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In Reply to: Awesome info, thanks posted by mopar guy on April 03, 2003 at 07:32:49:
"Thanks for the in-depth reply"Glad I could help.
"On second check I can barely see the number 35, possibly followed by a W, on the mystery tube."
Yep, a 35W4 7 pin rectifier. So look for a big resistor like I mentioned. Be sure it's in good shape.
"Someone started to make a tube layout diagram on the inside of the cabinet, but the only value filled in is the 12AU7, and of course I pulled both other tubes without noting their original locations."
The 12AU7 number is probably 12AU6. All the sockets are 7 pin, right?
"How would I figure out which tube goes in which socket?"
Okay, if you know where the 12AU6 came from, then the 35W4 goes to a socket that connects to one side of the line cord - probably through that big resistor I've been talking about. If you find that, then the 50C5 goes in the only remaining socket.
"The sockets are on either side of a smallish paper-sided transformer (is this the "choke?")."
No, that's part of the filament supply for the 12AU6 IIRC.
"There is also a bigger, all-metal transformer, "Tranco 44 36 NOV 1963", and a 3-section filter cap can. Is this a power trans?
No, that's probably the output transformer. And the triple cap is a power supply filter cap.
If it'll help, here a link to a site that has a schematic that's probably pretty close to your amp's:
http://www.deltabluesman.com/
Scroll down and click on "Bargain Bin Amps", then click on the Harmony H-400 amp. That schematic is pretty typical of the old transformerless 3 tube amps.
Be safe, but have fun!
Follow Ups:
What am I looking for when searching for the isolation transformer you recommend installing? I have a bunch of old amps that don't seem to have power transformers - I'd like to make them safe - can I plug them into an external isolation transformer? Does a variac perform the same isolation function?
A variac does NOT perform the same function, you can't use any autoformer (tapped single winding) type tranny. You need an isolation (discreet primary and secondary windings) tranny. Hammond makes some, and I'm sure others do too. They are often made with male and female AC plugs for easy use with a variety of different gear.Buy a large enough VA (volt-amp) size, and you'll be all set.
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