Why do some amp manufacturers bias their amps COLD?

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AndyK

AndyK

Well-known member
Is there a sonic advantage that these builders hear to having their tubes run cold, with lots of cross-over distortion? My Splawn QR was sent back to the man for repairs. He biased it at 30ma (EL34's). When I had Dave Friedman mod my Marshall, he biased it just as cold.

Could some of the high gain be reliant on crossover tube distortion? I thought it's better to bias the amp warmer - always?
:confused:
 
one argument could be for greater tube life on store shelves?
 
So the cheap tubes they put in last past the typical 3 month warranty.
 
True, but doesn't that effect the tone in a bad way? Or does it not matter? I always read that wamer bias, warmer tubes, means warmer, less sterile tone. Is my whole biasing philosophy flawed?
 
They don't do it for any other reason than tube life, like the others said....
 
I would expect that from the big manufacturers, like Marshall or Fender. But a small, boutique type manufacturer like Dave Friedman wouldn't sacrifice tone for tube life, would he?

Does everyone re-bias hotter when they get their amps home?
 
One reason that I've heard is bias-drift. With the case of my Bogner, it's biased very cold. And if you take a bias reading at idle, it'll be cold. But when you crank it, it'll drift up.

So people who bias it at 70% at idle will risk blowing tubes.
 
Who said 30 ma was low? really in a Marshall style amp 30 to 40 ma is fine anywhere in there that sounds good to you is fine. Throw out the numbers and really listen to the tone. I find I like the way 30ma sounds harmonically. Certain tubes I like at 40ma like winged c tubes. Now marshall biases there amps at about 15 to 20 ma now thats cold. Also another big factor is how loud you play a hoter bias might not sound that good loud to mushy. There is my 2 cents.
 
RACKSYSTEMS":3srml5m9 said:
Who said 30 ma was low? really in a Marshall style amp 30 to 40 ma is fine anywhere in there that sounds good to you is fine. Throw out the numbers and really listen to the tone. I find I like the way 30ma sounds harmonically. Certain tubes I like at 40ma like winged c tubes. Now marshall biases there amps at about 15 to 20 ma now thats cold. Also another big factor is how loud you play a hoter bias might not sound that good loud to mushy. There is my 2 cents.
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Cool Dave, thanks for the response! That makes sense, depending on how an amp is cranked volume-wise. At low volumes, what might sound cold would sound dead-on at higher levels. Does that mean the bias increases the louder you play - resulting in mushy tone if the bias is set higher at idle?
 
The bias current is set under "idle" conditions.
When you crank the amp each tube (depends on type) it goes up to 100mA and more of course.
 
duesentrieb":1xnhpy7f said:
The bias current is set under "idle" conditions.
When you crank the amp each tube (depends on type) it goes up to 100mA and more of course.
Click to expand...

So would it make more sense to measure/adjust bias at the range of volume the amp is expected to be operated at? If so, if at idle its 30mA, what would be a safe ceiling be to operate the amp wide open? If 100mA is what the tubes are flowing at maximum volume, is that acceptable or closer to where red-plating occurs?

Thanks,
Jimmie
 
As far as I know, the bias has the most to do with a balance of how much amplification the power amp applies vs. how much headroom is available. And a general tone rule of thumb is that what sounds great at bedroom practice volume will tend to fall apart at gig levels with few exceptions.
 
JakeAC5253":2dq86xwc said:
As far as I know, the bias has the most to do with a balance of how much amplification the power amp applies vs. how much headroom is available. And a general tone rule of thumb is that what sounds great at bedroom practice volume will tend to fall apart at gig levels with few exceptions.
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thats what I was getting at, but you defined it better.
:thumbsup:
 
30ma at idle for EL34s certainly isn't cold. And it's not in crossover distortion. Crossover would become obvious at 0ma at idle.
 
fek":k383mzsm said:
I bias my RM100 with =C= 34's at 30ma. Sounds great to me.
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me to, with both of my RM100's! cept, when am pounding? circutry starts to get way hot and tonal quality, unstable. To keep everything intact, Master Volume has to be set no higher than 12:00 and I use the level on the module as the volume adjustment. Doesnt seem designed right, but its still loud as hell, and holding together. Only getting about a year out of the tubes before they start shorting out.

Is why I was asking about bias at higher output...
 
BIAS DOES NOT MATTER.

That's right. I said it. The bias number, itself, means nothing. 30mA is neither cold nor hot. It 100% depends on what your plate voltage is. The bias setting, on its own, tells you nothing. It must be read in conjunction with your plate voltage. And that number will change depending on the mains voltage.

Let's say that 30mA is actually right a 70% dissipation for an EL34-equipped amp. Plug the amp into a different power/wall outlet and you might find that that same 30mA setting is getting you 80% dissipation due to a different mains voltage.

Now, I HIGHLY doubt the mains will fluctuate that much. But I'm just using it as an example.

Moreover, one of my amps has independent bias adjustment for each tube. I've played around with vastly mismatched settings. Both hot, cold and right where is "should" be at 65-70% dissipation. It all boils down to what sound sounds good to you and how long you want your tubes to last. The higher your amp's plate voltage is, the lower your bias setting will be. But it's all RELATIVE. If your amp is seeing 500V on the plates, then you'll probably be biasing in the 20's. If your amp is seeing 460V on the plates, then you'll probably be biasing in the low 30's.

You can't just look at the bias setting and assume 30mA is cold. You need to take plate voltage into account as well. Otherwise the bias number means absolutely nothing.

Now, if you know what your plate voltage is, given a specific mains voltage, and the math tells you 30mA is "cold," then that's another story. But if it sounds good, it sounds good!
 
FourT6and2":2ud44yu1 said:
BIAS DOES NOT MATTER.

That's right. I said it. The bias number, itself, means nothing. 30mA is neither cold nor hot. It 100% depends on what your plate voltage is. The bias setting, on its own, tells you nothing. It must be read in conjunction with your plate voltage. And that number will change depending on the mains voltage.

Let's say that 30mA is actually right a 70% dissipation for an EL34-equipped amp. Plug the amp into a different power/wall outlet and you might find that that same 30mA setting is getting you 80% dissipation due to a different mains voltage.

Now, I HIGHLY doubt the mains will fluctuate that much. But I'm just using it as an example.

Moreover, one of my amps has independent bias adjustment for each tube. I've played around with vastly mismatched settings. Both hot, cold and right where is "should" be at 65-70% dissipation. It all boils down to what sound sounds good to you and how long you want your tubes to last. The higher your amp's plate voltage is, the lower your bias setting will be. But it's all RELATIVE. If your amp is seeing 500V on the plates, then you'll probably be biasing in the 20's. If your amp is seeing 460V on the plates, then you'll probably be biasing in the low 30's.

You can't just look at the bias setting and assume 30mA is cold. You need to take plate voltage into account as well. Otherwise the bias number means absolutely nothing.

Now, if you know what your plate voltage is, given a specific mains voltage, and the math tells you 30mA is "cold," then that's another story. But if it sounds good, it sounds good!
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When I hear people biasing their tubes over 35, I always figure they either have a pretty low B+ voltage or they've got some pretty stout NOS glass. It's the joy of forums where people hear about some technical detail and obsess a little about it without knowing all the variables that tie together to get the end result. I don't profess to know as much as I would like about tube amps but recommend to anyone curious to build a kit. My Metro plexi I built is still my #1 after almost 4 years and the basics I learnt along the way have become really useful.
 
Real men use o'scopes to bias amps :D

And just to clear some things up, the idle bias is always what you set it at, regardless of volume. Think of it as a 'center point' that shifts up and down as the amplitude of the wave goes through the tube. It's much easier to understand if you look at it on paper.
 
wolfeman28":2ugafn0j said:
Real men use o'scopes to bias amps :D

And just to clear some things up, the idle bias is always what you set it at, regardless of volume. Think of it as a 'center point' that shifts up and down as the amplitude of the wave goes through the tube. It's much easier to understand if you look at it on paper.
Click to expand...

So after reading my post, what do you think I'm experiencing?
 
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