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

[pdf64]

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).

[dchang0]

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.

[pdf64]

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.

[dchang0]

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

[dchang0]

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.

[dchang0]

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.

[dchang0]

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.

[dchang0]

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.

[dchang0]

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).

[dchang0]

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).

[dchang0]

Added 100 ohm & 100 ohm virtual center tap on the the heater wiring.
Added speaker jack and attached old 8 ohm speaker.
Wired up power cord, power switch, main 3A slo blo fuse, voltage selector. There are bits of heat shrink tubing pre-placed (but not yet shrunk) on the leads for later.



I left tons of spare length on the transformer leads as was suggested by Tom (especially if the transformers were bad).

With the light bulb current limiter suggested by pdf64, it passes tubeswell's first two power-on tests here: New Build: Start-up procedure I also kinda followed Paul Ruby's start up testing, but not exactly.

1) no rectifier tube, installed pilot lamp, heaters work
2) add rectifier tube only, B+ goes high and climbs, quickly shut off

So far so good. I can't test more tonight.

[Tom Phillips]

...With the light bulb current limiter suggested by pdf64, it passes tubeswell's first two power-on tests here: New Build: Start-up procedure ...

I'm really glad to hear that you are doing the incremental startup live power testing before wrapping up all the wiring.
Cheers,
Tom

[dchang0]

Thanks for the advice.

The amp's normal channel worked perfectly on the first power up attempt. Almost no hum, so I lucked out with the heater wiring and grounding scheme. Total volume did seem quiet, but I am running through the light bulb current limiter right now (the 120VAC bulb was dimly lit). Question: does a light bulb current limiter reduce the amp's volume by about half? I'm assuming yes.

The tremolo channel, though, sounds like it is turned off, with barely-audible volume. I can hear the tremolo in the sound that is audible. It may be the jumper wire that I installed to replace the foot switch. Question: does the Vox footswitch disable the tremolo when it closes the circuit or enable it? I will remove that jumper later and give it a try again.

So far so good, though. I'll post up voltages later.

[Tom Phillips]

...Question: does the Vox footswitch disable the tremolo when it closes the circuit or enable it?...

When closed it enables the tremolo in the old style AC15.

Can't comment on the Light bulb limiter. I don't use them. Voltage readings W/O the limiter may give a clue to the problem.

Cheers,
Tom

[dchang0]

Thanks for confirming the footswitch behavior, Tom.

Thinking about it, the most likely cause of the low tremolo volume is in the two tremolo pots and the push-pull switch I installed. That's one of the few areas I deviated from the original Vox schematic to splice in a Ted Weber modification (from his 6V15 schematic). Also, I triple-checked the wiring on the tag strip boards against the Vox schematics but did not scrutinize the pots.

Ted copied his 6V15 tremolo mods from his AC30-derived clone, the 6V30, so we know they work in practice. Most likely I simply miswired something up there.

I'll measure voltages without the light bulb current limiter later--gotta pace myself so as to not take shortcuts.

Confirmed that the Cut pot as shown in the latest photos AFTER switching from Turret's layout and schematic to the original OA/031 schematic is wired correctly and cuts the top end as the pot is turned clockwise. The jumper across the pot legs is on the left and center legs, and C7 is connected to the left leg. Visually, this is the same as for Turret's layout, so it basically was just a wire swap back at C7 and C9.