BLOWN FUSE??????

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BLOWN FUSE??????

Postby 601blues » Mon Jan 29, 2007 1:02 pm

GEZZ!! yesterday my loyal and faithfull Friend ,my Classic 50,4/10 I have had since 1992, 2 weeks after I had got her she was in a fire,covered with black soot,and hosed down by the FD, after dryin out for a month she was back in action, She was always there with her humble self willin to take over the show after The new Marshall failed twice in 2 shows, Now after gettin a new Classic 30 Delta Blues a couple weeks ago, Iwas gonna hook the 2 up yesterday for a little Stereo action with my 345, The 50, upon warm up had a lotta cracklin sound comin from the speakers I have heard this a time or two over the years and will go away after warm up,anyway it got worse!!and stopped all of a sudden and I had no power I checked the fuse and it was Blown!! I know you guys here are Purdy darn smart on this stuff, whatcha think,Refuse and try again ???change tubes??? I was thinkin bout takin it up to Peavey In Merdian,if you get there when they open they will jump on it while you wait!!
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Postby ricochet » Mon Jan 29, 2007 5:27 pm

Refuse once. If it blows again, you'd better find out why.

It's not uncommon for power tubes to get gassy and "run away," blowing a fuse. If you see 'em when they're doing that, the plates will glow red. Fairly common for filter caps to short out and do the same. Sometimes a grid wire will break and short a tube internally, or shorts can develop elsewhere in the circuitry for lots of different reasons.
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Postby 601blues » Tue Jan 30, 2007 1:21 pm

:shock: Rico, explain Bias, last night Joel my rythym player was explaing why I shoulod make sure they check the bias cuz it gets wild and runs away, ????????? Man Iam stupid about electronics!! I know you got Bias Tires, and you can be Bias in your thoughts, but how does Bias fit into my amps???
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Postby ricochet » Tue Jan 30, 2007 5:12 pm

Bias is the DC voltage applied to the grids (or cathodes) of your tubes to control the level of idle current through them, and also the signal voltage levels where distortion starts.

"Fixed bias" means there's a fixed negative bias voltage applied to the grids all the time. The signal voltage varies around the average level set by the fixed bias voltage. Confusingly, this fixed bias voltage is often made adjustable. Amps with adjustable bias always have fixed bias. Sounds like an oxymoron.

"Cathode bias" means there's a resistor in series with the cathode of the tube. As current flows through it, there's a voltage drop that makes the cathode go positive. Since the grid stays at ground potential while the cathode goes positive, that has the same effect as a negative grid throttling back the current going from the cathode to the plate. Since that positive voltage increases as the current increases, this is a self-regulating system. The current won't vary a whole lot with tubes that differ a bit in their characteristics, so it's a plug and play system that needs no adjustment. A lot of people mistake this for fixed bias, since it's not adjustable.

So far as adjusting fixed bias goes, there are many, many pages to be found on the Web about adjusting bias and how it ought to be adjusted, and IMO a lot of them are wrong. It doesn't work to come up with arbitrary formulas like "Adjust the bias current to 70% of maximum plate dissipation." (Those kinds of recommendations abound online.) If you're interested, I can find you some detailed analyses of why this doesn't work. I don't want to reinvent the wheel.

The bias generally needs to be adjusted on an amp with adjustable bias when tubes are changed, because the new tubes may draw more or less current at the same fixed voltage compared to the tubes being replaced. The most important thing is that you don't want them drawing too much current and overheating, which can lead to "runaway" and fuse blowing. Then there are the issues of tonal preferences with regard to where the bias is set. If the bias is set "hot," with a low negative voltage on the grid and more current flowing, distortion will begin earlier, develop more gradually, and the overall power output from a Class AB amp will be less. If it's set "cold," with a higher negative voltage and less plate current at idle, there will be much more clean headroom, the maximum power output will be greater, but distortion will occur more rapidly at a higher signal level. It seems that many players now want their amps biased "hot," and there is a limit to how "hot" you can safely go. If you're running close to that limit, new tubes can easily push you over to being too "hot."

I like 'em biased cold, myself, and like the old system everyone used up till the '60s when the hi-fi freaks got involved with guitar amps, where the fixed negative bias was set high enough that any tubes meeting the normal specifications for the type will operate safely. Plug and play, baby! Leave that "Be All That You Can Be" stuff to the Army. I don't want to have to tweak bias in search of some mythical golden tone. And I'm damn sure not going to pay an amp tech a service bill, haul the amp in and leave it for some unkown period of time till he gets around to it, just to change tubes! That's ridiculous, it ought to be like plugging in a light bulb, it's not rocket science.

That said, many guitarists are on that tone search, and an amp company that developed an amp with a simple user adjustment for bias that couldn't fry the amp, with a knob and a meter for idle current with green, yellow and red ranges on it, would make a ton of money IMO. Wouldn't cost much to build it that way.

But if anybody wants my advice, I'll tell them to avoid amps with adjustable bias like the plague, unless they either have a full time amp tech on their staff or are prepared to acquire the equipment and knowledge to do it themselves. Don't make yourself dependent on techs for normal routine maintenance.

Peavey builds a number of amps with truly fixed bias.
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Postby 601blues » Tue Jan 30, 2007 6:33 pm

:shock: HUH!!! Now I understand???? :lol: OHHHHH now muh heads hurtin!! :cry: Thanks for makin that clear :lol:
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Postby ricochet » Tue Jan 30, 2007 7:35 pm

601, I'm trying.

In a nutshell: If your bias won't adjust, change your tubes as needed, play and be happy.

If you've got an amp with adjustable bias, either learn how to adjust it right yourself (and then all of what I said above will make sense), or go to an amp tech, bend over, smile and take it like a man.

If you don't want to do either of the above, buy only amps with nonadjustable bias.
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Postby 601blues » Tue Jan 30, 2007 7:59 pm

:shock: K!! Non Adjustable Bias!! gotcha!! one good thing about livin in mississippi,Iam only an hr. away from Peavey Factory, and they treat me like a king !! I will spend more on gas going there,next fri than they will charge me for redoin my amp!!
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Postby ricochet » Tue Jan 30, 2007 8:12 pm

The Peavey folks are a good bunch. They make good gear.
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Postby ricochet » Tue Jan 30, 2007 8:15 pm

BTW, I may have said this before, but the basic problem with tubes running away is power tubes that contain more air than they should. A rather common problem with the tubes of today. It's cured by replacing the tubes.

I believe your Classic 50 and Delta Blues both have nonadjustable fixed bias.
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Postby 601blues » Tue Jan 30, 2007 8:28 pm

ricochet wrote:The Peavey folks are a good bunch. They make good gear.
They really are a great company,I love the Classic series amps and the Delta Blues series. I have a set of stage lights by peavey, they fold into a suite case easy to use, they have long ago discontinued them ,anyway last year I was reading about a footswitch controler in my owners manuel, that made them do all kinds of neat stuff,I called peavey to see about gettin that controler, They didnot have any anywhere and for the next month or so I would get calls from them letting me know they was still lookin!!! after about 6 weeks they called and said they were making me one it might not be great lookin but it would work, they sent it to me and it worked,!!!! didn't charge me nuttin,!!!they do care about there customers,and will bend over backwards for you, If you have any peavey stuff and have trouble don"t go anywhere else go straight to the factory service!! thats what has sold me on Peavey,you can get HELP!!
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Postby ricochet » Tue Jan 30, 2007 9:21 pm

Now that's a good recommendation!
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