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Rented backline and bias

naf

Member
I have a question for the amp guys. I do lots of fly dates and consequently I am usually playing through rented backline and the amps are almost always new fenders, either DRRIs or some kind of Hot Rod. Sometimes they sound good and sometimes they really don't. My question is this-how big of a difference does bias make in this situation? Is it worthwhile to learn how to bias these things? And what is the best way to go about doing this on a nightly basis? I have room to carry a multi meter but not too much more than that. I have not done much work on amps buy I think I know a couple of techs who would show me. Thanks for your help.
 
You'd have to drop the chassis out of that rented gear to bias it. Bias could cause an amp to sound bad from one venue to the next, but it could be power, the tubes in that specific amp for that backline, and 100 other things. I wouldn't be concerned with biasing rented amps on the fly.
 

doc

Supporting Member
You can bias without removing the chassis using a Bias Probe. There are some commercial ones out, here is a link to how to make a DIY version.
http://www.jt30.com/jt30page/micKdiy/DIY-Bias-Probe.html
I personally think it probably would be worth the trouble if you have the time to fool with details like this on your gigs. Bias can make a significant difference in the tone of an amp, although as Frankiesixxxgun said there are a number of other things that could mess with the sound as well.
 
You can bias without removing the chassis using a Bias Probe. There are some commercial ones out, here is a link to how to make a DIY version.
http://www.jt30.com/jt30page/micKdiy/DIY-Bias-Probe.html
I personally think it probably would be worth the trouble if you have the time to fool with details like this on your gigs. Bias can make a significant difference in the tone of an amp, although as Frankiesixxxgun said there are a number of other things that could mess with the sound as well.
No, you can only check milliamperage with a tube socket bias probe. You still need to know plate voltage to get the full bias equation; moreover, you need to get into the chassis in order to turn the bias adjustment pot. Some amps do have the bias adjustment pot on the back of the chassis, but you still gotta measure the voltage to the tubes or that mA reading won't mean squat.
 
Ok,lets not get all bent out of shape worrying about plate voltages.
If you use backline equipment,try using a Hot Rod Deluxe or Hot Rod Deville.They both will take similar bias settings.You remove the back cover of the amp and use a multi-meter set on the mv setting and it has a bias test point that sums the two power tubes together.So if it reads 60mv,that translates to 60ma divided by two = 30ma a piece.Ballpark for that amp.Same with a Deville.
If those amps don't sound good at that setting then there are lots of other reasons,like speakers,tired tubes,poorly serviced amps.
Yes,the bias makes a difference in tone,but you can't just show up at a gig with your multi-meter and expect a simple bias setting to make the bad tone good.
You plugged into a great sounding HRD in Seattle and a nasty one in Portland.A bias adjustment is most likely NOT the answer to the bad tone.
The best bet for a travelling musician to get consistent tone is to get a small amp head and just rent va cabinet when you get to the gig.That way YOU are in control and you can travel with the amp head.
I built a pair of 50 watt heads that fit into the backpacks of a touring band.They toured Europe and Asia with the amps in tow.They simply rented cabinets wherever they were at the time.
 

Ronsonic

Member
My initial suspicion is that static bias doesn't explain the differences you're finding in those amps. I see a lot of HRDs on the job and don't recall seeing any where the bias adjustment would be an issue. They get to me with the nominal 60mA bias set anywhere from 58 to 65, but with some other problem.

My experience is that an ESR meter would do a better job of finding the differences between the good and bad sounding amps. The Illinois Capacitor branded caps that Fender has been using for years have taken a serious downturn in quality. I'm getting a lot of bad caps lately. When the decoupling cap in the preamp goes the amp turns into a 60 watt analog synth of amazing tone and character. Less spectacular failures will do all sorts of bad things to your tone. Anyway you won't be replacing caps on the job so just request a replacement.

My understanding is that backline companies vary wildly in quality of gear and maintenance. The guys I fix for are of course good, but under serious price pressure, others are less good and under serious price pressure.

Mr Psychonoodler has a good idea, a packable amp.
 

naf

Member
Thanks for all your input. I definitely understand that there are many variables that can cause an amp to sound bad, I am just looking for things that I might be able to have some control over. Obviously changing caps is a little much but I was thinking biasing would be relatively easy if it would have a noticeable impact. I have certainly thought about a packable amp but i sure like having real spring reverb and that could get a little big. Plus i already carry on a guitar. I have a friend who carries a tube pre and plugs into the power section of a Hot Rod, which would be another option. Thanks again-any more options or ideas let me know.
 


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