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Old 27th March 2013, 08:35 PM   #1
1108 is offline 1108  United States
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Default Replacement PT question

I have a Knight KN-728B that needs the power transformer replaced.

The original PT is: 640VCT 130ma
6.3VCT 5.5a

All I can find are a couple of Edcor's that are close but not exact.

- 600VCT 200ma
6.3VCT 6a

and

- 660VCT 200ma
6.3VCT 6a

Can I use either of these?

Thanks
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Old 27th March 2013, 09:03 PM   #2
PMI is offline PMI  United States
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These are both very close... -6% on one hand, and +2% on the other... These are tiny differences when compared to normal line and load variations, component tolerances, power output into different speakers, and so on. Either one should be fine.
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Old 27th March 2013, 09:06 PM   #3
1108 is offline 1108  United States
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Thanks PMI.
Does it make any difference if I go a little over or a little under?
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Old 27th March 2013, 09:39 PM   #4
PMI is offline PMI  United States
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Frankly, no.

A few percent plus or minus in voltage is not something you would hear. You might ask someone if that manufacturer rates their transformers to be on the conservative side, or at least look up what primary voltage they specify (meaning what voltage do they use to rate their product).

For example, if they use 115 VAC, I would choose the lower spec, because the line voltage in a typical home is higher than that most of the time, so your actual output will be proportionally higher. If they use 120VAC, I would choose the higher spec.

However, these are tiny differences when it comes to audio power supplies which are almost always rated very conservatively. In either case, you are buying a transformer that is rated at a higher current, so may be a bit bigger, and I would check the dimensions too.
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Old 27th March 2013, 09:48 PM   #5
1108 is offline 1108  United States
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Thanks again.
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Old 28th March 2013, 01:27 AM   #6
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The warning about making sure a replacement fits mechanically is quite correct.

Go with the 300-0-300 model and switch to SS rectified B+. The 150 mA. 6CA4/EZ81 is "in over its head" powering a PP 6BQ5/EL84 stereoblock. Forward drop losses in SS diodes are considerably less than those found in vacuum rectifiers. That way, a rectifier winding that's somewhat low in the voltage dept. gets compensated for.

BTW, the 130 mA. OEM part is undersized for the job at hand. That may account for the failure observed.

P.S. Post the schematic if you have it.
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Old 28th March 2013, 02:02 AM   #7
1108 is offline 1108  United States
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Thanks Eli.

I'll post the schematic tomorrow when I get home.
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Old 28th March 2013, 12:13 PM   #8
1108 is offline 1108  United States
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Hello Eli,
Here is the schematic. I'm interested in your suggestions.
Thanks
Attached Files
File Type: pdf kn-728b.1.pdf (219.9 KB, 59 views)
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Old 29th March 2013, 01:01 AM   #9
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The schematic shows that quite a few corners were cut. Besides the undersized power trafo and rectifier, the power supply takes O/P tube B+ from the 1st filter capacitor. No doubt about it, this is a cheap design.

Another penny pinching thing that was done is sharing a single bias network among all 4 O/P tubes. That requires a very closely matched quad and, of course, more expense to the unit's owner.

1108, how far do you want to go in getting this "beast" in proper working order? There's a "laundry list" of changes that can be made, besides correcting B+ PSU flaws. Modernization of source selection capability is 1 of them.
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Old 29th March 2013, 01:03 AM   #10
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Quote:
Originally Posted by 1108 View Post
Thanks PMI.
Does it make any difference if I go a little over or a little under?
Going over is fine.

I wouldn't go under or the amp wont give out full power.
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