So, you show up to the gig, plug in, turn on your Marshall JCM800 series head/combo and... nothing... nada.... zip... the amp is DOA without signs of a struggle. What happened? Well, the heater fuse(s) just gave up on you, and it happens pretty often. Happened today to a friend of mine in fact and that's why today's blog is on fuses. The trouble is that the heater fuses are only accessible by opening the amp. Inside a JCM800 amp there are up to 4 heater fuses, 3 hi-T fuses, and an "IDIOT" fuse which is rated just slightly higher than the mains fuse and will blow if someone tries to "fix" a problem with tin-foil or a 20 amp replacement.
The heater fuses are there to protect the output power tubes and the transformers in case of problems. They are in series with the (6.3v) tube heater filaments. Tubes need to be heated by these filaments in order to get them hot enough to operate. When the amp is cold and you turn it on there is a sudden surge of current to these filaments, not enough to cause the fuses to blow but enough to cause them to soften up and melt slightly. As time goes by these fuses suffer from metal fatigue and finally one day.... today... the amp dies on the starting line.
Sudden Death Syndrome got to be so bad that here in Canada the national distributor would hard-wire bypass these fuses altogether in all new amps before they reached the stores. In retrospect, (the JCM800 series came out in 1982) I would suggest to leave these fuses in, as they serve a very useful purpose. I would recommend using 7 or 8 amp Slo-Blo values instead of the 5 amp that comes stock. The 50 watt model has 2 such fuses, the 100 watt version has 4. They are clearly identified as heater fuses on the small PC board inside the amp.
To make your amp more road-worthy always change ALL your fuses before a tour. Do NOT change the rating of fuses other than the heater fuses as this will put your amp in jeopardy, and you will suffer the disgrace of needing the IDIOT Fuse.
Friday, October 22, 2004
Monday, October 18, 2004
Metal Prevents Hearing Loss
Tests performed by the Institute of Noise Hazards Research of the Israeli Army show that oral magnesium intake reduces permanent hearing loss induced by noise. 300 young, healthy, and normal-hearing recruits who underwent 2 months of basic military training were tested. This training necessarily included repeated exposures to high levels of impulse noises while using ear plugs.
Hmm... this sounds a lot like the kind of abuse you can be subjected to nightly on the stage. With a loud ride cymbal close to one ear, your amp next to the other, and vocals monitors right in your face, earplugs barely help. Breaks are just as bad with people screaming to be heard above the music. Then there's that "noise hangover" the next day when you feel totally brainless. Ever had to leave the TV blasting in your motel room so you could drown out the ringing in your ears just to get to sleep? Ever thought the TV was still on after you had turned it off?
Noise has the potential to cause stress reactions. Chronic noise-induced stress accelerates the ageing of the heart and thus increases the risk of heart attack. Recent studies support the importance of noise as a risk factor in circulatory and heart diseases. (we're talking LOUD here.) Noise causes a depletion of magnesium in the body.
Studies show that death rates from coronary heart disease are higher in areas where the water is low in magnesium. Men living in high magnesium water areas had a 35 percent lower chance of death from heart attack than those who drank low magnesium water.
As if it wasn't bad enough the the body's absorpion of magnesium is hampered by alcohol and fatty foods! Excuse me, I need to take some vitamin supplements.
Friday, October 08, 2004
Fender Master Volume Fiasco
Few innovations in tube amps were as ill conceived and as misunderstood as the master volume control found on Fender amps of the 70's. To most people this control signifies the end of an era. Pre master volume amps are more sought after and command higher prices. This master volume just sounds too thin when you turn it down and just doesn't seem to be right. And what about the pull switch, what exactly does it do?
Well we have to back-track here a little to understand what is going on. The Fender design utilises a 1 megohm pot inserted between the preamp and the power amp. This is pretty much the standard convention and is common to Marshall circuits and most other designs as well. The 1 megohm value is chosen because it is high enough that it does not load down the circuit and alter the sound when it is set full up to 10. The problem is that as you lower the control you loose high frequencies and the sound gets muddy. At low setting to 1 or 2 the high-end loss is not so much of a problem but at mid settings of 4 to 8 it is extremely apparent. In order to counter this effect Fender utilises a special pot that has a tap at about 70% of its operating range, a small capacitor is connected from this tap to the top of the control which acts like a bright cap and prevents the loss of highs. This is fine if you are playing clean and with no distortion but most people don't do that. The whole idea of having a Master volume in the first place is to make the sound dirtier and to crank up the preamp gain. When this happens in a Fender the sound is too shrill due to this bright cap.
Another problem that arises with high gain settings is that the reverb circuit, which is before the master volume, is easily overloaded and sounds just terrible. Fender tried to resolve these problems by means of the "pull switch" connected to this control. When you pull the switch out it lowers the drive to the reverb to prevent it from overloading, while at the same time connecting a treble cut capacitor from the top of the master volume to ground. Neither of these ideas really work very well and in modern designs the reverb circuit is almost always after the master volume to prevent this overload.
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Well we have to back-track here a little to understand what is going on. The Fender design utilises a 1 megohm pot inserted between the preamp and the power amp. This is pretty much the standard convention and is common to Marshall circuits and most other designs as well. The 1 megohm value is chosen because it is high enough that it does not load down the circuit and alter the sound when it is set full up to 10. The problem is that as you lower the control you loose high frequencies and the sound gets muddy. At low setting to 1 or 2 the high-end loss is not so much of a problem but at mid settings of 4 to 8 it is extremely apparent. In order to counter this effect Fender utilises a special pot that has a tap at about 70% of its operating range, a small capacitor is connected from this tap to the top of the control which acts like a bright cap and prevents the loss of highs. This is fine if you are playing clean and with no distortion but most people don't do that. The whole idea of having a Master volume in the first place is to make the sound dirtier and to crank up the preamp gain. When this happens in a Fender the sound is too shrill due to this bright cap.
Another problem that arises with high gain settings is that the reverb circuit, which is before the master volume, is easily overloaded and sounds just terrible. Fender tried to resolve these problems by means of the "pull switch" connected to this control. When you pull the switch out it lowers the drive to the reverb to prevent it from overloading, while at the same time connecting a treble cut capacitor from the top of the master volume to ground. Neither of these ideas really work very well and in modern designs the reverb circuit is almost always after the master volume to prevent this overload.
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Sunday, October 03, 2004
Buffer Madness
Ever notice that guitar tracks sound brighter and thinner than in the old days? Listen to Robin Trower or Jimmy Page tracks and see how round and mellow they are. Albert Collins used a 100 foot lead cord in order to walk through the audience and this gave him a unique sound. Hendrix would string Marshalls together like Christmas tree lights using cheap, lossy, coiled cords.
In the struggle to be heard above the cymbals on overcrowded stages and fighting the gradual loss of hearing from night after night of loud music and people shouting in your ear in bars, guitarists seek to counter the muddiness by increasing the top end. Modern pedals mostly all employ active switching these days and this acts like a buffer in order to prevent loss of high end. For years I bypassed the tone controls on my Strats to get more bite. I used buffers before my Univibe, Ecoplex, Wah pedals, Boss Chorus Ensemble.... pretty much everything. I even had buffers in my guitars.
Now I'm going the other route. I like the sound of the guitar plugged into these old effects without any buffering. I find that the Univibe is rounder and richer, the wah less shrill, and the Echoplex just fine. I reconnected the tone controls back into my Strats and, hey, the sound is much richer. The trick to the whole thing is to use a buffer when the pedals are bypassed so as not to lose too much high end from all the extra wiring that is necessary to hook-up the pedals together as well as to prevent the loading effect that these old pedals have on your sound..... not always an easy thing to accomplish.
In the struggle to be heard above the cymbals on overcrowded stages and fighting the gradual loss of hearing from night after night of loud music and people shouting in your ear in bars, guitarists seek to counter the muddiness by increasing the top end. Modern pedals mostly all employ active switching these days and this acts like a buffer in order to prevent loss of high end. For years I bypassed the tone controls on my Strats to get more bite. I used buffers before my Univibe, Ecoplex, Wah pedals, Boss Chorus Ensemble.... pretty much everything. I even had buffers in my guitars.
Now I'm going the other route. I like the sound of the guitar plugged into these old effects without any buffering. I find that the Univibe is rounder and richer, the wah less shrill, and the Echoplex just fine. I reconnected the tone controls back into my Strats and, hey, the sound is much richer. The trick to the whole thing is to use a buffer when the pedals are bypassed so as not to lose too much high end from all the extra wiring that is necessary to hook-up the pedals together as well as to prevent the loading effect that these old pedals have on your sound..... not always an easy thing to accomplish.
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