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PostPosted: 10 May 2010, 22:13 
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Joined: 03 Jan 2009, 10:44
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Hopefully this will be my last dumb question. I received my transformers from Edcor a couple of weeks ago but life has interfered with finishing my amps. Today, I hoped to make the final connections but ran into an obstacle, maybe.

In an earlier post regarding the KT88 amp's output transformers, Bruce said: "Use the yellow wire for the speaker ground, the orange for 4 ohms and the white for 8 ohms. Be sure to put the NFB connection on the white wire and the ground going back to the amp on the yellow if you do this."

According to the diagram received from Edcor for my CXPP25-MS-8K transformers, the white wire is identified as "common," the green as 16 ohms, the yellow as 8 ohms, and the orange as 4 ohms. For now, I'm only planning on having 8 ohm outputs. What should I do here?

Thanks in advance for your help! Please answer asap since I'm ready to listen to these babies!


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PostPosted: 11 May 2010, 11:03 
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Joined: 10 Sep 2009, 16:06
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Location: Sweden, Båstad
I would twist all the wires, and tape the ends of the 16 and 4 ohm taps and connect the 8ohm and common to the speaker-connecters. And as Bruce stated NFB goes on the common tap.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.


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PostPosted: 11 May 2010, 21:44 
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Joined: 28 Oct 2009, 02:34
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Location: Utrecht Nederland
WRONG!

Hallo, I am Ben from Holland.

Please stick to the Edcor diagram, yellow 8 ohm, orange 4 ohm and white is common ground, NFB goes to yellow, 8 ohm. I just looked at the inside of the real OddBlock's on picture for verification, and I am certain about that. Will send Bruce an e-mail for verification.

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Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 12 May 2010, 10:26 
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91lieb wrote:
And as Bruce stated NFB goes on the common tap.


I'm sorry, I must have been sleeping when I said that, feedback from "live" tap i.e. 4,8,16 ohm tap depending on which one is used.

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.


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PostPosted: 12 May 2010, 15:08 
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Hi Everyone, Sorry for the delay, I've been swamped with other things. Let me see if i can clarify this. First back to transformers 101. How a transformer company labels its connections is somewhat irrelevant. What has happened on many builds is that the phase of the primary windings (going to the tubes ) and the secondary windings (going to the speakers) are out of phase. With a NFB hookup this will cause positive feedback instead of negative. The amps will work that way, but sound poorly. So the easiest way to fix it is to swap the polarity of the output windings. That is, use the common wire from the transformer as the hot and the hot as the common. The alternative is to swap the plate and screen wires from one tube to the other in the pair (more difficult and I like easy solutions). Now how can you tell if the polarity is correct in the first place. With no NFB, the amp will be a certain loudness, with it in place and correct, the volume (at the same input level signal) will drop. If the polarity is backwards it will generally go up slightly. The ultimate power output with or with out NFB is the same, just the overall gain is a little less with NFB. Hope this helps.

Good listening
Bruce

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PostPosted: 12 May 2010, 15:47 
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This is helpful, Bruce.

Part of my confusion is based upon looking at photos of Gio's build of your kit. Since I'm using the same OT, I figured I'd match colors from the OT output. However, there is a white wire from the PCP that I know not whence from which it comes. (How's that for really bad grammar?)

Now I understand that I should use the common (white) wire from the OT as the positive speaker terminal and the yellow wire (8 ohm) for the negative speaker terminal. I assume (wrongly perhaps) that the NFB goes to the positive terminal? Since these connections are not soldered (ring terminals for the moment) it will be an easy matter to swap 'em around to confirm optimal sound. I just don't want to destroy my expensive, hard-to-get transformers! Sneaking the first set into the house was hard enough!!!
Thanks to all of you for the input. Or would that be "output?" Lofton


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PostPosted: 12 May 2010, 16:26 
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As always thanks for the informative answer Bruce. :)

/Ebbe

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"I'm gonna take my time on the next amp....." on First tube project.


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PostPosted: 13 May 2010, 23:07 
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Hi, Glad to assist. I used to go by the manufacturer's color codes on trannies, Now I always tack the wires the way there are supposed to go and check both polarities. It is easier to do this than to clean up mistakes later. You are very unlikely to harm the transformers if they are hooked up in reverse phase. The amp just won't work as well. The only time in over 50 years of messing with electronics that I harmed a trannie was when I hooked up the 6 volt heater leads to the AC mains instead of the correct ones. It was a 350 VA trannie and wham! Sparks, Arcs and Smoke, worse it knocked out the main 100 amp circuit breaker on that side of the house - the rest of the family was not amused. It also (acting briefly like a huge step up trannie) fried a bunch of parts.

Good listening
Bruce

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