is to drill a new #6 hole out towards the right edge of the chassis (when the power transformer is to the right side and the output transformer is to the left) along with a matching alignment pin hole.



(Photo is rotated--right edge of chassis is at bottom of photo.)

[dchang0]

Next up is drilling the holes to mount the Schurter rotary voltage selector switch (p/n 0033.1109) under the hole where the vintage selector is meant to go. The two holes are for 4-40 x 1/2" flat head screws. The copper faceplate will cover the screws. A countersink bit will be necessary in order to make the flat head screw fit flush under the faceplate. You can't "poor-man's countersink" this with a larger drill bit.

Schurter is pretty much the only maker of high-end voltage selector switches. You can get them from European suppliers (ebay) for about $20 each. It is possible to select a six-position voltage selector that has the voltage markings that fairly closely matches your power transformer--I got four out of six to match here. (The ClassicTone power transformer doesn't have 115V but does have 230V--the other four markings match. I will assign 230V to the blank 6th position.) Most of this effort is actually useless--how many people actually fly around the world enough with their amp to need all these different voltages? But the fake non-functional reproductions of vintage JMI voltage selectors are so expensive and rare, and the working original vintage JMI voltage selectors are unsafe and illegal and even more rare, and the huge hole needs to be filled somehow, and the power transformer has all these extra voltage taps, so it is worth installing the Schurter.

Note also that the Schurter switch is meant for through-hole chassis mounting through a 30mm hole, but the aluminum chassis' hole is not that large, and the switch is mounted under instead of through the chassis instead.

[dchang0]

The 4-40 fasteners I ordered arrived, so I finished mounting the tube sockets and populating the upper tagstrip board. This board sits very low using 3/16" standoffs, so I had to replace the 3/8" 4-40 screws on the V1 socket (turned 90 degrees relative to the other sockets) with much shorter ones (see picture--the upper screw has been replaced with a 3/16" 4-40 pan head screw while the lower one has yet to be replaced with a 1/4" 4-40 pan head screw). Otherwise the tagstrips would short to ground.

[dchang0]

I'm temporarily installing the front panel controls to see if there will be any fitment issues. The biggest problem is that all but one of the Bourns pots came with a locking washer or second hex nut, so they all stick up far too much out of the panel. I will have to order 6 locking washers and 5 hex nuts before soldering the bus wire to the backs of the pots. Ugh.

If I had to do this all over again, I'd choose smaller pots too, so there's more room inside the chassis.

The second problem is that the square cage nut at the left side of the aluminum chassis is very close to the nearest guitar input jack--if the machine screw for the cabinet is very long, it WILL intersect with the jack and may short out some of the solder tabs. I will have to use heat shrink tubing on them to protect them and/or select exactly the right length of machine screw.

Note: the pilot lamp is actually an expensive Dialight unit that includes a through-panel socket and a removable incandescent light cartridge. What you see is the empty socket.

Also, I plan on using pots for the 3-way switches originally meant for the Bright, Tremolo Speed and Tremolo Depth (with push-pull on/off). Obviously not period-correct, but way more useful. Most people went with the 6-way Brilliance switch, but I will run a bass-cut pot instead (from pages 233-4 of Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass" book, figure 10.7b). I did order the 6-way Brilliance switch in case it turns out to sound bad when combined with the stock treble-cut control. The use of pots for the Tremolo controls are validated by Ted Weber's 6V30 and 6V15 mods--they are known to sound good and work well.

[pdf64]

I suggest that you consider adding a B+ fuse.
A F200mA in the B+ winding CT would offer a good degree of protection, especially combined with series diodes on the rectifier plates.
If you've not got one, make a light bulb limiter asap.

[dchang0]

Thanks--by F200mA, do you mean "Fast-Blo" fuses at 200mA rating?

[pdf64]

Yes, F is fast / quick blow, T is time delay / slo-blo

[dchang0]

Interesting--I've never seen that notation before. Thanks for the tips! I've got the spare fuse block but no diodes. Will have to order those--might as well order them at the same time as the missing hex nuts and lock washers for the pots.

[pdf64]

See http://www.fmicassets.com/Damroot/Or...schematics.pdf
That shows the F and T fuse thing in use, and also an implementation of the protection diodes, in series with the rectifier plates.

[Tom Phillips]

See http://www.fmicassets.com/Damroot/Or...schematics.pdf
That shows the F and T fuse thing in use, and also an implementation of the protection diodes, in series with the rectifier plates.

It is interesting that the linked schematic shows that the heater AC supplies are fused but the B+ is not fused.

[pdf64]

Yes, I've posted before that I think it's strange Fender don't have a more consistent approach to the issue of tube amp circuit protection.
I'm aware of one Fender, the EC Vibro Champ, that has line, heater and B+ fusing, as well as the tube rectifier protection diodes.
The TRRI may be fitted with fuses that aren't shown on the publicly available documentation.
The only common, non-statutory protection strategy seems to be the line NTC surge limiting thermistor, which may allow F type fusing of the mains circuit.