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Old 16th June 2004, 02:24 PM   #1
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Default help with peavey 5150 bias adjustment

Hello, I have little to no knowledge on tube amplifiers.
My band is scheduled to tour beginning July 8th, and I'd like to adjust the bias in my 5150 to get the plate current up to around 34-35mA...I barely even know what this means. The point is to be able to use JJ Electronics E34L tubes in it, not to mention to get decent distortion out of the amp (not crossover distortion).

How possible is it for me to do this myself? I do not necessarily require adjustable bias, but I do need to alter mine. It can stay fixed once I do so.

Now, I know having no experience makes this that much more daunting...but I'm relatively intelligent and as such could maybe get it together with the right information.

Thanks in advance!!
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Old 17th June 2004, 03:25 PM   #2
benny is offline benny  Australia
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ok, i know you say about the intelligent thing, and the right info and all, but i am going to say that you should take it to someone who knows what they're doing... ok.

Quote:
and I'd like to adjust the bias in my 5150 to get the plate current up to around 34-35mA
firstly, where did you get the figure of 34-35mA from???

at this current, you are going to be getting nasty crossover distortion at a plate voltage that i would say to you, well give it a go, but watch out for this this and this and make sure you do this... but an EL34 @ 35mA plate current is going to be running with a plate voltage excess of 500V to avoid nasty x/over dist... and don't get me wrong, all voltages are dangerous... but once you start working with voltages at this level and higher, they become more dangerous... too dangerous infact, unless you are 100% sure of what you're doing. seeing as you are needing instructions have said you lack experience, then you are not safe... no one is safe actually, not even someone who sits and probes those kinds of voltages all day every day as their job... the only difference is... some are more safe than others... and this only comes from knowing what you're doing... that means, having the experience which you say you lack... and as you are going to have to probe a LIVE amp to bias the tubes, this just makes it out of the question. ok, cool???

it's like driving a car... someone could write a book on how to drive a car and what you have to watch for on the roads, but it is a slightly different experience driving it in real life... only experience makes you a good driver.

sorry to dissapoint you... but a bit of money spent on using an experienced tech is a fair trade for your life i reckon.

cheers
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Old 17th June 2004, 09:53 PM   #3
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First off, well said. Even if the guy I was getting all this information didn't say the same thing, I'd probably listen to you.


I got the information from this tech that does the specific mod regularly; its not that Im cheap at all, but my band has precious little time before we leave for tour and I was looking for good ways to upgrade my head. I would absolutely send it to this guy to do it if I had the time. And the only reason I even considered doing it is other than the required measurements, it seems the only actual changes to be made are clipping the 15k resistor, adding a 5k one, and adding a 10k pot. He showed me a picture of it and it looked pretty simple. But from the inevitable abyss of questions I threw at him he said pretty much what you did.


Oh yeah, and this isn't an option for me to have it done around where I live because it seems all shops in the area send amp repairs to a single person, who doesn't seem educated in it enough to be willing to do any more than what a manufacturer specifies.
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Old 18th June 2004, 03:15 PM   #4
benny is offline benny  Australia
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if you're keen, read this

Safety Practices, General and Ultra-High Voltage

there are some very good points in there... i only read the whole thing tonight just then for the firt time, and i even learnt something... like wearing rubber gloves... so obviously beneficial to safety, but so easy to overlook as another precaution...


Quote:
it's like driving a car... someone could write a book on how to drive a car and what you have to watch for on the roads, but it is a slightly different experience driving it in real life... only experience makes you a good driver.
and Sy makes the same point i was making here about being able to understand things on an intelectual level doesn't mean you can do them in real life... but he gives a real life example that is related to electricity, not some silly little analogy like i made.

hope this helps, and i wish you luck (and lots of it) if you atempt the mod... hope it turns out for you.

cheers
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Old 8th March 2012, 02:01 PM   #5
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Exclamation Answer to the question for 5150 bias mod/EL34 tube switch.

Quote:
Originally Posted by benny View Post
ok, i know you say about the intelligent thing, and the right info and all, but i am going to say that you should take it to someone who knows what they're doing... ok.



firstly, where did you get the figure of 34-35mA from???

at this current, you are going to be getting nasty crossover distortion at a plate voltage that i would say to you, well give it a go, but watch out for this this and this and make sure you do this... but an EL34 @ 35mA plate current is going to be running with a plate voltage excess of 500V to avoid nasty x/over dist... and don't get me wrong, all voltages are dangerous... but once you start working with voltages at this level and higher, they become more dangerous... too dangerous infact, unless you are 100% sure of what you're doing. seeing as you are needing instructions have said you lack experience, then you are not safe... no one is safe actually, not even someone who sits and probes those kinds of voltages all day every day as their job... the only difference is... some are more safe than others... and this only comes from knowing what you're doing... that means, having the experience which you say you lack... and as you are going to have to probe a LIVE amp to bias the tubes, this just makes it out of the question. ok, cool???

it's like driving a car... someone could write a book on how to drive a car and what you have to watch for on the roads, but it is a slightly different experience driving it in real life... only experience makes you a good driver.

sorry to dissapoint you... but a bit of money spent on using an experienced tech is a fair trade for your life i reckon.

cheers
I have a Marshall I run at 33mv/ma EL34 and it runs perfect with out any crossover its running about 480v. Bias by ear at 33mv/ma sounds like Marshall should sound. If you use Weber's or another bias calculator on the web they give different dissipation settings 60%-70%-80% etc. The amp runs at 480 or so volts so 35ma would be 70%. Take into account this amp ships with 6L6GC 30w tubes and uses 100ohm screen resistors. Sounds fair to me but if you are using EL34 you need to upgrade the screen resistors and check the Power Transformer. I suggest phoning Peavey repair support and asking them which resistors to use and about the Transformer. Just for grins I already did this and just repaired a XXX with 1k Wirewounds. I don't know about the PT though. This is what a Peavey Tech suggested. The XXX can run both types so if the XXX has the same transformer then you're set. Make sure your PT has enough juice to run EL34s or you might kill it. I just modded my 5150 with a bias pot and setting the bias by ear sounded right at about 34ma. The tone changes from spongy and muffled to gritty and low-fi and your ear can hear the sweet spot as well as any tech's equipment or better. bump the ear I am using a GT Bias tool which I am assuming has a 1ohm shunt. So the question remains, what is needed to bias and switch to EL34s in a 5150? 1. Install 1k Screen Grid resistor upgrade, 2. Install 10k 1/2w pot and 5.1k 1/2w resistor and two 6" pieces of wire bias adjustment upgrade, 3. Make sure the Power Transformer can supply enough current for EL34's. To eliminate crossover distortion you might need a dummy load, occilloscope and a signal generator and specific knowledege required to eliminate the dreaded "Bump" Just bias the amp and rock out what are you going for clean with a 5150? I don't really know what you were trying to say @benny? The bias figure he quoted was a question. I think it's perfect but you should check your voltage and do you own calculation. Watch out for high voltage you can get shocked like I did.
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Old 8th March 2012, 11:54 PM   #6
Enzo is offline Enzo  United States
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I assume this will be moved to the instrument amps section.

There is a lot being left out here.

The Peavey 5150 has no adjustment for bias. The amp as designed gets its tone from the preamp, the power amp is loud and strong, but the tubes run very cool. The only way to adjust bias is to modify the bias supply. yes, wiring a variable in place of the 15k lower leg of teh voltage divider will work.

Next, the use of EL34 in this amp will require modification as well. The amp is designed for 6L6s, which do not use pins 1 and 6. For design reasons, the plate leads on their way to pin 3 are also connected to pin 1. On a 6L6 this won;t matter, but the EL34 needs pin 1 to be grounded, or at least connected to pin 8. This can be done, but will require a lot of trace cutting and jumper wiring. Also, the screen resistors will need to be changed, and for sure the bias made adjustable. Now if someone is used to doing this, fair enough, but I don;t recommend this to the novice.

The Peavey power transformer is more than able to handle the extra heater current, that part is not an issue.
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Old 9th March 2012, 02:01 PM   #7
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just like was said the bias is fixed. now there is some people that does a mod in the bias area, but that would be a little over your head.

on cold bias amps like 5150 and mesa boogie 50/50 I recommend replacing the output tubes before touring. That away you have a fresh 1000 hours of output tubes.
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Old 9th March 2012, 06:17 PM   #8
kevinkr is offline kevinkr  United States
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Per forum directive since this an MI amplifier it belongs in Instruments & Amplifiers where I am moving it. Please read the headings at the top of Tubes / Valve forum and Instruments & Amplifiers.
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