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

[dchang0]

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.

[dchang0]

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.

[dchang0]

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).

[pdf64]

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.

[dchang0]

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

[Tom Phillips]

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 don’t 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 don’t 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. It’s 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

[dchang0]

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.

[Tom Phillips]

...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. Don’t want to be like some old cars that required removal of the starter to get to the last spark plug.

...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 don’t because I work alone so the list is in my head.

...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)? .

Don’t 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. That’s why, especially for a build that will take a long time, it’s advantageous to confidence test major parts sooner rather than later.

[dchang0]

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!

[ThermionicScott]

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.

[dchang0]

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!

[Tom Phillips]

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.

[dchang0]

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.

[pdf64]

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.

[dchang0]

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!