Vox Essex Bass Grounding Question

[The Dude]

I've clipped the death cap and installed a grounded plug on this amp. It has a separate power amp and preamp configuration. Power is fed to the power amp module, so that is where the grounded AC cord was installed. The power amp is inside the cabinet/not accessible when the amp is assembled. The preamp, of course, is accessible. My problem is that the preamp, apparently does not share a ground with the power amp, so it remains ungrounded. I tried a clip lead from power amp chassis to preamp chassis to ground it. There is an audible hum in this configuration, but not with the preamp ungrounded. I can't find a schematic- only a gutshot here:

The VOX Showroom - The Vox Essex Bass Amplifier - A Look Under the Hood

Anybody with experience on this amp? Any ideas why the above? I'd prefer to have the preamp earthed, but the amp doesn't appear to want me to do that.

[52 Bill]

My rememberance is that the ground switch and cap were all in the preamp box, so the chassis must have been (is still) connected to the power amp ground. I'm sure that R.G. will be able to tell you how the circuit is grounded. TO Vox used all sorts of odd grounding schemes in these amps. Maybe you will need to run a ground wire from the ac cord directly to the preamp box and disconnect it at the power amp chassis.

Which model is it? I have paper copies of one of the Essex amps around here.

[MarkusBass]

I think that you can buy it here for $5: US Vox Schematics - Essex Bass, Kensington Bass, Sovereign Bass, Westminster Bass (when they return from holidays next week).

Mark

[R.G.]

I have seen the schemos for the Essex, but the only live one I've ever seen was back in about 1970, so my experience with them is limited. I have some suspicions about the grounding issues, gleaned from some work I've done with the replacement PCBs for the "Big Head" line, the Buckingham, Guardsman, and Beatle.

Thomas did all the grounding and safety protections they were required to at the time - which wasn't much, by modern standards. If this is done like the "Big Head" models, the preamp chassis was grounded to the signal ground by the shield braid of the signal cable that carries the signal to the power amp. The power ground is carried to the preamp by the power cables, and is not tied to the chassis, excepting through the signal ground to the jacks on the preamp chassis.

This setup will not pass modern safety standards. It's also prone to various hums on its own. Grounding the chassis of the preamp with safety ground (I think...) causes a ground loop with the power, safety, and signal grounds. Connecting safety ground makes this worse.

I have some theories about how to fix this. One is to isolate the preamp chassis ground from signal ground by using isolated jacks and lifting signal ground from the chassis on the shield of the cable carrying the signal to the power amp, perhaps with a small value resistor. Another is to use a small isolation tranformer (my current favorite is the Triad TY-250P, $5.00) to isolate the preamp signal from chassis on the way to the power amp.

Unfortunately, I've had a several month setback on trying these things out; my personal life keeps interfering with the bench time I like.

You might try the isolated jacks and isolating the signal from chassis ground on the preamp. You might also be able to "float" the preamp chassis off safety ground with the diode-bridge trick.

[J M Fahey]

Grounding the main amp is the right thing to do, and you already did that.
Preamp IS grounded, or no signal would be able to pass between them.
Grounding path is most probably the audio cable shield, although is a bunch of cables joins them, there might be a dedicated ground one; adding another may create a ground loop, what you seem to be doing.

In my view, you do not need an additional safety ground in the preamp, it´s covered by the one in the power chassis.

[Mick Bailey]

When I've faced a need to install grounding that could potentially create a loop I use a pair of back-to-back diodes rated at 6A/600v. This floats the second grounded section by one diode drop and usually eliminates noise.

[R.G.]

Yeah, that's a variant of the diode-bridge trick. If you take a diode bridge and short the + and - terminals together, you get two diode pairs back to back when considered from the AC input terminals. This is usually coupled with a 10 ohm or so resistor paralleled with the diode bridge and a 0.01uF 1kV ceramic cap across both of them.

The cap "shorts" the two ends together for RF, the 10 R keeps much DC offset from building up, and the diode bridge steps in when the voltage gets over about 1.4V. A bolt-down diode bridge module is handy because it's easy to mount, and offers terminals to mount the 10R and cap on. A 20A diode bridge is usually under $2. The high current is just one of those things you do because the official safety test requires passing 25A of AC through the "ground" connection and having it not burn open. A 25A bridge would pass that. About all this won't pass is the voltage section of the standards, which requires ... um... remembering... I can't remember. Something trivial like 100mV or so.

[The Dude]

FWIW, it's a model V1043. I'm a bit hesitant to buy the manual linked in post #3 because it's actually for a V1042, but maybe it's close enough? And thanks everyone for chiming in. I'll read through all of this and give some of these things a try. For whatever reason, my internet connection is at the speed of smell today, so navigating is difficult.

[nosaj]

FWIW, it's a model V1043. I'm a bit hesitant to buy the manual linked in post #3 because it's actually for a V1042, but maybe it's close enough? And thanks everyone for chiming in. I'll read through all of this and give some of these things a try. For whatever reason, my internet connection is at the speed of smell today, so navigating is difficult.

To ask the obvious, does it still hum once preamp and amp are remounted? You know how on some fenders they have the foil on the chassis for shielding? Does it apply here?

nosaj
This was on that site. Is the cable in good shape as well as the connections?

Also just using the preamp ( I guess a line out tac'ed ) do you get hum into another power amp grounded or not?
A shielded cable terminated with RCA plug drops down from the preamp to provide audio signal to the power amp.

[Mick Bailey]

The high current is just one of those things you do because the official safety test requires passing 25A of AC through the "ground" connection and having it not burn open.....

A few amp manufacturers use the diode trick to elevate the signal ground - Fender and Marshall spring to mind. Fender uses a pair of 6A4 diodes with a 10R or 15R resistor (no cap though) and Marshall uses a 22R resistor. Neither would pass the 25A test current unless the diode failed short. I suppose this would depend where the test point ground was made to the chassis and in the case of a PAT test this comes down to operator discretion. If the test was carried out to an accessible part of the chassis that was connected to the mains supply earth it would pass. If the operator connected the test clip to the input socket ground (though unlikely) it could fail, depending on whether the diodes shorted or not. 25A seems to be the default test current, but it used to be the test current was specified as 1.5x the rated current of the equipment under test. Some testers used 2x the mains fuse rating, up to a maximum of 25A. There was a lower current test for certain categories of equipment (IT included) and I don't know if this still applies. The main chassis was tested to the full current - up to 25A - and any other accessible grounded connections tested at 100mA.

[52 Bill]

Dude, I have a copy of the V1042 schematic, etc. if you want it, I'll scan it for you.

I also dug through my pile of stuff and found that I have an Essex chassis. I'm not sure which model though.

[nosaj]

Dude, I have a copy of the V1042 schematic, etc. if you want it, I'll scan it for you.

I also dug through my pile of stuff and found that I have an Essex chassis. I'm not sure which model though.

I'd love a copy I've been collecting various ones trying to build up a gdrive with ones vintagekiki comes up with and the ones from nickb.(just didn't have the latest ones before he pulled them.

nosaj

[The Dude]

Thanks to one of our members, I now have a schematic for the V1042. Unfortunately, I'm about ready to hit the road for a couple weeks so I won't be able to get to it until I get back. Fortunately, it's sort of a "project amp" for a customer and he's not in any hurry to get it back. I'll update when I get back into the thing and study the schematic. I'd post it here, but I'm not sure I'm supposed to or what the legal issues might be. Thank you all for your help!

[The Dude]

Ok. I didn't think I would have the time, but got into this thing tonight. After having a look at the schematic, there's nothing extra different about the grounding scheme here. Schematic shows the preamp and power amp do share a common ground. But, checking continuity on this one, it was not so. What I found was that the preamp is built on terminal strips. Each terminal strip has two of the terminals riveted to the chassis to carry ground. The rivets had loosened and corroded over time and were not making a good ground. So, I had some things that were grounded and some (that were supposed to be) not. There is a face plate over the rivets on the knob side and I didn't want to mess with that for fear of damaging something. I took a chisel and hammer and banged the rivets tight from the rear. Then, for insurance, I got out the gun and soldered the rivets both to the terminal strips and the chassis so that the connection is no longer dependent on the rivets only, of course scraping off the corrosion first. The thing is very quiet now and there was no longer a need to run a ground wire from power amp to preamp. Again, thanks for all the help and chiming in!

[52 Bill]

Sorry that this took so long to post. Seems that everything that I do nowadays takes twice the time it used to. Well my wife will probably disagree with that, but I digress.

Here is the scanned Essex V1042 Service file.