Just wondering how many of you guys replace your own tubes and Bias them yourselves? How often?
I've been thinking about it for a while, but don't want to die...yet.
I'm a fairly technical person and think i can grasp the concepts but did'nt know if it is a common practice amongst the guitar playing community...
Well replacing tubes isn't anything special, properly biasing is an art though. I'd have it done, personally. Too easy to die, and too tedious.
I do my own tube replacements with no problem.
But then again, I have a self/auto-biasing amp. Messing with a fixed-bias amp is another matter entirely. If you don't have a good oscilloscope, a VARIAC, a Bias-probe, and a thorough understanding of DC electrical principles, I suggest paying an expert to do it.
Hey, if you're serious about biasing your own amp, check out this thread. That's my buddy Joe, and he knows his stuff. He explains it well, clearly, and with pictures.
If your amp didn't have an external bias, you could get one installed, then get a bias probe, like a Bias Rite, or Bias King...these plug inbetween the amp and power tubes. Then it's just a matter of turning on the bias meter, being able to crunch a few numbers, and turn a screwdriver to get the right bias...all in all pretty safe.
I worked this biasing thing out over the last year. Started the process with much trepidation, which is a good idea, and then checked out a few places,books etc.
If your amp has bias pots for the valves the process is straight forward if done with due diligence and caution after appraising yourself of the risks in working inside the amp. If no bias pots fitted, get a tech to install some.
The only equipment required for basic biasing is a multimeter.
It is not impossible voodoo despite what many will caution and a competant handiman will sort it out.
The main caution is not to touch anything but the bias pots inside the amp, on or off, and to only touch the bias pot with an insulated screwdriver.
Is another good site for information. But google your amp model and "valve bias" and you are likely to get some hits that provide the information.
I don't claim to know much as Hobie....I've only been at it for around fifty years, but my advice is: If you know a really, really good tube guy, have him do it. If you don't know a really, really good tube guy, solder a 1 ohm resister from each cathode to ground and read the voltage drop as current. The thing about biasing is that the amount of crossover notch distortion, or lack of it, has a real affect on the tone of the amp. If you do it on a scope, you can dial in the tone. For instance, Surf music freaks usually like a little notch because that was the sound of the Blonde Showman amps that had no bias pots. Most contemporary players like to get rid of the notch because it sounds warmer, especially when the amp breaks up. You can also trim the plate resistors on the phase inverter to get dead-on symetrical clippng, which will enhance the musical harmonics, while cancelling the non-musical harmonics when the amp goes into distortion. Of course, you don't want to bias the amp too hot; you just want to tune out the notch. Unfortunately, most repair guys don't understand or appreciate the subtleties. Reading the current with a 1 ohm resistor and a voltmeter will get you safely in the ballpark with most tubes. BZ
Yes as BZ demonstrates there is a lot more to it if you are wanting to do the biasing to assist in setting tone.
Look around and you will find information on your amp as to what is the recommended reading etc.
And Billy, thanks for the contribution!
Here's a handy bias calculator.
FYI, some lower powered amps have a cathode bias, basically a 5 or 10 watt resistor between the tube and ground. Champs, tweed Fender Deluxes, Valcos, Ampegs and suchlike use them. You may not have to rebias the amp if it's this type of circuit.
If it's cathode biased, it's a good idea to check the bias resistor, especially larger amps like AC 30's or Tweed Pro's. If the resistor is within 10% or so of the specified value, you're good. It's also a good idea to check the screen grid resistors when you change tubes. I like to keep those within 5%. Resistors usually increase in value with heat and age. These resistors get really hot if a power tube goes. BZ
It's just like adjusting a carburetor. Turn it up until the tubes glow bright orange, then back it off a 1/2 turn.
I bias my own amp and do my own work/builds. As Billy says the 1 ohm cathode resistor method is the safest and most accurate. The most important thing is safety because the filter caps in your amp hold high voltage even with the amp turned off and unplugged. Touch certain parts of the circuit and you can get a deadly shock. To set the bias you have to measure it with the amp turned on so the danger is everywhere.
Agree its probably best to let someone like Billy or a local tech do it unless you have the time and patience to learn all the safety, how to use a multimeter, soldering iron, ect. There are classes but they are expensive.
Learning on your own can also be expensive. My first project was blackfacing a silverface twin reverb. To get to the chassis I had to unplug the speaker wires. When I was done, I turned the amp on the first time with a current limiter (don't have a variac) and everything was great. Later when I went to bias the amp I forgot to plug in the speaker... only took a few minutes to short out the OT. Was a $235 lesson (I call it tuition).
A competent tech can install an adjustable bias pot and inserts for leads on the outside chassis so you can insert your multi meter leads and adjust the bias. Problem is it requires drilling holes in your chassis (not a fan). Some sites sell bias tools (bias rite ect) that you can use an avoid the multi meter use but they are expensive and you still have to adjust the bias pot or change the bias resistor (same high voltage issue).
Consider Billy's advice and find a good tech that understands how to set bias to get good sound. If they don't ask you about how much headroom you need and how loud you play that's a good clue that they are going to just set the bias "by the book" and usually that's too cold.
Shetland, If you'd like to learn about working on tube amps, Lee Jackson has a DVD about it. The DVD is available for rental at a cost of $9.99 per week.
It should give you a good idea of what's involved in biasing and whether it is something you want to try.
... plate resistors, phase invertors - WTF is this - Star Trek? All I know how to do is plug the thing in and stick a lead in... even then I still run the risk of killing myself. I think I'll leave it to my tech thanks very much.
If you really want to learn about amps, go to your local community college and enroll in their electronics program. AFTER you've completed their courses, you can start reading or watching DVD's about amp repair, and it'll mean something to you. It's easy to teach someone half a dozen basic rules. What takes time is learning the two hundred exceptions to those rules. BZ
I guess I'm lucky. My Fender '57 Tweed TwinAMp has a "bias pot" for the power tubes.
I use a Weber "Bias Rite", look at the reading, and adjust.
BZ can probably explain the "in's and outs" better, but this works pretty well. I switch between 6l6 and 5881 power tubes, and the Bias Rite gets me pretty close.
You'll still need a "matched pair" of tubes, but this is another complete thread.
Anything deeper than this, I take to my "tech", who's as old as BZ and myself together.
This guy loves to work on PTP amps, because he hardly sees any of them anymore
When do you have to bias your amp? I was under the impression that you needed to bias your amp when you put in a different type of tube. Like a 6V6 instead of a 6L6.
I can put in a different brand of 6V6 and not worry right?
The amp shoud be biased whenever you replace output tubes, even with the same brand and type. You shouldn't be changing tube types at all unless you really know what you're doing. Different tube types are not meant to be interchangeable. Sometimes you can modify an amp to accept a different tube, but you can't just swap them. 6L6 to 6V6 is not a good swap. Sure, it can be done, but there are too many problems that need to be addressed. 6L6 and 5881 are the same thing.
I have enough trouble working on guitars...i'm not going near the guts of an amplifier. i take my stuff to Torres up in San Mateo, who even were able to bias my tiny 2-tube Electar. i totally changed the tubes, with a Mullard 12AT7 in the preamp to get less clipping in the preamp stage and a "small-bottle" TungSol 5881 for the power tube.
Nothing wrong at all with taking your amp to a competent tech and get it biased properly. The problem is tube circuits are "old" technology and not many techs are not properly trained. I've fallen victim to paying for a tech to bias my amp only to learn later that the bias resistors weren't even touched and left out of spec for those tubes.
For instance, Surf music freaks usually like a little notch because that was the sound of the Blonde Showman amps that had no bias pots. Most contemporary players like to get rid of the notch because it sounds warmer, especially when the amp breaks up.
If your tech doesn't know what that bit above means then they aren't using an oscilloscope to set your bias. Frankly, for the price of a bench fee and bias job, I bought my own quality Fluke meter, learn amp safety, measure voltage/current, and set the bias myself within a safe range each time I change tubes (which is rarely). That's what many tech's and online tech gurus seem to do/teach today anyway.
What I can't do is use an O-scope to dial in the tone so I would definately pay Billy or another tech for that if I could find one.
