Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dynaman1 on January 10, 2011, 06:17:21 pm
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Some time ago, I picked up a very old Webster integrated amp that uses a quad of push/pull 6L6s. Tag reads "No. A 11817 Type 53 B 50". It had no tubes when I purchased it, so I didn't bother firing it up. I immediately checked out the OTs specs. The speaker connections were labeled C, 5, 7, 10, 14, 20, 28, 100. Before I pulled the OT from the chassis, I applied a low AC voltage to the ends of its primaries and noted the results of the output terminals numbered 5-20. Assuming the "C" was for "common", all the numbers jived just right for a primary of 3K4.
I labeled the wires and pulled the tranny. Some additional wires found on the primary side were a thin black wire that was attached directly to the chassis and one very thin mystery wire that I simply labeled "?". I don't remember what it was attached to, but it shows a resistance of 3 ohms when measured against the previously mentioned "chassis" wire. No continuity with anything else. The primary side also has another wire that went to the "28" terminal (as does one on the secondary side). That wire reads 2.5 ohms in relation to the "100" wire with no continuity anywhere else. All said wires are on the primary side.
The secondary side simply has C, 5, 7, 10, 14, 20 and 28. Except for the single lead "C", all the secondary wires seem to be typical and are each made of a pair of copper leads.
Normally, I'd just ignore the extra wires and simply use the C, 5, 7, and 14 for something close to 4, 8, 16, but I'm having a problem replicating the results I noted months ago. Using the same methods as before, I can only get the primary to reflect 3K4 with the C, 5 and 7 secondary wires. These three wires all have the usual low resistance between them. However, 10, 14, 20 and 28 show an open short when measured against the common tap. But they all have very low resistance between them. What's up on that? Is there a short in the transformer that occurred while it......sat on my shelf??? I didn't handle the OT roughly.
No, I haven't been able to locate a schematic.
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However, 10, 14, 20 and 28 show an open short when measured against the common tap.
What do you mean by "open short"?
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Wrong term. Infinite resistance maybe? Meter reads nothing and won't "buzz" when set to "continuity".
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> The primary side also has another wire that went to the "28" terminal (as does one on the secondary side).
The primary and secondary should NOT be connected together.
> one very thin mystery wire
How thin? Size of primary winding wire or secondary wire? Enameled?
Offhand I'd *guess* that the mice peeled-open the connection between 7 and 10.
Can you make a sketch with resistances?
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What color is the mystery wire?
Some of the Webster schematics (TP14 and TP45) show a yellow wire as a negative feedback wire.
http://www.angelfire.com/vt/audio/index.html (http://www.angelfire.com/vt/audio/index.html)
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After hours of testing and scratching my head I sat down for a Pall Mall and figured out the prob on my patio. I remember reading somewhere that some transformers don't use separate internally connected "tails" for each tap. Rather, the winding is simply pulled out for each tap and ran back into the tranny to continue the winding. In essence, each tap is simply a loop in the winding and that's why each tap seemingly uses two wires. Since the ends or "looped" part of the wires were trimmed, one (including me) could make mistake of not realizing that the winding has been broken. Not a big deal if both wires of each tap are attached somehow. The #7 "loop" had completely separated in mine and the leads oxidized very badly. Sure enough, One of #7s leads had continuity down to the common tap and the other half of #7s "loop" showed continuity up to the #28 tap. Twisting the clean ends of #7s wires restored the winding and the tranny is now operating properly. The problem was right in front of my eyes the whole time!
Thanks for taking a gander at my post and trying to help!!