Acoustic 136 bass amp 2-prong to 3-prong conversion

[bryandowen]

I'm the lucky new owner of what I believe is a first-gen (early 1972) Acoustic 136 bass combo amp. It's wider than it is tall with the speaker on the left and port on the right, whereas the later models (mid-1972 and later) are taller than they are wide, with the speaker on top and port on bottom. Almost every picture you find on Google is of the latter. Here's mine: https://www.tapatalk.com/groups/unof...amp-t4953.html

Unfortunately this amp has a 2-prong power cord, and it (the cord) is in really bad shape to boot. I read enough on the Internet to scare myself about the low pass coupling "death cap" and decided that I needed to upgrade to a 3-prong plug.

I tried a naive update after watching a Fender amp conversion video. Essentially I disconnected the ground select (snipped the wire to the capacitor) and wired the new 3-prong plug ground (not neutral) to the chassis. The amp was super noisy after I did that, and on further reading, this was a really not smart thing to do for a vintage amp that doesn't separate chassis and signal ground.

So I'm thinking I'll put things back the way they were, with a few exceptions:

1. I'll keep the 3-pronged plug, solely for the sake of polarity. I'll have a 50% < N < 100% chance that polarity is correct, versus 50% with the 2-pronged plug. (I don't see a reason to revert to a polarized 2-pronged plug. Is it bad form to "imply" that my amp chassis is separately grounded because it has a 3-pronged plug?)

2. I'll replace the coupling "death cap" with a Y-rated cap designed to fail open (I know, not very heavy metal).

3. I'll rewire "hot" through the fuse, rather than neutral through the fuse, which is how it's wired today.

Two questions:

1. Thoughts on any of the above? Is there a better way to approach this (in terms of safety and/or noise reduction)?

2. In the case that a 3-pronged outlet is incorrectly wired (and "neutral" is not actually neutral), is there a safe way to put a lamp across neutral and ground to indicate reversed wall outlet polarity? (If I did that and polarity was reversed, I'd be leaking hot to ground by way of the lamp, which could have unintended side effects.)

I'm very hobby-grade when it comes to this stuff, sorry if my ideas here are shockingly dumb, unsafe, etc. I hope to learn from folks who know what they're doing.

Thanks,

-Bryan

[g1]

Is it bad form to "imply" that my amp chassis is separately grounded because it has a 3-pronged plug?)

If you mean you are going to have a 3 prong plug with the 3rd prong ground pin disconnected, yes that is extremely bad form. It is a major safety hazard and may also put you in legal jeopardy.

It seems odd to me that it would have more hum with the chassis earthed.

on further reading, this was a really not smart thing to do for a vintage amp that doesn't separate chassis and signal ground

Most amps tie the circuit ground to chassis and never have the kind of problem you are describing. Can you give more detail about how you did the 3 prong conversion?
It was definitely the cap going to the ground switch that you disconnected?

[Enzo]

Exactly WHERE on the chassis did you connect the earth wire from the power cord?

[bryandowen]

If you mean you are going to have a 3 prong plug with the 3rd prong ground pin disconnected, yes that is extremely bad form. It is a major safety hazard and may also put you in legal jeopardy.

So using a polarized 2-prong is a better way to go?

It seems odd to me that it would have more hum with the chassis earthed.

Most amps tie the circuit ground to chassis and never have the kind of problem you are describing. Can you give more detail about how you did the 3 prong conversion?
It was definitely the cap going to the ground switch that you disconnected?

Looking at the schematic you attached:

1. Re: the cap going to the ground switch -- On the diagram you posted, all the way to the left and working right, there's chassis, C403, then the ground reverse switch (S402). I severed the wire between C403 and the ground reverse switch. There are two identical caps next to each other in a bracket, and I've just verified that the one I disconnected is the one that sits between the ground select switch and the chassis.

2. More detail on how I did the 3 prong conversion: I de-soldered the hot and neutral wires from the old 2-prong cord, ran the new 3-prong cord in, attached the hot and neutral to the exact same points where I removed the previous ones (this happens to be the on the back of the 2-prong "convenience" A/C plug on the back of the chassis, which I'd love to remove or replace with a 3-prong one eventually, but one thing at a time.) Lastly, I crimped an automotive-style ring terminal to the ground wire of the 3-prong cord, backed out one of the screws holding the power transformer to the chassis, slid the ring terminal between the transformer and chassis, and inserted the transformer screw through it. The ground lug makes direct contact with the aluminum faceplate on the back side of the amp -- the same piece of metal that all the pots and audio jacks are mounted to.

So essentially disconnected hot/neutral (depending on ground reverse selection) from chassis, and wired earth to chassis.

[bryandowen]

Exactly WHERE on the chassis did you connect the earth wire from the power cord?

I crimped an automotive-style ring terminal to the ground wire of the 3-prong cord, backed out one of the screws holding the power transformer to the chassis, slid the ring terminal between the transformer and chassis, and inserted the transformer screw through it. The ground lug makes direct contact with the aluminum faceplate on the back side of the amp -- the same piece of metal that all the pots and audio jacks are mounted to.

[Jazz P Bass]

The ground lug should be it's own entity.
Ideally it should be mounted where the mains wires come in to the chassis.

[g1]

Yes, if you are not using 3 conductor AC cord, polarized plug is better than a 3 prong plug with no earth ground.

But I really think this should be able to run quiet with chassis earthed and 3 conductor earthed plug. Are you 100% positive your house wiring and wall outlet in use is all proper 3 conductor type with separate earth at electical panel?
I'm attaching the service manual for the 270, it has 3 prong plug. You can see it is otherwise set-up pretty much the same. It also has some jacks directly connected to chassis like your amp does. However, fuse and switch are both in hot line, and neutral is direct to transformer. That should be done to your amp as well.
You could try reconnecting the cap to the ground switch for test purposes and see if it kills the hum while you have the chassis earthed.

[bryandowen]

I did some more experimenting this weekend. The hum went away (almost) entirely without having done anything (except for re-plug-in my bass). So I started jiggling cords, and got all kinds of intermittent noise. I've loosened, jiggled, and re-tightened the 1/4" input socket for channel 1 and the interference seems to have gone away. There's still quiet hum, but very tolerable.

So it appears that the 1/4" input socket that I plug into had a flaky connection to chassis, and was introducing a huge amount of interference. This didn't happen before I did the 3-prong conversion, which is suspicious, but stranger things have happened.

To boot, I get to keep my 3-prong plug and earth-grounded chassis! Thanks everyone!