Adding a Top Boost to a 1962 AC30

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Adding a Top Boost to a 1962 AC30

Postby Milocj on Sat Jul 22, 2017 8:18 pm

I've done a search and can't find an answer, though maybe the best answer is found in the pictures at the top of this forum of the pre-made kits.

I know that generally you'd want to keep preamp and B+ grounds separated, however the AC30 schematic basically shows that the preamp ground where the Top Boost is added eventually connects to the B+ anyway. By the same token, the original TB add-on schematic shows that 32uF cap with a ground and it isn't the same as the new preamp tube.

Is it best to simply ground the main filter cap to the TB tag strip and run one ground to the amp? If not, is there a preferred ground placement?

Also, I see some pics that use shielded cable grounded on one end for the two signal wires, and others that simply use one wire. Does the shielded cable help keep noise and oscillation away enough to go to the small added trouble of using it?

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Re: Adding a Top Boost to a 1962 AC30

Postby dlvoots on Thu Aug 03, 2017 7:02 pm


I wrote this post upside down because it puts your answer at the top. After your answer comes the theory behind it.

The theory about keeping ground wires electrically separate is still excellent advice when you can do it. For example, you'll never go wrong if you ground the 32uf cap you're asking about with a separate wire back to the return circuit in the power supply, rather than on the tag strip. It might not be what a collector / restorer would do with the amp, but it could make a difference in how it hums or doesn't hum for a player.

Same thing for shielding of signal wires. If a shielded wire goes from point A to point B in an amp, one and only one end should be grounded to avoid creating a ground loop with multiple paths to ground. Shielded cable is excellent for small signals inside the amp (an AC-30 uses them from the input jacks to the first gain stage), but only one side should be grounded.

Often you'll find that when you connect several pedals to several amps at the same time, the lash up will hum miserably. That's a classic symptom for a ground loop. How do you fix it? Break the ground connections somehow such that things are grounded at one and only one place.

NOW for the explanation / theory:

The best way to understand grounding for the top boost circuit in your amp is to go back to grounding basics. As you implied in your post, when you go back to grounding basics, you realize there are several ways to ground anything. There is also a big difference between what should theoretically be done in a perfect world, and what CAN be done in the imperfect, practical world we live in. When it comes to grounding as an art and a science, that difference affects how you physically wire your house, how you connect your stereo and video equipment, and even how grounding wires are designed and wired internally in your stereo and video equipment (and in your amp, including your top boost circuit).

When design engineers speak of good grounding practices, there is a built in contradiction of sorts. The mantra, or holy grail of grounding says that you want to have one and only one path for current to return to the source it came from. The problem is that in an audio circuit, there are actually several sources that need to be intermixed as they are grounded -- the audio signal source (hot and ground), AC power source (hot and ground), the power supply source (hot and ground), and finally, the chassis and how it is connected to earth ground for safety and shielding purposes.

In a perfect world, ground return wires for all four sources above should be kept as electrically separate as possible, and connect at one and only one place on the chassis -- the common grounding lug. Such a configuration is known as a Star Ground, and in a perfect world, the ground wires from the three non-AC sources and the ground return wires from the circuit(s) should not connect to anything but that common grounding lug. Then a single earth wire should go from the chassis to earth (in the US, it is the green wire in a power cord). A star ground configuration will usually give a designer the maximum amount of self-shielding, feedback, and hum rejection. In reality, most manufacturers don't always use a star ground it because it's expensive and can be quite complex to wire properly.

Remember, in theory, a ground lead should have zero resistance, but the problem is that all wires have resistance. That resistance translates to voltage drop across the ground wires and means that somebody in the circuit that should be grounded at zero volts.....ain't. Star grounding minimizes the effect of that resistance and mostly keeps you out of trouble.

Manufacturers will sometimes daisy-chain ground wires together because it's cheaper to produce. Depending on the circuit, it may work, or it may give you ghost signals and hum in the ground chain because somebody that should be at zero volts......ain't.

Another basic mantra is that you never provide multiple paths to ground for ANYTHING. Multiple paths to ground invite the creation of what is known as a ground loop. Loops are very insidious critters to debug, because they can be perfectly fine at all DC voltages, but behave like a radio antenna if AC voltages are involved.

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Re: Adding a Top Boost to a 1962 AC30

Postby Milocj on Sun Aug 06, 2017 1:05 am

Thanks very much for the extremely detailed response, Dave.

I aware of the two basic theories of either star grounding or separating stages, especially since most of my experience is with old Fender amps. This is what made me wonder why the Top Boost add-on schematic didn't show a ground location for the power supply filter while showing how to string all the other grounds to the single point near the existing preamp tube. I just found it odd that when I tried to trace some of the other grounds they all ended up similar to a star ground at that same point (looking at the preamp portion of the chassis).

My amp only has a slight buzz/him as it sits so I'll probably leave that 32uF cap ground on it's own lead while I install the top boost and see if I find any area that tends to lower any noise before I decide where to tack it in.

Mine is a fawn, but it was painted many years ago and also had an IEC socket cut into the upper back panel. Still trying to decide whether to make a new panel or just cut the hole for the Top Boost in the original. It would be nice to be able to strip the paint, but the couple of things I've tried so far take a long time to simply get a fingerprint sized area almost back to fawn.
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