When do you start to think it's time to adjust bias?

chadh · « **on:** 27 Mar 2010, 10:11 pm »

I'm an electronics nincompoop, and have been dreading the time I need to open up my amplifier, stick the multimeter in, and work out how to adjust bias. What am I going to hear to make me think the time has finally come?

I ask mostly because I just made a change in my system, and something just doesn't seem right. Even at low volumes, I'm suddenly getting a little distortion when the music hits a dynamic peak. These are recordings I've heard many times before, and suddenly they sound as though therecording level was set too high. It would seem a weird thing to happen when introducing a new DAC, and so I began to wonder what else might cause this effect. The effect I'm getting is a little like you hear when your favorite blues guitarist overdrives the tubes in his Fender Reverb Deluxe. That's why I began to think it might be a bias issue with my amp.

Thanks in advance.

Chad

Niteshade · « **Reply #1 on:** 27 Mar 2010, 10:19 pm »

It couldn't hurt to rebias them. When was it done last? At least you can get biasing out of the equation. Have you tried other sources with the amp?

chadh · « **Reply #2 on:** 27 Mar 2010, 10:45 pm »

The last time the amp was rebiased was when I bought it, which was...wow...almost two years ago.

I actually just swapped the old DAC back in, and didn't notice the same problem. Maybe it's the new DAC afterall. Break-in issues?

Anyway, I probably should do the rebiasing thing.

Thanks.

Chad

decal · « **Reply #3 on:** 27 Mar 2010, 11:22 pm »

I check my Rogue Tempest II Magnum about every 6 months.I have only actually had to adjust the bias once in two years.Steady as a rock!!!

bunnyma357 · « **Reply #4 on:** 27 Mar 2010, 11:25 pm »

It's good to check bias every few months or so. Your distortion problem sounds more like the output of the DAC is overdriving the input to the amp. You might check the specs for each and see what values they are looking for. If the DAC has a variable output, you might give that a try.

Jim C

SteveFord · « **Reply #5 on:** 28 Mar 2010, 12:10 am »

I check mine every weekend which is a lot more often than is required! I keep the covers off and it only takes a few minutes. Once every 3-6 months should be fine, though; I check bias that often because I like doing that kind of stuff.
It's easy on my VTLs. Give the amp around 10 minutes or so to warm up, have the volume on the preamp all the way down and make sure to get a long all plastic screwdriver from an electronics store.

Steve · « **Reply #6 on:** 29 Mar 2010, 01:13 am »

Quote from: chadh on 27 Mar 2010, 10:45 pm

The last time the amp was rebiased was when I bought it, which was...wow...almost two years ago.

I actually just swapped the old DAC back in, and didn't notice the same problem. Maybe it's the new DAC afterall. Break-in issues?

Anyway, I probably should do the rebiasing thing.

Thanks.

Chad

Hi Chad,

Yes it is past due for a rebias. If the idle current becomes low, for me the sound becomes thin and as it becomes even lower starts to sound distorted.

Hope this helps.

Steve

FullRangeMan · « **Reply #7 on:** 29 Mar 2010, 01:24 am »

Hi guys,
Nice to read these tube brands have a such stable BIAS as you just informed.
My doubt maybe is out of topic: in the Joule-Electra Site they inform this brand amps have Bias recommended adjust to each 20-30 hours.
Wonder why Joule-Electra circletron OTL circuit need this frequent Bias adjust and these others brands is once or twice a year??
Could be the ouput transformer lack (OTL)?
Thanks for any info, Gustavo

SteveFord · « **Reply #8 on:** 29 Mar 2010, 10:13 pm »

I can't answer that one but I think that you'll find that it's more of a bias check than an adjustment most of the time and that adjustments will be due to the voltage coming out of the wall/power conditioner at the time you're doing the checking.

FullRangeMan · « **Reply #9 on:** 30 Mar 2010, 03:44 am »

Quote from: SteveFord on 29 Mar 2010, 10:13 pm

I can't answer that one but I think that you'll find that it's more of a bias check than an adjustment most of the time and that adjustments will be due to the voltage coming out of the wall/power conditioner at the time you're doing the checking.

Thankyou for that info.
Gustavo

Frihed91 · « **Reply #10 on:** 7 Apr 2010, 09:14 am »

Biasing tubes in an amp is usually a breeze. Just follow the instructions from the mfr and set up your DMM accordingly. I keep my DMM permanently plugged into my McAlister PP amp and check the bias at least once a week. It takes 2 minutes.

JeffBrown · « **Reply #11 on:** 7 Apr 2010, 12:20 pm »

to me it takes far less then 2 mins. I keep the dmm in a drawer not far from the amp (maybe 3 feet away?) it is just a matter of turning it on and touching 2 leads to 2 test points! Ussually adjustments are not required, total time for both amps is around a min to 90 seconds. I probably do it between once a week and every 2 weeks.

Jeff Brown

Wayner · « **Reply #12 on:** 7 Apr 2010, 12:34 pm »

Quote from: Frihed91 on 7 Apr 2010, 09:14 am

Biasing tubes in an amp is usually a breeze. Just follow the instructions from the mfr and set up your DMM accordingly. I keep my DMM permanently plugged into my McAlister PP amp and check the bias at least once a week. It takes 2 minutes.

I built a dual voltage meter to check my bias. It's plugged into the amp 24/7 and with a flip of 2 switches, I'm turning the unit on, and connecting the bias test points.

Wayner

SteveFord · « **Reply #13 on:** 24 Apr 2010, 12:36 pm »

Well, ChadH, did adjusting the bias alleviate your problems?

chadh · « **Reply #14 on:** 24 Apr 2010, 12:46 pm »

Quote from: SteveFord on 24 Apr 2010, 12:36 pm

Well, ChadH, did adjusting the bias alleviate your problems?

I did adjust the bias on the amps. They didn't seem to be so far off - maybe 10% lower than ideal.
But it didn't fix my problem. It seemed some adjustment of the DAC was required.

Thanks for asking, though.

Chad

macrojack · « **Reply #15 on:** 24 Apr 2010, 12:58 pm »

Where is your volume control? On DAC? On Preamp? On amp? Does it seem to produce the same volume in the same position for both DACs? The output level in volts from one DAC to another can vary. The suggestion above that the second DAC might be overdriving your amp was my first thought as well. You can determine this comparing output voltage spec for both DACs and by noticing how high you need to turn up the volume control with each unit.
Maybe your new acquisition is faulty. Maybe you have discovered the opposite of system synergy. Have you done a double blind test? I've read that those are very reliable and can solve any problem.

SteveFord · « **Reply #16 on:** 24 Apr 2010, 01:42 pm »

Have you done a double blind test? I've read that those are very reliable and can solve any problem.

Well, not always...
If someone finds this offensive, please remove this post.

decal · « **Reply #17 on:** 24 Apr 2010, 06:25 pm »

I bet Ray would think that it's funny.

chadh · « **Reply #18 on:** 24 Apr 2010, 07:07 pm »

Quote from: macrojack on 24 Apr 2010, 12:58 pm

Where is your volume control? On DAC? On Preamp? On amp? Does it seem to produce the same volume in the same position for both DACs? The output level in volts from one DAC to another can vary. The suggestion above that the second DAC might be overdriving your amp was my first thought as well. You can determine this comparing output voltage spec for both DACs and by noticing how high you need to turn up the volume control with each unit.
Maybe your new acquisition is faulty. Maybe you have discovered the opposite of system synergy. Have you done a double blind test? I've read that those are very reliable and can solve any problem.

Volume control is on my pre-amp. And the two DACs put out considerably different voltage. The first was putting out 1v, and the new one was putting out 2v. However, this effect still existed when I used the pre-amp volume control to adjust them to the same output level.

There really was something wrong with the DAC. I'm hoping that it's fixed now.

Chad

nunhgrader · « **Reply #19 on:** 8 Jun 2010, 06:06 pm »

Wayner - that is too cool!