Standby

Most people don't use the standby switch on their amp correctly, at least in the way that I understand things. I wonder why these are still being put on amps. Yes, I understand the reasoning, but I haven't really seen tubes wearing out that much faster. Maybe it's a subtle difference.

The idea behind a standby switch is to give the tube heaters a chance to warm up to operating temperature before you apply the B+ voltage and the tube starts working. Failure to do so leads to cathode stripping (whatever that is), and is detrimental to tube life. What I've read, and believe enough to follow myself, is you should

1. turn on the power switch
2. wait at least 30-60 seconds
3. turn on the standby switch, and make noise.

Powering down is the opposite:

1. turn off the standby switch,
2. wait at least 30-60 seconds,
3. turn off the power switch

But where most people go wrong is the time in between those two events. At the end of the first set, don't use the standby switch. Leave the amp on. Unplug your guitar or turn the volume down at the end of a set, but leave the amp on the whole night. Component failures are most likely to happen at start up--caps are uncharged and there's a large current surge, things are cold and need to heat up, etc. Think about a light bulb. How many times have you seen one fail when it's been on a while? Most fail when you turn them on.

Leaving your amp on the whole night is easier on the amp because there's only one startup. After everything is warmed up and ready to go, it coasts along until the end of the night. If you're turning it off because it's getting warm and you don't want to cook the components, consider installing a small fan. Even a little air movement will help immensely. I've installed fans in my Fender amps because I don't like the idea of the downward hanging tubes cooking the components in the chassis above them. I think this is a drawback for combo amps, but then it can't be too big a deal because there are so many combo amps doing just fine. If you what to install one, you can run a 12 volt fan off the 6.3 volt heater supply. Sometimes you can hook it up directly, while other times you may need to add a diode and capacitor to give it a DC voltage. It will run slower than it was designed to, but if you can feel any air movement, it will move the heat out. Running it slower has the benefit of making it quieter.

If heat is a concern, you may have bigger problems. The tubes will get hot enough to burn you, but nothing else should be so hot that you can't touch it for a few seconds. Especially watch the transformers, as they are the most expensive items in the amp. Heat means too much current is being drawn, and you need to find out why. Sometimes it's as simple as a bias adjustment, other times it might be rusty laminations in the transformers causing eddy currents and robbing power.

Fender Pro Tube series Twin repair story

Got a Fender Professional Tube Twin for repair, the owner says it's lost a lot of its volume. Here's how the repair unfolded.

Most common cause of failure is the tubes. Checking the bias on the power tubes will show if they are all working (a dead tube doesn't draw any current), and also see if the amp is set up correctly. The amp is set up a little cool (460V plate, 26-30mA/tube), but this isn't terrible and isn't the cause of the problem. It's time to look inside.

This amp is a full-featured amp. Lots of settings and switches. 8 preamp tubes and 4 power tubes. Inside there are 4 circuit boards, 2 of which are mounted component side down so you can't see the parts. A schematic would be helpful, but I couldn't find one online so I started with the basics.

Checking the plate voltages on each tube is a good diagnostic test for the overall circuit. Pins 1 and 6 are the plates on 12A_7 tubes, and you don't need a schematic to find those. You should also get a pop as you touch the probe to the plate to indicate that the signal is going through the amp. If it doesn't pop, something is wrong downstream. Unfortunately (or fortunately), all the plate voltages were reasonable for a Fender amp.

This amp has two channels, plus a gain boost on one of the channels which probably adds in another tube stage. Since the low volume problem is happening on both the channels and with the gain boost, the problem has to be either after the preamp or in a shared tube stage. Often the first preamp tube stage is common to all the channels. Checking the first tube would just involve substituting a known good 12AX7 tube to see if the problem went away, but I decided to check the power amp first for no good reason--had to start somewhere.

This amp has Preamp Out and Power Amp In jacks. There are switches in these jacks that can oxidize, and it's possible that some resistance built up which could cause a volume drop. Usually this problem shows itself as variation in volume, or crackles, not a consistent volume drop. A quick check is to hook a guitar cable between these and get around the switches. If this brings the volume up, then some DeOxit will usually clean the switches. Doing this had no effect, so that wasn't the problem.

I have some experience with the plate resistors on the phase inverter of some newer Fender amps opening up. If one opens up, you lose half the power. This would have shown up as no voltage on the plate when I checked it earlier, but I still re-checked it. The failure on the amps I saw was due to what I think is a too-low power rating for the resistor. (As a side note, I've learned that if one of these resistors opens, BOTH plate resistors should be replaced while you're in there. It's usually just a matter of time before the other one goes.) But on this amp, the plates are 1 watt resistors which is a good design upgrade on Fender's part.

What's interesting on this amp is it seems that the two tube stages needed for a long-tailled phase inverter are split over two tubes. Normally one 12A_7 tube with its two tube stages is used. I wasn't sure what the other tube stage in each tube was doing, so I spent a little time trying to follow the circuit path with a continuity/resistance meter. In doing so, I found one resistor that looked fine but was open. In looking at the circuit, it appears that this tube stage is a cathode follower, and this is the cathode resistor. That made sense because the plate had voltage on it, but the signal is coming off the cathode, something that I hadn't checked earlier. This is where a schematic would have saved some time in identifying this.

Since the resistor was open, I bridged it with a good resistor and the volume came back. Looking at other Pro Tube schematics, this tube stage is possibly the recovery tube for the effects loop.

To repair it, I decided not to remove the circuit board to get at the solder traces underneath. Due to the complexity of the amp, this would have added time (raised the cost), and there is also some risk to flexing the ribbon cable connectors as you remove the circuit board. By clipping out the resistor, leaving as much of the leads intact as possible, you can form the leads into loops, and use these as sockets for the replacement resistor. The replacement resistor's leads wrap around the loops and provide a good mechanical connection. When soldered, the repair is very strong. I feel good about doing it this way on my amps, so I think it's the best way to go. Total billable time: 2 hour

Amp distortion

In the '50s, guitar amps were available in what is now considered to be the lower power range for amps. This means they had less headroom and were more prone to distortion. Since there was less clean headroom, when you needed a little more volume, or you had hotter humbucker pickups, you entered the operating range where distortion was present. So unintentionally, amp distortion was part of the sound to which we grew accustomed.

But distortion is also an interesting sound, more harmonics, a little bit sax-like, and players liked intentionally exploring it. (That, added to the fact that it was also a bit rebellious to be loud enough where the grown ups yelled at you to turn it down.) I've read stories of John Lennon, Eric Clapton, and Keith Richards fighting with the recording engineers to leave the distortion in the recording. The Beano album, with Clapton playing a Les Paul through a cranked Marshall is still today considered a hallmark of guitar tone, but it wouldn't have been had the "common sense" engineering of the time won out.

So the situation was one where distortion was present and being explored. Imagine though if more powerful amps were available during rock and roll's formative years. There would have been more clean headroom, less distortion, and distortion would have been harder to come by without being really loud. The sound of rock and roll would be quite different today. By the mid 60's, when 100 watt amps were finally readily available, it was too late for this effect to happen. Distortion had been dyed into the fabric of rock and roll, it was part of the tone. So in order to get the tone with the bigger amps, you had to turn up, and instead of becoming cleaner sounding like it would have in the 50's, in the 60's rock and roll got louder. The same bit of technology introduced at a different time had a different effect.

There was still some uncertainty about the role of distortion in the '60s. Around 1965 Leo Fender sold his company to CBS. CBS brought in engineers that tweaked the circuits with the intent of making them more hi-fi--they hadn't caught on to what was happening--and the CBS silverface amps got a bad reputation among musicians. Pre-CBS blackface amps were sought after then and still command a higher price than the silverface models, although enough time has passed where this is starting to fade. (A lot of silverface amps are modified to be more like the blackface circuitry, and used to be a bargain if you didn't care about the cosmetics.) Modeling amps still pay homage to the sound--nearly every digital modeling amp has a "blackface" or "California" sound on it.

In the '70s a watershed moment for distortion happened--it was accepted. Randall Smith was heavily modifying Fender Princeton amps to add more gain stages. In a December 2005 Guitar Player article, he said at the time he was doing this because the blues and rock players wanted more distortion at lower volumes. The cascaded tube stages he added produced the distortion, and master volume controls limited the volume going to the power amp. So the power amp was no longer distorting, since it was working less hard, while the preamp tubes were. But not all distortion is the same. When they distort, the small preamp tubes sound different than the larger power tubes. In essence, this is the sound that the Mesa Boogie Mark I amp brought to the table. and later Mark II amps made adjustable with the amp's knobs. Distortion tone had become more adjustable.

So the palette of distortion currently available in amps now involves several approaches. Bands with a vintage sound either play really loudly (AC/DC) to get the power tube crunch, or use power attenuators to dump some of the volume as heat. Metal bands rely more on cascaded tube stages to develop the sound (although they play loudly as well). Master volume controls give control over the character of the distortion. And all this is without considering the huge role played by stompboxes in front of the amp.

Cascaded gain stages had another effect on amps. The drawback of cascaded tube stages is you lose touch sensitivity; as you play lighter on the strings, or roll back your volume, the amp doesn't clean up the way it does with a vintage amp, the tones available from your fingers are reduced. I find this to be a limitation because it reduces dynamics and is more compressed, and a series of notes played on the guitar comes out all with the same tone. On a vintage amp, the dynamics of your playing influence the tone of the notes. But to each his/her own.

In order to get back the clean tones on a  cascaded amp you need to have different circuitry. This is done by having multiple channels. Most high-gain amps have more than one channel, usually footswitchable. The channels have increasing amounts of gain and have names like "clean," "crunch," "overdrive," etc. So when you want the clean sound that you can get with a lighter touch on a vintage amp, you step on a switch to change the channel. The advantage of this approach is the range of distortion sounds available is much greater than you can get with a vintage amp.

An electric guitar without its amp is only half the instrument. The foundation of your tone starts with your fingers, but it's shaped by your guitar and amp--and the settings you choose.

Playing as a band

By Infrogmation of New Orleans,
via Wikimedia Commons

Some people listen too much to themselves when playing with others. You've worked on your part, you've practiced, and you want to play it right. The last step is asking yourself "does this fit?"

In a 9/05 Guitar Player magazine article, Mike Stern was talking about some guitarists getting all excited about playing with a horn player because they can haul out all their chord inversions. He basically takes a less-is-more stance:

"When I'm comping and I'm deciding which chords to play, a lot of times I'll play the least amount of notes possible…I'm often playing no more than two notes from each chord."

I've been in situations where:

• in a 7 piece band, everyone was playing multi note triads--the guitarist was playing a 5 note triad, the keyboard added a 7 note triad, etc. There was no sense of anything but a close chord voicing because every note was played in all the octaves starting from the root.
• the backing vocal harmonies were louder than the lead vocals, and the harmonists were not listening to the phrasing being used by the lead vocalist, a situation I call "lead harmony."
• an instrument would play the melody along with the vocals, so like the lead harmony situation, there were two competing interpretations for the phrasing
• people playing "their part" when there are substitute players who don't know about, and aren't playing, the counterpoint that makes "their part" fit

Some situations that I like being in are the opposite:

• a 7 piece band playing 5 note chords, each person taking one note and 2 people playing rests
• 2 people playing 4 and 5 note chords as stacked chords. For example, a C maj7 chord can be played as a C triad and an Emin chord. This idea can also be applied if you let the bass take the roots, and you play the upper parts of a rootless chord.
• taking the above idea for creating solos in a band setting where the chords are triads and 7ths. With the backing tracks playing the chord, build a solo on arpeggios that add the upper end--the 7ths, 9ths, 11ths, 13ths.
• people that aren't afraid to play rests.

As with many things, less is often more.

More on using your ears

By Iain, via Wikimedia Commons

As a follow-up to my last blog about listening to your sound, I've seen people get caught by the mix of instruments in a band changing the way you hear your part. Typically this happens when the guitarist gets a new distortion pedal. You dial in what you think is a great sound (at home), go play with your band, and the mix is good when the pedal's off. Then your solo comes, you step on the distortion pedal and...disappear from the mix.

Here's a case where using your ears isn't enough by itself, the context also matters. When the disappearing-distortion-solo happens, it usually is because there is too much distortion dialed in. All the new harmonics generated by distortion change your place in the EQ spectrum of the mix, you find you're competing with the frequencies being generated by the other instruments, and there's no clarity. Reaching for the volume knob doesn't help, it just makes the mud louder.

When you can't hear yourself in a band, sometimes reaching for the tone knobs is more effective than reaching for the volume knob. Recording engineers know this. If you isolate instruments in a good mix, they may sound thin or strange, yet together the sound is working. For example, the bass and bass drum live in the 45Hz-250Hz area. The other instruments can stand to lose everything below 250Hz to stay out of their way. A heavy guitar player is not going to like the sound of this by itself, but allowing the low end of the guitar to overlap the bass will just generate mud and the mix will suffer.

Last year I got a 7 band EQ pedal that I have set for a slight mid boost. If I don't think my solo is standing out enough, I've been surprised at how well this makes it cut through when I step on it--same volume, more presence.

Playing around with home multi-track recording is a great way to see this in action, and gives you full control over all the variables.

On using your ears

I've been doing repairs for a while and have noticed that some musicians like to mark "their setting" on the gear. Sometimes there's tape with the setting numbers written on it, other times there are lines drawn on the faceplate and knob that are meant to match up. This hits me as strange, since your sound is not an absolute. There are so many variables that affect your sound, to list a few

• the size of the room
• is the room full of people or empty?
• are you outdoors?
• have the speaker cones been in a humid environment and absorbed some moisture?
• is your amp coupled to the stage, sitting directly on it?
• with some speaker cabinets, where are they pointing?
• How old are your strings?
• etc, etc

To assume that you can dial in your sound in your bedroom or rehearsal space, and then transplant it somewhere else is wishful thinking. At best it can be a starting point for dialing it in, but there's still work to do. The best advice I've heard is to adjust your amp settings like you are in a dark room--in other words, don't look at the numbers on the dial. Even better, get rid of the numbered knobs and face plate markings and just twist the knobs.

I got a callback on one repair that I did saying that the amp now had more treble. What had happened was I had taken the treble knob off the amp and put it back on in a slightly rotated position. This person was locked in to the notion that their treble setting was "7" and to their credit their ears were good enough to hear the difference. They could have adjusted it it by turning the treble down to 6.5, but it wasn't right for them until I pulled the knob off, unrotated it a click and put it back on. I guess there's nothing wrong with that, it just makes me say "hmmm..."

A bright idea

In the early days, it was a struggle for the guitarist as venues grew in size and volume levels went up. Amps grew in power and were EQed to cut through the mix with brightness and presence features.

Many amps at this time had a "bright" channel. The brightness is often achieved by routing the treble around the volume control so in effect, the high frequencies are always passed, and the volume control only affects the lower frequencies. In other words, the cap is a high pass filter.

A good example is the Marshall Super Lead. There are two high pass filters in the bright channel (see schematic), but the cap across the volume pot is the major brightness contributor. At low settings, the tone is very bright, but around 5 on the dial, the tone is much more in balance. However with a 100 watt Super Lead, you're also really loud!

So the amp designers set out to solve one problem with this cap, and inadvertently contributed to the volume at which rock was played. It's not the only or even the biggest factor in the volume climb over the years, but if you look at the amps being used at the time and realize they need to be turned up for tone, you may start to see a cause and effect situation.

The Super Lead is probably the most obvious use of a high-pass filter, but the Vox AC30 with a Top Boost preamp and most of the Fender amps also employed it. Many of the Fender amps made the high pass filter switchable. (The Deluxe Reverb is an exception where the cap is hardwired into the Vibrato channel.) Fender also used a more moderate value for the cap so the effect wasn't as great as it is on a Super Lead.

For many people today the bright channel on a Super Lead actually does its job too well which is why the normal and bright channels are often jumpered together and run in parallel. There is also a simple modification to clip one of the leads on the cap to remove it from the circuit. You still have the second high pass filter, but it's a more subtle effect.