JMI Vox AC30 refurbishment

[Rhodesplyr]

I'm starting my first Vox AC30 repair with a ~1964 (according to the owner) unit. The owner has only the amp chassis at this point. He intends to install it in a cabinet he's making himself to use as a head.

I'm going to start with standard electrolytic replacement, but I thought I'd ask if there are any particular things I should look out for when refurbishing an AC30. This one looks to have had a slightly rough life as witnessed by corrosion on the chassis, etc...

Someone installed 12AT7s for V8 (Mod) and V2 (Phase Inverter), and the output transformer is a Woodward-Schumacher OTV30 replacement. The PT and choke are Woden.

I've found two capacitor date codes, 5282 and 5503, but I'm not sure how to interpret British date codes. SN is 10737, and it has the Top Boost circuit with 6 preamp tubes.

How do the resistors Vox used tend to hold up?

[pdf64]

'How do the resistors Vox used tend to hold up?'
I've seen at least a couple of types used in early to mid 60s Voxs. Most common are chunkier than normal with a wax coating. No noise problems with them (yet) and they hold their value unless their dissipation gets pushed, as in the 56k cathode resistor which can have ~175Vdc across it on AC50s. I just check their value and leave them in if within 5%. The other type are the smaller 1/2 watt carbon comp, as per Fender; best just to replace them.
My woden OT has been pretty tough, surviving at least a couple of speakers burning out at full whack.
Be aware of the potential for dry joints due to the stiff, single core hook up wire used.
Consider some protection mods to mitigate for tube failure:-

Si diodes in series with the GZ34,
fuse on the B+ (in line with the B+ CT ground return is the simplest way),
cool the EL84 bias a little (the wirewound cathode resistor doesn't tend to drift up, unfortunately) - try 60ohms.

I prefer individual cathode resistors / bypass caps on each EL84 (240 ohm / 220uF); it allows them all to settle to the same static current, and accommodates for tube failure - if one dies then the others keep their normal bias, rather than ruper super hot, as with a common cathode bias resistor.
Don't expect more than 25 watts clean.
Pete.

[Rhodesplyr]

Interestingly, it appears that this this amp was designed to be three-wire/earth grounded, but this was defeated at some point, at which time a Sprague death-cap was installed.

[Steve Conner]

All British amps (that I know of) had the 3 wire grounding system. It would probably have had to be converted on importing to America.

As far as I know the first two digits of the date code are the year, but 1952, yikes! Would JMI really have used caps that were already 12 years old? What brand are the electrolytics?

The only thing I know about Woden is, I have an ancient schematic somewhere for a radio transmitter that calls for a "Woden modulation transformer", as if every British radio ham in the 50s was supposed to know which one. Maybe they only made one.

[Rhodesplyr]

I'm told by a contact of mine who worked for JMI/Vox in the 1960s that British-made components were labeled with manufacturer-specific "batch codes" rather than date codes. Thus, unless you have company records, you can't determine component production dates directly from those codes the same way you can with American-made components of the same era.

[Rick Erickson]

JMI Vox Rebuild

The best advice I can give you regarding a JMI AC30 is to be extremely gentle when attempting to remove components from the terminal boards. The terminals will snap off with the slightest provocation. I usually remove the solder with a professional vacuum pump (not a Solda-Pult, which will instantly snap the terminals) and then clip the leads after making sure they are loose, to allow the part to be removed without putting any stress on the solder terminals. All electrolytic caps and carbon comp resistors should be replaced without prejudice. Leave the original coupling caps alone unless they exhibit problems. It is typical for the original 500pF 1st stage coupling cap in the Brilliant channel to be bad. I recommend a 1KV ceramic here. A silver mica will eventually break down and become noisy. Been seeing a lot of those in newer Marshall and Vox amps, and even boutique builds lately. If you are unsure of what you are doing or not certain you are ready to take on a repair of this magnitude - refer the job to a specialist who does these often.

[pdf64]

'be extremely gentle when attempting to remove components from the terminal boards'
Agreed; in that respect they are more akin to PCB than turret or eyelet board.
Well, perhaps that's over-egging the fragility / consequence, but you get the message.
Pete.

[Rhodesplyr]

Rick,

Thanks for the advice. I regularly rebuild Hammond Organ preamplifiers, which also have easily-damaged terminal boards, and I have a professional solder vacuum. I personally can't recall having seen a mica capacitor fail, though it's certainly possible. Is there a particular brand you've seen go bad? I have several tube tuners from the 1960s that have lots of mica caps in them (maroon epoxy-dipped), and I've never had to replace one. On the other hand, I think that C0G/NP0 ceramics are comparable, and you can get them in larger values these days.

The main change I've noticed in current-production mica capacitors is that most now have copper-plated steel leads. Possible interaction between the steel and the silver? I don't know enough chemistry to speculate, but considering the premium price of mica caps, I'd think they could throw in solid copper leads.

I do appreciate the advice, but no guitar amp I've ever worked on approaches the challenges of the combo organs I work on from time to time, including 1960s Vox organs. The majority of guitar amps are refreshingly simple by comparison. Similar warnings apply to these organs in terms of how easy it can be to lift solder traces, etc... if you aren't careful.

And, yes, the 500pF cap in the Brilliant channel is bad/noisy and will be replaced.