Mod Entry: To everyone posting in this thread, please post only tips on using a tube amp in here. Any questions involving amps should be asked their own new thread.
Here are some tips that I have been getting from Harmony Central's Guitar Tip of the Day regarding tube amps that might answer some questions for you. There may be more to come:
Here's a tip regarding the 'care and feeding' of your tube amplifier. Tube amps are generally going to be just as reliable as their solid-state counterparts if you just observe a few things about them. Take a little extra care in transporting them. While you can generally bang around and lightly toss solid-state amps into trunks, back seats of cars, truck beds, etc., with tube amps you have to be a little more careful. Set aside a special place in the car so that the amp isn't going to be jostled around with hard shocks, e.g. abrupt movements with the car or potholes. Generally this means the back seat or the trunk with some towels or clothing around the amp. When you get to the gig, just make sure that you take special care not to bump the amp into other things, or to just drop the amp on the floor, if you're in the habit of doing that.
Make sure that there is proper ventilation for the tubes. Tube amps run hot (especially class A amps) and they need proper ventilation. Generally this just means that you shouldn't put your polish cloth or set lists over the ventilation holes, and you shouldn't set the amp right against a wall so that there is no air to get to the vents. Some players even keep a fan on the amp, directed so the airflow cools the tubes.
Change the power tubes regularly. If what you do is gig and rehearse at club volumes regularly, then changing the power tubes after six months to a year is fine. However, if all you do is play the amp at home at bedroom levels, the tubes can last two years or more.
If your amp is bias-adjustable, then you need to make sure that the bias is set when you change power tubes. If your amp is fixed-bias, it's a good idea to stick with the same type of power tubes that came with your amp. This way you can't go wrong.
Make sure that a speaker is always plugged into the amp when it's powered up. Tube amps need to see speakers plugged in at all times, so you cannot run a tube head or tube combo without the speaker plugged in. This is a big mistake and can cost you a lot of money if your amp blows a transformer. Even after you replace the transformer, the amp often sounds different, so don't blow it.
Make sure that the speakers are plugged in at the proper impedance, or at the very least, a mismatch in the 'safe' direction. The best case is always to match the impedance of the speaker or cabinet with the amp, i.e. if your speaker cabinet is 8 ohms, your amp should be set for 8 ohms. If you plug this same 8-ohm cabinet into the 4-ohm setting on your amp, that will also be safe, but you lose about half of your power as a result of the mismatch. If you take this same 8-ohm cabinet and opt for the 16-ohm setting on your amp, you're going to likely blow the head up as it tries to produce twice as much power.
Always use the standby switch when powering up and powering down your tube amp. The standby switch allows the amp to sort of "warm up" and allows the tubes to settle into a sort of equilibrium state before they get slammed with high voltage. If you allow the amp to warm up or down in standby for about 30 to 60 seconds when powering up or down, you'll find that the tubes last longer.
Here's a tip regarding things to look for when contemplating the purchase of a used tube amplifier. If the amp is a combo, take a look at the speaker(s). Give the cone a gentle poke with your finger to ensure that it's firm and not beginning to deteriorate. The same will hold true for closed-back cabinets, although on some of these it's a little harder to get at the speakers.
Here's another tip regarding the evaluation of a used tube amp. Turn the amp on and look to make sure there's no orange plate glow on the power tubes. A blue glow is fine. Be careful not to confuse the plate glow with the normal orange glow of the tube's heater filament. Orange plate glow indicates that the tubes are dangerously under-biased. This isn't necessarily a failure, but would have to be addressed right away, as it might burn a tube or transformer quickly.
Another thing you should take a look at when evaluating a tube amp for possible purchase is the condition of the electrolytic capacitors. Check the electrolytic capacitors for "bubbles" or residue on the ends of the cans. They'll need to be replaced, if so. It's not unusual for this to happen as electrolytic capacitors "die" after years of use and must be replaced.
Here's today's tip:
When looking at used tube amps, always ask if the amp's been played regularly, or stored away. In a weird sort of irony, amps that are played regularly often sound better than those that have been stored do do. The reason for this is the electrolytic caps need to be run regularly to prevent them from drying out.
Originally Posted by little_squishy_
1 i dont know
2 yeh he was talking about cabs
3 i dont think you can change them ~david~
Here's some info on biasing and ohms:
Active Device Biasing
In linear electronics the term 'Bias' is short for "set of bias conditions', it normally refers to a state described by the terminal voltages around AND current flow through an active device when zero-signal is applied at its input terminals and when it is placed in it's so-called linear mode of operation (... which is actually a highly non-linear realm in relation to dynamic amplitudes) ... there is some confusion about this for tube amp users because of the historical appearance of bias voltage figures in schematics - these we're in relation to the tubes of the day but now, especially with the advent of Russian hybrid tubes, these numbers should be taken as "loose" reference - not that they are invalid per se ...
But fact is, in signal amplifying devices the mathematical quantities from which transfer specs like signal Gain and Load/Drive Impedances are directly derived from the current flowing through the device acting as main variable ... engineers who study the behavior of gain stages under different bias conditions typically play with equations that have output current as the primary equation variable for all small-signal parameters used in first order amplification transfer calculations ... when biasing tubes in guitar amps amp techs typically shoot for this theoretically supported Idling Current - their numbers are learnt emperically, through measurement and listening to a broad range of tube amps over time ...
From a AC signal point of view the job of a vacuum tube is the same as a transistor in the common-cathode/emitter/source circuits ... FET, Bipolar or otherwise their job in the common-CMS circuits is to convert input AC voltage signal to output AC current signal, where the rate of voltage-to-current conversion is known as the small-signal Transconductance - typically abbreviated 'gm' in academic engineering ... the AC current that flows through any vaccum tube can then be used to drop an AC voltage across a resistor (in order to develope an AC voltage version of the output signal) or it can be used to set the AC current in a coil, as in an output stage (single-ended or push-pull) or a reverb coil driver circuit ... either way, the rate of transconductance increases with DC (static) bias current and consequently their associated voltage gains do so as well ... the driving point impedances at the device terminals also change in response to bias variation as well, always with idling current playing the role of dominant reference parameter ...
Biasing Tube Amps
The 'Biasing' of tube amps commonly refers to setting the idling (or small-signal average) current in the Power Tubes of Push-Pull output stages - no-signal Static (=DC) Current Levels are the target spec and the surrounding voltage conditions are adjusted accordingly if possible ... here lies a BIG CRUX thing: the multi-variable voltage-to-current bias relationship that exists for a power tube under a given bias set will respond differently (in terms of output current) from tube to tube AND over time of use also ... some amplifiers have mechanisms for automatically setting the bias of these push-pull circuit tubes - they typically employ some sort of servo-circuit to set the current levels ... this servo circuit can be passive as in the case of Cathode-biased circuits (these tubes self-bias the same way the smaller triode tubes of the preamp and phase-driver circuits bias themselves) or by electronic means as in the case of some modern tube amps like the new Yorkville Traynor amps (in order for this later arrangement to work perfectly there has to be absolutely zero amount of signal feedthrough that makes its way through the servo circuit, and this over the whole audio range, or else transient response can be compromised) ... in fixed or variable bias push-pull amplifiers a separate power supply circuits is employed to generate a negative voltage with respect to chassis ground - this voltage is fed to the grid terminal of the power tubes through bias feed resistors (typically 100k to 220k ohms) ... this is the negative Bias voltage that is typical written in amplifier schematics, in variable bias amplifiers this voltage can be "swept" through a potentiometer ...
As the bias voltage in the grid is brought more negative with respect to the chassis ground (say, from -42volts to -52volts) then you'll typically find that the gain of the output stage will drop along with the drop in idling currents ... if the bias voltage is made less negative (opposite) then you'll find the gain of the output stage going the other way along with the increase in idling currents ... Note: some players mistakingly see the bias control as merely another volume control - the Red-Knob Fender Twins are notorious for this because of their availability of external control ...
The Bias Limits
There are two limit sets that need to be observed when biasing power tubes ... the lower Biasing limit, when the amp is underbiased, produces a cross-over notch when observing AC waveforms on an oscilloscope ... shown here is a waveform of an Under-biased set of mismatched tubes in a Fender Super-Reverb amplifier ... notice the diverging turn-on/turn-off levels between the two half-cycles ... when a push-pull output stage is underbiased (bias voltage is too negative) it often sounds fuzzy and weak ...
Bottom line is - take it to someone who knows what they're doing. Anyone who's been here awhile knows I recommend Traynor amps and notice this information mentions that Traynors are self-biasing.
Ohms are a measure of resistance as explained below: http://www.rexc.com/services/ohms.htm
Here's another one. It is also a good argument for an attenuator (i.e., Major Tom's).
Try to play tube amps near their saturation point -- this brings out the power tube sound and engages the guitar speakers. To play quieter, use a lower-power tube amp such as 5 watts. Strive to bring out the physical, real, tangible tone of saturating power tubes and hard-driven speakers. This includes using feedback, room noise, room reverberation, and hum and buzz. Clinically sterile tones such as a guitar effects processor and "speaker simulator" (treble-cut) recorded straight into the mixer sound cheap and too convenient and easily reproducible.
As far as the electrolytic capacitors, they should be change after about 12 years. They will last about 15 years at the most. This is something that a professional should do as these caps are the ones that store the high voltage (enough to kill you). Its worth the price to have an amp tech do this.
Here's the tip I got today:
If you're experiencing problems with your tube amp, remember that not all things are equally likely to fail. Experience with tube guitar amps has shown that failures are most often in the following order: 1. Operator error (a control or something is set or switched wrong); 2. Tubes (the most likely thing to have gone bad on a once-working amp); 3. Power supply components (they handle lots of power and get hot); 4. Resistors and capacitors (especially electrolytic capacitors - see post above by Dickson amps); 5. Tube sockets; 6. Switches; 7. Cables, cords and jacks; and 8. Internal wiring. I would also add that blown fuses are usually a symptom of a problem and are not likely a cause.
Accordingly, suspect problems in this order. First make sure you are operating the amp correctly, master volume is turned up, cords plugged in, etc. Then, suspect that a tube has failed, and so on.
Here's another one.
Believe it or not, but the amplifiers we have been talking about up until now are only a small percentage of the whole story. The guitar you play, the string gauge, even the pick you use all make their own subtle differences that can add up. Most of these differences 'on their own' however, won't be very noticeable when you play a multi-amplifier setup at concert volumes. Yet these are still differences that can explain why you are having trouble duplicating 'on the nose' that vintage vibe from a boutique tweed covered amplifier. It may not be the amplifier's fault.
This applies to all amps really, to eliminate some or in a few cases all the hum or static in an amp use a 3-prong to 2-prong adapter to plug in to your power source with.
I dont think this was mentioned, but if it was, I apologize.
I thought I would add this link into this thread because I have found it useful many times when asked simple and advanced questions about tubes. http://studentweb.eku.edu/justin_holton/tubes.html