12 watt Heathkit amp gave up the smoke

skychief · #1 01-22-2012, 12:38 AM

Back in '54 my Dad built a 12 watt Heathkit amp when he was in graduate school at MIT. He gave me the amp when i was a 11 or 12(1971), I think. I ran a tube-FM tuner and a phonograph (i know-you kids are thinking *** is is a phonograph?) for a coupla years until i had to get a stereo like all the other kids. Put the thing in a box and stashed it away and basically forgot about it.

Fast-forward 41 years.

Today my wife was rummaging thru the attic for who-knows-what and she puts the box on the ladder. "hon, whats in here?"

When I saw that old dilapidated cardboard box, i knew right away what was inside it.

Couldnt wait to run it to a cab and plunk out some notes on it!

Quickly cobbled a RCA -> 1/4" plug cord to play the J bass. The thing sounded so fat and saucy. I was in tube heaven. For about 2 minutes. Then, I noticed the dreaded "component cooking" smell, and shortly after noticed wisps of smoke eminating from the back of the amp. Before I could kill the power, the pilot lamp went dark and the output faded to a really nasty 60 cycle hum.

I quickly popped the cover off and theres an aluminum can ( a cap?) that was so hot, when i touched it, it made that "tsssssss" sound. So Im thinking thats where the smoke came from.

So, my question is, what the heck happened? Its really late; ill post pics tomorrow.

will33 · #2 01-22-2012, 12:47 AM

50 year old capacitors. Re-cap the amp and it'll probably work fine, although the big ones are usually the power supply, if that went, it may have took put some other parts but start with caps. .Sitting that long, they dry out and don't work anymore.

Slowgypsy · #3 01-22-2012, 06:15 AM

Heathkit. Wow... that name brings back memories. Makes me even more fond of my Lafayette amp that powers the cd player in my studio. Best....

craig.p · #4 01-22-2012, 06:21 AM

Yeah, me too. Built a 5 MHz (wow) dual-trace scope kit and an audio generator kit back in the late '70s. Also used one of their dummy loads for amp repair for years. Great stuff.

rickdog · #5 01-22-2012, 08:57 AM

Quote:

Originally Posted by will33

50 year old capacitors. Re-cap the amp and it'll probably work fine, although the big ones are usually the power supply, if that went, it may have took put some other parts but start with caps. .Sitting that long, they dry out and don't work anymore.

+1. Old gear that hasn't been powered up in decades should be brought up slowly on a variac (variable transformer). Sometimes this will save the capacitors; sometimes they're too far gone.

Old Heathkit amps have wonderful tone! I have two W5M (25 watt) Heathkits (with matching WAP2 preamp), one of them needs to be recapped, but the other is working. I can practice in the living room, or play small coffeehouse gigs, and crank it up to the edge of sweet distortion.

Your amp is definitely worth restoring! Usually when a filter cap goes it shorts out the high voltage and the fuse blows before anything else is damaged.

**lowendfriend** · #6 01-22-2012, 09:27 AM

Quote:

Originally Posted by will33

50 year old capacitors. Re-cap the amp and it'll probably work fine, although the big ones are usually the power supply, if that went, it may have took put some other parts but start with caps. .Sitting that long, they dry out and don't work anymore.

Right on diagnosis....old caps just break down and short out.

skychief · #7 01-22-2012, 10:51 AM

OK, took a few pics this AM.:

What IS that aluminum can?

skychief · #8 01-22-2012, 10:52 AM

Heres the under-the-chassis view of that component:

Chainsaw Willie · #9 01-22-2012, 11:02 AM

That is a multi cap can. It has 3 or 4 caps built into it, all with one end tied to ground. On the side you will see the values of each cap with a square, triangle, circle, or other little icon. On the bottom of the cap where the lugs are you will see these icons identifying the lugs for what cap they are.

It can be difficult to find exactly the correct replacement can. It is really common to just buy individual replacement caps. The individual ones have less vintage mojo, but are much cheaper and don't have the parasitic coupling of multiple caps inside one can.

**B-string** · #10 01-22-2012, 11:21 AM

Try Antique Electronic Supply for a replacement. It is the filter cap and shorted out.

Chainsaw Willie · #11 01-22-2012, 11:44 AM

Also be very carefully in the power supply section of the amp! This cap can (and the resistors from lug to lug) is the power supply filter. These caps can store 300 to 500 volts even when the amp is off and unplugged.

This can kill you.

Make sure and short each lug to ground to discharge the caps. Some folks will use a junky screwdriver for this. If you can rig up a probe with a resistor, that is safer and will produce a smaller ZOT when you dump the voltage to chassis.

Buy replacement resistors too. You may break them while removing the cap can.

Weber speakers is another source for tube amp caps. They are probably the cheapest I have seen. These hi voltage caps will not be found at your local radio shack and maybe not even at mouser or digikey.

BassmanPaul · #12 01-22-2012, 12:26 PM

Quote:

Originally Posted by Chainsaw Willie

Also be very carefully in the power supply section of the amp! This cap can (and the resistors from lug to lug) is the power supply filter. These caps can store 300 to 500 volts even when the amp is off and unplugged.

This can kill you.

Make sure and short each lug to ground to discharge the caps. Some folks will use a junky screwdriver for this. If you can rig up a probe with a resistor, that is safer and will produce a smaller ZOT when you dump the voltage to chassis.

Buy replacement resistors too. You may break them while removing the cap can.

Weber speakers is another source for tube amp caps. They are probably the cheapest I have seen. These hi voltage caps will not be found at your local radio shack and maybe not even at mouser or digikey.

Very good advice. Safety should always be #1.

Never ever drain a cop by shorting it with a screwdriver. You can damage the cap that way. I use a 10W WW resistor for this purpose. Just about any value above 1KΩ is suitable.

I can't speak for the Weber parts but the ones from Antique Electronics have never let me down. One is running in my B15 without a problem.

**B-string** · #13 01-22-2012, 12:29 PM

Yes good points as there are probably not bleeder resistors in that power supply!

skychief · #14 01-22-2012, 01:03 PM

Great tips guys!! Especially about discharging the can cap. I knew to do that, but I prolly woulda just used a screwdriver.

Ill build a proper discharge probe like the one BassmanPaul described.

Now, to procure the parts!!!! *rolls up sleeves*

seamonkey · #15 01-22-2012, 01:04 PM

Keep the "Can" and replace the caps

As everybody says - Careful with that amp.
In 1954 there was not safety standards of today.
Get an isolation transformer if you want to work on it.
Get it wired with 3 prong plug.

Sneakypete · #16 01-22-2012, 01:18 PM

Quote:

Originally Posted by skychief

Heres the under-the-chassis view of that component:

Proper resistors with colour codes you can see! Definitely worth resuscitating - good luck.

skychief · #17 01-27-2012, 07:56 PM

Quote:

Originally Posted by seamonkey

Keep the "Can" and replace the caps

As everybody says - Careful with that amp.
In 1954 there was not safety standards of today.
Get an isolation transformer if you want to work on it.
Get it wired with 3 prong plug.

Would love to do this (stuffing the new caps in the can), but i need 4 caps - they just wont fit in that space.

Gonna have to replace each cap individually.

But I'll leave the can there for the visual impact!

**lowendfriend** · #18 01-28-2012, 12:22 PM

Yup, electrolytic caps.....famous for shorting out with age....

Chainsaw Willie · #19 01-28-2012, 12:23 PM

I have a similar project on my workbench right now. A 1951 Magnatone Melodier that hasn't been powered up since the '70s. I tried "re-forming" the filter caps with a variac but it didn't work. I spent a few hours slowly ramping up input power but when I got to 30 V the breaker tripped in the house. Repeated attempts also tripped the breaker. Oh well, at least the can didn't burst and spit goo all over.

I am also planning to leave the can in place so the amp looks correct, but disconnect it from the circuit and add modern caps inside the chassis where they cannot be seen.

When you order caps you may want to get a terminal strip (see photo) to build the new filter circuit on, as you can't use the cap cans lugs anymore.

skychief · #20 02-01-2012, 01:13 AM

Found the correct replacement can cap at tubesandmore.com

Woohoo! should be here by Friday.

Hey Chainsaw, check them out; they might have the proper replacement cap for your MagnaTone.! They have tonsa multi-cap can replacements for your vintage amp.

The voltage spec is way higher than is called for, but they managed to pack it all into a nice, small enclosure ( 2.5" high).

Check 'em out!