hello all-
i am looking to get into the tube amp world. i have tried many of the "tube emulation" amps but they don't impress me.

my concern is with maintenance- retubing and biasing.

is biasing necessary every time you change tubes?

i see that some amps have external ports to plug in for biasing. is this a real help?
i know tube amps can be fussy, but any help i can get would be appreciated

If you worry about bias, there are amps that set it automatically.

http://www.tubecad.com/2005/April/blog0043.htm

Cheers

David

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"Humans think they are smarter than dolphins because we build cars and buildings and start wars etc...and all that dolphins do is swim in the water, eat fish and play around. Dolphins believe that they are smarter for exactly the same reasons."

I'm really into amp work and maintenance. I rebiased my Marshall JCM800 combo lately, around a couple of months ago.

If you're replacing Tubes on an amp that doesn't have self biasing then you usually need to reset the bias after you replace the output tubes. You can happily replace preamp tubes without any worry as they are self biasing.

Also some amps make the bias adjustment vert easy, usually from an adjustment pot on the rear panel of the amp. These are really handy as you don't need to access the chassis of the amp by removing it from the cabinet.

Not so lucky with my Marshall! to get to the adjust control you have to completely remove the chassis from the cabinet which is a bit of a pain. Not too bad though.

Don't listen to the people who say biasing doesn't matter, it does big time! set too low you get a cold horrible sound, set too high you run the risk of burning your tubes out faster. If you are replacing a set of output tubes with the exact same type of tube and it's a matched set then you can usually just plug those in without adjusting the bias.

Ok now here's the disclaimer!!!! Don't remove the chassis from an amp and try to do this before you build sufficient knowledge. There are lethal voltages present inside tube amps that can kill you. Basically there are points in the amps circuitry that have the job of storing power. This means that an amp can be carrying hundreds of volts in it's circuit even though it's off and unplugged from the wall!!!

If you're going to be working in the chassis of the amplifier you have to drain all excess voltages from the amp before you go poking around in there. This is pretty easy to do when you learn it, nothing complicated.

I'll put up a link to a tech doing a bias adjustment on an JCM800, same one as mine that you might find informative. Once i dig out the URL.

http://www.eurotubes.com/eurotubes-Generic-How-To-Bias-Video-Marshall.htm

http://www.eurotubes.com/eurotubes-how-to-bias-video-probe-Pro-One.htm

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Luck won't do it and ignorance can't

George Van Eps.

Take those amps to someone who knows what they are doing! I have repaired more amps because of home repairs done improperly. You need a multimeter and know how to use it and where to check readings one ohm resister resistor method or what. And yes Capacitors will light you up!

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

CV is absolutely correct. It's wrong to buy an amp based on whether you'll be able to put your fingers inside it. Far better to buy an amp because you like the sound of it, then learn about the innards if you really have to.

You could just go for a different kind of amp circuit. Inquire of the manufacturer as to whether the amp is fixed bias or better still go with a cathode biased amp. Then you won't have to worry about biasing again unless you drastically want to alter the sound of the amp. In which case it's a repair place job. Unless you want to tackle circuitboard soldering.
Certain Mesa's have a bias point and adjuster round the back of the chassis that makes biasing a doddle.

Or do you really need enough volume to warrant anything other than a single ended amp?

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No no and no

i want to get an amp in the 40-50 watt range. i know that tube amps can be a bit more work, but for the tone i feel it is worth it.

i guess my concern is this: when it is time to replace a singl tube- or all tubes- can you just replace with the same tubes and play on, or will i have to take it in the shop.

if an amp comes with an external bias connection, is it easy yo use and accurate?

thanks for the great info so far

Only the power tubes need to be biased, and only on amps that are not fixed bias.
power tubes are replaced in pairs or quartets (depending on the amp)
Biasing is not normally difficult, but If you don't have the equipment or skill best to let an amp tech do it.

Biasing needs to be done with the power on and
the fat caps in a tube amp can kill you if you touch the wrong thing.

All the other tubes, (preamp, phase inverter, rectifier) can be swapped at will nothing to adjust.

external bias / test point are no different than any other amp, just easier to access, you still need a good multimeter and know how to calculate the setting for any given tube combination..

Modern tube amps are mostly discussed in this forum
viewforum.php?f=13

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Keep on Truckin, Going full speed ahead down the highway to hades.

You can usually replace output tubes without adjusting bias if it's the exact same tubes you're using as the originals and if the tubes are properly matched. If they're not properly matched tubes then you need to adjust the bias.

My amp has the bias control in the chassis and you use a small screwdriver to adjust the amount up or down. It's very easy to learn.

I've never adjusted bias on one of the more modern amps with the control mounted externally but from all accounts it's a lot easier because you don't have to remove the whole chassis from the amp.

Another thing to remember is always have a speaker load on the amp when you are doing this. Failure to have a speaker load on the amp while you've got it up and running will destroy the output transformer leading to mega expensive repairs.

All this advice sounds wordy and makes tube amps appear complicated, they're not really, they're less complex than solid state amps. It's just it's like everything new, it seems complex to start with.

If you get a tube amp you'll be able to play it for ages without worrying about this stuff anyway. While you're enjoying it you can read up about the amp you get and learn about it's intricacies.

Don't be put off with all this technical stuff we're talking about here. In a good tube amp very little ever go's wrong. In the 25 years or so i've owned my Marshall i've only ever had to replace tubes, that's been it. Most other quality tube amps are the same.

Usually all being well you would have to replace tubes now and then and that really should be that. They say that the filter capacitors should be replaced every 10 years or so but i know of amps still with the original caps that are a lot older than that. Filter cap replacement is a more tricky job, bearing in mind the high voltages involved and you have to solder them in so you need a bit of soldering skills. But this is very rarely needed.

My advice would be to get a tube amp, you won't regret it. They are far and away better than tranny amps or hybrids. You want top quality sound, tubes are the only way to go IMO. Then learn about your amp, read up about it and learn where everything is.

Get a book such as Dave Hunter's "Tube Amp Handbook" this book is solid gold and go's through amps part by part explaining what everything does and where everything's located. It even has a project at the end about building your own tube combo!!! They show you loads of amps opened up and trace the current through all the different parts of the amp. Well worth getting for anyone with a tube amp. Tell you all about resistors and capacitors, the power transformer, the output transformer. Everything.

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Luck won't do it and ignorance can't

George Van Eps.

Hey man,these guys have given some great advice especially Gorgon.
I would suggest that while you're thinking about this,visit us in the Vintage Amp section and tell what type of sound you're after and get some more advice...vintage amps aren't for everyone,but you might just become interested,they're all I use,but you do have to learn to care for them in some small ways if they're gig worthy,no big deal....you can leave the tech stuff to real techs and have great tone.

You do have to Bias fixed bias amps. Fixed bias just means just means the tube is biased by the Dc voltage and most of those amps have trimpots. It is different the cathode biasing where you may have to change the cathode resister but not often.

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson