Adjusting Bias

krimney

Super Member
I got a new tube from NOSvalves this weekend and went to bias the amp last night, AQ1003DT from ASL. My multimeter is an old analog unit from RS and is supposed to read from 125mV and up. From what I know about the amp I should be setting it a little over 250mV. However I can't seem to get much of a reading at all. The needle just barely moves. I do get some minor differences in the readings when I switch from one tube to another so I'm thinking it may be time to spring for a digital multimeter and keep the old one for checking impedance of speakers and checking outlets for 110V.

Do these #s sound right? If my unit does read down to 125mV then I should see the meter half way up the scale rather than just barely moving into the scale.

Ken
 
If the meter has a 125mV range, then full scale would be 125mV. If the minimum it can read is 125mV, then no, the needle might barely move with 250mV. You need something with a 200mV range; a DMM is what you need. Drop $100 or so and get a Fluke DMM.
 
Ken, Are you sure the bias shouldn't be more like 30-40mV not 350mV? Do you have the manual? Does anyone? I have spent the last 45 minutes Googling for specs or a schematic. NADA.

I see the newer models have a built in bias guage. That's handy.

I emailed my question to another ASL1003 owner. He may answer here.

Rick
 
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I would also be surprised if your amp should be biased to 350mV what does the manual exactly say?

Craig
 
Ken

Joseph Lau ( designer of the amp ) recommends a value between 250 and 450 millivolts. The readings are taken across a 10 ohm resistor and translate to the equivalent of a range of between .25 and .45 milliamps of grid bias.

The higher the bias the closer the output tubes operate to pure Class A. The downside is that the higher the bias voltage the more stress that the output tubes undergo. I run Klipschorns which require very little power and therefore I have biased my EH6CA7's at about 300 millivolts ( ie. .30 milliamps). If I were running speakers of relatively low sensitivity say in the 90 db/watt/meter range I might be inclined to increase the the bias setting to something in the 400 to 450 millivolt range.

You have my phone number. I would be happy to walk you through setting the proper bias for that amp.
 
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Craig

"I would also be surprised if your amp should be biased to 350mV what does the manual exactly say?"

Unfortunately,the "manual" supplied with the ASL AQ1003DT does not provide any information regarding the proper bias values for the amp.

I love my amp but must admit that the so called "documentation" that comes with it is virtually useless. Fortunately I was able to contact Joseph Lau who was able to provide the appropriate values for his design.
 
lynnm said:
Ken

Joseph Lau ( designer of the amp ) recommends a value between 250 and 450 millivolts. The readings are taken across a 10 ohm resistor and translate to the equivalent of a range of between .25 and .45 milliamps of grid bias.

250mV / 10 ohms = 25mA. E / R = I (said Mr. Ohm.)
 
dr*audio said:
lynnm said:
Ken

Joseph Lau ( designer of the amp ) recommends a value between 250 and 450 millivolts. The readings are taken across a 10 ohm resistor and translate to the equivalent of a range of between .25 and .45 milliamps of grid bias.

250mV / 10 ohms = 25mA. E / R = I (said Mr. Ohm.)


Makes perfect sense to me.

Craig
 
Got a DMM last night

I set the bias in the 250-275 range and things seem to be working ok. I checked my speakers resistance and it turns out they are 4 ohm and not 8 ohm so I switched to the 4 ohm taps on the amp. The seems to be very revealing and is picking up turntable noise I hadn't noticed before. I also seem to have alot of "electrical noise" coming through the amp. I've turned off the lights and other things in the house but still hear it.


FYI I am using the DC voltage setting in the 1000mV range.

Lynn, I have the instructions you PM me regarding setting the bias. I'll check for your phone # in my PMs as I still have a couple unpacked boxes in my audio room and I think all my "notes" are in one of them.
 
Biasing for the AQ1003DT

I have resent the following to Krimney. I am posting this here for the benefit of others who might have this amp too:


To bias that amp:

(1) Choose a bias value. Initially I would suggest something around 250 mv. ( .0250 volts). You can always increase it later if you wish. Joseph Lau ( the designer ) recommends a value between 250mv. and 450 mv. The trade-off is that the higher bias forces the amp to stay in pure class A for a larger portion of its power range but the higher the bias the more stress on your output tubes and thus the shorter the life of your O/P tubes. A final value of somewhere between 300 mv. to 350 mv. is adequate.

(2) Turn it on and allow it to warm up for about 30 minutes;
With no signal being fed to it.

(3) Ground the negative lead of your meter to the chassis and plug the positive lead into the hole to the right of the 4 holes and set the selector switch to the V1 position.

(4) Insert a small standard head precison screwdriver into the first adjustment hole and watch the meter as you turn the adjuster ( bear in mind that you are making an adjustment of a few thousands of a volt here so go VERY gently ). Adjust that setting as near as you are able to the initial value you have chosen but remember that you are dealing in thousands of a volt so that if you are aiming for 250 and the closest you can manage is in the range of say 240 to 260 don't start hyperventilating.

(5) Repeat that procedure for the other 3 tubes.

(6) Leave the amp powered up for another 15 minutes and recheck the bias for each tube and increase/decrease the bias for any tube that seems to have drifted significantly from the others.

(7) Put on some music and enjoy !

I find that this procedure while a PITA results in a pretty stable amp and I only need to check bias about every 3 months. It is also a good way to monitor the health of your tubes and amp because if more than occasional minor readjustment of a given tube's bias is required then perhaps there is a problem. That said I would get too excited about a tube whose bias drifts 30-40 mv. between checks. If on the other hand the bias drifts something more than say 80-100 mv. on a couple of checks there is a problem.

As a general rule the more efficient your speakers the lower the bias required to have your amp feed them sufficient output in Class A mode. The good news is that few of us will ever really hear the difference between the amp's output in Class A or in Class AB.

Keep me posted on your impression of that amp.

Lynn
 
noise update

Last night I disconnected the NAD receiver that I'm using for a phono pre and hooked up a CD player. The noise disappeared!.. So it's not from the electrical lines or the amp. It must be from the TT or the receiver. I'll swap a few mor receivers/pre/tt in and out and see if I can't get the vinyl to be as noise free as the CD.

The Dyncao ST-70 I had seemed to "warm up " after about an hour. The ASL seemed to getter better sounding after 2. Maybe the new tube is settling in or the new speaker cables are getting broken in?? Regardless, it sounded excellent. Does anyone know the efficiency of the mission Cyrus 781 speakers?
 
an update

I decided to check the bias againa after a few hours of playing and here is what it looks like.

I had set all of them to about 275 the other day. One is down to about 140, one is at 275, one around 300 and the other I can only get to read about 35 and I can't seem to adjust any higher.

Should I not bais after playing the unit? I'm also having some trouble getting at the adjustment dial as it is a bit offset from the access hole, I pulled the little plastic piece out and will try again tonight. It was sounding good prior to my attempts at checking. What exactly is the bias adjustment affecting/ input tubes, output tubes or ??

Ken
 
It sounds like the amp has some serious issues or your output tubes are wore out. I would quit running it until you get this Ironed out.

Craig
 
could it be....

that the 10 ohm resistor is gone and that's why my reading is only about 1/10th of what it should be? There is a range I can adjust it in but is is between about 20-40 mvolts. If this resistor was bad would I hear any sonic difference or is it only in the adjustment circuit? From what I can hear there is no significant difference beetween left and right channels. I switched tubes around and it the problem does not follow the tube so I think the tubes are ok.
 
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What do you mean by the "10 Ohm resistor is gone" ? I think you should hold everything until this gets ironed out.

Craig
 
From what I've been able to gather the bias readings(for this amp) should be in the 300 range. "normally" that would seem high as previous post indicate. In this amp the bias readings are take "across a 10ohm resistor". If the bias is actually 30-40 mv before entering the resistor then it would seem to me the resistor is upping the bias on the output side of it. My thinking is that if the resistor was removed then the bias readings should be in the more common 30-40 range. If the resistor is not doing it's job that mey be why the reading is low.

I don't have the technical knowledge to know what the resistors purpose is but logic tells me that if because it is there I get readings in the 300 range instead of 30 then if it isn't there I would expect 30.

When checking the bias I also noticed one side of the amp warmed up more than the other.

I'm gonna take the amp up to Rockmonton's on friday and have hime poke around a bit.

Just did some searching and according to the designer of the amp the bias should be .4V-.5V or 40ma- 50 ma.
 
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