Given that these amps use active components that pre-date the invention of solid-state electronics, is it still feasible to tweak our tube-based electric guitar workhorses for better sound?
By: Ringo Bones
Like it or not, tube-based electric guitar amplifiers are here to stay because high-speed and wide-bandwidth power transistors touted to have better power efficiency and “sound” than their vacuum tube / valve counterparts often have an average production life-span of two weeks. Especially when declared obsolete less than a month after being featured in “trendy” electronics periodicals – if you believe their press coverage story. Maybe those non-ferric bodied power transistors are relegated to stealth aircraft seeing RADAR use for the military?
If the corporate world of solid –state technology doesn’t give a damn to us electric guitar enthusiasts, then maybe these tips of upgrading your trusty-but-rusty tube-based electric guitar amplifier will at least add another 50 years of enjoyment. If you possess the requisite knowledge of the science (and art) of thermionic circuitry, then the following suggested tweaks could prove very valuable.
Going Triode – No ifs or buts, this tweak easily transforms the tone of any push-pull type electric guitar amplifier into something that sounds closer to a single-ended Black Face-era Fender Champ. 6L6 Beam tetrode-type tubes and EL34 pentode-type tubes that constitute the majority of push-pull output electric guitar amps can easily be converted into triode mode by strapping the screen grid of every output tube to their corresponding plate – if your amp uses multiple pairs of output tubes. This method also works with pentode-type tubes. Electric guitar amplifiers manufactured during the late 1980s to mid 1990s often feature a triode / pentode (beam tetrode) selector switch that allows you to switch between operating modes. But you should take precautions on both types in avoiding parasitic oscillation – i.e. using grid stoppers. The advantage of going triode includes lowering of the output impedance resulting in the amplifier sounding more dynamic and musical – i.e. gorgeous harmonics and tone. The disadvantages are the output power is usually divided in half, and the “spongy” tone will not be the liking to those “Heavy Metal Guitarists” weaned on full pentode / tetrode volume.
Use High-Mu Output Tubes – If you have grown fond of the “conventional” 12AX7 or ECC83 double-triode preamplifier tube phase-splitter stage of your electric guitar amplifier, then using high-mu output tubes has the advantage of retaining your stock preamplifier stage since high-mu tubes are so easy to drive. The high frequencies would improve and you now have the advantage of lowering the negative feedback without your preamplifier stage going dull (high-frequency roll-off). And even if the output impedance increases a little, your guitar amp’s musicality also increases. The problem is high-mu tubes require a higher power supply voltage or H.T. than your standard 440volts DC, so power supply circuits and the output transformer needs to be redesigned to handle the higher power supply voltage of high-mu tubes – usually 1,500volts DC - or the speakers will go ballistic. Those don’t come cheap you know. The benefit of having a good tone via this route usually outweighs the hardship of an almost complete guitar amp overhaul.
Using Beefier Tube Preamplifier Stages – Those 12AX7 and ECC83 tubes used in “conventional” double-triode phase-splitters are not exactly designed to drive banks of pentode or beam tetrode power output tubes. Not to mention that conventional double-triode phase-splitter has an inherent problem of high input capacitance caused by the Miller Effect. This causes high-frequency loading on the input tube and reduces bandwidth. Making it very difficult to use appreciable amounts of negative feedback without instability due to the phase shifts incurred, and negative feedback is “somewhat” of a necessity with “modern” pentode and beam tetrode output power tubes. Beefier preamplifier tubes like the 4-watt anode dissipation 5687 preamp tube, which can be used as an "improved” input tube. So does a 6AU6 low-noise R.F. pentode because it can maintain stability when used in a “modern” tube amp design with appreciable amounts of negative feedback. Or a dedicated phase-splitter tube, like the ECF80 triode-pentode tube. The problem with these types of tubes is that preamplifier circuits using these designs that are already relegated into the “public domain” are few and far between. For example, your friend from college will not be willing to take it likely that you are making millions of dollars from your friend’s vacuum tube preamplifier designs that he formulated during the 1990s - Especially when your friend from college is currently one of the millions of victims of the on-going subprime mortgage crisis. Lastly, using beefy preamplifier tubes might sound great for domestic hi-fi tube-based amplifiers, but in electric guitar amplifier applications, they tend to make the guitar amp sound “dry”. Like those hard-sounding Carlsbro transistor-based guitar amplifiers that became widely available during the early 1990s.
So there you have it - A few ways that you can tweak the sound of your electric guitar amplifier. If you have other ideas that I haven’t discussed here, please feel free to drop me a line.