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Dirty-South Blues Harp forum: wail on! >
Bias Settings and Distortion
Bias Settings and Distortion
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wolfkristiansen
173 posts
Apr 16, 2013
10:26 AM
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This is from greenbulletmics.com:
"One good thing about playing harp, is that you don't need to run tubes real hot as many guitarists do to get good tone because lower bias settings will make your amp distort sooner, and, make the tubes last longer too!"
This is counterintuitive to me. I had always thought that a higher, not lower, bias setting would make your amp distort sooner.
Anybody care to educate me? Rick Davis? 5F6H? Greg Heumann? Anyone else?
Cheers,
wolf kristiansen
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dted
46 posts
Apr 16, 2013
11:16 AM
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Wolf,
I agree with you that a high bias setting can make the tubes run hot and distort.
I think it may be also true that a low bias can run the tubes too cool and clip---- too cold and a tube can short.
I think that biasing within a normal range, and listening to the difference between the high and low end of this range---- you will probably not pick either extreme. Or if you do pick an extreme, you may get tired of it because you may loose some dynamics when you attack notes and fullness of tone.
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dted
47 posts
Apr 16, 2013
11:26 AM
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A harp microphone typically has alot more output than a guitar, so you can drive the tubes harder and run out of headroom, into distortion.
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tmf714
1680 posts
Apr 16, 2013
11:32 AM
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There are many factors to consider here-amp design,manuafactuer,tubes,etc.
The general rule of thumb for harp players is to run the tubes at the midrange of the bias point.
I run my Super Cruncher at 32 ma-the mid point.
Hotter may distort and crunch sooner,but at the risk of tube life.
My HarpKing 6-10 runs at 40 ma-Rod played through it last week,and told me that it sounded better than the 6-10 he used on the Blues Cruise. Not saying it was the bias on the HarpKing,because it is not adjustable,but could be the tubes-the two amps do have different power tubes in them-you can see why I say there are many variables.
I prefer a mid point bias for most of my amps-YMMV-.
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Rick Davis
1637 posts
Apr 16, 2013
12:45 PM
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Gerald Weber did the early work on this.
Harp Amp Secrets and Tips
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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5F6H
1608 posts
Apr 16, 2013
2:02 PM
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Wolf - assuming we are talking about fixed bias amps, bias current has a direct effect on tone & feedback rejection, low currents (<15mA on 6L6) more fuzz & crunch, less feedback. Recesses the high end fidelity. Not all brands of tubes sound good cold though...Russian tubes seem happiest, maybe 15-25mA min for Phillips WGBs, more for NJ Tung Sols.
Mid currents (>15mA up to mid/late 20's depending on tube brand) will give the most "normal" tone, hotter currents get greasier, more singing, brighter.
In a typical 6L6 amp, even for guitar, there's not much point, or necessarily sense in going over 40mA. The JJ6L6 are pretty sturdy and can take more than this in some amps.
Speaker configuration has an effect too - 4x10" holds together (sonically) well & can take lower currents, 2x10" can sound thin & washed out at lower currents, I tend to go regular current with these.
Dted - "I think it may be also true that a low bias can run the tubes too cool and clip---- too cold and a tube can short." If your tubes are biased too cold the amp won't make sound, or hold long notes. Tube failure is so rare as to be unheard of...I guess because once the amp stops making noise, there's not much point in keeping playing it.
Some early 60's brown Fender amps were biased cold at the factory & still run cold withsome tubes today, at stock circuit config...it's not unusual to stick a pair of 6L6 in a brown Concert & see 7mA or 8mA.
The Kendrick TC 35 runs a little cooler than described in Gerald's article (~55mA), but it's a cathode biased amp anyway...different ball game...I can run one of my cathode biased amps at 100mA per tube (big tubes).
Many of the players mentioned in this forum run/have run cold biased amps, whether they know it or not.
I had a pal round recently for an amp check up...he described the amp in question as "the best I have heard"...we looked at the bias (he's a stickler for 35mA per tube), one tube was 6mA, the other 18mA! ;-)
Whatever the logic, or instrument, bias for a tone you like, just keep it at a level that won't eat tubes, or push the B+ voltage over what the tubes and filter caps can stand.
EDIT: To put it simply, should have done this first, there will be a point where the amp is cleanest. Going significantly higher or lower will cause distortion, but differing kinds.
Higher is closer to saturation and the tube hit's its ceiling of current & dynamic, lower is closer to cut off/crossover. All push-pull, fixed bias amps go "cold" (relatively speaking) if you push them hard enough...they may draw lots of current under signal, but when the amp exceeds the clean range it is because the tubes can't provide enough current to keep the signal clean & crossover sets in eventually. With a hot biased amp & harp, you just might not get into the power range to make this happen as the big mic signal is often, effectively clamped at the preamp stages and has little effect on how much W the power tubes can potentially make (regarding W RMS as described by the manufacturer, though of course, different mics can yeild differing volumes from the same amp).
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Last Edited by 5F6H on Apr 16, 2013 2:25 PM
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wolfkristiansen
174 posts
Apr 16, 2013
3:36 PM
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I've read and considered what everyone has written or pointed me to. Thanks to all. I've still got a basic question, though.
For what it's worth, my amp is a '95 Fender Blues Deluxe with a 2X10 speaker configuration instead of the stock 1X12. The original power tubes have been replaced by two TAD 6L6WC tubes, per 5F6H's suggestion in an earlier post. I've not done the resistor changes recommended in that post; perhaps I will someday.
A tech friend installed a Torres Engineering bias kit, made specifically for '90s Blues Deluxes and Blues Devilles. The bias is currently set at 31 mA.
I wasn't asking specifically about how to tinker with the onset of distortion in my particular amp. I.e. does it kick in when the volume is at 3 or 6? I'm happy with the onset of, and degree of, distortion in my amp.
My question was simpler. Maybe it's so simple it can't be answered without qualifying caveats. Here it is again:
Do lower bias settings will make your amp distort sooner? (As per Dave Kott at greenbulletmics.com). Or has he got this backwards?
5F6H you may have answered when you said this:
Low currents (<15mA on 6L6) more fuzz & crunch, less feedback. Recesses the high end fidelity
Mid currents (>15mA up to mid/late 20's depending on tube brand) will give the most "normal" tone
Hotter currents get greasier, more singing, brighter
If "more fuzz and crunch" denotes distortion, then Dave Kott is right. If "greasier, more singing" denotes distortion, he's wrong. Maybe I'm wrong in assuming that adjusting bias affects amp distortion? I know it affects tube longevity.
As always, I am thankful for both the musical and technical knowledge shared on this forum.
Cheers,
wolf kristiansen
Added-- Mark, I hadn't read your added comment when I posted the above message. You needn't answer this message, you've answered it with your added comment.
wolf k
Last Edited by wolfkristiansen on Apr 16, 2013 3:38 PM
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MP
2711 posts
Apr 17, 2013
12:18 PM
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Wolf, those amps weigh a ton. i owned a 1/12 job.
sounded great in my living room. sold it right quick.
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