Anyone here get there 5150 bias modded?

AdamWathan

AdamWathan

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Apr 12, 2002
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Anyone here get the bias mod done on their 5150? I know they are biased really cold from the factory but they are fixed bias so you have to do a little bit of work to switch it over to be able to set the bias yourself... A lot of guys seem to swear by this mod, but I'm just wondering if anyone here has it or if any of the pros have it done on their 5150s? If all of my favourite recorded 5150 tones are from factory biased 5150s, I think I'll leave mine alone, haha...
 
I've had 5150's with the FJA mod and Bias mod done... I don't like 'em. It takes away from what a 5150 is, to me. It makes them warmer and more dynamic, sure, but they're less brutal sounding. They don't have as much grind on top and aren't as tight on the bottom.
 
I have a friend that sent his to FJA mods and sold it on ebay when he got it back. he says the same thing as dss3. Sucked the balls right out of it. Said it made it less brutal. He said it didn't sound warmer to him just less over the top gain.
 
BLOODROOT said:
I have a friend that sent his to FJA mods and sold it on ebay when he got it back. he says the same thing as dss3. Sucked the balls right out of it. Said it made it less brutal. He said it didn't sound warmer to him just less over the top gain.
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Since the bias mod makes the bias ajustable why didn't he just try changing it around? Sure the tubes would ware out quicker but at least that way you could get it back to where it was. Plus was the tube change (usually done with this mod) to his liking? Maybe the tubes weren't suited to his needs.

Just a thought.
 
In our rehearsal room, that we share with with other bands, there are 4 5150's. One is ours.

Two of them have different (5881) tubes and a biasjob. Those 2 sound way better than the untouched 5150's, in my opinion.

The grit is rawer, less fizz and the dynamics are better.
 
GuitarGodgt said:
Since the bias mod makes the bias ajustable why didn't he just try changing it around? Sure the tubes would ware out quicker but at least that way you could get it back to where it was. Plus was the tube change (usually done with this mod) to his liking? Maybe the tubes weren't suited to his needs.

Just a thought.
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You are probably right. He's a guy who doesn't know anything about stuff. He just plugs in and plays. Takes it to the tech kind of guy. he wouldn't know one tube from another. I just remember him telling me he got it back and a few days later he said he sold it on ebay.
 
BLOODROOT said:
You are probably right. He's a guy who doesn't know anything about stuff. He just plugs in and plays. Takes it to the tech kind of guy. he wouldn't know one tube from another. I just remember him telling me he got it back and a few days later he said he sold it on ebay.
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Yeah it was probably biased cold.... I always bias my amps hot, and I end up having to change power tubes once a year, which isn't bad IMO at all. Sounds killer too
 
I had always thought that 5150's sounded like shit, till i had the one in my studio re-tubed and bias moded. i put a set of ruby 6L6s in the power section and a mix of Groove tubes and german new old stock 12AX7s in the pre amp section. the amp now has a hi-fi quality to it that stock 5150's just don't have. the murky buzzy distortion that i hated is gone and in its place is a tone that is more defined and clear with a much tighter bottom end. but i don't think doing the bias mod alone made my amp sound good, i think it was more of a tube thing. as well as the ability to properly bias the output tubes.
but thats just my opinion your results may vary
 
I did the biasmod on mine (as Hack said) and it's great!
I mean you yould still bias it cold if you want (you won'T!) but you can just play through it and adjust the bias till it's perfect...mine ended up pretty hot and I LOVE that amp!
 
My combo sounds fucking rad, with the stock sheffields. Apparently the heads were biased (cold) at 30-34ma? The combos were biased much hotter (correctly) at 44-46ma so right off the bat a combo is supposed to sound "right" whereas the heads did not... which is probably true, but even biased fucktedly, the heads still kill.

I've played every incarnation of high gain peavey: rockmaster preamp (which i still love!) through a mesa 50/50, XXX, 6505(+ and non), 5150 head through various cabs... this goddamn 5150 combo is magical... i have her toob'd with some nos mullardos in the pre and a pair of new sensor mullardo reissues in the juicebox... its the bomb factory.. sick.. does everything i need and sounds like gods ballz or albert lees nutsack depending on my pickup or guitar volume knobber.. no pedals mind you.. just right into the high gain input.
 
EtherForBreakfast said:
My combo sounds fucking rad, with the stock sheffields. Apparently the heads were biased (cold) at 30-34ma? The combos were biased much hotter (correctly) at 44-46ma
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Ha, both heads I modded came WAY colder, one at 15mA the other one at 8mA !!!!
Peavey seems to take tubes that run as cold as possible so they don't have any tubefuckups in the first 2 years of warranty....
 
mine is stock.
still got the original (14 years old) valves in it, and it sounds great.
 
Could anyone explain me really quick what it means to do a bias-mod and what is the difference of having it modded cold or not?
Thank you!
 
Emdprodukt said:
Could anyone explain me really quick what it means to do a bias-mod and what is the difference of having it modded cold or not?
Thank you!
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Preetz, ha, that's like in front of my house ;)
Damn, I bet we made music with the same guys ;)

BIas is the idle-current of a tube (think od it as the car's engine running idle) the hotter it is (more "Satandgas") the quicker it jumos at you and the more dynamic it is, it also sounds "warmer" but tubelife decreases.
biasing tubes too cold (low idle current) introduces crossover distortion, a harsh and unpleasant kind of distortion the amp is less dynamic and sounds "colder" but the tubes last longer.

it's not always "the hotter the better" there's something like a safe range (no crossover dist and anodes not glowing) in this range everything's fine and you gotta try what you like best.

for the 5150 it would be something like 35-45mA or so for 6L6 (not sure now since I'm not totally sure about the Ua (anode-voltage) now.

so a properly biased amp would be somewhere in that ranhe where 35mA would be considered "cold" and 45mA "hot".....

so you see why a 12mA biased 5150 can't sound perfect ;).



usually the idle-current is adjustable but some companies (Mesa and Peavey in some amps) spare that trimpot and install a fixed resistor instead.
the reason:
cold biased tubes last longer-->less warranty claims
reason number 2: mesa can sell ridiculously overpriced tubes that are matched (colorcoded) so that they run in their specific range in a boogie amp.

to make the bias adjustable (as it should be, you don't wanna get rid of the possibility to adjust pickup and string-action on your guitar as well, do you?) you have to replace that fixed resistor with a resistor/minipot-combination.
 
Actually I was in Lübeck yesterday because we got two cats from a friends mother and had to pick them up. You bet you're the first choice in the case we EVER have the money and the skills to record an album :).

and thanks for the info! I'm playing a krank rev1 and don't know if it has a trimpot. I should know more about tubes and my equiptment, I know :). I mean... when I have to get a full set of new tubes for the powersection (4x 6l6), when does the bias matter? When I change the brand?
 
Emdprodukt said:
.....and thanks for the info! I'm playing a krank rev1 and don't know if it has a trimpot. I should know more about tubes and my equiptment, I know :). I mean... when I have to get a full set of new tubes for the powersection (4x 6l6), when does the bias matter? When I change the brand?
Click to expand...

Here's some info I posted a while ago about biasing my Rev for 6L6 Tubes:

.....I re-tubed my amp last week using JJECC83S Preamp Tubes and JJ 6L6GC Powertubes. Since I hadn't biased an amp before I took it to a tech. With the JJ Tubes in, he measured the Plate Voltage which was quite high at 526Vdc (I had expected around 480 from info I had researched). One method to set the bias current is to use the following equation:

Tube Wattage / Plate Voltage X 70% = bias current.

Therefore:

25/526 X 0.7 = 0.03327 or 33.3 mA

He then used a bias probe which plugs in between the tube and the tube socket on the amp. The particular one he used was a Weber and it had 4 bias probes connected to an LCD display with a rotary switch to select which probe's measurement was being displayed. He looked at the readings on all 4 tubes (which was close since they were a matched quad) and he adjusted the bias pot until the average was around the desired setting. They were as follows:


Tube1 - 32.6
Tube2 - 32.6
Tube3 - 33.6
Tube4 - 33.6

Attached are some photo's of the inside of the REV and of an access hole that was drilled in the bottom to save from removing the chassis from the box next time the bias needs to be adjusted/checked.

Original Sovtek 5881 currents were around 36mA, didn't measure the Plate Voltage for the 5881's though.

http://sneapforum.kazrog.com/GuitarHack/Bias Photo's.zip
 
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