Will raising the bias of my amp give me more clean headroom?

  • Thread starter Thread starter elenore19
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elenore19

elenore19

Slowing becoming un-noob.
I was watching videos on youtube on a vox ac30 and it got to a part on the biasing of the amp. It said something along the lines of putting the bias hotter gives you more clean headroom.
If I do that with my Super Champ XD will I get more clean headroom?

This could solve my problems...

Thanks,
-Elliot
 
Going to a higher rating tube and then rebiasing will. The lower the rating number the earlier the tubes will distort. A high rating will stay clean longer.
VP
 
just raising it, no. but, setting the bias correctly, that is where clipping occurs equally, is the bias point that allows the largest signal before clipping.
 
elenore19 said:
I was watching videos on youtube on a vox ac30 and it got to a part on the biasing of the amp. It said something along the lines of putting the bias hotter gives you more clean headroom.
If I do that with my Super Champ XD will I get more clean headroom?

This could solve my problems...

Thanks,
-Elliot
Click to expand...
aren't you looking for something for live gigging? (I may not be remembering right).
If so, nothing is gonna make that Champ loud enough for gigging.
Well, you could mic it, of course. But for my taste I need a certain amount of sound to be coming from my amp and there's no way that Champ could do it for me regardless of how you bias/tube it except for the very softest of gigs.
 
Sonixx said:
just raising it, no. but, setting the bias correctly, that is where clipping occurs equally, is the bias point that allows the largest signal before clipping.
Click to expand...

Biasing an amp can be tricky. There isnt really a definate bias point. When you use an oscilloscope to set the bias you increase the bias until the crossover notch dissapears. Depending on your O-scope it might not show clearly. After this point the signal doesnt really increase, but it provides for the tubes to be running at their optimum level. there is a window that can be increased where the amp will sound "better" but noise and distortion can result. These days there are alot of Bias Probes used to measure the idle plate current to set bias. Typically it is about 35ma for most tubes. setting bias this way keeps you in the safe mode. Tubes can actually go higher but tubes made today are not as robust as vintage tubes.
VP
 
Victory Pete said:
Biasing an amp can be tricky. There isnt really a definate bias point. When you use an oscilloscope to set the bias you increase the bias until the crossover notch dissapears. Depending on your O-scope it might not show clearly. After this point the signal doesnt really increase, but it provides for the tubes to be running at their optimum level. there is a window that can be increased where the amp will sound "better" but noise and distortion can result. These days there are alot of Bias Probes used to measure the idle plate current to set bias. Typically it is about 35ma for most tubes. setting bias this way keeps you in the safe mode. Tubes can actually go higher but tubes made today are not as robust as vintage tubes.
VP
Click to expand...
While bias can vary significantly w/o really changing the tone significantly, there is a bias point at which symmetrical headroom is achieved. That is the bias point where a sine wave's positive and negative peaks begin to flatten at the same time as the sine wave's level/magnitude is increased.. This is the optimal bias point. It takes an oscilloscope and load soak to determine this.
 
Sonixx said:
While bias can vary significantly w/o really changing the tone significantly, there is a bias point at which symmetrical headroom is achieved. That is the bias point where a sine wave's positive and negative peaks begin to flatten at the same time as the sine wave's level/magnitude is increased.. This is the optimal bias point. It takes an oscilloscope and load soak to determine this.
Click to expand...

Interesting, What would the typical idle plate current be at that point?
VP
 
Victory Pete said:
Interesting, What would the typical idle plate current be at that point?
VP
Click to expand...
IDK. the current will vary somewhat based on component tolerances, so there's no direct correlation. replace one tube, and my bet is the bias will need to be adjusted to get to this point again. but, is it critical that this be the bias point, no, but it is the best and most correct bias point from a circuit's design point.
 
All this aside, EL84 amps cannot be biased...so Unless your ac30 is one of the rare ones, It has 84s in it.
 
OK, I am certainly no expert on anything, but here's my take after crashing the party and eavesdropping. Just remember what you paid for this opinion.
  1. The Vox AC30 is Cathode-biased. While it is possible to adjust the bias slightly (by substituting resistor values), there are only so many stock resistor values out there, so you cannot bias it to infinite possibilities. With no potentiometer, you get what you get.
  2. In a fixed-bias amplifier, hotter bias can give a little more clean headroom, but that isn't going to be night-and-day different unless the bias was really cold to begin with. And remember, hotter bias = shorter tube life. When you see a thirty-year old Deluxe Reverb with the original tubes that test good and sound great, do you think the bias was hot or not so hot?
  3. The tube type isn't indicitive of whether or not any amplifier is Class A, Class AB, fixed-bias or Cathode bias.
Now the Super Champ XD is Class AB, but I haven't seen inside one to know if they have a bias potentiometer. If they do, check the bias. I have seen many, many, many amplifiers come from the factory and on the music store floor that were biased cold. A friend bought an Ampeg Super Jet. Right out of the box. Thought something was wrong with it. I told him before he took it back (this was a Sunday), bring it over, and I'll tell him what to tell the store so he sounded intelligent about his displeasure. The 'clean' was thin and bright, and the 'dirty' was much too dirty, and not in a pleasing way. Checking the bias, the Groove Tube 6L6's were idling at something like 8mA each :confused:. Cranking it up did the trick, and he loves the amp now. So don't think it came from the factory perfect. And 'perfect' is subjective, anyway.
For some guaranteed headroom, put 6L6's in that amplfiier. If the bias potentiometer has enough range, you'll like the results. There are other ways to get more clean headroom, but you'd have to be intimately familiar with the amplifier to know your options.
 
Factories often will overbias their amps so there will be little chance of tube related problems. Overbias is just the opposite of what you would think, an underbiased amp will run hot. I see this all the time when I have a new amp to retube.
VP
P.S When you tweak the bias control to increase the plate current you are actually decreasing the bias voltage. But the bias voltage is negative so reducing the negative voltage will increase the plate current. confused yet?
 
Lt. Bob said:
aren't you looking for something for live gigging? (I may not be remembering right).
If so, nothing is gonna make that Champ loud enough for gigging.
Well, you could mic it, of course. But for my taste I need a certain amount of sound to be coming from my amp and there's no way that Champ could do it for me regardless of how you bias/tube it except for the very softest of gigs.
Click to expand...
Yes for gigging. Micing hasn't been an option yet, so I assume it won't be an option for upcoming gigs. The last show I was loud enough, just not clean enough. I had my super champ running through a 2x12 and 4x12, and most shows I should be able to borrow someone else's 4x12 to make all that happen. So I just need cleaner.
More and more I'm leaning towards a new amp.
Looking at the Twin Reverb.
 
elenore19 said:
Yes for gigging. Micing hasn't been an option yet, so I assume it won't be an option for upcoming gigs. The last show I was loud enough, just not clean enough. I had my super champ running through a 2x12 and 4x12, and most shows I should be able to borrow someone else's 4x12 to make all that happen. So I just need cleaner.
More and more I'm leaning towards a new amp.
Looking at the Twin Reverb.
Click to expand...
well that's what I mean. If you can't get it loud enough clean then it's not loud enough.
 
And micing small amps for some 'big' tone at a large venue gig is one of those urban myths that won't die soon enough. I did try it waaaaaaaaay back in my young and naive days. Pretty much every sound man reminded me you want clean and clear vocals coming through the monitors, not my teeny weeny guitar and Big Muff Pi. Eventually I believed them, and now use my small amps just for recording. You want some headroom, and bigger amps have more of that. It seems simple, but Guitar Center probably sells more Blues Juniors than they do Twin Reverbs. Could the $1,000 price difference have anything to do with that :confused:?
 
Turning *down* your pickup volume on your guitar might give you more clean headroom.


If you're like me, you're using really hot pickups - I put them in all my guitars, and then keep forgetting I need to turn them down if I want clean sounds. I was bumming out on my blues jr. the other day, because it started breaking up with both the master and the preamp volume on one or two - then I remembered I had honkin' P94 pickups in my guitar - enough copper to make a crackhead drool (a couple of years ago - I guess prices have gone down recently). They sound pretty good at or even below half-mast, and I can crank the amp way up (both master and pre) and stay fairly clean. You also have to have decent pots of course, or you lose the treble, but those aren't too hard to come by - I just use the CTS 500k ones (I think that's it)
 
Sonixx said:
While bias can vary significantly w/o really changing the tone significantly, there is a bias point at which symmetrical headroom is achieved. That is the bias point where a sine wave's positive and negative peaks begin to flatten at the same time as the sine wave's level/magnitude is increased.. This is the optimal bias point. It takes an oscilloscope and load soak to determine this.
Click to expand...

Biasing on most adjustable bias amps affects the crossover point overlap in class AB operation but the center point doesn't move; all the power tubes get the same bias voltage and changing the bias moves the power tube output waveforms away from or closer to the center point symmetrically.

Some tube amps have an output tube balance pot that may do what you are talking about (example at http://www.schematicheaven.com/fenderamps/super_reverb_ab568_schem.pdf), but I believe that's kind of unusual.
 
we mic drums guitars and keys through the PA all the time. As long as the PA can handle it there are no problems with the vocals.
 
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