Help! I tried to recap my amp!

[Structo]

Careful about touching anything in a live amp Doc.

I don't know what you meant when you said you touched the coupling cap but hopefully it wasn't with your bare fingers.
Hundreds of volts inside the amp looking for a way out, if it happens to be you, it won't be nice.

[docz]

I used a chopstick

Docz

[martin manning]

Great that you constructed the center tap. Very bad that you joined it to the mains ground. You must move it. Move it anywhere else but there, preferably to it's own spot on the chassis. As I suggested a transformer bolt is OK. One of the ground lugs on a terminal strip is OK, as long as nothing else is there.

The reason you don't want to use the mains ground is that you now have a potential connection between the mains supply and the filament winding. One function of a transformer is to fully isolate the mains supply from the amp's power supply. You've potentially defeated that if something should go wrong. Fix this NOW, please.

Phil, you are correct in that the mains ground (Earth connection) should have it's own dedicated attachment to the chassis, so the artificial CT should indeed be moved to another location. The reasoning is that the mains ground connection should be very secure, with little to no chance of failure. I don't see how this defeats the isolation of the power transformer, unless you were thinking that the CT was connected to the neutral, or could be through a wiring error outside the amp?

Docz, in the picture back at the beginning of this thread it looks like you've soldered the earth lead from the power cable to the end of one of the transformer mounting bolts, and that is not the best practice.

You should get some solder lugs to attach the wires to (the earth wire and your CT), and mechanically attach these to the chassis, including "star" locking washers to secure the nuts and to help make good electrical contact.

The best arrangement for the earth connection would be to drill a new dedicated hole in the chassis, right next to the place where the power cable enters. Leave some slack in the earth lead so that it would be the last thing to break free if the power cable were pulled out. You can use the power transformer bolt nearest to the 6.3V winding terminals for your artificial CT.

I still suggest that the OP build a simple SE amp for several reasons
To get a feel for what goes where
To see what parts do what
To get the feel good of making an amp work
To gain some experience

Seems like this exercise is filling all of those needs with a zero initial cost, and this amp has some sentimental value to its owner. It's very good preparation for taking on a kit or scratch build, IMO.

MPM

[docz]

Ok thank you for your input again guys.

Please note that we don't have neutral/live here in Norway, both are live. Just so we are clear on that.

Mcrracer, wow that is pretty generous of you! I would love to build a small SE amp like a champ or a JTM45 or something like that, but it does in fact cost a lot, mainly the transformers. I'm not sure where to source good transformers that are not very expensive. II would hate to ruin an expensive transformer because of my inexperience, therefore I have been a bit hesitant to take on such a task. But it is most certainly a dream. read only one guy who had just taken two wall warts and hook them up to each other in reverse, I'm not sure if that is safe.

I have spent some money on parts for this project, the first parts I got here in Norway, and they were really expensive, those filter caps cost me 30$ a piece. Then when I was going to buy tubes which all cost from 80 - 200$+++ I decided to give Ebay a try. I think I have spent about 150$ on Ebay, but now I have a lot of parts!! A lot! I just need to figure out what I can do with all of the leftovers after fixing this amp.

I will fix that ground thing, I just need to buy something to drill with, bolts, lugs and that solder star thingy you mentioned.

I'll report back later once I get more work done.

DocZ

[Phil_S]

Phil, you are correct in that the mains ground (Earth connection) should have it's own dedicated attachment to the chassis, so the artificial CT should indeed be moved to another location. The reasoning is that the mains ground connection should be very secure, with little to no chance of failure. I don't see how this defeats the isolation of the power transformer, unless you were thinking that the CT was connected to the neutral, or could be through a wiring error outside the amp?

Martin, the problem is a very remote possibility of a short.

With the artificial CT tied directly to the mains ground, there is a straight path to the source of the line voltage.

Both of these are grounds, so what's the problem? As long as they are functioning as grounds and not "returns" everything is OK. The reason for the mains ground is to provide a redundant neutral. Most of these redundant neutral grounds are never actually called upon to do anything but sit there, just in case they are needed.

Against all odds, someone steps on the line cord and breaks the internal connection so that the line feed is touching the chassis, making it live. All of a sudden, the redundant neutral is called into service. There go the electrons, looking for a quick path to ground. The surge arcs over one or both of the 100 ohm resistors (unlikely but possible) to find a 0.5 ohm filament winding. Now there is unfused mains voltage on both the primary and the secondary of the PT. Are you prepared to hold a guitar plugged into this amp?

The practice of keeping the redundant neutral (mains ground) isolated is for safety. This is probably twice as important in Norway. Docz has reported that he's got a two-prong line feed where a reversible plug is used. Both wires are considered "hot". This suggests the mains should be switched with a DPST, not the typical SPST we use in the USA and should probably fuse both legs. Further, it makes the mains ground the only ground. Because it is not a redundant ground, it is even more important to observe all the best practices: isolated bolt near where the line enters the chassis, soldered to a ring lug, use a locking washer, apply some lock-tite to the nut to double secure it.

Is there a qualified licensed electrician in the house who can comment further? I'm perfectly content to be told I'm all wrong.

[docz]

I think a DPST switch is a good idea to.. I will see if I can find one that fits on Ebay.

DocZ

[martin manning]

From what I can learn, the most wide-spread system in Norway looks like the attached, a so-called IT earthing system. New installations are evidently including a protective earth and a neutral (as one or two additional conductors), which connect to the common point of the three phases, and the local earth is deleted.

Seems like what Docz is dealing with would be like connecting a 1:1 transformer to the mains here, and then using the secondary of that and a separate local ground rod. With a new power cable installed, his chassis is now "earthed."

There's lots of stuff on the secondary side of the PT grounded to the chassis, including the input jack sleeve. Safety depends on keeping the earth connection intact from the chassis to the local earth ground. If it fails, and the chassis becomes line "hot" I don't think it matters where or how anything in the circuit is grounded, does it?

I agree a two-pole power switch is a good idea... that must be standard with this earthing system.

MPM

[Cornelius]

If you want to build a 5W SE amp like a Champ; try to get hold of an old all-tube radio at an antique shop - or at your local garbage dump... Then you have a PT and an OT which is ok for a first project.

Then i suggest that you order a 'small parts kit' and a fiberboard from Mojo Musical or some others; should cost you around $60...

Then the only problem would be a nice and practical chassis to put everything in.

Edit:
I just ordered a Small parts kit from Mojo Musical, and it contains all the 'small parts'; resistors, pots, knobs, caps, screws, sockets, pilot light - all but chassis, tubes, speaker, on/off switch and trannys; a fiberboard are around $15.
I had an old Sony stereo tape recorder which i used as a donor; excellent!

[Cornelius]

A note on earthing here in norway...

You're correct Martin.
Measuring between any of the phases, we have 230V; measurting between any of the phases and earth, we have 115V...

It is mandatory to have an 'earth-fault' fuse in the fusebox in every house here; if any current close to lethal (actually, a good bit below) are detected between any of the phases and earth, that fuse trips... (and the whole house goes dark... Happens often if one forgets an extensioncord out in the rain... )

[Cornelius]

Sorry, Quoted instead of edit...

[docz]

would this thing work?
[img:575:385]http://cache.finn.no/mmo/6/203/986/16_-683678599.jpg[/img]
That would make a cool cabinet as well wouldn't it?

DocZ

[Structo]

That is a cool old radio.
I would not destroy that.

Grundig is a well respected brand. I used to have a multi band radio made by them but it didn't look like that.

Another thing to salvage for a guitar amp would be any old tube powered PA amps. Like Bogen, Masco, Stromberg Carlson, etc.
Or old organ amps from Hammond, Baldwin, Lowery.

These types of amps are usually pretty easy to convert to a guitar amp.
Or you could just use the transformers from them and get a new chassis and build it around that.

[Cornelius]

Looks nice.

What kind of output tube are there? EL84?

And you should measure what high voltage there is, before making a decision. If it is close to what you want to build, i guess you're there. Although, it IS a nice piece of a radio in its own right.

If you decice to use it as a donor; try to trace the wires to the OT and PT, and make good notes of what's what of the wires.

I'm sure there is some here that have something to say about these OT not being a guitar amp tranny etc, but for these small wattages; my experience are that they really sounds suprisingly good.

[Aurora]

MArtin - and others...
Just a comment on the hot/neutral/grounding situation here in Norway.

The norwegian electrical code underwent a major change about 10 years ago, and the work of adapting all the subsections is still ongoing.
Pre 2000 , grounded outlets were only required in "technical rooms" , - i.e. kitchens, bathrooms, washrooms and such. There were no grounded outlets in normal domestic rooms, and this is the practical problem Doc will face in regular use of the amp. Add to that , that most electricians following the old code, would also cut the ground wire as they went along with the cable, thus making it impossible to achieve a grounded outlet without a more or less complete rewiring of a house. Since the 70s or thereabouts, the use of single wires in hidden conduit increased, and in this case it's easy to upgrade to modern standards. My house is built in 1980, following the old code, but with hidden conduit, so this is easy to modernize. The new code demands 30mA GFIs on all technical ( "wet")rooms and a max of 100mA on all other circuits as fire safety.
A typical new installation will use only 30 mA combined breakers/GFIs.

Your drawing is essentialy correct, as it depicts most of norwegian transformer substations built up until ca 5 years ago. The mains wiring shown in his early pictures, are typical of quite a lot of electrical apparatus, where one phase is switched, and the other is fused. Ideally, of course, both phases should be switched. The grounding was/is correctly attached, and moving it to another point on the chassis is not going to solve anything, as the resistance between two reasonably close grounding points is just a mere milliohms, and provides nothing in terms of increased safety. What it will do in beating any hum, is another story. Seems like his rewiring fixed that problem.....?

[martin manning]

A, Thanks for the additional explanation. Interesting to know something about how these things are done in foreign lands.

The reason for the dedicated attach point for the earth ground to the chassis is more mechanical than electrical. I think DocZ needs a better strain relief on the power cable, too. The nylon tie-wraps will slide off or pull through the grommeted opening if the cable is jerked hard as a result of the amp falling over or someone stepping on or tripping over the cable. I think there are some regs about this in EU and here.

MPM