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Old 7th December 2012, 09:13 PM   #1
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Question Channel voltage mismatch

Hi,

i've recently finished s simple preamp based on 6N6P tube (6x4 LCLC rectifier). i really like the sound but a mystery (to me) appeared.

in breadboard version, both plate voltages were desired 160V (470 Ohm cathode resistors). in final wooden chassis i got reading from one channel 160v, but other only 75 or so. just before plate resistors they are both 280, which is right. one side of double triode glows a bit less as well. i checked cath resistance - both 470 ohm. i changed the tube - same result. i also tried to insert the tube cca 5x to get rid off some possible oxidation in the socket. hearing-wise there is just a subtle difference but i'd like to solve this.

thanks for any opinions in advance.

all the best,

misok
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Old 7th December 2012, 09:21 PM   #2
DF96 is offline DF96  England
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Cathode voltages? Missing grid resistor?
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Old 7th December 2012, 09:25 PM   #3
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Quote:
Originally Posted by DF96 View Post
Cathode voltages? Missing grid resistor?
cathode voltage is correct 6v on the 'ok' channel, on the wrong one is around 10.

there are no grid resistors, i use cowon player as a source and it worked fine (= equally and with good voltages) breadborarded. so my idea is to tune it as it is (hum wise) and then add a stepper attenuator that i have.
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Old 7th December 2012, 09:43 PM   #4
DF96 is offline DF96  England
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So what is grounding the grids for bias purposes? You can't use a step attenuator unless you can guarantee that it is make-before-break and will not fail.
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Old 7th December 2012, 10:05 PM   #5
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Do you have a schematic handy?
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Old 7th December 2012, 10:23 PM   #6
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Originally Posted by DF96 View Post
So what is grounding the grids for bias purposes? You can't use a step attenuator unless you can guarantee that it is make-before-break and will not fail.
thanks ! i just soldered 100k resistors across both channels and the voltages are both right. i still don't get it since before there were no grid resistors and voltakes on plate/cath matched, but now they do again and i can sleep well. one side still glows a bit less but it has to be a matter of a certain tube. it sounds good now.

i used this (well known) schematic http://diyparadise.com/web/images/stories/yeo5687.gif but with a 6N6P double triode. it sounds great even before fine-tuning values of cathode components.
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Old 7th December 2012, 10:51 PM   #7
DF96 is offline DF96  England
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Your breadboard version must have provided a conducting path for the grids, either deliberately (resistor) or accidentally (leakage).
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