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Old 13th January 2007, 11:24 PM   #1
OKBill is offline OKBill  United States
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Default Fender Bassman PT Rewind

For reasons that escape me at the moment, I bought a genuine, burned out 1962 Fender Bassman PT off EBay several months back. After looking at it and smelling it for several months I finally decided to rewind it, and have to questions about what I observed during disassembly.

1. At first glance, the 6.3V CT heater winding looked like it was 32 turns of 16AWG magnet wire, but after looking at the winding and its terminations it became obvious that it was really a 16 turn center tapped winding made of dual paralleled 16 guage magnet wire. Other than allowing a more flexible and smaller diameter magnet wire to be used, why was this winding done that way?

2. The 320-0-320 secondary winding was the inner winding, and it had an electrostatic shield of copper foiled wrapped around it, underneath the primary and heater windings. This shield had a drain wire attached to it, which was stuck in between two of the core laminations. How does that work, since laminations are supposed to have a non conductive surface coating?
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Old 14th January 2007, 02:09 AM   #2
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Default Re: Fender Bassman PT Rewind

Hi,

Quote:
Originally posted by OKBill
1. At first glance, the 6.3V CT heater winding looked like it was 32 turns of 16AWG magnet wire, but after looking at the winding and its terminations it became obvious that it was really a 16 turn center tapped winding made of dual paralleled 16 guage magnet wire. Other than allowing a more flexible and smaller diameter magnet wire to be used, why was this winding done that way?
Gives perfect balance.


Quote:
Originally posted by OKBill
2. The 320-0-320 secondary winding was the inner winding, and it had an electrostatic shield of copper foiled wrapped around it, underneath the primary and heater windings. This shield had a drain wire attached to it, which was stuck in between two of the core laminations. How does that work, since laminations are supposed to have a non conductive surface coating?
If you look closer, the wire probably contacted a mounting screw. At least all the transformers I rewound that had that kind of shield did.

If you put the filament winding between the primary and HV secondary, you can eliminate that shield too, as the filament windings will have the desired effect.


16 turns for 6.3V? That must be some core!
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Old 14th January 2007, 02:41 AM   #3
OKBill is offline OKBill  United States
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I had pretty much decided to connect the new drain wire to some such ground, it made no sense to connect it anywhere else.

That 16 turn heater winding turns out to have been a predicted feature also:

Before I tore out the burned windings etc., I wrapped an arbitrary length of 40 gauge magnet wire I had l around the core, it turned out I was able to wrap 15 turns . Then I used a variac to apply voltage to the 320-0-320 secondary. Before reaching the point where my safety fuse blew I measured the induced voltage across my 40 AWG secondary as well as the factory heater winding. I was astonished how close the two measurements were...so close that it was obvious without even calculating that the factory winding had to be 16 turns.....no such thing as a fraction of a turn!

But seeing the two heater leads soldered to two 16 gauge wires each, and the center tap lead soldered to FOUR really made me scratch my head until figured out what was going on.
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Old 14th January 2007, 03:46 AM   #4
Trout is offline Trout  United States
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Hey,

If you want more practice, I have one here in need of rewinding.

one day it worked, the next day it blew fuses, no smoke no stink.
Trout
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Old 14th January 2007, 02:51 PM   #5
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Two 16 gauge wires make the equivalent of a 13 gauge wire. The 16s are much easier to wind... also, that gauge might make a single full layer, putting the connections at the edge - you DON'T want a tap to fall in the middle...
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