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DC meter on power amp

My 300B power amp has two DC (mA) meters.  I know a VU meter moves along with the music, but this DC meters doesn’t.  I realized one has a lower reading caused by the 300B power tube, 0.64 mA and 0.72mA.  I switched left and right, and the lower reading follows.  I tried another pair of tubes and the readings are matched.  Does it mean the lower reading tube needs to be replaced?  What is the effect of one reading is lower?  I bought them used, and they are tested with no leak and shorts, and they are pretty closed in terms of percentage life left.  Would this mismatched in DC reading reflect in a tube tester?

Thank you very much in advance.


gte357s
rodman99999
6,206 posts
 
Here's hoping your solutions come quickly!
oldhvymec
5,101 posts
 
Turn the amps on and turn the volume to ZERO. Wait 10-15 minutes.

Turn the pot that has a HIGHER reading counter clock wise very slowly until it matches the LOWER reading.. BE VERY, accurate..

LOWER is better, HIGHER and HUMMING is NOT..

Contact the manufacture and ask what the bias range is on the pots and recommended setting for your amp and valves.

Normally lower bias setting =  a lot longer valve life. Changes the way they sound when they are cranked up too. 8W. Needs to be right..

Enjoy
gte357s OP
281 posts
 
those pots on top of the 300B doesn’t change the readings, no matter where the volume is.  
But interestingly, I just try change the 5AR4 on the high reading side, now the readings match, both about 72mA.  I tried switch the left and right of the 5AR4 earlier and the readings doesn’t follow the tube, so I thought the 5AR4 doesn’t matter.  But it seems it has an effect on it.  
rodman99999
6,206 posts
 
That rectifier is (obviously) faulty.                                                                                                   KUDOS, on your experimentation and discovery!                                                                     The VU readings, still seem a bit high, for a 300B's bias setting.                                             Even if things currently SOUND glorious: I'd advise getting as much info/instruction on the amp, as possible, from Soundgate.                                                                                                      Too much knowledge (regarding one's gear) still isn't enough!
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