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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 8:23 am
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My transformer has:
two red/yellow leads that provide B+
a yellow/red (I put the dominant color first) that goes to ground
a red/black (or blue) that has no connection
two yellow leads that are 5v
two green with 6.3v
a green/yellow that goes to ground
a white wire that goes to the bias control
black and black/white leads that go to the 120v power.

Yes, I'm using a rectifier tube in this project. I've tried a 5Y3GT, a new 5AR4C and a solid-state adapter from one of the standard vendors. All with the same results.

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 8:26 am
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shimmilou wrote:
Still sounds like your reference is not actually ground. Does the AC supply from the transformer that feeds the high voltage rectifier, have a center tap that should be connected to ground?


My black probe is clipped onto the chassis so it had better be the actual ground point. :)

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 8:47 am
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jimolson1963 wrote:
My transformer has:
two red/yellow leads that provide B+
a yellow/red (I put the dominant color first) that goes to ground
a red/black (or blue) that has no connection
two yellow leads that are 5v
two green with 6.3v
a green/yellow that goes to ground
a white wire that goes to the bias control
black and black/white leads that go to the 120v power.

Yes, I'm using a rectifier tube in this project. I've tried a 5Y3GT, a new 5AR4C and a solid-state adapter from one of the standard vendors. All with the same results.


Are you sure about that white wire for the bias power supply? Red-Blue is the usual tap wire for the bias. Who made the PT?


Thanks!


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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 8:55 am
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I have no idea, I searched the web for days but never figured out who made it and couldn't find a schematic with the color codes on it.

I thought so too but earlier in this thread someone said the white wire was the bias tap and that indeed is what went to the Selenium rectifier that it had in the chassis.

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 10:30 am
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jimolson1963 wrote:
I thought so too but earlier in this thread someone said the white wire was the bias tap and that indeed is what went to the Selenium rectifier that it had in the chassis.


Yeah, that was me. That heat sink device attached to the white wire sound like a selenium rectifier. Does it go to the bias supply circuit? Then, to the grids of the 5881's? Your B+ voltage is simply too high for any dual 5881 amp. It's in the range of old 211 amps. Which is definitely lethal. There is no chance that the DVM is incorrect?

Any serial numbers or other ID? Can you post a photo?

Thanks!


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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 10:45 am
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I'll post pics when I get home, this is a home-brew from the parts from that DuKane amp. It's entirely possible I got something hooked up wrong but both meters reading the same makes me think they're not reading it wrong. I could be wrong there too, I'm not afraid to admit that. It's all part of the learning process, right? lol

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 11:04 am
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I found a schematic of another Dukane amp on the Net. This amp's PT has a white wire off the secondary, that is feeding the bias power supply.


http://www.ohio.edu/people/postr/bapix/Duk1a475.gif


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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 11:49 am
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I have that one too, I just wish I could read the writing for the colors on the primary taps.

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 11:51 am
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jimolson1963 wrote:
shimmilou wrote:
Still sounds like your reference is not actually ground. Does the AC supply from the transformer that feeds the high voltage rectifier, have a center tap that should be connected to ground?


My black probe is clipped onto the chassis so it had better be the actual ground point. :)


Maybe I wasn't clear, but the chassis is just a piece of metal, a connection surface. The reference to the power supply has to be connected to the chassis in order for the chassis to be a legitimate reference, otherwise you can read high voltage where none exists.

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 12:03 pm
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I have three ground lugs on the chassis - one for the green/yellow, red/yellow, bias ground, and something else, one for the ground wire on the power cord, one at the front-right of the chassis for the circuit board, jacks and pots. I was getting good contacts on all of them.

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Post subject: Re: Power & output transformers
Posted: Wed Mar 26, 2014 12:23 pm
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I think I just figured out the color codes on the primary after looking at a few different schematics of that 1A475 - Black = A/C, Black/Yellow = 117v. Black/Red = 127?v (it's hard to read that schem)

I wonder if using the Black/Red instead of the Black/Yellow would give me lower voltages on the secondary since my outlets run 121-122v?

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