The Quest For Hum--my SET learning project

[wiredbecker]

Check it out, here's my first SET tube amp. A cheapo clunker re-purposed from an Admiral console.



Once I forget about the hum, I'm completely blown away by the clarity and sound stage. Don't laugh! I've been having revelatory experiences listening with this thing. Songs I've heard a million times over have new depth. I can hear where the performers are standing, imagine what kind of environment they're playing in, hear harmonies I never knew were there before.

After reading everyone's kind responses in the other thread, I went out and picked up some 10 ohm / 10W wirewound resistors to act as the dummy load. The good news is that these puppies are staying cool as cucumber. The bad news is that I am still experiencing amplifier hum as well as increasing distortion and it seems to be getting worse. Is the iPod at fault? The sound does not change as I move the mp3 player away from the unit (4') and I haven't tested any other sources yet.



The amp is plugged directly into the wall socket. No other appliances are on the circuit. The capacitors were reformed over two days time, 5-10 volts at a time.

Where to start my search?

[TerrySmith]

First of all, that is a HOT CHASSIS amp! One side of the A/C line is grounded to it. Get a small isolation transformer (cheap on ebay), and it will be safe to use and a lot of that hum may disappear.

[TomMcNally]

Next thing you want to do is replace that waxy looking tubular cardboard filter capacitor. You won't be able to find a multi-section like it, so just use individual capacitors with the same or bigger values than what is on the case. Put some tubing over the leads (wire insulation will work) and fit them in under the chassis. Replacing all of the capacitors wouldn't be a bad idea, and will only cost you a couple of bucks.

Just curious, what tubes does the amp use ?

[EWBrown]

My own SWAG, from looking under the hood, is two 50C5s and a 12AX7, filaments wired in series. Definitely a "hot chassis" job... Bzzzzzzzzztttttttt......... :o

/ed B in NH

[wiredbecker]

Toss the large waxy cardboard filter cap--check. Probably a stupid question but do they have polarity? I'm not sure if I'm completely up to a full cap job just yet.

Good eye EWB, that's def a 12AX7 but the other two are 50eh5 probably just a minor difference right?. I'm well aware of the BZZZT! factor and have been exceptionally careful in all my dealings with this beast. Which brings me to a few more questions...

What's the safest way to ground this thing?
What's the proper way to swap out the RCA input cable for female RCA ports which I will add to the chassis' backside?
Don't those traditionally ground to the chassis?
How do I test my vintage caps and resistors? I've been hoarding scads of them, some NIB. I'm probably just better off buying new caps for safety's sake.

[wiredbecker]

Isolation transformer? http://search.ebay.com/search/search.dl ... ransformer
Man, I have no idea where to start. Is that like what I used to use with my Lionel train set? What value am I looking for? Where would I even wire it up?

[mesherm]

http://www.jameco.com/webapp/wcs/stores ... tId=181315

An isolation transformer is just a 120 to 120 volt transformer. What it does is breaks the connection from the house wiring hot lead. The amp is then connected by a magnetic field only, effectively isolating or floating the output 120 volts from the house wiring anb ground.

[wiredbecker]

Very cool. Do these suckers need to breath or can I create some sort of sexy housing for this thing..

[mesherm]

How hot depends on how much current you draw. I would leave a few slots or holes in the case for some ventilation though.
I would be tempted to mount the transformer on a metal plate with the amp or else put them both in a hammond case with a cage. It wouldn't look much different from any other tube amp that way.

[wiredbecker]

I would be tempted to mount the transformer on a metal plate with the amp or else put them both in a hammond case with a cage. It wouldn't look much different from any other tube amp that way.

Yeah, it looks like it could fit over on the left hand side easily enough.



It's just a shame--I really loved how spare the layout was on top. Not to mention how cheap the thing was for the sound it put out. Only $44 including shipping! Obviously my safety and the way it sounds is more important.

[mesherm]

the el cheapo way would be to buy two inexpensive 12, 18 or 24 volt filament transformers and wire them with the low voltage sides together.
A pair of 12v @ 5amp, 18 v @ 4 amp , 24 volt @ 3 amp ought to be enough power.
That way you have 120 in and 120 out. Presto.

[TomMcNally]

You could mount the chassis on top of a wooden box, and move the ugly
transformers underneath, shorten the shafts on the controls, put some RCA jacks and speaker connectors on it. You'd have a nice place to hide a small isolation transformer too. Some of them are in boxes with power cord and ac outlet, some would get wired in.

Capacitors do have polarity, negative would most likely be grounded on all of yours. The cardboard cap on top should be replaced too. New caps are cheap enough, those are probably only 250 volts ?

... tom

[wiredbecker]

Sweet. I'm a big fan of cheap. I'll keep my eyes peeled thanks.

Found a useful capacitor FAQ at Phil's Antique Radio site



http://www.antiqueradio.org/recap.htm

Good resource with pictures for new comers like myself.

[wiredbecker]

You guys rock.

Thanks. Yeah, those transformers on the side are sort of ugly. Then again I could leave them there--they kinda reminded me of the bolts on Frankenstein's head. The chassis is made of copper and if I'm not careful, it'll oxidize and end up resembling his skin as well.

I picked up the female RCA connectors and speaker terminals yesterday but got nervous when I saw that the outer ring of the RCA housing seems to ground to the chassis which I knew was hot. Probably won't do any drilling tonight--the stock ST70 project is almost at 120 volts! This time I'll be using the recommended dummy loads instead of speakers.

[TerrySmith]

The isolation trans I have has a cord on one side, and a plug on the other. That way, you can leave the amp alone and place the iso tranny away from it. If you get into old radios, tv's, etc you NEED an isolation transformer so you won't get KILLED!

Another upgrade is install a full wave bridge rectifier, getting rid of the half wave selenium. 200v caps out of an old computer power supply would also work great here.