Input and Output Jacks

[CraigGa]

I'm on with my first build based on the Benson Monarch thread. Today I was mounting all the hardware onto the chassis and realised that the open type jacks I have are going to ground the input and speaker output sleeves to the chassis.

Is that going to cause any problems? The schematics show the input grounded to the bar across the controls then back towards the preamp filter caps.
Also the speaker output is floating straight from the OT secondary.

It's easier to change them out for insulated ones now if it's going to cause a problem?

Thanks
Craig

[Stevem]

Before we even talk about the grounding layout of the amp I need to ask what you mean by open input jack?

If you mean they are not shorted when there is no jack in them, then that is not the type you want , and actually you want shorting type jacks for the speaker output(s) also,

[CraigGa]

No I meant the open metal ones as opposed to the black plastic ones that would insulate the sleeve.

Craig

[xtian]

The output jack you typically want to be grounded to the chassis. Input jack may or may not make a ground loop when grounded, but you can drill a slightly larger hole and use fiber shoulder washers to isolate the jack if needed.

[Stevem]

In that case I have found the the input jacks are fine ground into there mounting holes, but all the grounds for the first two gain stages should get grounded at the input jack(s).

Be sure to have the speaker jacks mounted far away from the input jacks and the circuit components of the first two gain stages.

One of the most important things grounding layout wise is to split the other preamp ground runs from the output screen filter grounding point.
In other words you do not want to daisy chain the high current grounds from the output section with that feeding the power supply filter grounds for the preamp grounds.

While your designing things note that it’s good practice to not have more then two gain stages powered off of any one filter node.

[thetragichero]

as others have mentioned, output jack should not be insulated and for the input jack "it depends"
IF you are not going to insulate the input jack, make sure to use a toothed washer to ensure the nut stays tight

[CraigGa]

Thanks guys,

So is using a shorting jack on the output a good thing? It seems counter intuitive coming from my solid state background.

Craig

[ChopSauce]

From my readings here and there: having the secondary circuit closed avoids running the primary shorted, which would be the worst. It seems that having a (220R/1W?) resistor between output jack connections is safer however for it doesn't rely on the jack switch.

Back to the input jack and toothed washer - if you don't mind.

For some circuits, the input jack provides the physical connection to ground for the pre-amp ground bus and if you have a single jack - or don't fear small loops - grounding through the input jack is pretty convenient.

One question, now:

does anyone know how superior is the grounding with and without a toothed washer? I've assembled an amp without toothed washers for its ground points and I am a bit troubled with the resistance to ground reading a few ohms, rather than a fraction of an Ohm...

[Stevem]

Running a tube output transformer with no load on it basically makes it act like a cars spark coil and that extreme voltage with no place to go arcs over the the internal windings and shorts out that expensive chunk of copper and iron.

On the other hand if the amp is played into a shorted speaker jack, no harm comes to the output transformer, and yes far different then a SS output stage that has no protection circuit as part of it’s design.

If you want a added layer of safety then across the speaker jack install a 250 ohm 7 watt resistor.

Should the speaker your playing thru ever blow open then this resistor will still provide a load to the OT and it’s 250 ohm value does not interfere with the audio output.

[R.G.]

Using input and output jacks that are grounded to the chassis can work well, as proven by the generations of amplifiers that do this and work fine. But (you knew there was a "but" coming, right? )

You might want to isolate the output jack from the chassis and run its ground wire back to the OT's output winding common lead to keep the current out of the chassis metal. And the input can be grounded to the chassis by the jack with good results if there are no other currents flowing through the path from the chassis to the input tube circuit's ground node, but you can make sure of this by isolating the input jack from chassis and running a ground wire to the input tube's local ground. If you do this, you'll want a small ceramic cap from the input jack's ground terminal to the chassis to bleed off any RF coming in down the input cable's shield.

If this sounds like a lot of trouble and fuss, yeah, it is. But it offers a way to get low grounding noise on the input and output jacks independent of where the rest of the circuit's grounding happens to go. That does not mean that you can't also get low noise by just tying the jacks to the chassis, but this approach depends on being lucky about where the currents flow through the chassis metal.

Grounding is tricky because it's all about where the current flows, not where the voltage is. The current flows where there is the least impedance (that is, resistance at low frequencies, inductance and capacitance at high frequencies). In unlucky setups, speaker current flowing through the chassis can cause voltages on the input jack ground, and this is then amplified and ... the amp is noisy, or has resonant peaks and dips in response, or can oscillate. Any AC current flowing into/through the chassis can cause hum voltages that the input ground point turns into an input voltage and the amp can hum. In extreme cases, this can happen from capacitive leakage from the PT into the core and chassis, flowing to the AC safety ground attachment point.

My preferred setup is to isolate both input and output jacks from ground, then run a wire from the jack ground to where I want that "ground" to go. This forces the input ground reference to be at the input tube's ground, and forces the output ground current to flow back into the OT without making a voltage across the chassis. Yep, it's a PITA, but it works every time if you do it right, and you don't have to go hunting for magic ground points on the chassis to get hum to cancel. I also prefer to put the AC safety ground wire attachment to the chassis very near the transformer to stop any AC mains capacitive leakage on the chassis.

Star lockwashers are the way to go for any attachment to the chassis you want to be reliably attached for years. The screw-down force is transmitted to the points of the teeth and this causes enough pressure concentration at the tooth contact points to make what the connector folks call a "gas-tight high pressure point" where the metals push so hard they flow locally into a contact that does not corrode because air/oxygen can't get into that tiny point. The AC mains safety people insist on toothed lockwashers for grounding for this reason. Works for signals too, if you happen to want to force the signal to be attached to the chassis for some reason.

[ChopSauce]

Thanks R.G. That seems to be something of a valuable answer...

... isolating the input jack from chassis and running a ground wire to the input tube's local ground. If you do this, you'll want a small ceramic cap from the input jack's ground terminal to the chassis to bleed off any RF coming in down the input cable's shield.

Any value recommended for the small cap?

I did that (grounding the input jack to V1 socket) once and it worked just fine. I'm glad I received your "endorsement" afterwards, because I was not sure I would do that again, until now.

[Stevem]

This arrangement will shunt radio interference.

[CraigGa]

Wow thanks for all the great info,

Seeing as it's my first build and I don't want to let any smoke out, I'll go with the safety resistor across the output then if it hums I know where to start looking.
I've seen those Switchcraft fibre washers but with the price of getting some in the UK it'll be cheaper to get different jacks if that proves to be the problem.

One last question, R.G. mentioned the AC safety ground, would that typically be the same chassis bolt as the ground connections or a seperate one?

Thanks again
Craig

[pdf64]

…Any value recommended for the small cap?…

I think a ceramic cap in the range 10nF to 100nF should do the trick.
The chassis connection for the cap should be very close to the jack.

the AC safety ground, would that typically be the same chassis bolt as the ground connections or a seperate one?

Electrical equipment standards require a separate, dedicated fastener for the chassis safety ground connection.

[CraigGa]

Electrical equipment standards require a separate, dedicated fastener for the chassis safety ground connection.

Thanks,

I'll get my drill out again

Craig