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Old 20th February 2010, 05:02 PM   #1
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Join Date: Jul 2007
Default Already found problem with new amp

I was giving my new Chinese SE amp a quick once over to at least make sure there aren't any glaring safety issues when I found a rather obvious disparity between my two FU50 output tubes. Looking at the socket boards from the bottom I noticed the left side 12.6v heater circuit (pins 4 & 8) are bypassed twice, once to ground at pin 4 then again at pin 8 via two 50 ohm resistors (no capacitors). The right side board which is otherwise identical is not bypassed at all though the silk screening is present for the 2 50 ohm resistors. I have read about bypassing the heater in other threads but there are usually capacitors involved to filter out noise. I'm not sure what good a low resistance short to ground would do other than waste electricity. The fact that one has it and not the other suggests there is a problem but I need some guidance on what to do.
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Old 20th February 2010, 05:08 PM   #2
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Join Date: Aug 2009

the resistor connection is ok that way, you only have to connect the ac heater in one point each over 1x 50-100Ohm+ resistor to the ground. Without the resistors connected this way, you may notice an increased amount of hum..

Edit: check the attachment

Click the image to open in full size.
Best regards,


Last edited by Gorgomat; 20th February 2010 at 05:15 PM.
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Old 20th February 2010, 05:46 PM   #3
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Yes that is essentially how they have it wired, here are pics of the two sides. As you can see R2 and R3 are missing (as well as a grounding wire). It seems like 50 ohms is such low resistance. I'm not sure if R5 is missing, probably not as it would ground the pentode's screen to ground and it is already strapped to the anode via a 100 ohm resistor.
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Old 20th February 2010, 07:52 PM   #4
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Location: Nelson, NZ
It might be the lighting, but R6 on the left pic looks suspect (cracked?) as does it's solder joint RH end.
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