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In Reply to: old Lindell guitar amp - no info on web - tube id - ?'s posted by mopar guy on April 03, 2003 at 06:12:12:
Let me see if I can help..."The tubes light up but no sound, not even a buzz. One of the wax and paper capacitors is blown apart, can't read the values, but there is another capacitor that looks the same and it is marked ".01mfd +-20% 600 W.V.D.C. Dumont 157." It doesn't have a line around one end or a + marking."
"- is this an electrolytic?"
No, it's a coupling cap. Replace both of them, if you haven't got proper caps (or tubes) drop me a note privately.
"- does this cap being blown up point to a likely suspect?"
Hard to say, it's so old it may have just come undone on its own. The tubes are definitely suspect, as are the filter caps.
"The rectifier tube is a 12AU6, then there are two other tubes, one is marked 50C5, the other is indecipherable, but looks to be similar in construction, except it has a clear top.
Actually the 12AU6 is not the rectifier, it's a small pentode that's used as a voltage amp. The 50C5 is your output tube, and the mystery tube is likely to be the rectifier. It might be a 50DC4, but...
The amp probably does not have a power transformer, and the tube heaters are run right off the AC line. So if you add 50 volts for the 50C5 and 12 volts for the 12AU6, then the rectifier needs to drop about 50 volts, that's why I mention the 50DC4 (50+50+12=112 volts-close enough to 115). Some units also used a resistor in series with the heaters, and they often used a 35W4 rectifier. I'd bet you find a large resistor of 100 ohms or so in there, and that rectifier is a 35W4. The 35W4 was VERY common.
The B+ is developed with a 1/2 wave rectifier runnning straight off wall AC too. This is a DANGER! Don't run this amp or test this amp while it's plugged in unless you know EXACTLY how to handle it. The chassis may be "hot", so a nasty shock would occur if you touch the chassis and an earth ground at the same time!
I STRONGLY recommend you use an isolation transformer to work on or run this amp!!
Replace all the caps you can find, replace the tubes (they're cheap!), clean up the controls and tube sockets, jacks, etc., GET AN ISOLATION TRANNY, and you'll probably be good to go. You might find some resistors that act up or are cracked/damaged, but the caps and tubes are a must, as is the ISO tranny. Be sure the line cord insulation is 100% too!
Follow Ups:
I have been shocked by an amp very much like yours... not fun. Be careful.
Thanks for the in-depth reply - On second check I can barely see the number 35, possibly followed by a W, on the mystery tube.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. How would I figure out which tube goes in which socket? The sockets are on either side of a smallish paper-sided transformer (is this the "choke?").
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?
"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!
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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