Is biasing really nessasary?

[sm5150]

I have a JCM900 4100, and it needs new tubes. Ive never replaced tubes before because i was useing SS or sold my other tube amp before it needed new tubes.
So anyway, is biasing really required? what happens if I dont bias?

[Racerx2k]

Let's put it this way: Would you like your amp destroyed? If they answer is "no", then, please, have it biased.

[toejam]

If using the same type and brand tube, the tolerances will probably be pretty close that you might not have to, but it's always best to check.

[sm5150]

Alight thanks guys.
I dont know how to do this myself, and i didnt want to bring it in because they take liek 2 months. I guess its worth it then..

[DumpyWumpy]

Man everyone makes it seem to hard to change tubes... I've got a 5150 so it's fixed bias and I can't adjust it anyways without modding it, but I just unplug the old tubes, plug in the new tubes, fire it up and give 'er... No problems so far.

[scotty]

That is one thing I like about my Randall RM100 is has the test points right there in the back.

[lynchfan6]

I never biased my old Marshalls when changing power tubes. Now, my SLO is about due for some new ones, and when I called Soldano, they told me I didn't have to bias as long as I used the same type of tubes. Then , when I talked to Bob at Eurotubes, he insisted that it is highly recommended. So who's right??? Is it just a "tone" thing? Unless of course your are going to a different kind of output tube,,,,,then it is more than just a tone thing,,,

[roodyrocker]

That is one thing I like about my Randall RM100 is has the test points right there in the back.

I know what you about the RM 100. I re-tubed mine a couple months back. With the external test points and all the info in the supplied manual, it is simple to perform this on a Randall RM 100. I wish the rest of my tube heads were that simple.

[Rsmacker]

I never biased my old Marshalls when changing power tubes. Now, my SLO is about due for some new ones, and when I called Soldano, they told me I didn't have to bias as long as I used the same type of tubes. Then , when I talked to Bob at Eurotubes, he insisted that it is highly recommended. So who's right??? Is it just a "tone" thing? Unless of course your are going to a different kind of output tube,,,,,then it is more than just a tone thing,,,

It's not necessarily a "tone" thing, because the bulk of that work is done by the pre-amp valves and they don't need to be matched or biassed, just plug and play to your tastes.

It's important to have correctly biassed output valves, but, as has been said above, provided you are using similar sets of matched valves, there should be no real difference in the settings of the old and new sets.

To help visualise what valves do, in the most basic and non-scientifically, not-to-scale, drawn in crayon kinda way, so as Mr Average can picture it is to think of the output stage of your amp like a set of bellows :

It consists of what boils down to a "pull" and a "push" part* The first pulls the signal you want amplified, and the second pushes it out nice and loud. Now, if the pull is taking huge great breaths, and the push is pushing out little gasps, something is obviously mismatched. The same if the push is blowing out huge breaths but is only taking tiny gasps. When you bias your amp, you equalise the voltage to balance out these two halves of the output stage (which is why output valves are in pairs or multiples of pairs)

Mismatches cause heat, which destroy valves. However, it isn't necessarily a death sentence to run an amp slightly off-bias. A knowlegeable person can set the bias slightly off to get a particular sound, but at the price of shorter valve life. These technicians know what they are doing, have probably never kissed a girl on the lips, and know how much is too much! These are the techno-wizards sought out by guitarists down the ages, you know the type of chap I mean (cos I'm fucked if I can recall any of their names!!!) It's a black art, and the way these fellows found their magical way to set these amps probably involved many long nights over a scope and several fucked amplifiers.

Personally, if I wanted an amp sounding like a Marshall JMP, I'd buy one of those, not frig around running valves too hot in a Dual Showman to get that sound. In this day and age anyway, so many different amp voices can be reached via preamps and fx units, there's no need to alter the way your amps run to get the sound, in the same way that you simply wouldn't take a razor blade to a speaker cone to get that Kinks guitar sound (which is how they did it back in the 1960s).

The problem is that if the biassing is way off, as well as destroying the valves, the output transformer of your amp will go puff. Get a price on replacing one of these fellows and come back and tell me it's not worth having the biassing checked when you re-valve, just for peace of mind!

Leaving the bellows analogy behind, I suppose you could say that if your car has AWB, when you change the pads, it would be wise to bleed the system through, make sure they all are exerting the same pressure, and one isn't going to yank on super-fierce when you have to brake suddenly. The chances are if all pads are equally worn, you just bang some new ones in and that's it, but if there is a problem......the first thing the crash investigator would say is "the brakes were bled and working correctly, right?".

Re-biassing is not really the kind of job for the layman, (apart from on amps like the RM100, or newer Fender Twins, which have sexy easy to use probe points), possibly because the consequences of fucking around inside a valve amp can be a. expensive, b. deadly. Manufacturers don't want Tom Dick and Harry tinkering to "try out new sounds" and killing themself or burning their house down. There's no need for it.

If I were using an amp regularly, full-on, gig volume, I'd have it re-biassed when re-valved. Me, I play a little here, play a little there, by myself (with myself?), so it's not so important. I'd only re-valve with matched sets, so, as the amps had previously matched sets, there's not much likelihood of them being out. The amount of time they are used is miniscule, so I'm content to pay attention to the valves visually, and am happy I'm OK. If I decided to lob in a couple of old valves I found in the shed, then I would absolutely want to have it biassed so I didn't blow up any of my old toys!

The choice is yours chaps - how much is peace of mind, and how much is a new output tranny? It's a ball-ache to find someone who can do it, I grant you, but when you find that person, and there are plenty of people about who know their shit, it will take 5 minutes for him to check it. You'll have made a good contact for the day you need a valve amp specialist in a hurry (like 5 mins before a gig!), so keep him sweet, go chat about guitar sounds you love, smile politely when he goes off on one about scope graphs of certain valves made in 1965 Czechoslovakia, have a cup of tea with him, and pay him the few quid he will want to stick the meter across it. Ask if you can watch, just for your mental well-being, not because you want to know how to do his job. Chances are, he'll welcome you into his cave, erm, workshop. Four hours later you'll be trying to get away while he tells you about valves on Vulcan Bombers (yep, there are several of these blokes around here. Very very knowledgeable, but not the kind of blokes you want to get stuck in a lift with)

Sorry for the essay, but ah fuck it, I'm only supposed to get my tax return in today! Yikes!!!!!




* Before some cunt pipes up about different Classes and Voltages - please shut the fuck up, you'll confuse things!!

[Rsmacker]

I forgot to add:

I'd not take my amp to somewhere like, erm, a large chain of guitar shops staffed by 19 yr old guitar heroes who are more interested in posing, and where the gaffer is a 45 yr old bloke in a sleeveless t-shirt with a mullet. You know the kind of place I'm talking about, they have a Huge, Unrepeatable Blowout Sale every other week.

No, not one of those places, because they take the amp in, tell you to come back next Tuesday, and it sits there till Monday night, someone lobs a matched set of valves in and they gamble the settings are going to be balanced enough. If I pay for my amp to be re-biassed, I want someone who knows what they are doing to meter it! If I don't know how to do it (and there are plenty of my amps I wont even think about it), I can be absolutely 100% sure that the staff of these sheds won't have a fucking clue.
The chances of them having a dedicated geezer doing valve amps? Negligible, there won't be enough work for a specialist like that, and he will probably be a miserable old fart compared to his know-it-all ex-student workmates who can flog Squier Strats a-plenty. Not very employable.

There will be exceptions, of course, just don't trust the kiddiwink on the counter when he says he'll get their man to have a look at it, come back tomorrow. Smaller guitar/amp specialists are the places to find these chaps, or they will at least point you in the right direction.

[axehunter]

Rsmacker, thank you for this post....not only did I learn a lot, but it was HIGHLY entertaining! :ROTF:

[Infernal Death]

I never biased my old Marshalls when changing power tubes. Now, my SLO is about due for some new ones, and when I called Soldano, they told me I didn't have to bias as long as I used the same type of tubes. Then , when I talked to Bob at Eurotubes, he insisted that it is highly recommended. So who's right??? Is it just a "tone" thing? Unless of course your are going to a different kind of output tube,,,,,then it is more than just a tone thing,,,

Well they are both right. It never hurts to check the bias settings, even only to confirm they are right. But Soldano is also right, if you stick in tubes that have the same values then it's ok. That's exactly how fixed-bias amps work. You need to drop in tubes with the same values because the bias settings work for exactly those tube. Same type of tubes doesn't mean you are ok as long as you use the same brand and model. Tubes themselves have different values, even from the same production run. So if you want to have drop-in replacement tubes for your SLO you need to buy tubes with the same values. But for this you need to know the values of the tubes. So basically with non-fixed bias amps it's best to get the bias settings checked. Since you don't need to retube every month i think the greater expense of having the bias settings checked should be ok to deal with.

Rsmacker already said much about the "tone thing". And of course i agree with him: If you don't know exactly what you are doing, never "attempt" on biasing or for that matter repairing a tube amp. Always consult a tech who knows what he is doing and has experience with what he is doing. Even more with an expensive amp like the SLO.

Flo

[Madness]

The choice is yours chaps - how much is peace of mind, and how much is a new output tranny? It's a ball-ache to find someone who can do it, I grant you, but when you find that person, and there are plenty of people about who know their shit, it will take 5 minutes for him to check it. You'll have made a good contact for the day you need a valve amp specialist in a hurry (like 5 mins before a gig!), so keep him sweet, go chat about guitar sounds you love, smile politely when he goes off on one about scope graphs of certain valves made in 1965 Czechoslovakia, have a cup of tea with him, and pay him the few quid he will want to stick the meter across it. Ask if you can watch, just for your mental well-being, not because you want to know how to do his job. Chances are, he'll welcome you into his cave, erm, workshop. Four hours later you'll be trying to get away while he tells you about valves on Vulcan Bombers (yep, there are several of these blokes around here. Very very knowledgeable, but not the kind of blokes you want to get stuck in a lift with)

I know exactly what you are saying. My tech moved last year. He was exactly the kind of dude your talking about. As soon as I expressed intrest in what he was doing I could not get out of his shop. He seemed so excited that I was interested in the specifics of what he was doing. He would just talk and talk and talk. I was having him convert my JCM900 from 5881's to EL34's.

[Spivonious]

I wish guitar amps were more like my dad's stereo amp (Conrad Johnson 5?). Below each tube is a red LED and a bias adjustment pot. When you retube, you turn the pot until the light is a solid red instead of blinking. Bam, biasing done, no multimeter needed. If they could do this in the 70s, why can't they do it now?

[roodyrocker]

I wish guitar amps were more like my dad's stereo amp (Conrad Johnson 5?). Below each tube is a red LED and a bias adjustment pot. When you retube, you turn the pot until the light is a solid red instead of blinking. Bam, biasing done, no multimeter needed. If they could do this in the 70s, why can't they do it now?

The Randall RM 100 we were talking about earlier is as close as you'll get to that. The back of the head has bias test points for each tube and a little dial that is adjustable with a small screw driver. Instead of a blinking light in your wish, you use a $25 meter from Radio Shack and Randall provides the different values in the manual for the type of tube you wish to use. Very simple. Oh yeah, the RM 10also has a light for each tube so that you don't have to guess which tube blew in the event of a failure plus, it will continue to run on the other tubes anyway so you can finish your gig. Very thoughtful of Randall to put such features in the hands of the player instead of the need for a bench tech!