Input jack grounding

[EFK]

R. Aiken writes, "If you are using non-isolated input jacks, you can instead directly connect their shield to chassis directly with a short wire (don't depend on the nut to make contact, because they corrode over time). If you do this, you cannot also ground the main power supply star node to the chassis, or you will likely develop massive hum."

I personally tend to prefer isolated jacks anyway and implement a star system, which is usually foolproof for dummies like me, but if one is forced to use non-isolated jacks, then my question is where DO you ground the main power supply star node if you can't ground it to the chassis? I'm having a dense moment as I can't see where else you could ground it; it has to end up at the chassis eventually, somewhere, doesn't it?

[tubeswell]

if one is forced to use non-isolated jacks, then my question is where DO you ground the main power supply star node if you can't ground it to the chassis? I'm having a dense moment as I can't see where else you could ground it; it has to end up at the chassis eventually, somewhere, doesn't it?

Use a toothed washer between the input jack and the chassis, then do it up really tight and make the input jack your ground. It might well rust eventually in years to come, but you can clean it up at that point if it stops working. I use non-isolated jacks all the time and I don't give a hoot about rust.

[Joey Voltage]

R. Aiken writes, "If you are using non-isolated input jacks, you can instead directly connect their shield to chassis directly with a short wire (don't depend on the nut to make contact, because they corrode over time). If you do this, you cannot also ground the main power supply star node to the chassis, or you will likely develop massive hum."

I personally tend to prefer isolated jacks anyway and implement a star system, which is usually foolproof for dummies like me, but if one is forced to use non-isolated jacks, then my question is where DO you ground the main power supply star node if you can't ground it to the chassis? I'm having a dense moment as I can't see where else you could ground it; it has to end up at the chassis eventually, somewhere, doesn't it?

If you use a non-insulated jack, this should become the only connection point to the chassis for the circuit returns. If you do this though, I would also make another bolted connection to the chassis from the sleeve lug of the jack, directly near by for some added "insurance" in case it comes loose or corrodes. The un insulated jack method may help more effectively in shunting any rf riding on the instrument cable straight to the chassis directly.

I also recommend taking a look at Merlin's article here: The Valve Wizard

Look into his book as well, it is pretty nifty

[EFK]

So you would essentially use the input jack chassis ground - whether a star washer or a secondary point via wire close to the jack/jacks - as your star point? You run all your power supply grounds back to that point as well? For some reason, that doesn't sound right.

[tubeswell]

I mount a 'string' of 4mm eyelets at one end (nearest the input jacks) of my eyelet board so that the eyelet flanges are all overlapping, and I solder them all together, then I run all my pre-amp grounds to that eyelet string, and one short thick wire from there to the input jack ground lug. Works for me

[DIY_Guy]

I mount a 'string' of 4mm eyelets at one end (nearest the input jacks) of my eyelet board so that the eyelet flanges are all overlapping, and I solder them all together, then I run all my pre-amp grounds to that eyelet string, and one short thick wire from there to the input jack ground lug. Works for me

I did pretty much the same thing on the SLO build I did. Works great.

[Chuck H]

Hmmm...

All leads and even the chassis have "resistance", albiet tiny. But this should be taken into account when placing grounds. Since these amps we use are often VERY high gain it stands to reason that very low level signals can be audibly amplified if they occur on the input circuitry. Shared grounds, daisy chained ground leads on boards and poorly placed star points can all have an effect on hum and stability.

The way I see it, the hum should be the first thing out of the amp. So I place power supply grounds on the same point as the AC cord ground. Then mid chassis I ground other in between stuff, like presence control grounds and PI circuit grounds. Then close to the input jack I make all preamp grounds EXCEPT the power supply filters. I know this goes against what others have said but by my observance it's working well.

Using a buss wire that grounds EVERYTHING at or near the input jack doesn't make any sense to me at all because you then have power supply filter, PT and output grounds that must share lead or chassis resistance with the amps input to reach ground potential. To my logic that means hum and at worst instability. Even if it's only a small amount. Unless you try a totally different ground scheme how can you know?

I almost never use isolated input or output jacks. Residual hum and noise on my builds is so low I almost can't even tell my amps are on until they're up loud enough to start to OD the power tubes. Until my way stops working for me I'm going to keep using it. Which could happen. My builds aren't that complex. I would think a more complex design could benefit from isolated input and output jacks.

Chuck

[tubeswell]

Well I don't get any hum at all in my own amps - absolutely zip.

[mooreamps]

R. Aiken writes, "If you are using non-isolated input jacks, you can instead directly connect their shield to chassis directly with a short wire (don't depend on the nut to make contact, because they corrode over time). If you do this, you cannot also ground the main power supply star node to the chassis, or you will likely develop massive hum."

I personally tend to prefer isolated jacks anyway and implement a star system, which is usually foolproof for dummies like me, but if one is forced to use non-isolated jacks, then my question is where DO you ground the main power supply star node if you can't ground it to the chassis? I'm having a dense moment as I can't see where else you could ground it; it has to end up at the chassis eventually, somewhere, doesn't it?

Well, you put the ground lug on one fixed location on the chassis, and then you wire the ground ; from your "isolated input jack" to that point. I usually find some point close to the main power breaker located on the back of the chassis. If you are using star grounding, or single point grounding, keep on doing that.



-g

[Merlinb]

So you would essentially use the input jack chassis ground - whether a star washer or a secondary point via wire close to the jack/jacks - as your star point? You run all your power supply grounds back to that point as well? For some reason, that doesn't sound right.

No, you wire the amp as you normally would with the star point closer to the PA stage, but the star point is not bonded chassis, it is sitting on turret board or whatever. Finally, the input jack is bonded to chassis- that's all there is to it!

When wiring in the strictest fashion, the input jack should always be the ground-to-chassis connection, never the start point, as this eliminates grounds loop hum when other equipment is plugged in. Unless you have more than one input of course (e.g., multi channel amp). Fortunately, earthing the star point often gives no trouble in practice, at least in valve amps where the PSU currents are meagre.

[Joey Voltage]

So you would essentially use the input jack chassis ground - whether a star washer or a secondary point via wire close to the jack/jacks - as your star point? You run all your power supply grounds back to that point as well? For some reason, that doesn't sound right.

Merlin already beat me to it

I wrote out a post last night, but there was some site maintenance so I couldn't post it. I saved it in text edit though. here it is, although a bit too little too late

No. the input jack would become the only reference to the Chassis, the second wire providing insurance against corrosion, or if the jack loosens, think of both as just one connection, nothing else touches the second wire bond point. All the star points would be just as they typically would be for any star/buss system - the negative lead of each filter cap, which are then bussed together (none of which touch the chassis independently, except for the one that decouples the input stage in this system, the negative terminal of which is then run to the sleeve of the input jack referencing all the other circuitry in the amplifier to ground potential). Note you cannot use uninsulated jacks for most anything else though, as it will risk pretty large loops.

[Russ]

Chuck H:

Using a buss wire that grounds EVERYTHING at or near the input jack doesn't make any sense to me at all because you then have power supply filter, PT and output grounds that must share lead or chassis resistance with the amps input to reach ground potential.

The way I understand it works is that all the amp circuits reach ground potential throught the PT center tap wire. This is where all the current flows back to, not to the chassis ground. The chassis ground just provides a ground reference, no current flows to this point.

Russ

[Wittgenstein]

Could you please provide a picture of this. I would like to give this a shot, thanks!

[Joey Voltage]

Could you please provide a picture of this. I would like to give this a shot, thanks!

sure, just click on the thumbnail:




On this particular one, I couldn't use an uninsulated jack because the chassis hole was a bit too large for it, but it is the same concept.. oh wait you mean the input jack method or something else...

[Wittgenstein]

Thanks for the pic. I have uninsulated switchcraft jacks on my 5E3 which are grounding out to the chassis, as well as back to the star ground. Do you have any suggestions for where to get plastic/rubber washers to insulate the jacks from the chassis. I do not have TONS of hum, but I'm hoping this might clear up the little is there. Would I be better to not run grounds off the jacks back to the star ground? And if so, what would be your advise for the best way of doing this.