The second Twin was the one which still had its original power amp valves. The tubes were RCAs, and seemed to be in working order. But, because we had already agreed with Silas that the power amp tubes should be replaced anyway, I took the old ones out without further testing. Albert had been playing this amp for a few years and Silas had also used the combo regularly, which meant that the original valves had probably reached the end of the road.
I had long been planning to write something concise about tube amp biasing. Would this be a good place to combine something beneficial with something amusing? In my view biasing has not been explained in an easy-to-understand way anywhere, yet. Let me try to demystify the process for your benefit.
Why do we need biasing?
Let’s hope I don’t disappoint you, but actually tube amp biasing is a much smaller, and much more mundane matter, when it comes to your rig’s tone, than many so-called gurus would make you believe! Basically, biasing makes your power amp valves work in their ideal range of operation. It is also a sort of security operation – if your valves run too hot, something they might burn out in just a couple of minutes. I’ve been there and watched it happen!
If the amp’s bias is set too low part of your signal will go missing-in-action, resulting in a weak sound. A too hot bias, on the other hand, will drive the power valves beyond their ideal break-up point – what is usually called the “sweet spot”. In this case your amp would lose a good deal of its dynamic range and touch-sensitivity. In most high-gain master volume amps players rely on heavy preamp distortion for their tone, though, making biasing more of a formality.
What does adjusting the bias mean?
If you imagine an amplifier as a sort of automobile, then biasing is the way to setting its engine’s idle speed. You adjust the bias level in order for the amp to “idle” smoothly and efficiently, without using up too much “fuel” (i.e. electricity). Just as a car uses up fuel when idling at a traffic light, a valve amp will use some electricity, even when you’re not playing.
Playing your guitar feeds more electricity through the tube. If your power amp tube were a jug of water, the bias voltage would make up approximately 7 dl (70 %), and your guitar-playing would then fill the jug up to the top, adding 3 dl (30 %). This whole litre would be the ideal maximum level of saturation (100 %) in our little metaphorical look at the vacuum tube.
- If you leave the biasing too low, say at 60 percent, you will not get optimal saturation, as your guitar signal would only fill the jug up to 90 percent. Your sound won’t be hot enough. There’s not enough water in the jug to quench all of your thirst.
- If you set the bias too high – for example all the way to 90 percent – you would push the valves into a rating of 120 percent with your guitar signal. The tubes would be driven into over-saturation, they would compress and get far too hot. The jug would run over, spilling water onto the table.
- Problem is a tube cannot flow over like a jug, it is much more like a closed container. If you have a closed container filled up to its full capacity (100 %), and you try to force an additional 30 percent of water in, chances are your container will break. This is what happens to a tube, too.
This type of scheme applies to all Class AB (push-pull) amplifiers. Class A -amps are self-biasing (at least in theory), or cathode-biased amps. Preamp valves need not be biased at all.
Biasing in practice
First off I measured the grid resistors soldered to the valve sockets. They are meant to work as protective “fuses” for the tubes. The resistors were all in working order and their readings within specifications.
Sometimes, when a valve fails catastrophically it can take down one (or more) of its protective grid resistors with it. Even if you don’t know how to change these resistors you should find out the resistors’ correct values and keep a couple of spares with you. Without these resistors changing a blown power amp valve is completely useless. If you’re at a gig you should be able to locate a repairperson to exchange the melted resistors for you in almost any city.
Next, I checked the heater supplies, which also were all within specs at 6.42 VAC. The heater voltage should be checked as a routine step. If the inside of a tube isn’t heated no electrons will be able to flow, because of the lack of conductive gases. Usually the heater supplies run somewhere between 6.3 and 6.9 VAC.
The next step was measuring each tube’s plate- (pin 3) and grid-voltages (pin 4) off of each socket. In our case the values were the same for each of the power amp valves. In case of a serious component failure inside the amp’s wiring you would get differing readings between the tubes. In such cases you would first have to get to the bottom of the underlying problem, before swapping tubes would make any sense. Here everything is hunky-dory, and I note down the values for reference during the actual biasing phase.
Negative grid voltage or bias
As a last step before biasing I measured the negative grid voltage at pin 5. This reading is connected to the grid inside the valve which regulates the flow of electrons from the cathode to the anode. This negative voltage can also be measured without any tube inserted. It sounds kind of confusing that the voltage measured at the socket pin is negative. Does a larger negative voltage mean a higher or lower signal level? When were biasing – with a tube inserted – we will use a different meter to gauge its performance. Then, the voltage readings will be positive. What does this mean?
Tubes, and their appetite for energy
Every vacuum tube is an individual case. The way a valve consumes and processes electrical voltage and current is crucial to its own, and the amp’s, functionality and performance. Even if you hated Ohm’s law at school, you should bear with me for a few moments:
A guitar signal is voltage. Each preamp stage adds voltage, until the signal reaches the power amplifier.
The power amp valves are geared to produce current (amperes). The tubes won’t put out any watts, unless there is sufficient voltage supplied by the guitar signal. For a guitar amp trying to amplify our tiny little guitar signal into a mighty roar this means (in simplified terms):
Volts (voltage) x amperes (current) = watts (power)
The larger all these numbers are the louder your amp will be. Power amp tubes are divided up into pairs of two or four by the process of matching. The tubes are measured at the factory (or by the supplier) and graded as “low amplification” (low current draw), “medium amplification” (medium current draw) or “high amplification” (high current draw) – often denominated by a number on the package. Biasing allows us to use the power amp valves at their optimal operating level, regardless of their grading.
A tube’s life expectancy
A good flow of current is a precondition to amplifying a guitar signal. Like us humans a valve ages over time, making it less and less efficient. There’s a large efficiency drop right at the beginning of a fresh tube’s working life. Because of this we can adjust a new valve to draw a little too much current. The first drop at the beginning will make the tube settle into its optimal operating conditions. When a tube’s efficiency starts deteriorating with age – leading to a drop in current draw – you can readjust its bias-idling to give it a new lease of life.
Many players swap their power amp valves too early, because of their amp’s tone and dynamics becoming lackluster. By readjusting the bias you can “widen” the grid to let more electrons flow from the cathode to the anode (increasing the draw), leading to the valve getting back to its optimum level of saturation. This step can very often be repeated a couple of times, until the valve is so worn out that it isn’t capable of drawing enough current anymore. When this happens it is time for a new set of power amp tubes, because – according to our rendition of Ohm’s law – if there’s no sufficient current draw, no amount of voltage will turn out a powerful signal.
I can see how “tight” a tube’s grid is “woven” by measuring the voltage at pin 5. The lower the voltage reads the tighter the grid standing in the electrons’ way is. A low voltage means less current running through the valve – it runs cool. The higher the voltage is raised – meaning less negative grid voltage (i.e. a smaller number) – the hotter the signal becomes, because the grid impedes the electron flow much less. For starters – and to stay on the safe side – we can set the negative grid voltage to -30 VDC, before putting a new tube into the socket. Once the valve is in place, we will be able to monitor its current draw and fine-adjust its bias using the amps internal trim pot(s).
The old tubes in the Twin didn’t seem to have been matched all that well, or maybe their biasing had been changed somewhere along the way, as the two pairs were reading slightly different voltages (-55 and -57 VDC). These voltages are so low that I can plug in new tubes without fear of damage.
The bias is calculated in relation to the anode-voltage. Some of my biggest amp idols, like Ken Fischer (Trainwreck Amplifiers) and Gerald Weber (Kendrick Amplifiers) do their biasing by ear, going for about 35 mA for a 6L6-type. I like to do things much more methodically, and use a TAD Bias Master.
A handy measuring tool
An easy way to measure the efficiency of power amp tubes is to insert specialized sockets between the valves and the tube sockets, and connect them up to a bias meter. Inside the Bias Master's sensors a special one-ohm resistor converts the amperes running through the tubes into easy-to-meter millivolts. I hooked the TAD Bias Master up to the Fender Twin and turned the amp on. I remembered the value of the anode-voltage I had measured before. As the amp warmed up, the grid values climbed to 25 to 27 mA.
You have to keep in mind that raising the bias will lower the anode-voltage. The term “Brown Sound” describes the sound of valves with a low anode-voltage. Usually a lower the anode-voltage leads to a crunchier sound.
You can achieve this effect by using a Variac to drop the amp’s mains power supply artificially. In this way the mains transformer’s secondary coil will send a lower voltage to the tube’s anode connected to pin 3. If you adjust your bias correctly the sound will be warm and crunchy. The drawback in this method lies in the lower heater voltage, which has its own impact on the valve’s efficiency. The best way to achieve a Brown Sound is to install a separate transformer for the heater, which is getting its power before the Variac.
Another way to achieve a crunchy tone is to use very powerful and efficient tubes, which draw a lot of current. Because such a tube needs more current the anode voltage will be lower, and the bias will be reset in relation to the changed parameters. The sound will seem crunchier to most people. It is important to get the relationship between all the settings right. It also depends on the amplifier in question, whether the bias pot will have enough of a range to be able to compensate for power amp tubes that are vastly different from its factory specs.
I measured the Twin's lowered anode-voltage and compare its level to the values on my bias value chart. The bias was still a good deal too low.
Understanding the bias value
In contrast to the negative grid voltage, with the bias value, more really is more. A higher value will result in a hotter-running valve. I turned the trim pot full on, but the meter still read too low. It seemed these tubes didn't seem to suck up enough power. You could call them low-powered. Usually a bias trimmer has a much wider range than this one here, allowing you to use a much more diverse field of different makes of valves. This type of narrow trim pot is rather unique.
Virtually all manufacturers bias their tube amps “cold” at the factory. This way the tubes run cooler, don't age as quickly, and the fail rate is much lower. At Backline Rental I used low bias settings for all amplifiers that didn't rely on power amp distortion for their signature sounds (read: almost all amps, expecially Heavy Metal and Thrash stacks). The amps sounded good and the valves didn't blow up. With non-master volume amps correct biasing is more crucial than with a master volume amplifier. I could just have given up and run the Twin at these low bias values without problems, but this was not the time for compromises. I wanted to get the biasing right for a number of reasons.
Selecting a different set of tubes
You're left with two options in situations like these – swap the trim pot or swap the valves. I opted for the latter. I shuffled over to the Custom Sounds shop and picked up a different set of matched power amp valves. The PC-values (PC = plate current) printed on their boxes were different. More is more in this case, so I was pretty sure these valves would raise my bias-readings.
I plugged them in, and I knew I had a winner! Now, the bias levels were much higher in relation to the anode-voltage. The bias was even high enough for me to use the trim pot to set it a little bit lower, leaving a wee bit of welcome leeway for adjustments further down the line.
Just to repeat it once more: As tubes lose their efficiency, when they get old, you have to raise the bias levels to keep them at the correct working level in relation to the anode-voltage. This way you can get a lot more mileage out of your power amp valves, before they start to get really “tired”. Have a look at my bias reference table. These days, you can even use clever online calculators to determine the correct biasing value, like the Weber Bias Calculator.
Mismatched tubes, good tone?
Let me play the devil’s advocate turn what we’ve already learned about biasing and matched sets of tubes on its ear: The early years of Rock spawned a whole wealth of great guitar sounds. At that time biasing was largely unknown by anybody outside an amp factory. This means that plenty of fantastic records have been made with unmatched, badly adjusted valves. You used whatever you could get from the nearest TV- and radio shop to make it through the next gig.
Unbalanced valves result in unsymmetrical clipping, which is an effect many guitarists like. Many overdrive-pedals are based on unsymmetrical clipping, like some of Mike Fuller’s Fulltone-pedals or some of the creations by Finland’s sound guru Miikka Paatelainen. You can still get this effect in your amp. If your tubes are badly-matched in just the “right” way (in the right relationship to each other), you will get odd-numbered harmonics, resulting in a juicy tone that will stand out in a mix. Personally biased and (mis-) matched tubes are another part of guitar amp mystique that can be exploited by the marketing departments of amp makers.
Time for a test run
Now it was time to try out both combos. I set up the pair of Järvinen-Twins side-by-side, hooking them up for simultaneous use. I was so excited I had difficulty remembering which Twin was which. Which amp would prove to be the better-sounding one – the JJ-equipped version Olli had biased or the combo I had loaded with a set of TAD's more saturated 6L6s? I felt so excited, like being 16 again. Silly, isn't it?
The clean channel
I plugged in a Nash S-63 and selected the clean channel. I was greeted by a very Fender-ish clean tone with spades of headroom. I instantly took to the bright and clear tone. The reverb had to be applied sparingly. It wasn't as full-sounding as a Blackface-era spring reverb, but sounded very good when applied with taste. The amp on the right had a good “ring”, as well as a tighter and brighter bottom end, which pleased me very much. Somehow the combo on the right sounded a bit more coherent – the differences between the amps were small, but noticeable. This might also have been down to differently aged capacitors. I simply had to peek at the Twins' backs, and was relieved to find that the combo I had equipped with new tubes and re-biased myself was my favourite. Placebo effect or no – time to switch channels!
The overdrive channel
The overdriven side of things sounded surprisingly lush – more Marshall than Fender – fitting a Strat-style guitar to a tee. There was a fair amount of bottom end, without the bass swamping the clear trebles. I remembered Albert once remarking that the Twin gave him “all possible sounds in the same amp”. Seems this wasn't blatant advertising speak at all, but the genuine truth! At least with singlecoil pickups…
As my next move I hooked up my Reverend Bob Balch guitar to the pair of Fenders – not such a good fit for the Twins, as it turned out. The clean channel's tone was just about passable, but I was in for a huge shock, when I switched to the overdrive channel. With this humbucker-equipped guitar the overdrive channel had no finesse whatsoever and sounded more like a fuzzy transistor amp than the valve combo the Fender Twin really is. Strange, as the Fender-style guitar had worked so well…
I returned with the Nash strapped on, and started sending Albert's riffs and licks from his Hurriganes-period into the darkening evening sky of Kruunuhaka. My personal favourite still is It Ain´t What You Do, which I remember having seen Albert play on TV sometime in the Seventies. He played the riff in a special way, using both the open low E-string, as well as the octave on the A-string. After going over it for some time, I finally managed a decent version of the song myself. My electric guitar-riffing days have been behind me for many years now, so I turned off the video camera to spare myself public embarrassment.
As the day turned into night I switched off the lights and stepped onto the sidewalk. As I walked home I remembered the words of Jukka Orma (who also lived in the district of Kruunuhaka): “Albert always sounded like himself, regardless of the gear he used!” And he should know, because he played with Albert during the time Järvinen used these very Fender Twins.
As I walked up the stairs I realized that the amps hadn't made me sound like Järvinen at all. The older I get the more I find truth in the notion of the tone coming from a guitarist's hands, and not his (or her) equipment. I sipped my evening coffee, while I watched this clip from the 1970s on YouTube, the music of my hero filling the room.
Thank you for all the music, Albert!
24.9.2014 Kimmo Aroluoma
(Translated by Martin Berka)
The author is one of Custom Sounds’ owners, and an incorrigible guitar and gear enthusiast.