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Post subject: What to listen for as a sign that your amp needs biasing
Posted: Tue Feb 22, 2011 10:30 am
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I am still in the learning phase regarding amp biasing, as im a rookie with tube amps. My question here does not entail the process of biasing, but rather just what signs to listen/look for so as to know if re-biasing is needed at a given time. i've been trying to google the subject, but i was hoping you guys could make it easier to understand. I also figured this would be helpful for many other people.

So how do you know by listening if your amp is biased too cold or too hot? Can you describe the resulting sounds/characteristics?

Not sure if tube type influences this, but in my case, EL84s in a pro jr are in topic.

thanks


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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Tue Feb 22, 2011 12:12 pm
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Biased too cold, your sound may be thin and lack character with a hint of icepick in the forehead tone with a lack of decent natural distortion. The tubes will sound like arse but last forever biased too cold. Too hot and you'll get pretty quick breakup, possibly red plate the tubes and burn them up. The trick is to find the balance. The best thing to do is buy a Bias Rite or similar device, read up and bias properly. Specs vary among tube manufacturers and unless you realize what you're listening for, you can smoke a lot of tubes by experimenting.

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Tue Feb 22, 2011 1:02 pm
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msvolpe wrote:
I am still in the learning phase regarding amp biasing, as im a rookie with tube amps. My question here does not entail the process of biasing, but rather just what signs to listen/look for so as to know if re-biasing is needed at a given time. i've been trying to google the subject, but i was hoping you guys could make it easier to understand. I also figured this would be helpful for many other people.

So how do you know by listening if your amp is biased too cold or too hot? Can you describe the resulting sounds/characteristics?

Not sure if tube type influences this, but in my case, EL84s in a pro jr are in topic.

thanks


Typically in non vintage amps, (although not all the time), an EL84 amp is cathode biased, and as long as you buy sturdy tubes, you can just swap them in.

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Tue Feb 22, 2011 7:12 pm
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SOLID!


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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 6:23 am
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TTSC wrote:
SOLID!


Be absolutely positive the amp is a cathode biased circuit. A fixed bias amp is not the same as a cathode amp. Many folks get them confused and think a fixed bias amp is plug and play, it's not. :wink: Some fixed bias amps have trim pots to adjust the bias, some don't and you need to change a resistor. For example, my Egnater Rebel 20 has bias pots for both set of my power tubes. It runs both 6V6 and EL84 power tubes.

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 9:12 am
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The Pro Junior does not have a bias adjust pot, and is likely biased very hot from the factory. For that amp, just swap tubes and done, same as the stock BJr. If you are so inclined, and have the necessary skills, you can add a bias adjust as it is not Cathode biased. :idea:

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 12:22 pm
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If the amp isn't cathode bias, you need to check it before installing new tubes and properly set it. Unless you like asstone or burning up tubes all the time. :wink:

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 12:40 pm
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63supro wrote:
Unless you like asstone...


Doesn't Behringer make a pedal for that?

:lol:

Arjay

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 12:46 pm
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Retroverbial wrote:
63supro wrote:
Unless you like asstone...


Doesn't Behringer make a pedal for that?

:lol:

Arjay


They have a whole amp line too! :lol:

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 2:45 pm
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msvolpe wrote:
I am still in the learning phase regarding amp biasing, as im a rookie with tube amps. My question here does not entail the process of biasing, but rather just what signs to listen/look for so as to know if re-biasing is needed at a given time. i've been trying to google the subject, but i was hoping you guys could make it easier to understand. I also figured this would be helpful for many other people.

So how do you know by listening if your amp is biased too cold or too hot? Can you describe the resulting sounds/characteristics?

Not sure if tube type influences this, but in my case, EL84s in a pro jr are in topic.

thanks


Check this out on some of the '84s they tested with PJs :

http://thetubestore.com/el84review.html

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Wed Feb 23, 2011 3:15 pm
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Yep, good reviews. And notice that there are no warnings about modifying the PJr for any of those tubes. The PJr, like the BJr, will be rough on whatever tubes are used with the amp's fixed bias. The only way to adjust the bias on either amp is to mod the amp to add an adjustable bias. You could simply change one resistor, if you could calculate the value needed for a particular set of tubes, but then you would have to do it again next time different tubes are used, so it would be foolish to change the resistor when it is nearly as easy to add the adjustable pot, and adding the pot only has to be done once. Either way the warranty will be void, Fender won't pay to modify the amp because someone wants to change the bias. I remember someone here on the forum that adamantly opposed modifying a new amp....

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Thu Feb 24, 2011 6:07 am
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If a certified Fender tech changes the resistor, you won't void the warranty. It's an adjustment to the amp. When you change the power tubes, any good tech will put the bias in spec for the tubes you chose. You'll just have to pay the tech to do the work. This info came from my certified Fender tech. He said adding a trim pot will probably void the warranty though because it wasn't there to begin with, so it's considered a modification. If an adjustment like that would void the warranty, I wouldn't have the amp if I had to constantly change tubes or just stick to one brand.

Changing resistors to adjust bias has been a common and necessary practice for decades. It's the way it's done on amps with a fixed bias and no trim pot, it's the way your supposed adjust the bias. The tech I use said he would do what I wanted and what was right. Good clients could still get it done under warranty. :wink: I wouldn't consider it foolish to change the resistor, it's smart insurance to get the most from your amp in regards to your tone and longevity of your tubes.

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Thu Feb 24, 2011 9:43 am
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I don't know whether to laugh or cry. :lol: :cry:

That must be one of the most ridiculous things that I've ever heard.

"If a certified Fender tech changes the resistor, you won't void the warranty."

But yet; "...adding a trim pot will probably void the warranty though because it wasn't there to begin with..."

BS! It ain't gonna happen! You will void the warranty. Any change to the amp, including changing speakers can void the warranty. And the tech isn't going to charge Fender for this change, along with the voided warranty.

Call a Fender authorized tech and see for yourself (I just did). :wink: If your tech is doing things not authorized by Fender, I wouldn't tell on him like that, you might get him in trouble. 8)

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Thu Feb 24, 2011 9:51 am
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No, it's not BS at lest with my tech. Maybe you need a new tech. I've changed speakers, and stock tubes and never voided a warranty on any Fender amp I've owned. My HRDlx was a great example.

Sometimes you like to disagree with me just to disagree or push my buttons. You don't know every tech on the planet. What did you expect from a phone call? I've had the same tech for 25 years and he's been a certified Fender tech since the late 60's. :roll:

No tech who wants your future business will void your warranty over a resistor. Also last time I checked, speakers aren't usually warranted as long as the rest of the amp.

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Post subject: Re: What to listen for as a sign that your amp needs biasing
Posted: Thu Feb 24, 2011 10:04 am
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63supro wrote:
No, it's not BS at lest with my tech.


Nor mine (he's the authorized Fender repair facility for a four-county area). He told me he receives several TSB's (technical service bulletins) from the company yearly which both detail and authorize deviations which may be performed on a warranteed Fender product to address a specific condition. In rare cases of a problem where no TSB has been issued, he requests (and usually receives) verbal authorization via telephone to perform a "field mod" to get the customer's amp back on the road.

Arjay

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