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Low Biasing on an ARC Ref75

Hi Audiogoners,

I need help with a question concerning biasing an ARC Ref75 amp.

I recently purchased the amp from an Audiogon member who promptly shipped the unit to me. Unfortunately, in the process of packing and shipping one of us lost the biasing tool. ARC is in the process of getting one to me but it will take some time.

The problem is that I have been running the bias too low (due to not being able to make any change) and I have just finished reading that it causes the tubes to run hotter. Intuitively I thought the opposite to be true (meaning high bias equates to low tempts and visa versa but I have learned the opposite is true).

The biasing adjustment is suppose to be at 65mA but is currently set at approximately 52mA.

My question is, of course, is the bias setting of 52mA too low and producing excessive heat and therefore I should leave the amp off until I receive the biasing tool? Or is such a low bias okay to maintain until I can make the proper adjustments? I am not so concerned with the tubes as I am damaging a tube socket or the main board. My plans are to replace the tubes in the near future just to be safe.

Any correct answers or advise is certainly appreciated.
okimike23
7 responses Add your response
lostbears's avatar
lostbears
538 posts
 

 I would not run the bias that low. Normally I set the bias on my Ref 110 around 63ma. I am not sure if my Ref 110 even came with a bias tool. My Classic 60 came with a red one which I still use.

 The bias tool is nothing more than a plastic screwdriver.  There is nothing special about it. I have a couple of them. You can get them at electronic parts stores. probably Ebay or Amazon would have them. My favorite has 2 size heads. A smaller head on one end and a little bit larger head on the other. I have only ever need the smaller head with ARC gear.. In a pinch you could even use a thin piece of plastic.



okimike23 OP
9 posts
 
Thanks much Lostbear. Hopefully I did not damage anything. I will look for a plastic screw driver until ARC sends me one.

My best to you.
czarivey
5,946 posts
 

for sake you can also use fingernail or guitar pick and finally, what's wrong with just a screwdriver? i never even knew that ARC provides 'bias tool' and adjusted bias with conventional long neck screwdrivers from wiha tools(Germany)

lostbears's avatar
lostbears
538 posts
 

 It is best to use an anti static plastic screwdriver ( I think they are actually called trim tools). You do not wan't to use anything metal in case it slips.


okimike23 OP
9 posts
 
Thanks to each of you for the good advice. I have ordered a plastic screwdriver set (yep, trim tool set is the correct term) to make the bias adjustments. 

Thanks again for your input.
ljs_fin
14 posts
 
My understanding is that 65ma adjustment for ARC amps relates to the idle current, not the bias voltage.  So if you reduce the idle current, you are not reducing the bias, and the amp will run cooler, not hotter.  I don't think this damages the amp though the sound may not be optimal.

It makes sense that if you reduce the bias voltage, the amp will run hotter.  But the 65ma measurement is not the same as the bias measurement.

That's my understanding, but I'm far from an expert and would like someone to clarify.  Thanks.  


okimike23 OP
9 posts
 
Hi ljs_fin,

Yeah, I am new to tubes so I am trying to catch up. I did receive the biasing tool from ARC and made the adjustments and all is fine. The amp sounds even better with it biased correctly--go figure!

Anyway, the amp is wonderful sounding and I am very happy with the Ref75.

Thanks to everyone who replied.
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