  |
| Hi Jeff, I just read your article in the February 2007 issue of Premier Guitar about swapping out 6L6s with EL34s in a Crate amp and what goes into re-biasing. I wonder if you might be so good as to help answer another biasrelated question. I own a Framus Cobra 100-watt head which has Electro-Harmonix 12AX7 preamp and EL34 output tubes. I read online that it ships from the factory with a “cold” bias setting of around 25mA and should be taken in to an amp tech to “heat it up” to around 33mA to minimize “crossover distortion and clean up some of the buzziness.” I then took the amp to a repair shop. When I got it back, the tech said it was true that the bias was cold and didn’t have a “clean” wave on his oscilloscope. He ended up “warming” the amp up to around 32mA and said that the amp should now have a lot less distortion. That took me by surprise because the Cobra is supposed to be a high-gain amp, not low-gain. Ever since, the amp has had a good hard rock sound on channel 2, but not a really good metal sound on channels 2 or 3. I understand that a clean sine wave on an oscilloscope indicates little or no distortion of the input signal, and you say in your article that you take into account the power output under load and the overall sound, in addition to the main bias current through the output tubes, when biasing amps to find the best balance. Is it possible that the guy who adjusted my amp didn’t take into account that I was looking for high-gain performance and instead cleaned up the amp as much as he could? If so, what kinds of things should I ask an amp tech in order to get better quality high-gain performance? Thanks for any help, Rene |
Hi Rene,
First, let me thank you for reading the column. It’s fun to write and I hope informative to all who read it. Now, let’s see if I can shed some light on your particular situation. To distort or not to distort, that is only part of the question! We also need to know what kind of distortion and where it should take place. To start things off, consider this hypothetical question.
The information that you read online is:
A) probably correct
B) not a bad thing to do
C) maybe not right for you
Okay, it’s a trick question. The correct answer is “all of the above.”
The amp may come with a slightly cold bias and may need to be warmed up for players that wish to extract better clean, bluesy or classic rock tones, but that doesn’t seem to be your particular focus. Since you mentioned that it’s supposed to be a high-gain amp, it may have purposely been designed and set up with what is perceived by some as a cold bias. I’ll try to explain why, but first, I’d like to point out the difference between gain and distortion in very broad strokes.
Gain relates to the amount that a signal level is increased. Most of the time, most of the gain in an amplifier is developed in the first preamp stages. Too much gain in a preceding stage can cause a subsequent stage to distort. Early on in guitar amplifier design this was considered to be a bad thing – now we know better. Anyway, when a guitar amplifier is considered to be “high-gain,” the term refers to the amount of gain and distortion created in the preamp section of the amp. It really has nothing to do with the inherent distortion characteristics of the output stage. So when your tech said that the output stage would now have less distortion, he was not referring to, nor did he lower, the front-end gain or change the front-end characteristics of the amp. If you could have listened to the characteristics of the preamp section in the amp prior to and after the bias adjustment, they would have been virtually identical. The effect you are now noticing is a result of the way the output stage is handling the preamp signal. While the 32mA measurement for the idle bias current is a pretty average setting, it shouldn’t be considered the correct or perfect setting – there is no such thing! It is a setting that would achieve less crossover distortion in the output stage and more faithfully reproduce the signal that was being sent to it by the preamp. This, as you have found, is a good characteristic when it comes to some uses of the amp but not for others. I’ve found that sometimes a colder bias on the output stage is better for players that focus mostly on very aggressive, high-gain styles of music. The additional crossover distortion developed in the output stage seems to add a little teeth to the sound. This may be the very reason that your amp ships with a cold bias. It is has been optimized for its target market – not all amps can be all things to all players.
If you want the characteristics of your amp to shine in the high-gain mode, have your tech readjust the amp back to its original bias setting. If, however, you can no longer live without the cool hard rock sound that the amp currently has, you could always add a stompbox for the over-the-top metal tones.
Now, go put the fangs back in your Cobra.
Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
jeffb@budda.com
www.budda.com
©2007 Jeff Bober
EBS BassIQ Blue Label Triple Envelope Filter Pedal
The EBS BassIQ produces sounds ranging from classic auto-wah effects to spaced-out "Funkadelic" and synth-bass sounds. It is for everyone looking for a fun, fat-sounding, and responsive envelope filter that reacts to how you play in a musical way.
Reverend Vernon Reid Totem Series Electric Guitar - Shaman
Silos features six presets, tap tempo, one full second of delay time, and three voices—two of which are styled after bucket-brigade and tape-delay sounds. In the $150 price category, it’s not unusual for a digital delay to leave some number of those functions out. And spending the same money on a true-analog alternative usually means warm, enveloping sounds but limited functionality and delay time. Silos, improbably perhaps, offers a very elegant solution to this can’t-have-it-all dilemma in a U.S.-made effect.
A More Complete Cobbling Together
Silos’ utility is bolstered by a very unintimidating control set, which is streamlined and approachable. Three of those controls are dedicated to the same mix, time, and repeats controls you see on any delay. But saving a preset to one of the six spots on the rotary preset dial is as easy as holding the green/red illuminated button just below the mix and preset knobs. And you certainly won’t get lost in the weeds if you move to the 3-position toggle, which switches between a clear “digital” voice, darker “analog” voice, and a “tape” voice which is darker still.
“The three voices offer discernibly different response to gain devices.”
One might suspect that a tone control for the repeats offers similar functionality as the voice toggle switch. But while it’s true that the most obvious audible differences between digital, BBD, and tape delays are apparent in the relative fidelity and darkness of their echoes, the Silos’ three voices behave differently in ways that are more complex than lighter or duskier tonality. For instance, the digital voice will never exhibit runaway oscillation, even at maximum mix and repeat settings. Instead, repeats fade out after about six seconds (at the fastest time settings) or create sleepy layers of slow-decaying repeats that enhance detail in complex, sprawling, loop-like melodic phrases. The analog voice and tape voice, on the other hand, will happily feed back to psychotic extremes. Both also offer satisfying sensitivity to real-time, on-the-fly adjustments. For example, I was tickled with how I could generate Apocalypse Now helicopter-chop effects and fade them in and out of prominence as if they were approaching or receding in proximity—an effect made easier still if you assign an expression pedal to the mix control. This kind of interactivity is what makes analog machines like the Echoplex, Space Echo, and Memory Man transcend mere delay status, and the sensitivity and just-right resistance make the process of manipulating repeats endlessly engaging.
Doesn't Flinch at Filth
EarthQuaker makes a point of highlighting the Silos’ affinity for dirty and distorted sounds. I did not notice that it behaved light-years better than other delays in this regard. But the three voices most definitely offer discernibly different responses to gain devices. The super-clear first repeat in the digital mode lends clarity and melodic focus, even to hectic, unpredictable, fractured fuzzes. The analog voice, which EQD says is inspired by the tone makeup of a 1980s-vintage, Japan-made KMD bucket brigade echo, handles fuzz forgivingly inasmuch as its repeats fade warmly and evenly, but the strong midrange also keeps many overtones present as the echoes fade. The tape voice, which uses aMaestro Echoplex as its sonic inspiration, is distinctly dirtier and creates more nebulous undercurrents in the repeats. If you want to retain clarity in more melodic settings, it will create a warm glow around repeats at conservative levels. Push it, and it will summon thick, sometimes droning haze that makes a great backdrop for slower, simpler, and hooky psychedelic riffs.
In clean applications, this decay and tone profile lend the tape setting a spooky, foggy aura that suggests the cold vastness of outer space. The analog voice often displays an authentic BBD clickiness in clean repeats that’s sweet for underscoring rhythmic patterns, while the digital voice’s pronounced regularity adds a clockwork quality that supports more up-tempo, driving, electronic rhythms.
The Verdict
Silos’ combination of features seems like a very obvious and appealing one. But bringing it all together at just less than 150 bucks represents a smart, adept threading of the cost/feature needle.
EarthQuaker Devices Silos Multi-generational Time Reflection Delay Pedal
