Tubes & Biasing

Category: Main -> Amplifiers



Q: My amp has started to howl or feedback by itself, what's wrong?

Certain brands of pre-amp tubes, especially the Chinese ones used by Boogie, wear out quickly and become very microphonic within 2 - 3 years. The elements inside the tube suffer from metal fatigue due to the changes of temperature that they undergo. When they become slack enough they start to vibrate and cause the howling sound you are hearing. Sovtek models are less prone to this as well as higher quality brands.

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Q: Should I turn my amp off between sets?

No, always leave the amp on standby. This will prolong the life expectancy of your power tubes. It will also prevent premature fatigue of your pre-amp tubes. Only turn it off when you are finished for the night. If there is no standby switch to your amp it is probably better to turn it off between sets if it gets very hot and the air circulation is bad.

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Q: I left my amp on (not on standby) overnight. What kind of damage this will do to my amp? (Morgan H)

Leaving your amp on continously is not a problem if there is adequate ventilation. It is more damaging to turn the amp on and off frequently than it is to leave it on. Power tubes are like light bulbs and their lifetime is rated in hours. This rating decreases the more they are switched on and off. Radio stations still use very high power tubes to transmit their signal and they leave them on 24 hours a day.

It's a question of metal fatigue caused by constantly changing the temperature of the elements in the tube. As they expand and contract the metal weakens causing the tube to become microphonic and stressing the welding points that join the elements together. Eventually the element can break off or cause a short. When the amp is on standby there is no plate current but the heater filament is still on in order to keep the tubes warm. This causes less of a strain when you turn the amp on and off between sets.

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Q:What is the correct or safest way to hook up two amps together to be used at the same time? (Don B)

Well, there are several things to consider with multiple amp hookups.

Signal Attenuation - In most Fender amps there are two input jacks for each channel usually labeled I and 2.These two jacks are hooked up together in a very clever fashion. Input 1 is the one that we normally use. When used alone, that means to say with nothing actually plugged into input 2, Input 1 connects directly to your amp and has an input impedance of about 1Meg ohms. Input 2 when used alone is an attenuated or "padded" input. Any signal applied to input 2 will be reduced by 50% before it connects to your amp. The input impedance is much lower, about 130K ohms.

When both inputs are used at the same time the situation is completely different. Now both inputs are mixed together equally through resistors at the input. Now it is possible to plug into input 1 and use input 2 as a send to connect to another amp or channel. This is the simplest way to connect amps together and has been done at some point by pretty much everybody and works fine, there's nothing wrong with it. The problem is that since the signal to the second amp is coming form an goes through these mixing resistors it will be weaker and duller sounding. This hookup is generally more effective when using an old Marshall Super Lead and slaving the channels together. For Fender amps it's better to use a splitter box instead

Phase - In Non-Reverb Blackface amps both channels are in phase and can be hooked up together. In Reverb models the Vibrato (Reverb) channel is 180 degrees out of phase. Connecting the Vibrato channel together with the Normal channel will cause them to cancel each other out. Hooking up the Vibrato channel of one amp to the normal channel of another will cause all kinds of weird phase cancellations and is not a good idea. You could always reverse the speaker phase of the second amp. A phase reversing speaker cable is good for Marshalls.

Capacitance - All the added cables and loading by multiple amps take their toll on the high frequencies. Passive A/B splitters can load your signal as well. It's best to use a buffer pedal first to preserve your tone

So, to sum it up, if you want the same signal sent to each amp use a buffer pedal first, then go to a splitter or A/B box then plug into each amp. If you like the sound of the second input feeding the other amp then that's Ok too .... whatever sounds best.

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Q: How can I eliminate hum from my Marshall amp. It is very noisy when I record

First make sure that only the Marshall amp is grounded in the AC wall socket. Any other FX or processing units should be ground lifted. If you are using an FX loop in the Marshall cut the ground wire from one of the two cables that are used to connect the loop and insert a 10 ohm resistor in series.

If you are connecting the amp directly to your mixer instead of micing it try using an audio isolation transformer between the amp and the board.

To reduce radiated buzzing noise from your computer monitor turn it off when you start to record the track or else use an LCD monitor

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Q: Is it worth converting my Silverface AB868 Fender Deluxe Reverb to Blackface AB763 specs to improve tone. (Max.K)

The most significant difference between these two designs is that Blackface amps used a much smaller input cap to the driver stage, a .001uf instead of .01uf, thereby rolling off some of the bottom end to the power amp. Its a matter of personal taste, really, as to which is better. I would certainly remove the caps across the grids of the output tubes, however.

All of these comments apply only to the specific models of Deluxe Reverb that you have referred to and not to all Silverface and Blackface Amps .

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Q: The 6.3v heater filament wires on one of the output tubes of my Fender Amp are reversed in comparison to the other output tube. This means that the output tubes are supposedly not hum-cancelling. Is it advisable to rewire the filaments to be in phase? (Max K.)

The output power tubes are generally not sensitive to hum from the filaments. Remove the 12AT7 driver tube and turn on the amp. If you don't hear any hum then don't bother to rewire the socket.

If you decide to do it anyway, be sure to take great care when soldering the connections. Heater filament wires conduct several amps of current and must be very well secured. It takes a fair amount of skill to solder properly.

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Q: What is Class A operation? (Jerry H.)

When a component operates in ClassA (either a transistor, preamp tube or power tube) the current through it follows the input signal at all times. Only one amplifier section is necessary to amplify the signal and it does so in a very linear fashion.

The other amplifier classes (ClassAB and ClassB) are all means of increasing the power efficiency of an amplifier by splitting the signal into two halves (positive and negative), sending each half to a separate ampliifer and then recombining the two signals once more. This technique often cause CrossoverDistortion but significantly increases the output power.

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Q: The reverb springs in my Fender combo are very sensitive to vibration, can I do anything about it? (Frohm S.)

Flip the bag that houses the reverb springs over so that the springs are upside down. Make sure that the jacks inside the tolex bag are facing the rear of the amp (away from the speakers) otherwise you will get hum from the power transformer. The open bottom of the reverb pan that is covered with a piece of cardboard will now be facing up toward the top of the amp.

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Q: My Marshall amplifier has recently developed a loud buzzing sound resembling an electric razor that seems to be emanating from the transformer. Is this dangerous? (Lee F.)

Don't worry about the buzzing, it's just a vibration noise caused by the power transformer. Try tightening the transformer mounting bolts.

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Q: I have a Marshall JCM 900 dual reverb 100 watt head. what would happen if the amp is run using only the recording compensated output and no speakers or attenuator? (John G.)

A tube amp must have a load connected to the speaker output whenever there is a signal applied to the power stage. The load can be a loud-speaker, a resistive / inductive power attenuator, or a combination of both. It is only possible to operate a guitar amp without a load when you are using the preamp stage alone and no signal is being fed to the power amp.

Using a tube amp without a load is SUICIDAL to the amp. You can arc the output tube sockets, short the power tubes, burn the screen supply, damage the filament supply of the power transformer, and fry the output transformer in minutes. I've seen printed circuit boards burst into flames!

Before ever using your amp without a load, connect a loud-speaker to it first. If you hear any sound through the speaker then you must leave the speaker connected (or use a power attenuator). If you are sure that there is no sound then you can remove the loud-speaker.

It is much safer to leave the speaker cabinet connected at all times just in case someone turns up the master volume or FX return to the power amp.

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Q: Recently I got an Ibanez dl5 delay pedal. Do I plug the pedal into the fx loop? Or should I just go straight into the amp from the front side input jacks? (Eddie)

You get different results when using a delay pedal before an amp. If you use the amp in a clean or slightly edgy setting, then using the delay pedal before the amp can give a good slap-back or Rock-a Billy effect that can fatten up your tone. Analog delay pedals are good for this.

If you use heavy overdrive settings on the amp or for high quality, long-delay effects in the style of Pink Floyd or U2 then it is better to plug the delay pedal in the FX loop. This will prevent distorting and overly compressing the delay effect as well as keeping the noise down.

Try both methods and see which works best for you. Also make sure that the FX loop signal is not too hot for the pedal and causing it to overload. If there is no clipping indicator then just listen for possible distortion when using the pedal.

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Q: The Power switch light on my Marshall amp kinda vibrates like it is going on and off very quickly, is there something wrong? (Karim H.)

Older Marshall amps used a 6.3v incandescent light bulb as a pilot light. There was no problem with these bulbs but they eventually would burn out.

In the late 70's Marshall introduced the big square rocker switches with built-in 120v neon pilot light. All Marshall amps have been equipped with neon bulbs ever since. This is the same neon light that is used in most power bars and is notorious for being unstable and flickering on and off.

There is no problem with the amp nor with the electrical supply. Simply upgrade to a better quality pilot light and the flickering will stop.

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Q: I have Fender Twin Reverb '65 reissue. Do you think it's OK to use both channels (Vibrato and Normal) together at the same time? (Diego S.)

Good Question. Because the reverb circuit in the vibrato channel requires an additional tube (compared to the normal channel) the two channels of a Twin Reverb are out of phase with each other. This means that if you plug a guitar into both channels at once some frequencies will cancel each other out and you will get a thin tone. This depends on how you set the volume and EQ, of course, but over-all it's not a good idea. If you plug different instruments into each channel then there is no problem.

This applies to all 2 channel Blackface and Silverface Fender amps that have reverb (Deluxe Reverb, Twin Reverb, Bandmaster Reverb, etc). Non-reverb models (Deluxe Amp, Bandmaster Amp, etc) do not have this problem since both channels are the same.

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