Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Scsoul on February 14, 2017, 06:15:32 pm
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I have a power transformer it seem to have 2 hv center taps is this possable, when I ohm out the hv red leads I get 60 ohm from red to red or hv and 30ohms from the red -yellow or cenertap too each of the red or hv leads. , problem is there's a second wire that read 40ohms when connected to the red or hv lead, it's doesn't connect with any other wire except the red or hv when I put my leads on both these. Wires or center taps they read 10ohm. I know this is a strange question and hard to follow any help would be greatly appreciated. I've figure out the other leads , but this hv central has me stumped. Any help appreciated. Thanks mark
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It's pretty common to have a PT HT secondary winding with a center tap and a bias tap.
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Secure the PT to a grounded piece of suitable sheetmetal/chassis, wire the primary to a mains fuse, switch and 3-cord plug, plug it in and measure the VAC between each pair of secondary winding ends. (Don't let the winding ends touch each other (or anything else when you do this test, and use insulted gator clips on your VAC meter).
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Ditto. In addition I made a simple breadboard: a slat of wood with screw type terminal strips attached. (I even attached a fuse holder.) Attach all the Tranny wires & power cord leads to a terminal so they'ere not loose and dangerous. Also, if you have a variac, you can reduce wall voltage by a factor of 10, down to 12VAC. Then the secondary voltages will also be reduced by a factor of 10, making the process safer.
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Ok so im not crazy, the PT is already in a chassis I'll just wire it up I won't need
The bias tap I'll just snip it out,
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For unused tranny leads you can keep future options open by leaving them long & intact. Cap them off with a wire nut & el tape; fold them out of the way with tape or a zip tie.
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> wire the primary to a mains fuse
Alternative to this full-power experiment:
Connect the presumed 120V or 240V primary to a 6VAC source, such as a heater winding on another transformer (can be in an amplifier). All voltages will scale in proportion. A 120V:700V PT fed 6V into "120V" will output 35V. So you are unlikely to have huge voltages or currents, or instant smoke.
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I already specks all the wires be just wiring a power cord and tested everything with 120ac wall voltage this is the way I've always done it, I know it's kind risky but after 10yrs I've gotten pretty good at it never smoked one or even arched on, that second CT had me confused this is the wire you would hook a reversed diode too for neg bias voltage?
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that second CT had me confused this is the wire you would hook a reversed diode too for neg bias voltage?
Probably. You can confirm by measuring the voltage between that wire and the red/yellow wire. Will be in the neighborhood of 50VAC if it's a bias tap.