47 Ohms Above Ground

[silverfox]

Working on X-Chassis today I noticed in the Peavey Roadmaster schematic there is a 47 Ohm resistor placed between the ground of the preamp board and the Chassis ground. The pi is tied to chassis ground so it only applies to the gain stages.

I assume this is a ground lift. Any comments on this finding? I thought it was an interesting way of lifting preamp above chassis ground but, why doesn't that create a ground loop??

Silverfox.

[jazbo8]

No, it is good way to isolate the noisier "power related" ground from the sensitive preamp section. Is the Roadmaster a good-sounding amp?

[pdf64]

Merlin has an excellent document which cover this The Valve Wizard

[eschertron]

Working on X-Chassis today I noticed in the Peavey Roadmaster schematic there is a 47 Ohm resistor placed between the ground of the preamp board and the Chassis ground. The pi is tied to chassis ground so it only applies to the gain stages.

I assume this is a ground lift. Any comments on this finding? I thought it was an interesting way of lifting preamp above chassis ground but, why doesn't that create a ground loop??

Silverfox.

I've looked at the schematic for the PV Classic 20, and it too has a small resistor between the ground sections. Based on the size (1/8W) and the specification in the schem for it to be "flameproof", my suspicion is that it is intended to be a safety feature that disconnects the input from ground if there is a high voltage differential between the user and the amplifier ground. I think I found information to back that up, but it's been a while back...

[Enzo]

If you look at both the Roadmaster and the C20, and a lot of other Peavey amps, you will find the sectional grounding scheme with 47 ohm resistors "between" them. But if you put the amp together and measure the resistance across those resistors, you will find them "shorted." The various grounds are tied together at chassis points, usually the jack bushings.

I refer to the symbols as the triangle and rake. To me the rake liike like a garden rake, and the trianlge is the one made of a number of small horizontal lines forming a triangular pattern. Then either symbol can be boxed or circled. If you have better or more descriptive names for these symbols, let me know.

Look at the Classic 20, and at the input jack note the triangle ground for the preamp joins the chassis ground there. Now look down to the external speaker jack, the circled triangle there is joined to the chassis. So when the amp is buttoned up, the two triangle grounds, circled and not circled, are joined together through the chassis. And that shorts across the 47 ohm resistor.

Look at the Roadmaster. Preamp triangle meets chassis rake at the input jack. Note below the OT, the rake and boxed rake are joined. Upper left of power tubes, the triangle and circled triangle are joined, and same up below the pump control. At center rake and box rake join at the chassis in the FS jack.

There is more, but this is enough. If you now look at the 47 ohm resistor, the two ends each are linked to chassis, and so with the amp all in place the resistor is shorted across.

So the resistor does no ground lifting. What it does is provide a path to the rest of the system ground if the jacks lose contact with chassis.

[jazbo8]

So I stand corrected, there is no ground-lift at all... it would help if I had bother to read the schematic

[daz]

I hope this isn't a stupid question enzo, but i'm pretty sure it will be. But hey, look who you're talking to. Anyways, are you saying that this allows the sections that are 47R above ground to NOT cause a ground loop with the input jack thereby allowing it to a be a non isolated jack?

If you look at both the Roadmaster and the C20, and a lot of other Peavey amps, you will find the sectional grounding scheme with 47 ohm resistors "between" them. But if you put the amp together and measure the resistance across those resistors, you will find them "shorted." The various grounds are tied together at chassis points, usually the jack bushings.

I refer to the symbols as the triangle and rake. To me the rake liike like a garden rake, and the trianlge is the one made of a number of small horizontal lines forming a triangular pattern. Then either symbol can be boxed or circled. If you have better or more descriptive names for these symbols, let me know.

Look at the Classic 20, and at the input jack note the triangle ground for the preamp joins the chassis ground there. Now look down to the external speaker jack, the circled triangle there is joined to the chassis. So when the amp is buttoned up, the two triangle grounds, circled and not circled, are joined together through the chassis. And that shorts across the 47 ohm resistor.

Look at the Roadmaster. Preamp triangle meets chassis rake at the input jack. Note below the OT, the rake and boxed rake are joined. Upper left of power tubes, the triangle and circled triangle are joined, and same up below the pump control. At center rake and box rake join at the chassis in the FS jack.

There is more, but this is enough. If you now look at the 47 ohm resistor, the two ends each are linked to chassis, and so with the amp all in place the resistor is shorted across.

So the resistor does no ground lifting. What it does is provide a path to the rest of the system ground if the jacks lose contact with chassis.

[Mick Bailey]

To see how resistors are used to raise the signal ground, take a look at the Fender '63 reverb reissue service manual. Marshall also uses a similar method in some amps.

The flaw in the method is if the equipment is connected to another piece of kit where the signal ground is connected directly to chassis ground, it shorts out the scheme.

[Enzo]

The point, daz, is that no section of those Peaveys is 47 ohms above ground, all points are grounded to chassis ultimately, the 47 ohms only enters the picture if the chassis connection is removed from one of the jacks.


Having multiple ground sections is itself a way to prevent ground loops.

[silverfox]

Is the Roadmaster a good-sounding amp?

The Road Master sounded okay. I'm using the chassis and power amp to build a bread board. Rather than the method used by others I'm using the amp chassis with the power amp rebuilt on perf board. I'm about to post more over on the X-Chassis topic in Theory and Design.

[Chuck H]

I'm reluctant to jump in here at the risk of demonstrating my own ignorance (I'll be disappointed if jokes don't follow ), but...

I think what daz (and myself and probably others) would like to know is, if the circuits are all ultimately grounded without the 47 ohm resistor why is it there? It seems redundant. It's not enough for me to know THAT it works. I want to know WHY it works and HOW to implement the ideology in design.

[daz]

Don't be afraid chuck....i will always be here to take the spotlight off of you.

I'm reluctant to jump in here at the risk of demonstrating my own ignorance (I'll be disappointed if jokes don't follow ), but...

I think what daz (and myself and probably others) would like to know is, if the circuits are all ultimately grounded without the 47 ohm resistor why is it there? It seems redundant. It's not enough for me to know THAT it works. I want to know WHY it works and HOW to implement the ideology in design.

[Enzo]

As i stated, it insures a complete ground path even in the event the jacks lose contact with the chassis.

[g1]

As Enzo mentioned, they are basically redundant, unless there is a ground lost somewhere, then the grounds are still connected via the resistor.
In my experience, measuring the resistance with an ohm meter will show a zero ohm reading if all the grounds are properly connected.

[daz]

I still don't understand why resistors instead of straight ground connections. The only thing i could see as a possibility i mentioned in my last post, but since thats not it according to enzo, what is?