A few questions of grounding and fusing on first DIY...

heekma

heekma

Well-Known Member
Well I'm in a holding pattern on the build, the Scott xfrmrs had dried out and cracked leads that couldn't be repaired so I've got some new ones from Heyboer on the way.

In the meantime I was thinking about some ground and fuse options:

I'll ground the powercord to chassis. That way no matter what happens I'll not have a hot chassis if I get a short to ground. But since the RCA inputs will be grounded to chassis due to how they install, do I run into any problems with the signal and AC sides sharing ground?

I was thinking about a floating ground on the signal side-even if something with high voltage shorted to chassis I'm still safe, right? But would I gain anything by having a floating ground on the signal side of the circuit going to centertap of the power xfrmr secondary?

Next question is fuses. The AC mains will be fused at the switch. But I was thinking it would be a good idea to have some DC fuses also. Reason being, if I'm goofing around in the amp and I happen to touch something I shouldn't wouldn't I have to have to allow more than 3 amps to pass through me to blow the mains fuse? And that's if I happen to be grounded to the chassis as well, right?

So I'm thinking about adding some in-line fuses under the amp, one on each filtering circuit of each channel, say around .5 amps. That way if I'm not grounded to chassis when I touch something I shouldn't those fuses should blow right away.

If I'm off track on this I'd appreciate being put right. I'm trying to keep this amp as safe as possible because I know I've got a LOT to learn and where high-voltage is concerned, I'd rather not learn the hard way. :no:

Best,
mojo
 
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If you use a grounded power cord all of the exposed metal on the amp, the jacks, etc. need to be grounded to the chassis. If the circuit ground is isolated from the chassis, and the chassis is connected to the powerline ground, you will get a voltage difference between the earth ground (chassis) and the shell of the RCA jacks which are connected to the circuit ground, and can get shocked if you touch them both at the same time.

Extra fuses are not a bad idea...you can use them in the filament and plate circuits to help provide extra protection against shorts.

You should not touch any HV while power is on, make sure you take steps while servicing so that this does not happen. Fuses will NOT protect against shock.
 
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Make sure you diferentiate between the HOT and NEUTRAL when connecting the power cord. and verify the power cord is polarized correctly white to neutral. The Ground (green) wire is a safety, but treat the neutral (White) as the return for the transformer. (Edited per Chads observation)

Fuses should be on the HOT (Black) wire.

You should also verify that your home circuits are properly polarized. hardware stores have a simple and cheap plug in device for testing polarity.
 
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The white wire should NOT connect to the circuit ground. The power line hot and neutral wires should connect to the primary of the power transformer only and that's IT. You would end up with a "hot chassis" by connecting the power line white wire to the circuit ground. Please someone else back me up on this, it can be very dangerous to have the power line connected incorrectly.
 
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Chad Hauris said:
The white wire should NOT connect to the circuit ground. The power line hot and neutral wires should connect to the primary of the power transformer only and that's IT. You would end up with a "hot chassis" by connecting the power line white wire to the circuit ground. Please someone else back me up on this, it can be very dangerous to have the power line connected incorrectly.
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Your right... I'm asleep, too darn early, I'll edit my post. Thanks! :thmbsp:
 
Thanks for the help Chad, always appreciated! That definately clears up my questions on AC vs. signal ground.

Like I said, I'm new to all this. I understand how the tubes work and why and what voltages go where, but that's kind of like just seeing the trees and not the forest.

Should I run a ground to each of the sockets just to be safe? I know they're grounded by being mounted to the chassis, but since the chassis is painted, they'll be grounded by the screws which may not have full contact with chassis, so is it a good idea to have a dedicated ground run to them as well or just sand the chassis a bit on the underside where the bolts mount?

I know where the HV is, I know to check the caps for charges and I've got bleeders in the rectifier circuit so I know where not to touch, but even so accidents still happen even if you're being careful. I've no illusions about how dangerous this stuff is, believe me. At least I'm starting to reach a point where I don't look inside and amp and go, "hmmm, I wonder what THAT does?"

It definately helps to know where all the nasty stuff is, helps the red light come on in the brain when you start getting close to it.

Outlawmws,

No prob on the goof-heck I'd be lucky to even find the keyboard let alone type before at least a half a pot of coffee! :D

Best,
mojo
 
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A couple pointers from the brain-dead maybe....IME....

I never dealt with 3 prong power cords, Just 2 pin hot and nuetral to the primary, line side through a fuse.
I've used a buss, a 10 awg copper wire for the grounds. If the Ps xformer has a CT, that would be the first place to ground to the buss wire. If it is a cap input PS, the cap should grounded right at the same spot as the center tap.
Then try to keep the higher voltage ground points at one end, (CT, cap..) working your way to the other end of the buss to the AC signal grounds.
If running input cable to the input jacks, ground the sheild of the cable to the buss. Then the the sheild to the ground pin of the jack, then to the chassis.

I wouldn't worry about grounding the sockets.

Been lucky so far.....knocking on wood......
 
Hey Mike, thanks for the tips-I can use all the help I can get!

On the ground wire itself, is 12 or 14 gauge big enough? IIRC 14 gauge is rated for 15amps and the insulation on the wire I have is rated for 600v.

My layout is similar to your suggestions: The AC gets fused and grounded right away. The power xfrmr secondary center tap and filament center tap gets grounded there as well.

As for the first filter cap off the rectifiers, should it physically be connected to the same ground point? My plan so far has been to have a major ground point at the back of the amp for AC and HV, then building separate filter sections using small lytics mounted on terminal strips rather than multi-sections and running a ground wire to each strip where needed, trying to keep most of the filters in the center and center/rear of the amp.

But to do it this way, I won't be able to connect the leads directly to that main ground point-I'll do it with ground wire. Will this still be OK?

Any info would be much appreciated.

Best,
mojo
 
In case you have missed my posting this before you need to put a filter on the end of the earth ground where it is soldered to the chassis. A .1 uF up to 1 uF 400V to 600V cap in parallel with a 100 ohm 5 watts or higher (wattage) resister will "float" the earth ground. I was told that this should be SOP when building with an earth ground. I guess if you don't do this it will effect the way the neg chassis functions. Electricity flows from neg to pos after all.
 
I would take a dumb guess that the cap and resistor on the earth ground to chassis would keep the current from bouncing around, I dunno.....

If I recall, sometimes using a tube amp with a chassis grounded to earth and hooking up components with a non-grounded chassis can cause a ground loop issue.
I've always went with two prong cords so I never had any experience with the issue.



""As for the first filter cap off the rectifiers, should it physically be connected to the same ground point?""

If it's a PS xformer with a center-tap, and a capacitor input PS, the first cap's negative lead should be physically grounded in the same spot as the center-tap. That's kind of a rule of thumb I suppose....


""My plan so far has been to have a major ground point at the back of the amp for AC and HV, then building separate filter sections using small lytics mounted on terminal strips rather than multi-sections and running a ground wire to each strip where needed, trying to keep most of the filters in the center and center/rear of the amp.""

Seem's like that would be fine, you could go star grounding with that idea in a way. The thing there is, is you end end up with a bunch of ground runs to the main ground point.
It isn't too bad with a simple circuit, but a little more complicated and you have many ground runs added in with all the other runs and then it gets ugly.....
Where you could take a buss wire and run it by the separate sections, (pre-amp tubes and whatever) and then ground those sections to the buss.

Or run a small buss by the section grounding the section to the buss, and then wire those sections to the main ground buss/spot...
That could cut down the ground runs.
There is many ways to do it, you just gotta keep things simple with layout and follow the rules and guidelines.

When I mentioned using a 10 awg buss wire, it's a 10 gauge exposed solid-core wire. The wire is not touching the chassis, and is mounted on stand-off's or whatever I can think of. It runs by each section, and the sections are grounded to it.
The buss is only grounded to the chassis through the ground of the input jacks.
This is in reference to a triode amp and kinda scratch layout in general.

I dunno what you are building or restoring. But sometimes with restoring the vintage units, sticking with the original grounds that the unit has can be a good thing.
Some units were assembled in factories, and they had to get them quiet. So you figure once they had a good layout and a quiet product I doubt they would deviate from it.
When I rebuilt my Magnavox PP 6BQ5 console amp I was thinking of doing a different grounding scheme. But I stuck with the way it was stock, and it has less line noise than of all my tube amps. Two-prong chassis.....


"But to do it this way, I won't be able to connect the leads directly to that main ground point-I'll do it with ground wire. Will this still be OK?"

Should be I suppose. The AWG wire you mentioned should be fine.
The first cap should be grounded to the CT, it'll make life easier......
 
Hey Thatch!

I must've missed it somewhere along the way, but it's good to know. You're talking an electrolytic cap, right? I'm guessing the cap smooths current to the chassis and the parallel resistor drains the cap?

I don't doubt your info, but my ignorance leads me to ask, what's the reason? Is this what I would do if I wanted to use a floating ground for the signal side and a chassis ground for the AC side?

Mike,

"If I recall, sometimes using a tube amp with a chassis grounded to earth and hooking up components with a non-grounded chassis can cause a ground loop issue."

If the chassis is grounded, and the RCA jacks are grounded to the chassis, then I should be avoiding the ground loop issue, right?

As far as the star grounding scheme, we're again both on the same path. My plan is to have two terminal strips mounted near the left and right channel. Each strip will have two lytics and two dropping resistors on it(multi-cap filter/voltage dropping sections). The negative leads of the lytics will share a lug which will have a ground running back to the main ground point at the rear of the amp.

Since each terminal strip will be mounted somewhat close to the gain and output stages, any resistors or ground wires will run to the grounded lug shared by the lytics on the strip. In a way, it's kind of like one big star ground at the back of the amp and then 2 "sub-star-hubs" spaced at equal points of each channel. Imagine the main ground at the back as the base of a "Y" and the v shape of the "Y" as the ground for the left and right channel.

I hope that makes sense. I'm working from a schematic for my first scratch build, so i'm in the dark about whether this is a good idea or not. Part of the fun of building from scratch: it's a good way to learn if this is a good or bad way of grounding. It makes sense to me, but that's not saying too much!

Best,
mojo
 
The cap shoulb be a poly, pio or something like that. I have a bunch of green Sangamos that would work. Remember the chassis as ground makes it part of the circuit. You have to put something to block and filter exchange between it and the earth to stop loops. Think of is a window, transparent, won't really slow down a brick, but a fly bounces off.

As you can see a 1 uF 400VDC cap is pretty large. You can go smaller in uF but not in Volts because you want that earth ground to work, not have the cap act like fuse if something goes wrong.

For a bus line it is best to use a tinned solid copper wire. Tinning a wire in sections isn't easy and you do not want cold solder joints on your filter caps, it will drive you crazy. You might be able to tin sections if you have one of those monster irons with the cloth covered cords from the 40s/50s. They have huge copper tips that are heated to around 750* and the size allows you to expell a lot of heat without cooling down to where the solder won't flow.
Of course if you can precut and preshape you could use a torch, paste flux and get the solder to flow like sweating a pipe. That is actually kind of fun, I like playing with fire.
 
Here is one I actually had put together but not used. A Cornell Dubilier .25 uF 600V and 5W100 ohm. one end goes to chassis the other to the earth ground wire. They do take up a bit of room.
 
Thatch, you da man! :yes:

Thanks for taking time to post some pix-worth a thousand words. I've got some poly caps that should work-.33uf orange drops, 630v and I've got a few 5w 100ohm ww resistors as well. That's the great thing-you do enough restorations, and after a while you've pretty much got your own parts warehouse!

One last question: Just for the sake of simplicity and a cleaner layout, since the chassis will be ground, is there any reason why I couldn't take the "sub-star-hubs" I described and just run a wire from the shared ground lug on the strip directly to chassis next to the strip rather than running a wire all the way from the previous ground point? That way the strip gets grounded right where it is and any other resistors/caps which need to go to ground can go to the shared ground lug. Simple and clean.

The reason for not using the dedicated bus line is the current layout was never designed around using one, it was designed for the "sub-star-hubs" I mentioned before. That and I think it will just look cleaner this way.

Thanks again for your help Thatch, I surely do appreciate it. I'm out here all by myself on this build with nobody looking over my shoulder. A scratch DIY is ambitious, a scratch DIY PP screen regulated amp with no previous kit experience even more so. I'm almost past the point of having enough knowledge to be dangerous, but now I'm getting to the point of having enough knowledge to see just how big the gaps in my knowldege are!

Best,
mojo
 
I think the star grounds will be fine, a ground is a ground. I assume some things shouldn't be grounded at the same point to avoid cross talk but the star grounds will keep groups isolated from each other.

I guess the only other thing I can think of is that you want distance between your PS and the input triodes. Using extensions and lowering the tubes into the chassis with enough room for air flow creats convection and you tubes will run cooler, but it also allows you to use nylon bolts and rubber grommetts in a way to provide some isolation for the tubes and having the tube socket under the chassis lowers the height of the tubes and looks good.

At this point your basic understanding of how amps operate exceeds mine. I just know a few things that somebody who really knows told me I needed to do. After asking why a few times and not understanding the answer, I now just do them.
 
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Thatch_Ear said:
SNIP
I guess the only other thing I can think of is that you want distance between your PS and the input triodes. Using extensions and lowering the tubes into the chassis with enough room for air flow creats convection and you tubes will run cooler, but it also allows you to use nylon bolts and rubber grommetts in a way to provide some isolation for the tubes and having the tube socket under the chassis lowers the height of the tubes and looks good.
SNIP
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Thatch, I'm not sure I follow this. Are you talking about making a bigger hole and leaving room for air from under the chassis flow around the tubes? What if there is no venting from under the chassis?

Also, I would think using ceramic sockets and letting heat conduct into the chassis would help hold the retained heat in the tubes lower. but that's just conjecture on my part.

I know my Motorola Chassis' get quite warm, but I was assuming most of the heat was from the transformer. Certainly that end is warmer that the output end.
 
Thatch,

"At this point your basic understanding of how amps operate exceeds mine."

Don't overestimate me-my capacity for stupidity is boundless-I've got the scars to prove it!

Seriously though, I appreciate the help. Without folks like you willing to take the time to give me a leg up I wouldn't get far at all.


Hey outlaw,

I think you're describing exactly what Thatch is talking about. IIRC correctly Bill _fort posted a pic of his diy and had done the same thing, sockets mounted on a plate below the chassis suspended by standoffs. I think it may also be a way of lowering the chances of microphonics as well-kind of a "suspension" for the tube. Looks damn cool to boot!

Best,
mojo
 
That is a great example plus he has holes drilled around the hole to provide more air for convection cooling. It used to be that tubes were cheaper than paying for the tooling on the chassis, in fact the transformers and capacitors were probably together the most costly things after labor. Good gear had to have the proper output transformers so cap values was a place to trim.
So if a .33 would just get the job done it was used instead of a .47. 4 to 10 caps a unit times so many thousand units and it is quite a bit of money, ratio wise more so back then.
DIY allows you to take the time to drill the holes a bit larger, mount the sockets under the chassis, use nylon to isolate, put in vent holes for better convection ( some people use a sub chassis to mount on!) use larger caps where better filtering can help......in other words not having to worry about bean counters. With the price of transformers and tubes being what they are, bumping the caps up a bit or adding the price of stand off mounting is trivial.
 
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