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Old 08-30-2004, 11:05 AM   #1
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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.

Quote:
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:

Biasing:

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.

From: surferdude9375
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


Last edited by thesteve; 09-24-2007 at 09:31 PM.
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Old 07-16-2008, 12:58 PM   #2
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I didn't see this, but after playing wait at least ten minutes before moving the thing around.

And this can't be said enough:

If you don't know what you're doing, take it to someone - messing around inside a tube amp will get you killed.
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Old 03-22-2009, 11:23 AM   #3
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Cold weather tip

I didn't see this tip in the article but for those of us in the Northern climates (I happen to be in Alaska), when bringing your amp in (from the car or whatever) let it warm up to room temperature before powering up. If the amp has been in freezing temperatures (in your back seat for example), it doesn't even like to put in Standby mode until it's thawed out. Powering up a frozen amp will shorten the lifespan of your tubes. I'll generally uncover an amp (blankets, covers, etc. are insulation that will keep the amp cold) and give it 20 minutes or so to warm up at room temperature. Then, put it in Standby mode.

And +1 for the post above. Tube amplification is OLD technology. Tube amplification is dangerous! Don't play around back where the juice is flowing--you can be KILLED!

Last edited by totallyfrozen; 03-22-2009 at 11:27 AM. Reason: no good reason
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Old 07-23-2009, 05:27 AM   #4
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more on fans... and more

(1) The idea of adding a small fan is a good one, but it's not to cool the tubes (if they were cool, they wouldn't be working). The fan is there to help the heat from the tubes away from all the other works. Many combos have the tubes mounted underneath an enclosed metal box containing all the other components and it's the stuff inside the box that's baking unnecessarily.

(2) Most tubes have more than one equivalent reference number. When looking for replacement tubes, you can sometimes find a bargain by doing an internet search for one of its less-used names (e.g. look for CV511 instead of the widely-known 6V6GT).

(3) The most common tubes in use are in current production - 12AX7, 12AT7, 6V6 types , 6L6 types, EL34, EL84. The world will not run out of these types in the foreseeable future and the vast majority of current-production tube amps use only these types. If you're buying an old amp which uses other types, take a moment to check you can find replacement tubes.

Also, if you're playing a solid-state amp and considering replacing it with a tube amp, it's worth remembering that (in sweeping generalities) a tube amp will sound louder than a solid-state amp of the same output power. If you're replacing a 30W solid-state combo but you're happy with its max volume, chances are you won't need a 30W tube combo. Try 15W for starters. The reason for this is; when maxed, solid-state amps tend to distort within their power stages in a way that doesn't sound nice. But if you push a tube amp to the same % of distortion, it probably still sounds reasonably clean, or at least distorted in in a pleasant, desirable fashion. So there's more useful power in (say) a 30W tube combo than a 30W transistorised combo. As I said, these are generalities, but at least consider them a warning which may stop you over-spending.

Yet more also, when buying an amp, IMO not every amp described as 'tube' really deserves the word. There are amps where

(a) the entire signal path and the rectification is tube
(b) as above but the rectification is solid-state
(c) transistorised preamp with tube power stage (eg late 80's Fender Champ 25)
(d) modelling preamp and tube power stage (Fender Super Champ XD)
(e) all-transistor except for one preamp tube (Marshall Valvestate and others)

... and other combinations I haven't thought of.

Any of these may sound good to you, and I am not saying you must buy one kind or avoid another. However be warned that type (e) above may not sound any better to you than an all-transistor amp no matter what the marketing blurb says. In a hi-fi environment, blind tests have shown that people prefer tube output stages but don't much care whether the preamp is tube or solid-state. But that shouldn't stop you using your ears when deciding between, say, an all-transistor amp and type (c) above. I'm just saying, IMO a 'real' tube amp has tubes in the power stage. Common power tubes include 6V6, 6L6, EL84, EL34, 5881. Peeking into the back of an amp and finding a lone 12AX7 will not automatically make it sound better, even though it allows the marketing guys to call it a "tube amp".

Last edited by Stratopastor; 07-21-2011 at 05:29 AM. Reason: thought of something else
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Old 07-02-2010, 03:43 PM   #5
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I've seen this asked a couple of times recently in *th*r f*r*ms so I thought I'd add the best answer I've seen.

The question is "how do I know if my power tubes/valves are losing their edge and getting close to replacement time?"

The best answer I've seen is, get yourself a set of spares and fit them, rebiasing as necessary. If the amp sounds like it's come to life, stop there, job done. If no real improvement, the 'old' tubes were fine. Reverse the process and keep the new ones as spares. Try again in 6 months, sooner for a hard-gigging lifestyle.

I take issue a little with Hopeful's suggestion that sticking to the same brand will stop you going wrong in some way... there could be variations in behaviour between different batches of the same brand. He's right to recommend checking bias every time, and I'd like to add 'even if they're the same brand as before'.

I made myself a simple bias probe and every time I acquire another pair of power tubes (probably because I got them part-used on eBay) I'm able to check their bias without dismantling the amp. If they're within a range of bias that's acceptable to me without adjusting the amp then I keep them, making a note of the idling wattage on the box. If not, I sell them on. That way I have built up a small collection of spares which I know I can plug straight in without further checking. I haven't had a power tube fail in front of an audience/congregation but if that happens, I know the 'travelling reserves' will plug straight in.
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Old 04-28-2011, 08:50 PM   #6
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Turn it up to 11!
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Old 07-14-2011, 05:29 AM   #7
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.... and get yourself some spare tubes while the amp's working; don't wait till it fails in front of people. You're going to spend the money one day, so why not now? You want at least one of each type of tube in your amp. If the power section is a pair of tubes or a quad, ideally you need a spare matched pair or a spare matched foursome.

In a real emergency, if a preamp tube fails, on some amps you can take out a preamp tube from another section of the amp and carry on playing minus reverb or minus a channel, thus freeing up a spare for the channel-you-can't-do-without. But that's only going to be necessary if you didn't have a spare (even a cheap brand or an old tired one... come on....)
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Old 07-19-2011, 01:24 AM   #8
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How about posting tips on different ways to use tube amps and explaining what is happening inside to make it react certain ways?
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Old 07-19-2011, 10:46 AM   #9
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OK, here are a few...

- for amps with a front-end gain and a master volume, some folks set the front-end gain knob to taste and then adjust the actual volume with the master. But the amp may well behave differently (and nicer/nastier to your ears) if you whack the master up to 10 (OK, picknpluck, 11 then) and adjust for desired volume using the front-end gain knob.

- You can get serious changes in tone from different brands of valves/tubes. That also goes for the character of any distortion the amp may create. It's like a plug-and-play, no-dismantling method of modding the amp. Example - old Brimar 6V6s give more bass than any other brand I know.

- You can change an amp's character by using different tube types SO LONG AS you know the substitiution is valid. The most common one is to reduce an amp's gain, maybe cleaning it up, by taking out a 12AX7 (usually the one nearest in the input) and substituting a 12AY7 (40% gain compared to 12AX7) or 12AU7 (20%).

- If the amp's tube rectified, you can sometimes change its behaviour by using a different type of rectifier tube. The most common thing here is to go for a lower-rated rectifier tube which makes the amp's internal power supply less capable of delivering current. The result is more springy compression as you approach max volume - the power supply can't deliver the current so the internal voltage begins to fall (this is called 'sag') and the volume limits itself. Under the right circumstances the tone and behaviour of the amp become touch-sensitive in a controllable, responsive way.

Is that the kind of thing you meant, Plexi?
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Last edited by Stratopastor; 07-24-2011 at 08:55 AM. Reason: added the rectifier tip
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Old 07-19-2011, 07:08 PM   #10
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Very nice Stratopastor!!!! I was especially intrigued by the bit about trying different preamp tubes. I've mostly considered the power amp tubes.

And YES!!!! Crank It To Eleven!!!! And this is why ... Amps that have gain knobs or pre vol or master vol knobs give you the ability to saturate the preamp tubes (smaller tubes). The tendency however is to get most of your overdrive from the preamp and to turn your power amp down to get lower output vol levels. It's nice if your in an apartment but preamp saturation alone is very buzzy sounding and not very rich harmonically probably because of the size of the tubes. By saturating the power amp you get much more depth in odd order harmonic overtones as well as a more responsive signal flow.

Why saturate the pre amp at all then? One word ... Well two ... Natural compression. When tubes saturate they also compress the signal a bit adding some sustain. This is especially true with the smaller preamp tubes and when you couple that with the harmonic complexity of a saturated power amp you get at the heart of some of the classic lead tones of the 80's to the present with eq being the major difference between them.

I don't necessarily crank my power amp to the max. Too much saturation makes the tone squashy and sluggish ... Not enough and the tone is thin and sterile ... This is the preverbial "sweet spot" and every amp is different even within like modles or sometimes even when you change tubes not to mention personal taste.

Last edited by Plexi8759; 07-20-2011 at 06:04 AM. Reason: My mixup (incorrect info)
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Old 08-16-2011, 05:37 AM   #11
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more on biasing

In an idle moment I thought I'd expand a little on Hopeful's original post concerning power tube biasing.

There are a few amps which don't need bias-checking when you change the power tubes. They are mostly low-powered, mostly with only a single power tube. Specifically they employ a bias method on the power tube called cathode-bias or self-bias. If your amp's spec or maker's blurb mentions this, then you can change the power tubes without checking. If you don't understand schematics but you've got the schematic for your amp, show it someone who DOES understand them and he/she will know at a glance.

At the rick of scaring folks by quoting techy stuff, here are some examples

http://www.vhtamp.com/pdf/VHT_Specia...ic_5-17-10.pdf

http://mercurymagnetics.com/images/p...VJ-schem1a.pdf

http://www.kilback.net/homebrewtweak..._5e3_schem.gif

see the power tube(s) on each? 2 use 6V6s, one uses EL84, doesn't make a difference for this discussion. Each one has a resistor with a wattage specified (1 or 2W) with a big fat capacitor, say 22uF, going to ground. That's cathode bias. The third schematic is actually for a medium-powered amp with 2 cathode-biased power tubes - they share that same setup, the resistor this time being specified as 5 Watts. In all 3 amps the resistor value in ohms sets the bias current; no adjustment necessary; it needs to be a larger-wattage resistor than elsewhere in the amp because it has to dump heat to the atmosphere all the time, even when the amp is idling.

If in doubt, get advice, and don't stick your fingers inside.
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Last edited by Stratopastor; 08-16-2011 at 05:39 AM. Reason: cahtode
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Old 12-11-2013, 11:27 AM   #12
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just by way of keeping this thread awake....

With currently-produced tubes, beware of criticising one brand and swearing undying love for another until you know who really made them. For lo, many are the brands thereof, but of factories there are only four. Two in Russia, one in Slovakia, one in China. It would be rather embarassing to praise one brand, damn another, and then find the only difference is the print job.

However there are some resellers who do extra testing so the customer has a lower chance of getting early failures, high microphonics, or whatever.
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