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Old 26th February 2012, 01:48 PM   #1
john65b is offline john65b  United States
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Default Audio Research Dual 75 - Low Bias

Hey all,

I have acquired an ARC Dual 75 from a local guy here and it sounds OK. Upon taking it home, I checked the bias for the hell of it and noticed it is only 1.5 to 3 ma on each tube. I have swapped other tubes and get only a few ma current...schematic says to bias it up to 50ma each tube.

The guy I bought it from had his tech look it over and said it was ok, but wonder if he really did anything if bias is so low.

All tubes (five 6550, two 12AX7, and four 6FQ7) test out good (two 6FQ7 were 65%) on my Knight 600 tester.

I put the settings in for the 6550 and it pegged out the 100% very quick, so I think the settings on the Knight 600 may not be quite right.

I have also tried other 6550 that I had, KT88 too, and cannot get more than a couple ma each tube.

6550 Plates show the correct 585v, the bias voltage is at -21v...and ideas?
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Old 26th February 2012, 02:10 PM   #2
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how you are measuring current in situ ?
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Old 26th February 2012, 02:16 PM   #3
john65b is offline john65b  United States
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I have the jack measuring across the cathode. There is a jack for all four 6550 output tubes
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Old 26th February 2012, 02:20 PM   #4
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Quote:
Originally Posted by john65b View Post
I have the jack measuring across the cathode. There is a jack for all four 6550 output tubes
Not sure what 'across the cathode' means. Is it 'across the cathode resistor(s)'? And are there four jacks or one?

A schematic would be helpful... Make sure the test jack does not measure imbalance instead of total current.

Greetings,
Andreas
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Old 26th February 2012, 02:23 PM   #5
john65b is offline john65b  United States
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Sorry, should have said through the cathode, not across it - there is no cathode resistor...Cathode is grounded - the plug beaks the ground connection so you can read current.

See schematic
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File Type: jpg ARC_DUAL75A_schematic.jpg (214.6 KB, 135 views)
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Old 26th February 2012, 02:29 PM   #6
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yup

sch will help - or at least few high res pics of amp inside

edit - I see that you posted sch
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Old 26th February 2012, 02:32 PM   #7
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what's happening if you fiddle with RV7 ?
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Old 26th February 2012, 02:33 PM   #8
john65b is offline john65b  United States
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With RV7, I can move the current a tiny bit, but still within a couple ma, not the suggested 50ma

I have tried with and without load resistor across 0 and 4 ohm tap, does not make a difference...

EDIT: Just noticed the 4 ohm tap is grounded (why?)...maybe put the load across the 4 ohm and 8 ohm taps?
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Last edited by john65b; 26th February 2012 at 02:41 PM.
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Old 26th February 2012, 03:02 PM   #9
limono is offline limono  United States
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John
You're supposed to measure current not voltage . Set the multimeter to mA range .(300mA is OK) plug the jack , turn bias to 55ma check the other tube and turn DC balance so it's approximately the same on both tubes than turn the bias again to get close to 55mA and so forth with DC balance to the point you have 55mA on both tubes . Than go to other pair.
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Old 26th February 2012, 03:06 PM   #10
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Did you check the value of the (regulated) screen supply? Everything as it should be there?

Greetings,
Andreas
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