A friend found this old Bell and Howell projector in the trash and gave me the amp out of it. I'm building a guitar amp using the chassis and xformers. I think I have the PT figured out but I'd like another set of eyes to verify before I blow it up.
The colors on the cloth wire are gone so I heat shrinked some colors on for identification. I have:
Green:Green
White:White/Black:White
Blue:blue/black:blue
Yellow:yellow/black:yellow
The schematic attached is confusing. The primary shows a CT at about 90%, connected to the hot lead. When I meter it out this seems to be the blue:blue/black:blue. I marked the wires so I know what connects together.
If I connect 12VAC to the blue:blue/black and blue I read
White:white = 54V
green:green = .48
Yellow:yellow = .56
So white is HV and will be about 270:0:270, green is 5V heaters and yellow is the 6V heaters.
Look good?
Unknown power xformer double check
Moderators: pompeiisneaks, Colossal
Unknown power xformer double check
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Re: Unknown power xformer double check
How lucky can you get? That schematic shows voltage and ohms for each winding. If you can meter the ohms and it agrees to the schematic, you are pretty much assured it will be 320-0-320 like it says. Also, since you are looking for 5v and 6.3v, in the context of the 12v test, it suggests some amount of inaccuracy at low voltage and that you will end up at the higher numbers on the schematic. You should be good to go with it!
Re: Unknown power xformer double check
double post. sorry.
Re: Unknown power xformer double check
The problem with the resistance measurements is none of the windings measure what the schem says so I took a best guess.Phil_S wrote:How lucky can you get? That schematic shows voltage and ohms for each winding. If you can meter the ohms and it agrees to the schematic, you are pretty much assured it will be 320-0-320 like it says. Also, since you are looking for 5v and 6.3v, in the context of the 12v test, it suggests some amount of inaccuracy at low voltage and that you will end up at the higher numbers on the schematic. You should be good to go with it!
Thx for the double check. I'm gonna go for it, wire of the AC and give 'er the smoke test.
Re: Unknown power xformer double check
So, not surprising that things aren't perfect. Since you started this, it would be nice to know what the result is at line voltage, including the actual line voltage. In my house, there is no typical, but I've seen it below 120 and as high as 129. Whatever it is, you can do some simple math to "normalize" the result to whatever you think the intended was. If it is an older tranny, 117 is probably "normal".
xformer wiring
Hello! One thing to consider is the 6.3 wires have a center tap, and the 5 volt windings don't. (According to the schematic that is) So the secondary wires should be fairly easy to figure out. The HV windings have a much higher ohms, then by finding which winding is center tapped, you can find the 6.3v and 5v windings. Likewise the primary has one large ohmic value section, and one smaller value section.
Re: Unknown power xformer double check
Thx everyone. I'll about 90% done with this little amp. Once I power it up I'll know if my measurements were right.
What I did was replace 2 of the octal sockets with noval and one is wired for an EF86 preamp, the other for a 12AX7 LTP. The tone circuit is a 6 way switch with a variety of coupling caps from 500pF to .047uF. That goes into a 1M volume and into the PI. The output will be 2x6V6.
What I did was replace 2 of the octal sockets with noval and one is wired for an EF86 preamp, the other for a 12AX7 LTP. The tone circuit is a 6 way switch with a variety of coupling caps from 500pF to .047uF. That goes into a 1M volume and into the PI. The output will be 2x6V6.