#1
So before someone says "Go google it" I have. Extensively. So much that my brains about to explode. Everytime I look up something I get so many different answers I don't know what's right so I decided to ask the people of UG. Hopefully this will help answer my questions instead of confuse me more. So here are my questions: (I have a peavey 6505+ head)

1. What units do you measure bias in? Some people say mV some mA. Do both work or is it specific to each head?

2. For all you 6505+ owners, is the plate voltage for a 6505+ around 500V? Cuz that's what I've read or should I check it anyway? And if so how do you check it?

3. The 6505+ has external bias testers so if I plug in my multimeter and just read of what it says (in the correct units) then that should be the exact bias it is set at?

4. When do you measure bias? When the amp is off or after it's been plugged in and on for a good while?

Sorry if some of these are stupid questions but I just want to get to the bottom of this already! Thanks alot!
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#2
1. Confusing, definitely! The short answer is that you're measuring both. You're measuring the mV across a resistor, which can tell you the mA, which is what changes when you bias. If you use a 1 ohm resistor, the units are the same since the mA = mV x the resistor value.

2. Newer amps don't vary that much in plate voltage. If the schematic says 500V, your amp is probably close enough to that.

3. Yes, that's what they're for. Use a multimeter set to mV.

4. You'll find out really quickly that there's only one time you can measure bias - when the amp is on. It will drift depending on how long the amp is on, and if you have a new set of tubes they'll drift after a few weeks, so it's worth checking the bias again after a while.
#3
Thanks Roc! So I'm guessin since this multimeter has no mV setting that I can see it would be bad to use to measure the bias using the external bias testers?

http://www.byramlabs.com/images/hi-res/SPE-sp-6a.gif
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#4
Yup, that one's no good for biasing. Try to get one with a digital readout if you can. Amazon has a ton of decent multimeters under $20 if you're in a place they'll ship to. Search "digital multimeter". Try to avoid very cheap ones that only have autoranging; you want one with a bunch of options on the dial so you can specify the range yourself.
#5
DO NOT use the external bias test points. They are very inaccurate. Also if you haven't read it yet, 311zosojhvh has a very good bias tutorial in his bio on here. Very informative.
Ibanez SIR27
Pod HD500x

RIP:
Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent
#6
Alright. Also I can't seem to find the plate voltage anywhere in my head manual. Is there some way to calculate it based on watts outputted or do I have to measure it on the head itself?
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#7
Thanks Blue I'll check that out!
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#8
http://eurotubes.com/euro-k.htm
According to Bob of Eurotubes its

A final bias setting of between 36 to 40mA of plate current per tube measured with a bias probe is a good setting. You only need to probe one tube, set the bias for it and you’re done.
#9
You can read about it in the xxx/3120/jsx thread on here cause I asked the same question this month pretty much. I read a few different places where those jacks were off enough to cause you to over or under bias your amp pretty bad.

Pick up a bias probe like this: https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=108&sub_category_id=109

tubedepot has some more expensive ones that look better but this is what was suggested to me. Also on that site I linked you to, there is a peavey biasing tutorial that will show you step by step what to do. the site is eurotubes.com if the link doesn't work

^he is the one that helped me out
Ibanez SIR27
Pod HD500x

RIP:
Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent
#10
It says on that page that the bias adjustment is pretty small. Does this mean that possibly I wouldn't even have to adjust the power tubes when i put em in if the adjustment is that small? or is it still mandatory?
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#11
I've read from Bob that Peavey normally biases their amps very low or cold out of the factory. Biasing is just a twist of a knob as long as you are making sure you are reading the amperage on the tubes. Biasing is also not mandatory but it helps with getting better tone out of the amp if they are biased low or increase tube life if the amp is biased too high or hot.
#12
I see. People have made it seem like biassing is a life or death thing. If you DON'T do it you'll kill your amp.
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#13
It is when they have been biased improperly and are running too hot. It is much worse to have speaker impedance mismatch than it is to have tubes running cold.

I believe its just proper amp maintenance.
#14
On some amps it's more necessary. Plexis often don't sound good, or will redplate and kill tubes, if they're not biased right. Peaveys generally come very cold from the factory so that you can almost always change power tubes without needing to bias, but you can bias hotter to get more power tube breakup if you want.
#15
Its possible I suppose if its biased really hot it could go through power tubes bad. It is a really good idea to do it though and its not hard at all. I didn't know at all how to do it and watched that vid I linked and did it no problem.
Ibanez SIR27
Pod HD500x

RIP:
Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent
#16
I see. I bought mine used off of ebay which is why I've become so interested because my local guitar shop doesn't bias so I wanted to see if I could measure the bias myself to make sure it's all good but honestly I don't see why anyone would mess up the biassing on purpose so it's probably fine. (knock on wood)
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#17
Quote by danimalhxc
I see. I bought mine used off of ebay which is why I've become so interested because my local guitar shop doesn't bias so I wanted to see if I could measure the bias myself to make sure it's all good but honestly I don't see why anyone would mess up the biassing on purpose so it's probably fine. (knock on wood)

If you do get new tubes though, its important that you do because there are differences in even same brand tubes that would require rebiasing. It's like putting new tires on a car, you got to make sure the pressure's right.
#18
There is probably a better chance that its not been biased at all than someone messing up the bias I would think. If no one has touched it then its probably biased cold so you might get some better tone if you biased it. But like sputnik said, its good amp maintenance. I don't see why your local guitar shop wouldn't offer that kind of service though.
Ibanez SIR27
Pod HD500x

RIP:
Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent
#19
I called and they said they don't but they gave me a number for someone who they know that does. It's a really small shop that's not all that good haha but I see. I think that it probably has been biassed AT LEAST when the first person bought it new. If i plug it in and turn it on and after like 10 minutes the tubes are either totally red hot or not that red at all is that when i know that they're either too hot or too cold?
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#20
Quote by danimalhxc
I called and they said they don't but they gave me a number for someone who they know that does. It's a really small shop that's not all that good haha but I see. I think that it probably has been biassed AT LEAST when the first person bought it new. If i plug it in and turn it on and after like 10 minutes the tubes are either totally red hot or not that red at all is that when i know that they're either too hot or too cold?

As long as they aren't red, they are fine.
#21
Awesome! Well thanks alot! If I do end up getting new tubes I'll most likely take it to a tech the first time so I can see how he does it on my head then after that I think eventually I'll just do it myself =)
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#22
From what I've read on the 6505+ they are too cold even with the bias pot set to max. Pretty much all bias googling on that amp relates to swapping out a resistor to heat up the bias. So unless the resistor has been replaced biasing amounts to setting the pot to max.
#23
Quote by fly135
From what I've read on the 6505+ they are too cold even with the bias pot set to max. Pretty much all bias googling on that amp relates to swapping out a resistor to heat up the bias. So unless the resistor has been replaced biasing amounts to setting the pot to max.

+1, looks like the knob has an incredibly small range.
#24
Ya that is something I've read up on too. I'll just have to see what it sounds like. Cuz it's also about tone =) but if that is necessary I'll definitely let a tech do that
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