Vox Buchingham Amp Ground Technical Question

[bbheli]

I have a Vox Buchingham amp that has a two prong AC cord. Amp hisses, and wonder if I should add a ground wire cord to the chassis and to the power supply, they are seperate. The RC plug I assume acts a a nuetural of sorts but not sure. Also is there a capaictor that needs to be eleimated when I do this. Touching a wire to the metal chassis to eart ground creates a rather good spark when it touches.

Thank You and Happy new Years All

Brett

[phatt]

If you get a spark from that earth wire to case then don't use the amp as something is wrong and likely dangerous.
Wait till the tecks get over the new year hangover as they will know more. :lmao:
Meantime maybe post some pics of the internals especially around the power inlet and main transformer.
Phil.

[bbheli]

If you get a spark from that earth wire to case then don't use the amp as something is wrong and likely dangerous.
Wait till the tecks get over the new year hangover as they will know more. :lmao:
Meantime maybe post some pics of the internals especially around the power inlet and main transformer.
Phil.

Thanks for the advice, I wonder if the spark is just the ground draining and transient voltage but not sure. I guess the tech guys are still hung-over cause no one has commented. 

[g1]

Are you in a 120V or 240V country?
Can you post the proper schematic for the amp?

[bbheli]

Are you in a 120V or 240V country?
Can you post the proper schematic for the amp?

120V USA

[R.G.]

Somebody call a tech guy?


First - don't mess with that any more if you are not already skilled and experienced enough to work on AC power wiring >> safely <<. It's easy to get a bad shock or get killed if you are not already skilled enough.

As a side thought, it's >possible< to get yourself killed with AC power wiring even if you are already skilled and experienced enough to work on Ac power wiring.

What you describe, a significant spark when connecting a wire between an earth ground and the metal chassis, sounds like more than just trivia, AC leakage. It sounds like there might be a significant fault in the AC wiring. This might be in the line reverse switch or the AC "death cap".

Before you do anything else, get out your digital multimeter, set it to read AC volts and read the voltage between the accessible metal on the preamp (that is, the knobs and jack bushings) and the earth ground connection you're worried about. Test this in both positions of the "line reverse" switch. One position of the line reverse switch should be significantly more voltage, but neither position should be more than maybe 20-30v. If one or both readings are more than that, there are additional tests to run, but those are in the "gotta be skilled" category.

I don't mean to be talking down to you if you already do have AC power wiring skills, I just don't want someone to get in over their head with something dangerous.

Having done the due-dilligence warning, there are some issues with three-wiring the Thomas Vox amps.  They are set up in a funny way compared to most guitar amps, with a cable between the preamp and power amp chassis that carries AC power and DC back and forth, and a somewhat delicate signal ground setup. the more I get into these amps, the more complex the situation appears. I have actually written a significant new set of tech literature on the Thomas Vox amps, both a main overall book, and dedicated per-model supplements, far more than the factory schematics information. The "Vox Owner's Safety Net" is the main book, and it talks a bit about three-wiring. You can find this service stuff at thomas-vox-repair.com if you're interested.

To three-wire them, you need to remove the wiring from the "line reverse" switch, attach the incoming safety ground wire from the line cord to the preamp chassis with a dedicated safety ground screw, then run an additional safety ground wire through the cable to the power amp chassis, and attach it to the power chassis with a dedicated safety ground screw to that chassis as well.

Doing this will make the amp safe, but also induce a certain amount of hum. This is because Thomas had some grounding problems that they solved by floating the preamp chassis on signal ground as carried to it by the RCA signal jack from the power chassis, not the main power ground in the power supply in the power chassis. There are some funny reasons for this, but it's a complex issue to me, and I've been working signal grounding for a few decades.

The best theoretical solution is to change that 9-pin connector between the two chassis to a 12-pin universal make-n-lok type and add additional wires to fix both the grounding and safety issues. This is again an issue for a skilled amp-tech.

[bbheli]

R.G

Thanks for the insight and safety measures. I am use to working with AC systems not in amps, household wiring. Was thinking about measuring from chassis to earth ground and forgot about it, gotta get my meter from the shop next week and I will update. Thanks for the help. Appreciate it.

[R.G.]

OK then. I didn't mean to talk down to you. It's just that there are hordes of people wanting to work on AC that really don't have any idea about how it can make them dead. Given that you're experienced, let's forge ahead.

Your comment actually set me off on some more grinding it out on a Thomas Vox Guardsman head that's on the bench. Same preamp setup as your Buckingham. Here's some fresh info.

It really is likely that your amp has either a partial connection to the chassis from the AC wiring or a fault in the line reverse switch or cap. It is possible that this is even a short from the primary side of the power transformer to the transformer iron. That's rare, but happens. Your ohmmeter will be your most valuable tool here, as it will just tell you both that there is a low-resistance connection to the chassis. It's also possible that there's some contamination or wire issues in the cable between the preamp and power amp, as the 120Vac wires run through that cable.

I have better info now on the grounding scheme. With the additional sleuthing I did, there are two wires I found that are apparently on every Thomas Vox "big head" amp that I cannot find on any of the factory schematics. These are a wire bringing power ground to the preamp circuits and a wire attaching power ground to the power amp chassis. They pretty much had to be there, but were undocumented (in the technical sense    )

Once you run down that AC fault, here's how to make your Buckingham head very quiet.

It turns out that I was wrong about needing the 12position connector. There are just enough positions in the 9-position connector in the cable to do a decent grounding job. Of course, this is made worse by the fact that I can't find where to get the terminals to modify that particular cable connector anywhere. So changing to third wire ground or any other changes to the cable wiring will need a new connector. Both Molex and Amp make polarized cable connectors suiltable for this. I like the Molex "MLX" series; You can get the full set of plug, socket, and pins for a 9-position connector replacement at Mouser for under $8.00 all told.

The biggest hum reduction is from cutting the input jacks loose from the preamp chassis. Switchcraft and Amphenol both make nylon body jacks that will fit the same holes as the originals, and Amphenols are only $0.80 each. So replace the input jacks with nylon-sleeve jacks, and run a ground wire from each jack to the section of the PCB it goes to. Thomas Organ actually provided a field upgrade kit for this to its service centers. No changes to the cable needed.

Next hum issue is third wire ground. For this, you need to bond the incoming AC power cord safety ground to the preamp sheet metal, then run an additional wire through the cable to the power amp chassis, and bond it to the power chassis. That means you'll need a new connector setup as mentioned if you do this. If you don't run a separate ground wire in the cable, third wire grounding only the preamp chassis will increase the hum you get at the end.

That leaves one position open in the cable connector. To really get lower hum, you can run a ground wire there that carries just the ground return current for the indicators. This removes the sawtooth ripple 120Hz current from the indicators, and cuts some more millivolts of hum out of the speaker output.

I'm still writing and drawing this up for inclusion in the Vox Owner's Safety Net, but you needed the tech info, as you're right at the point of fixing one with these issues.