OT Here is a schematic for Harmony H303B tube amp.

[RLBJR65]

Harmony H303B tube amp Schematic posted here. http://home.earthlink.net/~rlbjr65/

I picked this up really cheap at a thrift store
Google didn’t yield much info on it, no schematic just a couple of ratings.
So I was very happy to see that the schematic was pasted to the inside of the case! I know there are quite a few amp schematic sites on the internet but I have never corresponded with any of them, so if you know of a site that would host the schematic feel free to post it there.

BTW it sounds pretty good  
Fairly tame by itself, seems me that it is at least as loud as a Ruby and has just a little distortion when fully cranked. It really comes alive when I hook up an effect to it.

Richard

[puretube]

that schem makes me shiver a bit... :shock:

(no isolation from the mains!)

(no polarity designation on the PS caps)

(large cathode resistor on output tube with lovolt cap in parallell)

[Fret Wire]

"Death Amps" as people like to call them. Danalectro and Valco made a million of these budget amps for Sears, Monkey Wards, and under the names Harmony, Silvertone, etc. Point to point wired with buss lugs riveted to the chassis.

Many of them sound nice, but you really want to make sure it is completly up to snuff, ckt wise. Electro's should definately be changed, and the ground to chassis should be ckecked on those riveted lugs, as many of those riveted style lugs lose a good ground connection over time. They are so simple, and have such a low parts count, that there is no reason to not replace every component in them. No mojo components to keep. You don't want any component opening up or shorting out in these type of amps. I have two Harmony H400's that I re-built, but I only use them at home. One multi-cap, a 5w resistor, 1/2w carbon comps, and Mallory 150 style caps. Nothing special. I would not plug one of these style amps into any AC outlet until I checked the building main's wiring with one of those outlet polarity checkers. And do use a modern cord. Most of these types of amps were UL listed in the sixties, picture that happening today. :shock:

Can these amps be used today? Yes. Should they be completely up to snuff ckt wise? Definately. Should they only be plugged into mains wiring that has been checked? Yes. You must heed the above, plus all the standard safety precautions, because these amps are less tolorant of component failure/mains wiring problems than amps with a PT, and will kill you quick.  

The guy who runs this site would no doubt post the schematic, he only has the 400 series Harmony's (under bargin bin amps).
http://www.schematicheaven.com/

[puretube]

in that "400" schem (original), the electrolytics are shown polarized (although hard to see...),
and the cathode resistor is apparently a 1500 Ohm "WW" (probably stands for : wirewound), which is 100 times lower than the 150k in that 303 schemo, where the bypass cap (25V) surely will explode within 5 seconds...
:roll:

[Fret Wire]

The cathod resistor (r7) on the H400 is a 150 ohm. This is noted on the factory schematic pasted inside the amp (I took a pic of to work with). I also verified all values when I went under the hood. I used a 1 watt 150 ohm in mine. The schematics for some amps on that site are poorly scanned.

The 30uf @25v is correct on the H400 schematic. That should be 30uf @ 25v, and it should be part of the 100uf/50uf @150v/30uf @ 25v multi-cap (paper). On the H400, the multi is noted as C4a (100uf), C4b (50uf), and C4c (30uf).

Hard to read schematics like those are just another reason why you should always verify them against the actual values.

[Fret Wire]

Double checking my notes, I replaced the multi on mine with single 100uf @160v, 47uf @160v, and 33uf @50v electo's.

[puretube]

I`ve gotten that hard to read value of R7 from the "schematicsheaven"
.jpg, and thought by myself 150 Ohm would be more reasonable, though in the pic it looked like 1500 plus the letters WW;
maybe it was intended to read 150 5WW - hard to decipher...

my point was: a tube running on over 100V with an OT at plate, and a
150k at cathode, bears at least 100V at the cathode... too much for a 25V cap.


[EDIT] I looked at that schem once more (enlarged): it says "150 OHM".

[Fret Wire]

Point made.  The schematics for the Harmony's are real hard to read on that site. The factory schematic isn't much better. The 2.2m resistor (r5) is labled as 2 2 meg, with no period between the 2's. Another Harmony H400a schematic I have has a 15 ohm resistor (r2) listed as "15 om". It would be easy to mistake those for 22meg and 150 ohm. Amps can be dangerous enough to work on, let alone with fuzzy schematics.

[puretube]

yep - especially with these un-isolated amps,
and even more with people possibly trying to mod such things for european 230V mains voltage...

easier to get a kill-loop, than a ground-loop...  :shock:

[RLBJR65]

Thanks!

I had a few errors and revised the schematic :oops: http://home.earthlink.net/~rlbjr65/

The caps are polarized I just forgot to mark them. It is a multi-cap like the 400. I'll be replacing those, probably all the caps.
Nice catch Puretube! 150K is indeed a 150 ohm.
Fetwire, those rivets are definitely bad news! The isolation transformer was loose and when I first plugged the amp in and it gave me a nasty little jolt. I drilled all of them out and bolted everything down.

A lot of those schems are very hard to read that's why I drew it instead of trying to scan the original :wink:

I think I will email it to http://www.schematicheaven.com/

Thanks again for helping me get it right

[Fret Wire]

Fetwire, those rivets are definitely bad news! The isolation transformer was loose and when I first plugged the amp in and it gave me a nasty little jolt. I drilled all of them out and bolted everything down.
A lot of those schems are very hard to read that's why I drew it instead of trying to scan the original :wink:
I think I will email it to http://www.schematicheaven.com/
Thanks again for helping me get it right

Fender had the right idea with machine screws, washers, and star-tooth washers. I end up drilling those out all the time. Not only the transformers, but grounding busses and tube sockets also.

Go with a 1 watt 150 ohm on the cathode when you rebuild it. Since your not gonna find a three piece multi-cap with the right values, go separate and use what I did for the electro's. They are nice sounding little amps when they're running right. Weber and Jensen make cheap 6" ceramic replacement speakers that will give you more volume and low end.

You should send that to Schem-Heaven. A correct, clear, easy to read schematic would be doing others a big service down the road. I should do the same with my H400's.

[R.G.]

Do yourself a favor. Measure the AC line current, then go buy a small isolation transformer (that is, 120vac:120Vac) and put that isolator inside the box. Run the amp on the isolated side of the transformer. It's not perfect, but it will prevent your untimely death under many circumstances.

It can't be more than an amp or two. Isolating transformers in that range are cheap.

Unless you have a death wish...

[Marek]

W8, how exactly does a transformer protect you? Limits the current which would otherwise shake your heart muscles? Or how?

And, if it does protect, under which circumstances does it happen? Could somebody write a few words about it? Or maybe post some link?

Thanks,
Marek

[R.G.]

W8, how exactly does a transformer protect you? Limits the current which would otherwise shake your heart muscles? Or how?

And, if it does protect, under which circumstances does it happen? Could somebody write a few words about it? Or maybe post some link?

It has to do with how electrical power is transmitted. In a simplistic, hand waving sense, the power companies use the earth ( that is, the whole planet) as a return conductor. It's not that simple, but that's a useful way to think of it.

The electrical power that comes out of the wall in non-industrial applications is a power/hot wire and a neutral/cold wire. The neutral wire is connected to the earth/planet back at the power breaker box or power pole by a copper wire running down to buried copper conductors or metallic cold water pipes to connect it to the earth electrically. This is the two-wire system that was common several decades ago. The third wire safety ground has been included for safety reasons that we'll get to.

It used to be thought that two wires was enough. The second wire (neutral/cold) was after all grounded, right? So the current flowed out of the hot wire, through the equipment, and down to the cold wire, which was at 0 volts.

If only it was that simple. Since the hot wire is hot to the entire planet, you can complete a circuit through your body by just standing on dirt or concrete (concrete is a conductor for these purposes) and touching anything that is at line voltage. And since it only takes about 20ma of current through your chest to kill you, you don't need full line voltage, just some leakage. 20 ma is almost unnoticeably small compared to the power it takes to run an amp.

(Yes, I know, we're not to transformers yet. Hang on, we'll get there eventually.)

So if your amplifier is one of those with only two power wires and it has a failure inside that shorts the power and neutral power lines together, or just leaks from power to neutral,  then the amplifier is slightly hot. The neutral line conducts this current back to the true, really, no fooling earth ground back at the pole. But what if something opens up the neutral line?

With only two power wires, where do you put the power on/off switch and the fuse? Do you put them in the hot wire, the cold wire, or one in each? The only safe answer is "both in the hot wire" because if you put the switch to the cold wire, the amplifier is all hot and ready to go, floating on the hot voltage but with the switch preventing any current from flowing. Stand on dirt, touch something inside the amp that's connected to the AC line, and you complete the circuit and potentially die with the power switch "safely" turned off. This has been the cause of death of several good technicians. Same argument for the fuse in the cold side. A blown fuse sets you up for death if you reach inside.

Let's assume you wire it all correctly, with the power switch and fuse both in the hot wire. Now what happens if someone flips the AC line cord over the other way? Now it's a death trap again.

That's the reason for three wire power. The third wire is there ONLY to carry safety earth ground. Neither power wire has to be considered "safe", and three wire plugs only go in one way.

In a non-isolated amplifier like yours, the amplifier common conductor inside is connected solidly to the "neutral/cold" power line wire. All the isolation comes from the high impedance of the input and from an output transformer isolating the speaker and any other outputs from the amplifier power supply common. These amps are also known as "hot chassis" amps because the chassis is literally at the power line voltage if you get the plug plugged in backwards. Same problem if the AC power receptacle in the wall is wired backwards, which happens a lot. Don't get me started on electricians in general. Most of them are OK, but there are some that can get you killed.

So your amplifier chassis is hot to the AC line in some circumstances. Which ones? You can't tell until it shocks (or kills) you.

The isolation transformer (Hooray!! we're finally there) protects you by breaking the link between the AC power line/planet power setup and the amplifier circuitry. The amplifier chassis is now isolated from the planet and the AC power line by the insolation transformer. If there is an AC leak inside, it is not "leaky" to the planet because the two are separated by the insulation of the transformer. If you accidentally touch the planet (or any properly grounded equipment, same voltage) and also the hot chassis, no current can flow through you because the transformer isolation insulation is in the way.

The transformer breaks the conductive chain that could let your body be the conductive path. And that's how it protects you.

Note that you can still kill yourself with this thing, even with an isolation transformer. The insides still have parts that are hundreds of volts apart. If you accidentally contact two internal parts that are hundreds of volts apart, you can still get current flowing through your body and die, but now you have to make **two** mistakes at the same time, which does happen, but it's not a deathtrap like the un-isolated case.

Did that help?

[sir_modulus]

Rg...you are a true master!

Cheers,

Nish

[Marek]

Well, that was a VERY nice explanation, indeed. Thank you!!

BTW, in Germany, there is no law or regulation telling you to which of the two 'holes' on the wall AC outlet you have to connect blue(Neutral) and to which you have to put black(Phase). Green/Yellow always goes to the 'sides' of the plug (Protective Earth); but the other two you can wire as you wish.

Greetings,
Marek Gospodarek

[RLBJR65]

Thanks, R.G. Not only a "true master" but a mind reader as well!
Any suggestions for making it a little safer was going to be my next question Great suggestion and explanation. I think I will add that to the schematic as a HIGHLY recomened mod.

More OT - I had a strange experience once wiring a string of lights. Just plain old el-cheapo ceramic bases wired in series, there were 2 florescent lights already wired. Turn the switch on 4 out of 5 lights worked, turn the switch off and the 5th came on :? Turned out someone had switched the hot and nuetral wires around in a jbox, also there were a couple of stray strands of wire touching the conduit. That had me scratching my head and tracing wiring for about an hour. Obviously this was pre-ground fault protection era.

[Fret Wire]

What's really scary is that you still see a lot of used amps from the 80's and 90's with the 3rd ground prong cut off the cords. There are still a lot of older homes that aren't wired up to spec.

[RLBJR65]

Over a year and a half later I finally get around to finishing this up!
Actually I had replaced all the caps and resistors shortly after my original post and it sounded much better.
Last week while cleaning I found the isolation transformer that I bought for it so I decided to put it in today.

The transformer I picked up was kind of big 3" X 3" X 2" (NOS, only paid a buck for it ) so I had to move some stuff around to fit the transformer in which allowed me to make a some of wires shorter and re route a few others. While I was at it I replaced all of the old wiring and 1 of the tube sockets. (only cause I broke 1 of the tabs off it $#@!)

Well it's no AC30 but It no longer gives me a jolt if I plug it in the wrong way and most of the AC hum is gone, so it was well worth the effort!

Thanks Puretube, Fret Wire and R.G.!

Richard

BTW the schematic is here if anyone is interested. http://www.schematicheaven.com/bargainbin/harmony_h303b.pdf

[JonFrum]

The 50C5 power tube is not known as a great tube for guitar amps - better for table top radios and portable record players - remember those?    On the other hand, the 12AU6 and its brother the 6AU6 are nice pentodes for guitar preamp. You can get some nice compression out of them when you hit them hard. Its always nice to get an old guitar amp up and running again - you earned yourself a dollop of mojo for your effort.