What to do with that extra channel

Almost all the Fender blackface and silverface amps have 2 channels, a Normal and a Vibrato (which may also have reverb). This is a historical feature. In the early days, one amp was used for the guitar, microphone, and possibly an accordian. Now with the one-amp-per-instrument model, having two similar channels is not the most flexible setup.

For one thing, the Normal channel requires a 7025/12AX7 tube, and if you're not going to use it, why buy one? You can not install the first preamp tube, but there's a subtle difference in sound. The B+ voltage goes up a hair (probably not sonically noticeable), but because the cathode resistor one the second triode in the tube is shared between the Normal and Vibrato channel, the Vibrato channel's tube is now set up differently. Instead of the cathode resistor being 1500 ohms, it drops to 820, which changes the bias and makes it distort a little easier. Try it and see if you hear a difference.

What about running the channels in parallel by plugging into #1 on the Vibrato channel and running a short patch cord from Vibrato #2 to Normal #1? On the non-reverb amps like the Band Master and Dual Showman, this works, and is an interesting sound. But on the reverb amps, the two channels are out of phase and there is some cancellation which doesn't sound good. Each time the audio signal passes through a tube stage, its phase is inverted. The tubes used in the reverb circuitry invert the phase of the signal when compared to the Normal channel signal, so this isn't a good idea for most uses. (Maybe you're one of the few that has a use for the phase cancellation sound.)

-------------------------
SIDEBAR
Running the preamps in parallel works really well for Marshall 4-input amps. A lot of players jumper the two channels of a SuperLead as a standard way of setting up the amp. This came about for another reason (see my June 20, 2012 post) but creates a nice thick tone.
-------------------------
There's a simple mod that lets you use the Normal channel for finer reverb control (on amps that have reverb). Run a shielded cable from the output of the reverb tank to the input on the Normal channel (you'll need and RCA to 1/4" cable). The Volume knob is now similar to the Reverb knob in function, but you have tone controls. Easy enough to try without a big commitment.

For more intrusive uses, you can cascade the channels, which was the idea Randall Smith had in the '70's which became Mesa Boogie. This is a bit of an art, as you can't get a good sound by just running one channel into the other (see the Marshall One-Wire mod on my website). Distortion sounds better if it's built up over several tube stages, so you'll need to attenuate the signal. Plus bass muddies the tone, so you'll also have to add a high-pass filter to get something usable.

Another thought is to re-voice the channel so it sounds different. One idea on my website is to turn it into a Tweed style preamp. Other people change the tone stack components, or add a Marshall style preamp (where the tone controls are after the 2nd triode stage). Something to give a different sound and make the amp more flexible. By using an A-B switch box you can add channel switching to these amps.

Consider it--it might be a fun way to play with your sound. But be cautious about modifying these amps in a way that can't be returned to stock (like by drilling holes). As they get older they're becoming more valuable, and sometimes it's better to mod your amp ownership rather than your amp.

If it's too loud, you're too old

Made by Dr. Lex

I read a really well-done article by Dr. Lex describing a trend of reduced dynamic range in music recordings for the sake of perceived loudness. The graph here is from his web site, and shows this trend starting about 1990, and getting increasingly more apparent over the next 20 years.

I started paying attention to music seriously in the '70s which was a relatively stable period for dynamic range--it was present in recordings. The vinyl albums I was listening to had a lower possible dynamic range than CDs have, but still it was used to effect--there were quiet parts and drastically different loud parts. The first album that I remember being extra loud was Alice Cooper's "Killer" in 1971. At the time I didn't understand why or how this could be, but it fit with the nature of the album and was cool. I wasn't alone, my friends noticed it too. But this may have been one of the first shots fired in what Dr. Lex calls the "volume wars."

I think the ear training that I received in the '70s colors the way I hear contemporary mixes--they're less interesting. Younger people are growing up with this reduced dynamic range which will be their ear training that they will take forward with them. It will be interesting to see where it goes.

It's ironic that modern media has the ability to reproduce a greater dynamic range than was available in the '70s, but it isn't being used due to the nature of how the data is delivered and consumed--MP3s, earbuds, etc.

In the meantime, I must be too old, because it's too loud. I miss dynamic range.

What a difference a (tube) stage makes

Blackface Fender Deluxe amps sell for much less than blackface Deluxe Reverbs. An informal survey of completed Ebay sales shows about $1000 difference between the two.

This is due to more than the lack of reverb; the Deluxe Reverb has just a touch more gain to it, giving it a slightly different sound. There is one extra triode that's used to mix the reverb return with the dry signal, and even with the reverb knob set to 0, it's still in the signal path of the Vibrato channel. This, I think, is what endears it to a lot of guitar players.

When you add the reverb into the picture, the signal now goes through 3 additional stages, so the reverb adds additional coloration to the amp. I've read newsgroup posts where the Deluxe's fans feel they can get the same thing through a reverb pedal or other effect, but it doesn't duplicate the the signal path. There's reverb, but it's not achieved the same way.

The Deluxe is a great amp and has great sound. Although it's similar in name to the Deluxe Reverb, it's a different amp, and not as flexible. Its two channels are differentiated by one having tremolo, and the other not. A Deluxe Reverb's channels on the other hand, are almost like having two amps. The normal channel is a Deluxe with the tremolo turned off, and the vibrato channel adds a gain stage, reverb, and tremolo.

Another take on using your ears

Just came across a Gibson online article that is similar to two of my early postings, the July 2, 2012 On Using Your Ears, and the July 19, 2012 More On Using Your Ears. In the Gibson article, Dave Hunter talks about "home tone" vs. "gig tone." The take-away line for me is

"When you get “your tone” set up and ready to go in sound check, then find it is lost in the ether once the band kicks in, don’t be too proud—or too stubborn—to change it."

 And he sums it up with

"...get your tone out there to the ears that matter most to the live performance: someone else’s."

 It's a good read for another take on this topic.

Selection strategy

Knobs courtesy Derek Jensen (Tysto) WikiMedia Commons

A lot of digital gear comes with hundreds of preset patches or at least hundreds of spaces for you to record your own patches. For live playing, this is overwhelming; at best you can remember a handful of your favorites. One way to deal is to simplify and memorize a just a small bunch at at time. But you need a way to call these up quickly, and that's where things get interesting, how to map the list to a control device. I think the best schemes provide an analogy (visual or mental), and simplify (reduce) the number of choices.

I've heard that there are 128 pickup combinations on a Strat (pickup(s) chosen, series wiring, parallel wiring, phasing) yet the original Strat wiring simplified by provided just 3 with a lever switch. The switch pretty much pointed at the pickup that was being selected, providing the analogy. That's since been expanded to 5, and uses the same position analogy. But even if you only consider the parallel wiring that's so commonly used, there are 7 combinations, so some simplification is still happening.

In trying to get at more of the possibilities, I wired in a 3 position mode selector to one Strat that ups the combinations to 10. The 5 position selector switch is still the same, but the mode switch changes what 5 pickup combinations you get. This makes it a lot easier for me, since I only need to remember 5 things at a time, and the position of the switch tip uses the traditional analogy to sort of point at the pickups being selected. There is still some simplification going on, as this setup can theoretically have 15 selections, but wiring limitations lower that number. Also the groupings are meant to be functional not exhaustive. The modes group pickup selections into usable groups. Mode 1 is the stock wiring, so you don't need to remember anything new. Mode 2 gets at the missing 2 combinations--neck+bridge, and all 3 pickups on. The 4th and 5th positions are rewired, but positions 1-3 still give you the stock combinations. I think of mode 3 as a 2 pickup humbucker guitar. It's louder, and has only 3 pickup combinations.

SIDEBAR
Some people choose to implement pickup wiring with a set of 3 or more switches to individually control the pickups. This doesn't work for me, there's no visual or mental analogy that I can lock into. It also isn't simple, it takes several switch throws to select a pickup combination. A compromise is to provide a switch that just turns on the bridge pickup so it can be added to any combination selected by the pickup selector. I tried this for a while, and didn't like it because selecting pickups is still a 2 step operation--use the lever switch and check or throw the bridge pickup switch.

Another example came up with a Marshall JMP1 preamp. This has 100 patches. On its front panel, these are laid out linearly using a knob to sequentially scroll through them. I originally started out using a MIDI foot controller that allowed me to access any patch. I noticed however that I never used more than 5 patches in any situation. I think Marshall realized this too, because in addition to MIDI switching, they have a footswitch connection that puts just 4 patches at your disposal (simplification). You can map the 4 buttons on the footswitch to any patch. I made a footswitch for it and laid out the switches with the idea that the cleanest patch  was at the left side, and the highest gain at the right. This mental analogy, like the visual image on the Strat pickup selector switch, helps me keep things straight.

There's another way to remember patches that works for me, and that's a table layout. Patches are arranged in banks, with the patches in a bank being a variation on the theme of the bank. Way back when, I set up my Zoom 9002 that way. There is a distortion bank, a time based bank (chorus, flange), a weirdness bank (envelope follower, arpeggio generator), and a harmony bank (added octaves, fifths). The patches in a bank use other effects like distortion, reverb, and compression, but the main flavor of the patch was what got it added to a particular bank. Doing this, I was able to remember a 4x4 array, or 16 patches.

The MIDI controller for the JMP1 also used table layout. However remembering the large array needed for 100 patches (10x10?) misses the  idea of simplifying. Still this layout has the advantage of making it quicker to get at patches at different ends of the spectrum. On the JMP1 itself, going from patch 1 to patch 50 involves scrolling through 49 patches (still, it's fairly quick with Marshall's spin knob). With the MIDI foot controller, you step on the bank switch to get to the correct bank, then select the patch in that bank.

I think the inability to hold all the possibilities in our heads is what contributed to the decline in popularity of rack effects processors in live guitar rigs, and the return to stomp boxes with analog style controls. The stomp box provides the simplicity of a few knobs to control parameters, and a visual indication of where those parameters are set. This has extended into audio recording software where there is often an optional interface for effects plug ins that looks like a stompbox. Somewhere along the way the technology parted ways with the way we think and we went back to the tried and true.

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