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#1
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The big hum story
Hello everybody,
Next step this evening. I started to listen to the amp. after modifying it following your advices. Without GNFB : big hum, with GNFB hum (lot less of course). the gfb loop seems to work. I can hear a nice music with the hum (with a dummy speaker and a dummy CD player. Here is where we are now : http://cjoint.com/?emuwdkPsFE I have questions for you : - the transformer for filaments 6.3V has no center tap, so the filaments are not grounded, is it a problem ? - i have made the prototype on a piece of wood (a 50cm square), i have tried to separe : heaters, ground, HT, signal. I don't think so much hum can come from the path of the wires, can it ? - if i raise the volume the hum increases with the music, strange ? - when i took my measurements with my voltmeter (60Mohm internal resistance), at the 60V point in the first 6sn7, the hum decrease. what is the reason for that ? - I cannot increase the 4mfd, because if i increase it the voltage is too high. Maybe i can diminue the voltage by interting a power resistor ? (i have to buy it but i don't have idea of the value, power) And when i tried 47mfd instead of 4mfd the hum was still there. - i have burnt another 1.6A fuse... I have here 230V. 230*1.6 = more than 300w. I happens when i plug it when the tubes are hot. Maybe it is not a good fuse (slow fuse ?) I now use 2.5A fuse, but it makes me think if i don't take risks. Thank you, Best regards, Luc D. |
#2
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#3
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"Sander deWaal" wrote in message ... said: I have questions for you : - the transformer for filaments 6.3V has no center tap, so the filaments are not grounded, is it a problem ? Yep. Connect 2 100 ohm/2 watt resistors in series over the 6.3 V lines, and ground the junction. -- Sander de Waal " SOA of a KT88? Sufficient. " Do you mean in parallel across the 2 legs of the filament lines and gnd. the junction of the 2 resistors? west |
#4
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Do you mean in parallel across the 2 legs of the filament lines and gnd. the junction of the 2 resistors? west I don't have 100ohm 2W, but i have 1000ohm 1/4w, is it ok ? |
#5
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I smell troll .........
wrote in message oups.com... Do you mean in parallel across the 2 legs of the filament lines and gnd. the junction of the 2 resistors? west I don't have 100ohm 2W, but i have 1000ohm 1/4w, is it ok ? |
#7
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ok good, i will buy 100ohm 2W.
and at the moment i ground one side to see if it improves the hum reduction. Thank you. Other topic : I plan to but 50w 200 ohm reostat to reduce the HT (as i have too much HT out of my main tranformer). 200ma*200 = 100V So i can reduce the HT between 0 and 100v. (i plan to reduce by 60V) Is there a reason to avoid doing this ? Luc. |
#8
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"west" said:
I have questions for you : - the transformer for filaments 6.3V has no center tap, so the filaments are not grounded, is it a problem ? Yep. Connect 2 100 ohm/2 watt resistors in series over the 6.3 V lines, and ground the junction. Do you mean in parallel across the 2 legs of the filament lines and gnd. the junction of the 2 resistors? The resistors go in series, across the two heater lines with the junction grounded. Sloppy wording on my part, sorry. -- Sander de Waal " SOA of a KT88? Sufficient. " |
#9
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said:
I plan to but 50w 200 ohm reostat to reduce the HT (as i have too much HT out of my main tranformer). 200ma*200 = 100V 0.2 A X200V = 40 watts. A lot of heat to dissipate in a resistor/rheostat. You can use an additional transformer to reduce the AC HT voltage, per http://www.ibiblio.org/kuphaldt/elec...s/AC/AC_9.html Scroll down about halfway, look for the "bucking" config. -- Sander de Waal " SOA of a KT88? Sufficient. " |
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