Tube Bias Adjustment [Archive]

Early B.

03-07-2006, 12:40 PM

I biased my tubes when I originally procured my amp (about 18 months ago), so I decided to check the bias today to make sure everything was cool. Three of the tubes were OK, but with the fourth tube I had to turn the screw as far to the right as it could go. However, this only biased the tube to 39 mV and I was trying to get it to 50 mV.

Why did I max out at 39 mV? Is there a problem with the tube or the bias pot?

Thanks.

dorokusai

03-07-2006, 12:57 PM

I would say tube...swap one from another socket and see if you can spec it out.

Early B.

03-07-2006, 01:23 PM

I would say tube...swap one from another socket and see if you can spec it out.

OK, I swapped a tube from another socket and isolated the problem -- it's the tube. I'll order another one.

Thanks, Doro.

dragon1952

03-07-2006, 03:57 PM

Cool, since you've got your answer I'm going to piggyback onto your thread since I've been meaning to ask this for some time :D
Is the bias something you have to check periodically as the tubes age? I was under the impression that the only time you had to bias is if you changed to a different tube type, like going from a KT88 to an EL34. Does the bias adjustment ever get out of whack or is the need for adjustment strictly to dial in your tubes? If you get a brand new set of matched tubes of the type the amp came with is there really any reason to check bias?
I think I just asked the same question 3 different ways :o

Early B.

03-07-2006, 04:12 PM

According to the Jolida manual:

"In general, after biasing for the first time, you do not have to bias the SJ502A for one to three years."

Yeah, the bias adjustment can get out of whack for a variety of reasons, and you always have to re-bias your power tubes when you change them.

RuSsMaN

03-08-2006, 02:51 AM

My general rule, with new tubes (or a new amp to you) - go ahead and bias 5 minutes after power on, check and adjust at 30-45 minutes, re-check at 3 - 4 hours and you should be good to go.

I check bias frequently, once a week or so, but I'm anal about that stuff. Most gear you shouldn't have a problem doing it every 3-6 months. Depending on usage, I wouldn't go much longer than that.

Cheers,
Russ

dragon1952

03-08-2006, 04:21 AM

Rats! Thanks for the replys dudes but not what I wanted to hear :( This Chinese Ming Da I've got has internal biasing but I've got no freakin' idea how to do it.
I wrote an email to the company asking about biasing and got kind of a funny response.
Here's my email:
Hi,
I have a question about the Ming Da MC88-C integrated tube amplifier. Does this amp require tube biasing? Do you have any information on how to set the bias for this amp? I bought this amp but can find no information on how to set bias.
Thank you very much.

The reply:
Dear Sir,
I am sorry to hear that. MC88-C should be biasing 0.41V. Please look at the diagram of MC88-C attached, I hope you can able to repair it.
Best wishes,
Xiao

:confused: It's not broken! And what's he sorry about? That I'm to stupid to know how to bias it?:rolleyes:

ND13

03-08-2006, 09:35 AM

It's not broken! And what's he sorry about? That I'm to stupid to know how to bias it?:rolleyes:

No, that you bought that amp...:p ;) :D

J/K:)

BTW, I don't remember you ever letting us know what you think about that amp. Did you ever do a review? Is it all THAT??

Early B.

03-08-2006, 09:38 AM

Dragon --

If your bias pot is internal, it's possible that you have an autobiasing unit. This would explain, in part, the strange response you received.

For more info on your amp and a lead for acquiring a manual and schematics, talk with the folks at www.responseaudio.com. They perform mods on your amp.

madmax

03-08-2006, 04:48 PM

I think he meant he is sorry you could not find any info. Since he gave you the 0.41 volts I assume it is not self biasing.
madmax

Early B.

03-08-2006, 05:49 PM

organ

03-08-2006, 07:00 PM

One thing to keep in mind is to find out if your amp have a regulated power supply or not. Vintage tube amps do not but I'm sure there are some modern units without them as well.

If the power supply is not regulated, your bias will fluctuate along with your line voltage which is not always 120V. Sometimes it will dip and rise. My amp is suppose to be biased at 1.56V through a 15.6ohm resistor. This gives 50mA per tube. Some times the bias could drift up to 0.1V. So I always keep it around 1.4V to be safe.

I check bias every few weeks. For my amp it takes a fresh quad about one week to really settle down.

Maurice

Early B.

03-08-2006, 08:01 PM

For the tube veterans out there -- do you guys ever fool around with setting the bias higher or lower than the manufacturer recommended settings? If so, what has been your experience in terms of SQ? Is it worth exploring?

organ

03-08-2006, 08:57 PM

First you'll need to find out how many amps the tube will handle and do some calculations. The bias determines when the amp switches from Class A to Class AB. The more bias, the longer it stays in Class A. This is for Push-Pull amps. I don't think I'd go over 50mA per tube for EL34. The JJ E34L can be biased hotter than the standard EL34. I've played around with bias and don't hear the difference. However, I need to get my bias over around 1.25V or else it sounds sloppy and lifeless. Between 1.3 to the recommended 1.56V, I don't hear a difference. It's usually guitar players that play around with bias and plate voltage to get a desired tone.

Maurice

cmy330go

03-08-2006, 10:32 PM

Oops

Early B.

03-08-2006, 10:32 PM

First you'll need to find out how many amps the tube will handle and do some calculations. The bias determines when the amp switches from Class A to Class AB. The more bias, the longer it stays in Class A. This is for Push-Pull amps.

OK, I've heard that Class A generally produces a higher quality sound. If that's true, how do you determine the decibel level at which the amp kicks into Class AB?

Even if there's no audible difference, I'd be interested in knowing if my amp is operating in Class A or Class AB at my normal listening level.

dragon1952

03-08-2006, 10:56 PM

No, that you bought that amp...:p ;) :D

J/K:)

BTW, I don't remember you ever letting us know what you think about that amp. Did you ever do a review? Is it all THAT??

LOL! Very funny :p

The amp is awesome. Clean, clear, warm, non-fatiguing and beautifully dreamy sounding. I don't think I will ever have a desire to replace it, which is something I have never said before. I feel the same about the Jolida CDP and Vandersteen speakers as well as my Silent Audio interconnects. I could live with this combination forever. Power cords and tubes I might play with a bit though :D

dragon1952

03-08-2006, 10:57 PM

Dragon --

In case you haven't seen this, here's a review of your Ming Da:

http://202.186.86.35/audio/printerfriendly.asp?file=/2004/6/24/audiofile/24vega&sec=audiofile

There's a hint about the need to bias it. It says, "All tubes come factory-fitted so there’s no need for users to bias the tubes themselves."

Thanks for the link! I can't tell you how many searches I have done on this amp and never found this review. ;)

BTW, I did email Response Audio awhile back but they never replied. See if I ever do business with them!

organ

03-08-2006, 11:01 PM

I think you'll need some lab equipments (scope, vom, power source, etc) to find out exactly where your amp switches to Class AB.

You can't determine at which decible level the amp kicks into Class AB. This is done by power. I believe most AB amps can only do Class A for the first few watts. That's why those single ended Class A amps have low power output. So it is pretty hard to tell without knowing the circuit and we all know music is pretty complex. You might get Class A during soft vocal parts and the amp will kick to AB when a drum kicks in. But of course your speakers will play a big role in determining this. The more efficient the speakers, the longer it can stay in Class A compared to less efficient designs.

Best thing to do would probably to contact the manufacturer and ask them at what power output the amp goes to AB. You can also ask this question on audiokarma.org A lot of the folks there really know their shit.

Your 502 uses EL34 right?

Early B.

03-08-2006, 11:23 PM

Thanks for the link! I can't tell you how many searches I have done on this amp and never found this review. ;)

BTW, I did email Response Audio awhile back but they never replied. See if I ever do business with them!

You're welcome.

BTW, I also emailed Response Audio a few months ago and never got a reply, either.

Early B.

03-08-2006, 11:25 PM

Your 502 uses EL34 right?

The 502 uses 6550/KT88's.

I'm using EH KT88's right now until Max sends me his Tungsol's.;)

organ

03-08-2006, 11:37 PM

Tung Sol??? Now that's pricey!

I've heard great things about the Shuguang KT88.

madmax

03-09-2006, 12:48 PM

I'm using EH KT88's right now until Max sends me his Tungsol's.;)

I'll leave them to you in my will. :D
madmax

Early B.

03-09-2006, 01:40 PM

I'll leave them to you in my will. :D
madmax

Cool. I'll PM you with my full name just to make sure you spell it right in your will.

On a totally unrelated topic, can anyone refer me to a good hitman?

ND13

03-09-2006, 05:02 PM

On a totally unrelated topic, can anyone refer me to a good hitman?

Hey, I'm unemployed right now, what's it pay???:D :D

organ

03-09-2006, 05:21 PM

That's it! I'm going to report you guys to Dog the Bounty Hunter!:D

candyliquor35m

03-10-2006, 12:00 AM

fredv

03-10-2006, 12:15 AM

Hey, I'm unemployed right now, what's it pay???:D :D
Won't be more than a quad of TungSol for sure, but then you can't even recover the gas ;)

Tour2ma

03-10-2006, 09:50 AM

How many here have played with their bias setting?

I usually go a little below the suggested value as it supposedly extends tube life. I've played with +/- 10% and haven't heard a difference.