My Biased and un-biased opinion..!!!

beardog03

when I first got my SLI-80, the Bias was set at 85mA....It sounded sooo good !

After about 120 hrs, I was reading through the manual again, and saw that it say`s to set at 75mA..

And thinking I was doing the right thing, I set both sides at 75, and then checked it again after it had been playing for 5 or 6 hrs..
Of course it had changed, and so did the sound..

So I set it at 80...Thinking that if it`s between 75 and 85 that that would be a perfect compramise..!

Last night, after it had been on all day, and into the night, I checked it (tubes were real warm !), and it was at 91mA....so I backed it back down to 83...
This morning when I got up, the first thing I did (as usual) was check the board for new posts/threads...the second was to flip the switch and warm up the tubes and get some Java...
It was at 73.2 and 78.4....I adjusted it back to 83 and some change..

My questions is this....why does it keep changing, and does it really matter if it`s 75 or 85mA ?

The Bias is supposed to make it sound balanced and correct (I think !), but does a higher bias # mean the tubes are having more through them, thus working harder and not lasting as long ?

Last night while it was at 91 and change, it sounded awesome...It was late (2 am) and for some reason she was singing like never before...with great separation and tone.....and you could cook eggs on the tubes..!!

again...why doesn`t it just stay where I set it, and is it okay to run it hot (advanced)...

I`m kinda equating the bias to the ignition on my bike....withy the ignition slightly advanced it is snappier, and doesn`t ping so much, but you can also burn your valves or piston if you run it too hot for too long..:D

disneyjoe7

Staying by for the official response. RuSsMan?

Ricardo

^^^^^^What he said.

disneyjoe7

I'm going with normal to a bad tube, I'm 65% normal to 35% a bad tube. Any takers?

Ricardo

I don't know; I don't have a lot of experience with tube amps, but the times I've had them the bias was stable. maybe it's the type of circuit? I've also had tubes that didn't get to the specified bias value, that would be a bad tube. I really want to hear from the experts.

BlueMDPicker

What are you using for AC conditioning? If nothing (and assuming you are letting the unit fully warm-up before setting bias to manufacturer's spec), the differences could be in house line voltage.

Always make sure there is no input before biasing. I even go as far as disconnecting input ICs.

beardog03

I have no AC conditioning...
If I should, please recomend something and I will make it happen..!

I was not aware that I should unplug the sources when Biasing..

What I do is....I turn the unit on, wait 10 min., then check / adjust bias...
volume is all the way down, and sources are plugged in and turned on, but no sound through the speaks..

Then after a few hours, sometimes many hours (8-10), I turn the volume all the way down , and check it again

according to the manual, I should check when it`s just warmed up, then 10 min. to an hour later.....if I`m understanding it correctly

BlueMDPicker

beardog03 wrote:

according to the manual, I should check when it`s just warmed up, then 10 min. to an hour later.....if I`m understanding it correctly

They know most people just aren't going to wait more than 10 minutes, so the hour check is the real biasing. Even with no apparent volume from sources, there could be minor voltage. Try it both ways and see if it makes a difference.

I'm not advocating that you get a voltage regenerator (like the PS Audio Power Plants), but be aware that line voltage fluctuations will effect your biasing. If you get it set where you like it close to spec, and it isn't smoking, leave it alone and enjoy!

beardog03

My house is a 1955 model, and I`m not sure what the voltage is
I like to run it up in the high 80`s for some reason...the manual say`s 75, but 85 seems better....will that just make the tubes work harder and wear out faster, or does it affect the unit ?

Just for grits and grins (pc way to say it), what conditioner do you use, or suggest ?

I`m working on putting my main rig together and I would like to get some dedicated circuits for that room (and the office), but should be realistic about getting a clean/protected/regulated source of energy..

I`m running a SS amp and Tube prein the main rig, and she draws some serious juice !

F1nut

Some good reading for ya, http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#bias

Running your tubes "hot" will shorten the life span and could cause massive tube failure, like ka-boom.

BlueMDPicker

The Tube Depot has an excellent online resource regarding biasing: http://www.tubedepot.com/whisbipo.html

Basically, your higher than nominal biasing preference creates more headroom before clipping and requires more signal current to reach output level "x" than a nominally biased tube compliment.

I use PS Audio UPC-200's and the newer Quintet. Both feature clipping and surge protection, the latter has an under voltage cutoff. Neither regenerates AC.

beardog03

F1nut wrote:

Some good reading for ya, http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#bias

Running your tubes "hot" will shorten the life span and could cause massive tube failure, like ka-boom.

I`m gonna feel pretty stupid standing ther with little shards of glass all stuck in my grill, wondering.."wwhaaahoppin" ?!!



Thanks guy`s...I knew I could count on you

beardog03

BlueMDPicker wrote:

I use PS Audio UPC-200's and the newer Quintet. Both feature clipping and surge protection, the latter has an under voltage cutoff. Neither regenerates AC.

Is the regeneration of AC a good thing, or is it going to be just a matter o throwing cash at a problem ?

Might be a good idea to go to the source and update my panel outside and put in dedicated outlets...then get a conditioner...

beardog03

both good reads..!

Thanks for the lesson(s)

I dig learning about tubes, amplification, sound,etc.

I love this hobby !

disneyjoe7

Well did I win 65% nothing wrong / normal?

F1nut

Is the regeneration of AC a good thing

If you've got problems with over/under voltages, yes. However, I believe the PS Audio unit is limited to 1500 watts max draw, but double check that one.

beardog03

disneyjoe7 wrote:

Well did I win 65% nothing wrong / normal?

You win...nothing wrong...it`s normal
I will disconnect the sources tonight after it has been on all day, and re- check the bias..
I notice that the jam nut on one side needs to be tightened down, but I believe the heat from the tubes is what`s causing the slight fluctuation
in the bias reading...

She`s been sounding tight all day long....man I love this rig !!!!!

I have been on the reading / learning curve all day....
I enjoy getting my learn on....

beardog03

F1nut wrote:

If you've got problems with over/under voltages, yes. However, I believe the PS Audio unit is limited to 1500 watts max draw, but double check that one.

I think I will look into getting one for the office and one for the Main Rig...which is currently under construction..;) :cool: :rolleyes:

Crispycircuit

I had problems with bias fluctuation and it turn out to be Chinese tubes. As they warm up the plate and grid distance changes from the heat. I doubt these tubes will ever settle down. Now if it's an older amp the couple caps could be leaking and changing the bias voltage. What kind of output tubes??

beardog03

I was reading about the leaking cap thing today, but the Cary is only a few months old..

with no more than 2-300 hrs on it...:cool:

The tubes were matched at the Cary work bench when they built the unit....they are all Electro Harmonix....5u4, Kt88EH, 6SN7,and 6922

I think it may be the heat doing what you just described....possibly

Maybe if / when I Bias them correctly, they will stay put !!!

hearingimpared

..... subscribing....

organ

It's normal to have your bias drift during different times of day/night.
Most tube amps don't have voltage regulators, so as your wall voltage drops, so will your bias.
Use your multimeter and measure the wall voltage during different times of day and night and record your findings. I get my highest reading around 2 and 4am. This is when your amp should be biased. It's very difficult to keep your bias at one exact value.
However, I'm surprised you're able to hear a difference with very small changes in bias current.

Ricardo

Here's a probaby ignorant question; most amps have a "mA" bias method; some like my ASL-20, you measure mV's (300mV is the target)....any particular reason? Also, there are two output tubes and they are biased individually; the Sixpacs I had had 6 output tubes and they were biased together.
???

organ

HTR,
Pretty much the same thing. What you're doing is measuring voltge across a a resistor instead of measuring current directly.
It's just Ohms Law.
Current=Voltage/Resistance
Voltage=Current*Resistance

Hope this helps

Crispycircuit

Mr. Organ is correct. I don't find my sound changes with a few milliamps plus or minus. The more current the tubes draw the closer to Class A your getting. I also understand as tubes are used they get "gassy" and will that will cause bias drift. They test fine, look fine, but are gassy. I've found 6BQ5's have this problem the most. Some testers have a check for this. ..... Your coupling caps are new, it takes decades for these to leak. Check you wall voltage first like Mr. Organ said. Then if it's not drifting, and the drifting bias is bothering you, try a new set. ... I should have asked if this is a Class A amp?? If it is Class A, do NOT bias higher, your amp is at max.... Also Millivolts and Milliamps are two different things and CAN'T be substituted for each other... Bias can be a 10 page subject...

Ricardo

I understand mA and mV are two different things; just wondering why some amps are biased to one and some to the other.

RuSsMaN

Robert, when I first install a tube amp, I always check bias on first turn on - but as Mike said, an hour or so into it is what really counts. Bias can drift due to lots of things, tubes, bias pots, bias resistors, etc. Bias can also appear high, or to flucuate if you have a source 'on' - not just playing, but even just out of mute.

A few mA is nothing to loose sleep over. The fact is, most manufacturers run tubes WAY below their specs, and what they can handle. Start with a good matched set, and make sure they are all close in bias (and balance if you have that adjustment too), and you're fine. Now if you had an OTL amp, I'd be a lot more careful with bias, but I don't worry too much with conventional designs using output trannies. I myself will never run OTL, I like having that big transformer between the tube that's about to go tango uniform, and my speakers. It's built in protection if you will - and I can afford to keep replacement Spendor drivers and crossovers in the closet.

On a similar note - I'm actually starting to move to the camp of running tubes right on the ragged edge. I've been doing a lot of reading, and talking to amp builders, and a lot are starting to move towards the hot side of things.

For example, I've got a set of modded Allen Organ amps, where the only thing stock is the output tranny (Acrosound) and the chassis. I'm running a pair of 6L6GC in each monoblock, push pull - right at 500v to the plate. On the bench, they easily topped 50 watts each. Find me another 6L6 (2 tube) amp, that is anywhere near that power.

We actually used a tube rectifier at first, and the plate voltage dropped to a little over 400, and the sound was good, but kind of mushy and flat. Dropped in a solid state rectifier (right into the tube socket), and got it right at 500v (what a 6L6GC can run at) - and they came to life. Bass slam and control, and power like you wouldn't believe from a pair of 6L6 tubes. I planned on posting them, I'll try to get to it tonight.

Gary is playing with a design right now, to get 100w from a pair of EL34. Right on the ragged edge, but again, with QUALITY tubes, a pair of EL34 can handle it. He had a lot more technical info, some of which I don't understand - but amazingly tube life isn't really affected either.

Cheers,
Russ

hearingimpared

Nice write up Bro.

fredv

RuSsMaN wrote:

Gary is playing with a design right now, to get 100w from a pair of EL34. Right on the ragged edge, but again, with QUALITY tubes, a pair of EL34 can handle it. He had a lot more technical info, some of which I don't understand - but amazingly tube life isn't really affected either.

You ought to have stash of the good old Mullard's to go to 800V plate and 500V screen grid Any new production probably dies at power on

Crispycircuit

Why do some amps use milliamps to measure bias or Millivolts.... It's the choice of the designer. The cathode of the output tube goes to ground. This the easiest place to measure bias/current draw. It can be done at the plate but your messing with 300 to 550 volts! The cathode is low voltage. Anyway, by placing a 1 ohm or 10 ohm resistor in the cathode to circicuit, you can measure the voltage accross the resistor in millivolts. When measuring in Milliamps are actually inserting the meter between the cathode and ground and measuring DC current which in milliamps. Bias is set a idle/no signal, when the signal increases the current draw will be more except in a Class A amp where the tube is full on all the time. Everyone has their own idea about bias.....

beardog03

I was biasing the tube rig last night...
I unplugged everything (cdp,tuna)and let it warm up 15 min.
adjusted bias.....


waited an hour....re-adjusted

another hour later, the tubes are glowing and really warmed up, I`m adjusting and the wife flips on the vacume cleaner in the other room !

Just like you said...line voltage...!!


when she turns it on the Ma drops 2 points, when she turns it off....goes back up a couple points...!!

I waited for her to finish, and adjusted accordingly