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BIAS with a twist on MA-1 Silver Edition

87.11.211.17
Posted on October 10, 2009 at 17:56:57
Nuvistor
 I read a few months back Ralph suggesting how to properly use a digital multimeter to set bias and DC offset; I own a wonderful pair of MA-1 SE connected to a MP-3 pre. I always check every now and then the bias via the front meters but this time, after checking them both with fine readings, I went further and used my Fluke 87. Following Ralph’s instructions I put the red/black probes in the speaker posts, set the mA scale on the meter, depressed the 2 front switches and... I was not prepared to what I read on the display.

Instead of being both more or less at 0.750 mA, one amp was at +/- 0.600 and the other at +/- 0.850, while they checked the same on the analog meters! I was quite thrilled by the discovery, so I repeated the measurements a few times. No changes. So I adjusted accordingly both bias and offset, sat in front of my Avantgarde horns and listened. Hmmmm… not bad at all… more space, more air, less edge maybe… my favorite Al Green song, Simply beautiful, sounded wonderful for instance. Natan Milstein Chaconne was even more involving, and so on. I believed my set up was quite up to my expectations but this discovery has changed the whole picture. So the analog meters can be *so* wrong; or it’s rather the other method unreliable? And what about the +/- offset between the two amps? Is it just mebeing dumb? Please, enlighten me, or make a fool of me, since I don’t know what method to trust anymore.

Cheers, Nuvistor

RE: BIAS with a twist on MA-1 Silver Edition, posted on October 20, 2009 at 12:41:24
MLin
Audiophile

Posts: 1
Joined: October 20, 2009
 The meters on my MA1 have fine-tuning screws. I set the bias and DC offset of the amps using my Fluke digital meter. I then fine tune the analog meters on the amps accordingly (based on DC offset).

Cheers!

units, posted on October 12, 2009 at 20:35:48
Lew
Audiophile

Posts: 2685
Location: Bethesda, Maryland
Joined: December 11, 2000
 FWIW, the units of current in this case are either milliamps (mA) (e.g., 750mA) or amperes (A) (e.g., 0.75A). Sorry I cannot help you with your question as my own amps have no meters; they were built so that I can bias each output tube separately, which I do with a meter directly. I don't know how the analog chassis meters are wired in your amps, but a true test of their accuracy would be to read current with your multimeter leads right on the chassis-mounted meter in and outputs and then to compare the reading of the multimeter to what you see on the meter. Maybe that's what you did, in effect.

RE: BIAS with a twist on MA-1 Silver Edition, posted on October 12, 2009 at 14:02:33
acres verde
Audiophile

Posts: 209
Location: Big Easy
Joined: November 13, 2004
 Point of clarification. In your post you mention depressing BOTH switches. See the post below where MarcG asks how to check bias with a VM. Ralph responds that you place the plus and minus probes of the meter across the speaker terminals and then depress only the bias switch. This was how hw said to do it specifically on MA-1s and also how he told me to do it on my M-60s (which, BTW, are hooked up to Duos). By depressing that switch only the signal you read bypasses the meter itself.

RE: BIAS with a twist on MA-1 Silver Edition, posted on October 17, 2009 at 06:51:20
Nuvistor
 Thanks for the hint! I actually always depress both switches, as per instructions sheet actually ("Depress both the Test/Operate switch (edge of chassis opposite the power switches) and the Bias Test switch (next to the Test/Operate switch)"). I'll check this later and report the results.
Cheers, N.

RE: BIAS with a twist on MA-1 Silver Edition, posted on October 11, 2009 at 07:46:49
baileyler
Audiophile

Posts: 102
Location: Chadds Ford, PA
Joined: January 17, 2007
 I have a pair of M-60 (modified Mk1, upgraded from original M-50), and I do not trust my meters (have had some issues). I use them to do a quick screen for any changes, but always do my final set via the speaker posts.

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