Harmony H-400A Project

[svstee]

First off, I'm brand new to this forum, and pretty new to tube amplifier work as well, all I have under my belt are some mods to an Epi VJ and replacing/cleaning pots for some friends.

I recently bought a Harmony H-400A off ebay. Stupidly, I didn't take any picks of the thing before I took it apart, but here is an old add for it:
[img:343:475]http://www.geocities.com/harmonyvintage ... h-400a.jpg[/img]

And here is the schematic:
http://schematicheaven.com/bargainbin/harmony_400a.pdf

I decided I wanted to:
1: Replace the power cord with a 3-prong.
2: Replace the filtering caps.
3: Retube.
4: Replace the torn speaker with a line out, enabling me to use it as either a head or combo, wiring a speaker cable directly to whatever speaker I replace the torn 8-inch Jensen that was stock.
5: Add an on/off indicator L.E.D..
Gut shot of the unmodified amp:



All point-to-point, as you can see.

I took care of the line out mod initially, worked great, and allowed for a lot of flexibility.

Problem cam when I added the 3-prong power cord. For the ground, I drilled a hole in the amp chassis at a convenient point and soldered it on. now the amp won't even power on, no sound no tube light, no tube heat.
When I desoldered the ground wire, the amp worked fine. What am I doing wrong?

[Structo]

[IMG:319:239]http://i301.photobucket.com/albums/nn71 ... 001096.jpg[/img]

I dug into your album and reposted the picture.
It really is too small to tell anything.

You should change the image size in your photobucket account to 800x600 which you can find under the upload window.
You will probably have to upload the photos again because it won't change the size after the change unless you do.

This amp does not have a power transformer.
You should really install an isolation transformer which is a simple 1:1 ratio transformer that does not step up the voltage but rather isolates the house current from being directly on the amp.

This amp uses the chassis for it's ground and depending on which way the plug is turned when plugged into the wall will result in either the neutral or hot side of the house current to be present on the chassis.
Very dangerous and was outlawed years ago.

[svstee]

I lost the original pic, but I re sized that one with Paint.

I got shocked by the chassis when the amp was off but plugged in, not fun. Does that mean I have the hot and neutral wires mixed up?

Sorry, I can't seem to get photobucket too work, I copied the image location and put it between the image tags with no luck. It worked fine for that old ad I found on a google image search, though!

[CaseyJones]

I got shocked by the chassis when the amp was off but plugged in, not fun. Does that mean I have the hot and neutral wires mixed up?

Oh, the horror.

I try to inform people about transformerless junk so here we go again:

Transformerless as you may have noticed is just that, transformerless. No power transformer. It also means the chassis is frequently hot as in "connected directly to the mains". It's a fantastic Grade A opportunity for electrocution. Ground a hot chassis and amp no work. I'm surprised it didn't pop a breaker.

Don't screw around with this if you don't know what you're doing. I know what I'm doing and I don't screw around with transformerless, either. They were cheap amps built to be cheap amps using cheap parts as indicated by the extravagant $49.99 price in the pic above. Use the cabinet, the speaker and anything else you can salvage, write the rest off as the price of education. Cut the cord off and throw the chassis out. Beat it flat before you throw it out so some bum doesn't dig it out of the dustbin, electrocute himself and thus expose you to liability from his next of kin. If you haven't stood all your hair straight up due to a memorable shock... or worse... then count yer blessings. You've escaped the heavy hand of Darwinism and you get to contribute to the gene pool for at least another day.

[Wayne]

The isolation transformer thing sounds like it might work, too...

W

[CaseyJones]

The isolation transformer thing sounds like it might work, too...

W

Yeah, the isolation transformer thing might indeed work.

Except that in my experience no one spends the extra dough to buy an isolation transformer for a fifty buck amp.

CLICK ON THE SCHEMATIC. THE AC CORD CONNECTS DIRECTLY TO THE CHASSIS. NO FUSE. AS-IS IT'S A DEATHTRAP.

I see stuff every day where musicians have abandoned good sense. Stuff that was dumb to begin with, stuff that became dumb because someone implemented a "great idea" without thinking through the consequences. If there's a correct way to do something and there's a cheap (but incorrect) way to do something which way do you suppose it goes the vast majority of the time?

Read the subtext. "I want to add a line out". O.k., the O.P. doesn't really mean "line out", that would be horrible beyond horrible. Two hot chassis are better than one I guess or if you have a ground loop it might as well be a full 117 volt ground loop. (Many of them are.) O.P. means, "speaker jack".

"I grounded my (hot) chassis and now my amp won't work. Why?" The O.P. is blissfully unaware the chassis is hot even though he reports gettting shocked with the amp turned off.

The first thing they tell you in any technical writing course is to guage the aptitude and ability of your audience. Don't assume your audience knows about safety, also don't assume that your entire audience is at the same level. There will always be a few on the steep part of the learning curve.

[Phil_S]

Use something like this before you kill yourself or someone else. This is no joke.
http://tinyurl.com/82g4qg

The ground from the a/c wall supply is (functionally) nothing more than a redundant neutral. You are shorting a/c on the chassis. I am surprised your circuit breaker doesn't trip.

What you are doing is not much different than plugging a jumper into the a/c wall outlet.

[CaseyJones]

Use something like this before you kill yourself or someone else. This is no joke.
http://tinyurl.com/82g4qg

Heres' the thing with that isolation transformer:

Like many isolation transformers the amp plugs into the transformer. That usually means if the fifty buck amp gets sold the fifty buck isolation transformer doesn't go with the fifty buck amp... if the owner even ponies up the fifty bucks for an isolation transformer in the first place. The right way to do it is to bolt the transformer down to the amp chassis and hard wire it in so it can't be easily removed.

Add a fuse so the cheapo amp doesn't wipe out the fifty buck transformer when the filaments short in the cheapo tubes.

[Mach Schnell]

It always amazes me when I come across an old cheap amp that they were even allowed to do that. Thank heaven for three prongs and modern wiring.

[Structo]

Yes if you do buy an isolation transformer, make sure to buy one that is made to be mounted on the chassis with wires that you can solder into the amp.
It doesn't have to have a huge current rating since the amp is small.

What Casey and others have said, if you don't feel comfortably doing this type of work, find someone who will or junk the amp.

I almost said something about the "line out" text.

You mean a speaker out jack. You need to isolate that chassis from the house current.

Notice the ground symbol on the secondaries of the output transformer?
Or is it a field effect tranny? Anyways:

Anyway, what that means is if you plug the power cord in the wrong way and have a speaker out jack on it, the speaker jack and cord shield (read the plug) then would also have hot 120 volts on it as well.
Very dangerous.
So read up on safe working practices on amps and only stick one hand in an amp when powered up.
That may save your heart from getting a lethal
shock by the current going through each hand and across your chest..

[CaseyJones]

First off, I'm brand new to this forum, and pretty new to tube amplifier work as well, all I have under my belt are some mods to an Epi VJ and replacing/cleaning pots for some friends.

Anyway, what that means is if you plug the power cord in the wrong way and have a speaker out jack on it, the speaker jack and cord shield (read the plug) then would also have hot 120 volts on it as well.
Very dangerous.
So read up on safe working practices on amps and only stick one hand in an amp when powered up.
That may save your heart from getting a lethal
shock by the current going through each hand and across your chest...

Riddle me this: If I hand my Uzi to my pet monkey with a full clip... what's the probable outcome?!

[svstee]

I understand basic amp safety (discharging caps, one hand rule, ect.) I I can read schematics, although I don't know how I missed some of the vital info that was in that schem.

And yes the amp is a cheap piece of crap, but it actually sounds pretty good. A decent mount of people use them (check out how much they go for on Ebay and what some people say about them on Harmony Central.)
I would like to get this amp up and running safely and am not opposed to shelling out a bit of money.

And I would like to think I'm not as dangerous as a monkey a loaded Uzi. Maybe not by a lot, though.

[CaseyJones]

I understand basic amp safety (discharging caps, one hand rule, ect.) I I can read schematics, although I don't know how I missed some of the vital info that was in that schem.

And yes the amp is a cheap piece of crap, but it actually sounds pretty good. A decent mount of people use them (check out how much they go for on Ebay and what some people say about them on Harmony Central.)
I would like to get this amp up and running safely and am not opposed to shelling out a bit of money.

I'll pass along one of my trademarked anecdotes:

We were out (trading amps!) Saturday. At some point there was a squirrel takin' a "nap" on the white line of the road. Well, it was pretty obvious he wasn't actually takin' a nap, he was more like a decal really.

We humans are lucky. We get to make mistakes and live to learn from 'em. Critters ain't so lucky, how many generations of squirrels do you suppose it's gonna require to breed a squirrel that instinctively knows to stay the hell outta the road?

Anyway, Google "isolation transformer" and see where it takes you. I made a quick search of my usual sources and came up empty but it was only a quick search. Heyboer can probably wind one without a second thought, it's not a difficult application. Then proceed with extreme caution. Your voltometer is your friend, use it and learn to interpret what it's tellin' you.

If it were me and I were in love with that amp I'd shitcan the 35W4 and 50C5 tubes, I'd add a real power supply starting with a Hammond 270DX or 270EX either of which have a real 6.3 volt filament winding. Shitcan the tube rectifier and go solid state or use a 6X4. Use a 6AQ5 tube and a 6AU6 tube which is identical to the 12AU6 except for the filament voltage. By the time you got done you'd have nothing left of the old amp besides the chassis, the pots and a couple sockets. I expect it's a little out of your reach at the moment but done correctly you'd have something safe and solid. Cost-wise a power transformer and an isolation transformer are equal, you're buying a transformer one way or the other.

And I would like to think I'm not as dangerous as a monkey a loaded Uzi. Maybe not by a lot, though.

Wanna borrow my Uzi? We'll find out real quick. I'd set it down 50 paces from ya and run like hell!

Nah. I'm keepin' the bullets. I don't run as fast as I used to.

[svstee]

I keep on hearing that this amp has no PT, or that it is "Transformerless", but I see two transformers on the schematic, and two transformers on the amp.
You can only see one hear, but:

[CaseyJones]

I keep on hearing that this amp has no PT, or that it is "Transformerless", but I see two transformers on the schematic, and two transformers on the amp.
You can only see one hear, but:

One of the hallmarks of "transformerless" amps is that if you add up the tube prefixes they add up to approximately 100. It's a throwback to the early '50s when the line voltage was 110 volts or less. So: 35+50+12=97, close enough. That's so when the filaments are wired in series they use all of the voltage from the wall.

The other hallmark of transformerless amps is that they hummmmmm... You can't get away from the hum, it's a function of having a relatively high AC voltage on the filaments.

Ah, but the clever little designers at Harmony thought about that in days gone by. There's that stupid little isolation transformer for the 12AU6 filament, then there's the output transformer. It's almost as if they were makin' an apology for cheapin' out on the amp but then they had second thoughts...

They were clever indeed. See the notation for the "ground" plane, the old sideways "E" for "earth"? They thought they were clever and we aren't, that we wouldn't notice that the chassis is connected to one side of the zip cord. It's a pretty slick ruse. It works some of the time.

Look all you want, there's no power transformer on that thing.