#1
Someone said when I get new tubes I have to rebias my amp.. Is that true? And if so what the hell is that? And.. Can I do it? Or would I have to bring it in somewhere ?

thanks
My Gear:
Gibson Flying V (active Emg's 81, 85)
Peavey 6505 Amp

Avatar Cab /w Vintage 30's / G12H-30's
Schecter Damien-6
ISP Decimator
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Ibanez TS808 Tube Screamer
Roland Cube 60
#2
you dont necessarily have to rebias the amp if the tubes are exactly the same make/model you previously had, but it isnt bad to do regardless. i believe there is a link in the FAQ sticky, but i'm not sure about that. look up "amp rebias how to" in google in case it isnt.

also, if you arent that handy with amp repair/tube amp repair specifically, i would suggest getting a tech to do it for you. esp. since fatal voltages can be stored in a tube amp, and you dont want to get fried.
#3
Yes, you do need to rebias. You might be able to do it yourself, I'm not familiar with the insides of that amp so I can't tell you for sure. What I do know is, I have a Fender DeVille and to bias that you just need a multimeter and a screwdriver. It's really really easy.

Mine was easy because that amp has a variable resistor you adjust to set the correct bias, I think that is the trend in newer amps, but some I know have fixed value resistors in that case it will be more complicated because you'd have to actually remove and replace the resistor if the bias is too "off". google "setting bias in..." and go form there.
#4
You don't need to, not with a 6505, it's biased cold enough that you can just swap the tubes out and be on your way.

Skierinanutshel, tubes from the same company can vary widely so what you said there isn't necessarily true.
I don't give a shit if you listen to me or not
#5
Quote by Kevin Saale
You don't need to, not with a 6505, it's biased cold enough that you can just swap the tubes out and be on your way.

Skierinanutshel, tubes from the same company can vary widely so what you said there isn't necessarily true.

+1, you can't rebias it anyway unless you mod it with an adjustable bias. They are set very cold though, so it won't matter. Also, just like Kevin said, tubes can vary from the same manufacturer/model, even from the same batch. It's an old old technology.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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#6
Rebiasing isn't like...set in stone necessary like Kevin said. However, when it does come time to rebias, its best just to take your amp in and have a certified tech do it....it's never safe to do it at home when you don't know what you're doing...
#7
Quote by SayAnything
Rebiasing isn't like...set in stone necessary like Kevin said. However, when it does come time to rebias, its best just to take your amp in and have a certified tech do it....it's never safe to do it at home when you don't know what you're doing...


On that amp the bias is fixed and non adjustable, so without a circuit mod the bias is set.

I agree though, you need to know what you're doing. That being said, if the amp is adjustable 10-20 minutes of reading and a multimeter is enough to be able to bias an amp, hell I was able to modify my amp to be biased in about an hour (probably 20 minutes of actual work) and I've never modded the circuit on an amp.
I don't give a shit if you listen to me or not
#8
Quote by Kevin Saale
You don't need to, not with a 6505, it's biased cold enough that you can just swap the tubes out and be on your way.

Skierinanutshel, tubes from the same company can vary widely so what you said there isn't necessarily true.

That doesnt make sense....Biasing either "hot" or "cold" are terms used when referring to biasing a fixed bias amp. When you replace output tubes in a fixed bias amp you definitely gotta rebias.

The only time you don't bias is with cathode biased amps or you know your knew tubes are %101 the exact same as your old tubes (even still I would check).
#9
It's not so horribly expensive to get done by a tech that you should ignore it. I always have mine biased when I install new tubes, but that's because I like mine biased fairly hot, so...
no
#10
Sure, it would sound better if it was biased right, but I was talking about need. You don't need too since no tube will run hot enough in that amp to damage it.

Would modding it for a bias adjustment be a good idea? Yup, 'tis what I'd do, but someone who doesn't even know about biasing probably doesn't have the know how for a circuit mod like that, no matter how easy.

edit to bucky: If the amp has an adjustment do it yourself, it's silly to pay a tech to turn a knob in your amp, which essentially what biasing the amp is. If the amp doesn't have an adjustment then he's just popping new tubes in your amp and charging you for doing nothing.
I don't give a shit if you listen to me or not
#11
Actually it does need to be biased right. You bias tubes too "hot" they red plate which is not good, that can talk out your expensive output transformer and many other things along the way.
#12
Quote by kurtlives91
Actually it does need to be biased right. You bias tubes too "hot" they red plate which is not good, that can talk out your expensive output transformer and many other things along the way.



I totally agree, but 5150s/6505/peaveys in general are biased so cold that won't happen.
I don't give a shit if you listen to me or not
#13
Quote by kurtlives91
That doesnt make sense....Biasing either "hot" or "cold" are terms used when referring to biasing a fixed bias amp. When you replace output tubes in a fixed bias amp you definitely gotta rebias.

The only time you don't bias is with cathode biased amps or you know your knew tubes are %101 the exact same as your old tubes (even still I would check).


the 5150 is a fixed bias "non-adjustable" amp. There is a 15k resistor, that's it. That 15k resistor controls the negative bias voltage. They average about 420Vdc plate volts, and I've seen tubes run at between 9ma-20ma depending on the specific quad used. Mine ran between 17mA and 20mA with the stock 5150 fixed bias, with a plate voltage of 422Vdc, and that was with a quad tested to run hot already. I modified my amp to make it adjustable so they could run around 65% max plate dissipation, which is closer to 30-32mA.

TS, just order from a place like Doug's Tubes, Tupedepot, Eurotubes, etc., and tell them the quad is for a stock 5150/6505. They will test a set that should work fine with Peavey's fixed bias.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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Last edited by Erock503 at Dec 7, 2008,
#14
Quote by Kevin Saale
edit to bucky: If the amp has an adjustment do it yourself, it's silly to pay a tech to turn a knob in your amp, which essentially what biasing the amp is. If the amp doesn't have an adjustment then he's just popping new tubes in your amp and charging you for doing nothing.


Seeing as I don't have a multimeter or change tubes all the time, I'm good where I am.
no
#15
Quote by bucky_2300
Seeing as I don't have a multimeter or change tubes all the time, I'm good where I am.



Ten bucks at radioshack, it's all you need. You really should check the bias periodically since tubes and components drift over time.
I don't give a shit if you listen to me or not
#16
Quote by Kevin Saale
Ten bucks at radioshack, it's all you need. You really should check the bias periodically since tubes and components drift over time.

yup, it seems ridiculous to me not to invest in something as cheap as a bias probe, or at least a multimeter. Not if you plan on continuing to play guitar and use tube amps. It pays for itself in a couple years, that is, if you change your power tubes at the recommended intervals. Old power tubes have a tendency to catastrophically fail and take out other parts. They aren't produced at the same standards and quality as they were 40-50 years ago, where you could get away with running them into the ground until they actually died.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
amp clips
amp vids
Last edited by Erock503 at Dec 8, 2008,
#17
Quote by Erock503
the 5150 is a fixed bias "non-adjustable" amp. There is a 15k resistor, that's it. That 15k resistor controls the negative bias voltage. They average about 420Vdc plate volts, and I've seen tubes run at between 9ma-20ma depending on the specific quad used. Mine ran between 17mA and 20mA with the stock 5150 fixed bias, with a plate voltage of 422Vdc, and that was with a quad tested to run hot already. I modified my amp to make it adjustable so they could run around 65% max plate dissipation, which is closer to 30-32mA.

TS, just order from a place like Doug's Tubes, Tupedepot, Eurotubes, etc., and tell them the quad is for a stock 5150/6505. They will test a set that should work fine with Peavey's fixed bias.



I've already ordered the exact same tubes that came with the originial Peavey 6505. Which are Ruby 6l6GMC tubes. I just got them in today. Im still wondering if I should re bias my amp at Sam ash or some place or just put the tubes in because the bias is cold enough that it wont happen..
My Gear:
Gibson Flying V (active Emg's 81, 85)
Peavey 6505 Amp

Avatar Cab /w Vintage 30's / G12H-30's
Schecter Damien-6
ISP Decimator
Vox Wah Pedal
Ibanez TS808 Tube Screamer
Roland Cube 60