A New Project has come along!

[noredr2]

Greetings all. First time in on this forum, getting back in to some amp work. I've tinkered around with a Valve junior to varying degrees, but those came to me already working and fairly straight forward. This new one is a bit different. Just acquired an old Setchell Carlson Model 13B amp, and this will be my first restoration project. Power it up, tubes heat up, no sound output, then it starts smoking a bit. Got it opened up and it looks like the choke has melted a bit, but that's the only apparent problem after visual inspection. Mostly original components save for a capacitor and a couple resistors.

Here's what I'm thinking of doing with it. I'd like to add a master volume, of course replace the choke, put in a new output transformer to allow for more modern speakers (versus the 500 ohm the schematic shows), new power cable, and perhaps some sort of tone stack. Nothing too ambitions (I think)...Once it's up and running, then it's time to tap in to the woodworking skills and build a new cabinet for it. Looking forward to that as well.

For first steps, I need to figure out a new choke and output transformer. The current ones have no labeling whatsoever and I want to learn how to determine what's needed. Some questions that have come up in trying to figure that out: How does one check the OT and the choke? Is a choke even necessary? Some places I've read say that in low-powered amps filter caps are enough. And how do I even determine (roughly) the wattage of this amp?

Sorry if I ask some stupid questions. I've been away from amp work for a few academic years and all the reading/papers have pushed what little I knew out of my head. I'd rather ask than screw something up.

Schematic is available online here: http://www.theused.com/manuals/setch...tchell_13b.pdf

[Gunny]

before doing mods I think that you must find out the cause of the smoking components. My first guess is that you may have a shorted filter capacitor (or more than one) which draw excessive current, hence the smoking.

[loudthud]

The reason for the choke was it cost less at the time than bigger filter caps. If you use 16uF or 22uF caps, the choke isn't necessary. You may have trouble with the PT if there is very much rust. Remove the type 80 rectifier tube and see if the PT will run by itself without smoking. Carefully measure the AC voltage to ground at the rectifier socket. Two of the wires should be something like 300 to 350 VAC. If it's over 300 VAC you will need to use 500V filter capacitors. Otherwise you can use 450V caps.

Based on the 6V6 tubes, a tweed Deluxe OT should work fine.

[noredr2]

Alright, so I've had a chance to check these things out...

The smoke is definitely from the choke. Flip the switch, it starts sizzling in about 30 seconds. So that will get replaced with a capacitor as per loudthud's recommendation.

I was also able to check the readings with the rectifier tube pulled. 350VAC, and with all the tubes in, they all light up nice and strong. No rust, barely any paint chipping.

Another thing came up while checking through things today...the amp I have doesn't exactly line up with the schematic. Several capacitor values are off. C3 from the schematic (I believe the filter caps) are rated at 450V rather than 600V, and one of the caps is .01, 400V instead of 10, 50V. I'll have more time to check specifics later this week, but with all those differences and the age of the components (~60 years), would it be a good idea to just replace them?

Any thoughts on what I've said so far? Any questions that I should be asking? Thanks for all the input thus far...I'm looking forward to learning quite a bit more in this process.

[loudthud]

It's a good idea to replace most of the resistors and caps. At least you should have the parts on hand as you rebuild. Do you plan on keeping the circuitry more or less the same or changing it to something a little more modern? Regardless, don't expect any instant magic from a master volume control.