"How do all these knobs work?"
As we enter the 21st century knowledge from the 50's and 60's becomes more distant and obscure. Often we don't know
exactly how things such as tube amplifiers were designed to work, causing a bit of confusion in both terms and function. Maybe this simple page will make it clearer.
Volume Controls. The volume control isn't a "gas pedal," the numbers are not a measurement of how much power is being used. That is, on a 100 watt amp, 5 is not 50 watts.
In general, Fender and Marshall amps all begin to "break up" or distort just above four. The amp is putting out its full clean power at about three and a half. Early catalogs will bear this fact out.
Why have the knob go to 10 then? Owners were expected to know about amplifiers and be aware that you couldn't turn
it all the way up and expect a clean sound. The instructions given were "if you notice distortion turn the volume down." The other reason for the wide range on the volume control was because guitars have an extremely wide range of output levels. A Les Paul has about four times the output of a Telecaster and about six times the output of a Rickenbacker. The manufacturers wanted to have their amp put out the full RMS power with any guitar.
Master Volume Controls. Again, these are not gas pedals and not calibrated (5 isn't half power.)
Master volumes were added to the amp designs in the mid to late 60's. The idea was to be able to turn the preamp (the regular volume) up causing distortion, and then turn the actual volume of the amplifier down with the master volume so the distorted tones were available at a reasonable overall volume.
But master volumes don't work exactly like that as they are still within the amplifiers preamp circuit (and aren't controlling the final sound to the speakers..) Turning a master volume down causes some degree of distortion no matter where the regular volume is set. If you want your amp to be as clean as possible, turn the master volume all the way up to 10.
For clean tone - Turn the master all the way up, turn the regular volume down.
For distorted tones - Turn the master down, turn the regular volume up.
There are hundreds of combinations of master volume and regular volume.
Tone control circuits. Fender and Marshall amps share the same basic tone circuit. This is not an active circuit and the controls are not "notch filters." That is, they are not really tone controls that select a specific frequency response and boost or cut it. The treble control is more of a "mixer" between high frequencies and low frequencies than a treble control - it has a great deal of gain available. Midrange controls are actually tone circuit gain controls that, within the circuit design, appear to boost
The amps can become muddy sounding with all the tone controls at 10. It's a good idea to evaluate the tone of the amp as you hear it instead of trying to duplicate some settings you've read in a magazine. In general, any list of "settings" is very vague and inaccurate and would only apply to one person's playing technique with one particular guitar, plugged into a specific amplifier,
with an individual speaker, in the same exact room - you can see - pretty inaccurate to your playing conditions.
Boost Circuits used on Torres Amps.
Midrange boost. It increase gain, midrange and sustain. It works best with the treble control turned up to 6 or higher and is not active at all with the treble below 4.
Gain Boost. It increase gain, bass and overdrive. It gets its effect by deactivating part of the tone control circuit. When the gain boost is on, the tone controls are not as effective.
Bright switch. The standard guitar amp bright switch works if the volume is turned down a bit. If the volume is at 10 the bright switch is out of the circuit.
General information for all tube amps.
Most of the used tube amps we are using today were built quite a long while ago, 15 to 30 years. At that time everyone knew a bit about tubes because everything used them. Now, we are pretty much in the dark about the whole thing, and the Guitar Magazine's "Tube Secrets" articles don't help much.
Tube amplifiers need to have a speaker connected all the time. This goes for any tube amp, anytime. Even if you amp has a line-out, you still need speakers connected. Otherwise it will burn up the tubes and destroy the output transformer.
Some very small tube amps have a headphone jack built in. Usually these amps have a "speaker load" resistor switched in by plugging into the jack. Read the amp instructions. Note: without very expensive parts, this isn't practical at power levels
above 6 - 8 watts.
Tubes are made of Glass. Sure, no problem. But we forget sometimes and toss the amps around in the truck or back of the car. Its amazing how much they can stand, but you should be aware - glass breaks. And once a tube is broken, it loses its vacuum and won't work anymore. Usually the tube will turn white when it loses the vacuum. Discard "white tubes", they will never be any good
Tubes Don't live forever. One of the main reasons the "mainstream" world left tubes behind is the fact that they are temperamental. A brand new tube can last 20 years or 20 minutes and paying a high price for them doesn't really eliminate this.
It is a good idea to have spare tubes on hand. Buying matched power tubes will ensure that they have been tested and are good tubes, but it doesn't provide any kind of "any and all conditions" warranty.
New Old Stock (NOS) tubes, made in America can be very good, but are subject to the same temperamental nature of any item made of glass. Almost all the tubes come from Russia, China and Eastern Europe. The old folk-type statement "I just got them so they have to be good"
is not valid no matter what brand or who you got the tubes from. The tubes being "lit up" does not absolutely mean they are good -
12AX7 tubes. Many tubes had different names in the past. The distinctions have evaporated and some tubes with several names are all the same tube now. Most common is the standard 12AX7 preamp tube. It has quite a few names. 7025 (industrial equivalent) and ECC83 (British part number.) They used to be different, now they are the same tube. 50's and 60's Fenders specify a mix of 12AX7 and 7025 tubes. You can replace them all with 12AX7 tubes.
What's a "standby switch?" Fenders, Marshalls, Traynors, Boogie's all have one. Its purpose is to put the amp on idle while you take a break. The tubes have a filament that has to be heated up in order for the tube to work. Heating the filament is why the amp has to "warm up."
The standby switch leaves the filament heated up, and turns off the power and preamp stages.
You can flip the standby switch off after a break and play right away. This extends tube life, as the filament doesn't have to be reheated every 45 minutes.
For a short break in the bar or club, put it on standby. But, if you and the other guys in the band are going away - turn all the amps completely off. A tube can go bad at any time. If the amp is on standby when the tube shorts out, you may have some damage. Only use the Standby switch when you are still around the amp.
When you first turn the amp on, turn on the on-off switch, but leave it on standby for about 2 minutes. This allows the filaments to heat up and be working right before you start playing.
Technology upgrades. Older amps. When we work on older amps there are some parts that are becoming increasingly defective. The amplifier's power supply has "filter capacitors." If the amp is over 22 years old and hasn't been serviced we will, most likely, have to replace the filter capacitors in the amp.
'What's bias?" Well, I won't go into all the voltage potential, reverse grid bias etc. mumbo jumbo. But be aware that the amp has an adjustment that has to be made with different tubes, and as tubes wear out. Anytime you change the power tubes the bias should be adjusted or at least looked at. Again, no matter what brand etc.
The Bias is a negative voltage fed to the tubes. It makes them work right, and sound right. If it is off - underbias (too little voltage) the tubes will glow red hot, the amp will hum and sound lousy. If there is too much bias (overbias,) you will be low on power, and tone will suffer. We have "bias instructions ($5.00") to help you set the bias yourself. It's not terribly hard, learn
to do it and keep your amp working perfectly for years..
old amplifiers very rarely have had the bias "filter capacitor" replaced. If it is bad it sends a bad hum into the amp. If we find it bad we will upgrade and replace it if necessary.
Ground Switches. Ground switches no longer meet federal safety standards. We will begin omitting them from some custom amplifiers on 4/1/98
Copyright Dan Torres 1999
All rights reserved.
For one-time only non-commercial use.
May not be reproduced in any media existing now or to exist in the future.
Revised November 2, 1999
final notes on web page
Legendary Book "Inside Tube Amps" is now
available on Amazon/Kindle
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in business 42 years