Sunday, August 9, 2009

Does Cranking the Bias Current Make a Better Tube Amp?

Many swear that it improves high frequency performance, but does cranking up the bias current a sensible way to improve high frequency performance of your tube / valve amplifier?

By: Ringo Bones

This thorny issue started in my neck of the woods during the tube / valve amp revival of the mid-1990s. Where everyone with a still-working 6L6-equipped 1965 Fender twin of a EL34-equipped Marshall amplifier found out that these rock guitar workhorses can be made into very righteous hi-fi amps by jacking up the output tubes’ bias currents and connecting them to domestic hi-fi loudspeakers. And after the news arrived that a fresh batch of tubes / valves are only a plane ride away in neighboring Hong Kong, thus turning every tube-based guitar amp toting guitar hero wannabe curious about hi-fi into ad hoc tube / valve electronic experts almost overnight. The frenzy of everyone in my place born way after astronauts Armstrong and Aldrin first walked on the surface of the Moon scrambling in a Manhattan Project-like zeal to make their tube-based guitar amps do double duty as audiophile quality hi-fi amp-on-the-cheap. Is probably the most surreal manifestation of anachronism I will ever see in my entire life, but does the price of their blissful ignorance worth more than they bargained for?

11 years ago (in 1998), I was in the process of fulfilling my personal tour-de-force of constructing a Black Face Fender Champ on steroids. It was a single-ended guitar amp based on the Russian GM70 transmitter tube / valve that I found for sale rather cheaply – 2 for 15 US dollars – in a garage sale. This rather “gigantic tube” has an anode dissipation of 250 watts and requires a power supply of 1,500 volts DC. The only “substantially expensive” parts of my Black Face Fender Champ on steroids was an output transformer by Audio Note that can handle the GM70’s 1,500 volt DC power supply brought used from an “disenfranchised” hobbyist for about 50 US dollars. And a Japanese-sourced OEM Leslie-type organ speaker cab from the 1970s being heavily discounted by our local flea market because no one ever bought it since it went on display back in 1979. The flea market’s storekeeper very gladly sold it to me for about 20 US dollars. Luckily the other components required for my project amp had been lying around in my spare equipment trunk. It wasn’t long before someone in my neighborhood haggled me to part with it for about 1,000 US dollars.

At the time this was a very good deal because I’ve only probably spent about 250 to 300 US dollars creating my ultimate guitar amp, plus I desperately needed the money to bribe into a lucrative government job to pad my résumé. Sadly the new owner began cranking up the output tube’s bias current to accentuate the Black Face Fender Champ on steroids’ very beautiful high-frequency timbre. Allowing the big tube to fail 7 months later. Coming to his senses, he backed up the bias to normal and the “spare” GM70 tube still played to this day (2009).

By their very nature, tube-based electric guitar amplifiers don’t work particularly well as uncompromising audiophile-grade domestic hi-fi amplifiers. Especially if the musically-inclined user discovers the “virtues” of reducing the negative feedback levels of his or her electric guitar tube amp as a hi-fi amp set-up. Though zero-feedback tube / valve amplifiers are well known for retrieving the subtle nuances of the acoustic environment of recorded music, redesigning your guitar amp’s negative feedback to zero could destroy the tweeters of your domestic hi-fi loudspeakers. Especially since an overwhelming majority of guitar amplifiers output transformers are not purposefully wound to work in a zero negative feedback tube-based amplification circuitry. Let alone designed using a Fast Fourier Transform.

EL34 equipped Marshall amps are the most often tweaked by this method to adopt them for domestic hi-fi use. Most of these types have an H.T. of about 440 volts DC when measured between the plate (pin 3) and the cathode (pin 8) of the EL84 tube / valve. Most EL34 push-pull power amplifiers are fitted with access pins / ports where you can insert the + and – test terminals of your voltmeter / multi-tester / multi-meter to allow you to adjust the bias current of each tube. Usually this is composed of a high-wattage 10-ohm resistor – usually designated as Rs and is connected from the pin 8 of a particular EL34 to the circuit ground. When you attach your voltmeter at this test point, it is effectively in parallel to an internal 10-ohm resistor or the Rs resistor.

Setting your voltmeter / multi-tester to the 1-volt range, if the voltage reading across this point is 500-millivolts or ½ volt, you can use ohms law to compute for the bias current of the particular tube you are measuring – i.e. current = voltage / resistance. Which at 500-millivolts / 10-ohms = 50mA or 50-milliamperes of standing current or bias current. Using this data, you can now compute the EL34 tube’s power dissipation in the amp you are testing. Using Ohm’s law formula to obtain power P=IV or P = 50mA multiplied by 440 volts = 23 watts, which will be the power dissipation of the EL34 tube as used in the particular amplifier design you are measuring. Though the voltage of the H.T. and the resistance value of the Rs resistor could deviate by as much as 10% during normal use.

From a design standpoint, the EL34 tube / valve and its derivatives – like the 6L6, KT77, KT88, 5881, 6550, 6CA7 tubes – were rated by their original manufacturers during the “Golden Age of Stereo” for a maximum anode dissipation of 25 watts and exceeding this rating will drastically shorten tube / valve life. This is so because the excess heat generated by exceeding the tube’s anode dissipation will cause the anode to release what is known as occluded gas, which damages the delicate cathode coating reducing electron emission. Some types of EL34 tubes / valves – especially ones manufactured in Eastern Block countries and China during the 1980s – may also have a low emission to start with and cranking up the quiescent bias current / standing current could cause the tube / valve to saturate on peaks / high-level signals, causing distortion.

Many veteran tube / valve amplifier designers suggest that if you are after extended output tube life, you shouldn’t run EL34-type tubes at over 20 watts of total anode dissipation to lessen the heat generated inside the tube / valve. Unfortunately, cranking up the bias current on these types of tubes usually results in an “improvement” of their high frequency response – i.e. a subjectively much louder high-frequency sound output. Which can be addictive when it comes to tube amps because of their really sweet and grain-free presentation of high-frequency audio information. Some are even known to crank up the quiescent bias currents of the tubes of their EL34s up to 85mA, making the tubes less likely to last more than eight months when use at an average of eight hours a week.

If you want to make the high-frequency response of your tube-based electric guitar amp “seem” louder, do what Queensrÿche does during the late 1980s. Replace the 12-inch Celestion speakers of your Marshall cabs with JBLs. Or if you can afford them 15-inch Leslie-derived organ speakers equipped with a whizzer cone similar to the one used by Jimi Hendrix when he recorded Little Wing in the studio back in 1967.

For those with a more thorough knowledge in tube / valve-based electronics, another method of increasing the high-frequency output of your tube-based electric guitar amplifier is to modify the preamplifier section. The majority of tube-based guitar amps being sold today still use the “conventional” 12AX7-based double-triode phase splitters in their preamplifier sections. The problem with the conventional double-triode phase-splitter is it’s high input capacitance caused by the Miller Effect. This causes high-frequency loading on the input (12AX7-based?) tube and reduces bandwidth, making it very difficult to use appreciable amounts of negative feedback - especially with 6L6 Beam Power Tetrode and EL34 Pentode output tube designs – without instability due to the phase shifts incurred. By the way, negative feedback is a necessity when extracting the maximum output power obtainable of relatively “modern” output power tubes like the 6L6 and the EL34.

Low noise R.F. pentode preamplifier tubes can be used as a better phase splitter in the preamplifier section of tube-based electric guitar amps because a pentode has a very low input capacitance and high gain due to the shielding effect of the screen grid. This means that the capacitance loading on the input tube (12AX7) is greatly reduced, increasing bandwidth and decreasing troublesome phase shifts.

Or if you want the ability to experiment lowering the negative feedback of your electric guitar amp or operating the output power section in triode mode without your amplifier breaking into spurious oscillation. Which could destroy tweeters when you used your tube-based guitar amp as an ad hoc hi-fi tube amp, then use beefier preamplifier tubes like the 5687 tube. The 5687 tube can dissipate 4 watts, it’s dead linear, needs less drive current, and has a low output impedance. Making it able to be used as a driver stage of an inter-stage transformer (Does this remind you of the AN214 driving the inter-stage transformer of an MJ2955-based transformer-coupled booster amp?). If you go with this redesign route, the output tubes of your guitar amp could be configured in self-biased or auto bias mode due to the secondary winding of the driver / inter-stage transformer as opposed to their de rigueur fixed bias configuration.

Auto bias has the advantage of providing automatic compensation of tube variability characteristics, so tube matching and individual bias adjustment will no longer be necessary. Though matched pairs of tubes are still desirable. Auto bias gives that sweet, easy sound tubes possess. Fixed bias gives more power, but a harder sound. In my experience, output tubes of auto-bias configured tube amplifiers tend to last forever. My neighbor’s 300B-based auto-bias amp still uses tubes that he purchased back in 1996 without any degradation in sound quality.

Another way of retaining the de rigueur double-triode preamplifier – i.e. 12AX7 tube-based preamplifier – of your guitar amp is by selecting an output tube designed with a higher mu or a high mu version. I have tried this in the past and it does work but it needs power supply / H.T. circuit modification of your guitar amp. The advantage of this route is that high-mu version of your typical output power tube – like the EL34 and 6L6 – are easier to drive, thus retaining the 12AX7 pre-amp tubes. The caveats include complete redesign of the power supply since high-mu tubes require higher supply voltages than the 450 volts DC needed by your typical output power tube / valve. Some of them needs the power supply / H.T. voltage to be raised to 1,500 volts DC in order to enjoy the maximum benefits of high-mu power tubes. Which can be a problem since power supply capacitors capable of handling 1,500 volts DC are somewhat rare, plus the output transformer needs to be changed to a type that can handle such voltage or your loudspeaker will be turned into a dazzling pyrotechnics display.

Though if you change the choke / inductor filtering the 12AX7 pre-amp tubes to one with a higher inductance – therefore higher DC resistance to lower the incoming voltage supplying the pre-amp tubes, the 12AX7 tubes will be fed with a highly-filtered H.T. DC voltage whose hum is now vanishingly low. Another caveat of this design route is that the output impedance of your guitar amp will now be higher than before – i.e. reduced damping factor – which your amplifiers tonality will be readily be affected by a loudspeaker with a widely-varying impedance curve across the audio band. Although guitar amplifiers that uses minimal amounts of negative feedback and high output impedance are known for their excellent musical performance, so you could probably design an electric guitar amplifier that could double as an excellent sounding domestic hi-fi audio amplifier using this route.

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