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Old 9th November 2017, 12:35 PM   #11
20to20 is offline 20to20  United States
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OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.
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Old 9th November 2017, 12:48 PM   #12
wdecho is offline wdecho  United States
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Thanks for the response. Do you think the sound would be better with both feedback networks or left as is?
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Old 9th November 2017, 12:52 PM   #13
Demonkleaner is offline Demonkleaner  United States
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Originally Posted by 20to20 View Post
OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.
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Old 9th November 2017, 12:58 PM   #14
20to20 is offline 20to20  United States
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Thanks for the response. Do you think the sound would be better with both feedback networks or left as is?
You have to consider what electronic effect your changes have made. By eliminating the 2K GNFB primary resistor circuit you opened the parallel connection it had to the driver cathode, and the connection to ground through the output transformer, and that changes the overall cathode resistance. When you lowered the plate to plate 1.5M resistor you may have increased the voltage to the driver plate if there is no coupling cap to block it. Those changes will have moved the operating point of the driver and the input impedance to the output tube grid. The only way to judge how the amp was designed to sound is to get it working according the the builder's concept. Then tear it up.
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Old 9th November 2017, 01:00 PM   #15
wdecho is offline wdecho  United States
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Good point. I will give it a go this morning and report back. Thanks.
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Old 9th November 2017, 01:20 PM   #16
goldenbeer is offline goldenbeer  Germany
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"Since no UL tap on OPT, have you tried triode strapped ?"

No I have not but it may be something I may try at a later date.
6P3P = 6L6G = super weak output power triode strapped...
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Old 9th November 2017, 02:11 PM   #17
audiowize is offline audiowize  United States
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I'd also look for where you run out of gas on the bottom end. I like to look at 5% or 10% distortion power available at 30Hz, but that's pretty arbitrary.
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Old 9th November 2017, 04:01 PM   #18
artosalo is offline artosalo  Finland
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Quote:
6P3P = 6L6G = super weak output power triode strapped...
Yes, but with high supply voltage, say 400 V, some 5 W is possible.
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Old 9th November 2017, 04:02 PM   #19
wdecho is offline wdecho  United States
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Originally Posted by 20to20 View Post
OK, that is a perfect example of having the GNFB polarity backward. You can't assume any color coding will give you the correct polarity. I'd put the FB components back in place and then swap the tranny leads on the primary side to reverse the polarity of the tranny. You could also do the swap at the speaker leads as long as you make sure the FB connection stays on the (+) side. Make sure the (-) is chassis grounded.
Much thanks, problem, riddle solved. I reversed the wires on the primary side of the OPT transformers and installed both feedback networks with proper resistance and it is now as the designer intended. Short listening with a few songs and it sounds very good.
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Old 9th November 2017, 04:06 PM   #20
20to20 is offline 20to20  United States
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Cool, thanks for the update. Always like to know how well advice works out. The fun begins. 20
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