gringopig's guitars
Amp techs drive around in Mercedes electric vehicles, holiday in the Bahamas and gargle with Perrier all because we are all a bit scared and clueless about biasing valve amps.
Well, in some respect maybe rightfully so as the voltages inside the amplifier when on and potentially even when off and for some time thereafter can be lethal. I was once shocked by a Fender Champ and it wasn't a lot of fun I can tell you and ever since, I have been very wary about the wiggly bits inside an amp.
So, first things first. Lethal voltages exist inside amps and even when off, significant charge is stored in capacitors that when touched will discharge to ground using the path of least resistance and that might be right across your soon not be beating heart. So equip yourself with a means of measuring voltage up to 600V and a wire with a series resistor of a few Watts power handling and 500 ohm resistance to act as a conduit to chassis ground for the charge in the capacitors.
I use these: A Fluke meter with the ability to measure up to 600V DC and a cable I made up with clips on each end and a resistor in the middle, taped up.
In order to make sure the capacitors are discharged you will have to refer to a circuit diagram or probe between chassis and the plate pins. It takes a bit of experience. However, Lyle at Psionic Audio has a very good video about an easy rough and ready way to drain the capacitors and I can recommend him and his channel. See the video below...
Warnings and caution having been offered, I can say that fitting a new set of power valves is relatively easy with some nice tools. You need to at least check the bias after installing new valves but first, what is bias and why do you need to set it and how?
A valve (or tube as our American friends call it) is an amplification device. Like all amplifiers, it has a small signal input and a large signal output. The reason you get a large signal as an output is that there is a DC voltage present across the output that the amplified signal can exist in. This is an alternating signal in an audio amplifier and it has to exist in the space between extremes of polarity. The small signal input causes a large signal to be present at the output and the increase in power comes from the power supply.
However, the amplifier must be set into a condition where is amplifies in a linear way without distorting unduly on either positive or negative excursion of the output. There are graphs of performance for these various idle condition of the amplifier and these define the bias point of the amplifier.
The bias is the 'balance' of the amplifier which allows the amplifier, the valve, to amplify linearly and not to draw large amounts of current when no input signal is present. Think of it as a brake on the flow of current through the output.
In a valve amplifier, the bias voltage is negative with respect to ground to restrict the natural flow of current from cathode (negative) to anode (positive)
If there is no bias voltage or it is not correctly negative, then electrons will shoot across the vacuum from cathode to anode uncontrollably and bombard the anode with charge. This will cause the metal plate to get hot and start glowing red which is known as red-plating. Too much current and too many electrons bombarding the metal.
In other words, the bias and therefore idle bias current must be set within certain limits to avoid damage to the valve and also to provide a centre point to the excursions of the AC output signal.
Relax!!!
You don't need to know all this though. All you need is a bias meter, a screwdriver, a sense of whether the amplifier is fixed bias or cathode bias and in the former case, where the potentiometer is to adjust it and an understanding about the voltages present and the potential dangers present.
Adjusting the bias is only a requirement of fixed biased amps. In cathode bias, it is mostly automatically set by the circuit and the point of bias is much nearer 100% than fixed bias which is typically lower. Note that pre-amp valves don't require manual biasing!
I'm going to go through the procedure I used recently to replace the 6L6GC valves in my Fender Twin Reverb to show you how possible it is to do the work yourself.
I have 2 meters I bought from Eurotubes in the US. Not cheap and TAD in Germany do one as well but the Eurotubes one is more sophisticated than the TAD ones and I use the version which shows plate voltage, plate current and plate dissipation on the one meter. This is super easy as you don't have to do even the most simple calculation Power = I (current) X V (voltage). All you need to know is the maximum plate dissipation of the valve, in my case 30 Watts for a 6L6GC and select a suitable real world value of this - I assume a 6L6GC is a 25 Watt valve and a percentage of this dissipation to achieve at idle with no signal present. I will use 60-70% or in my case 16 or 17 Watts at idle on the meter. The power calculation is internal to the meter but the Wattage can be calculated from P=IV with V being the plate voltage and I being the idle bias current. You can use a TAD meter and a mulimeter to do the same thing.
You can see that the meter has a valve base that the valve sits in which itself fits in the amp valve socket to allow one valve to be checked. I have 2 meters and do pairs of valves: 2 outer then 2 inner on the Twin reverb
Inner pair of valves on meter
Outer pair of valves on meter
Plate voltages on meter
I pulled the chassis out slightly after taking out the top straps and other screws so as to access the bias potentiometer at the back of the chassis. I was careful not to touch anything inside or any of the pins in the valve base, so didn't need to discharge the amp. In any case, the Twin reverb drains charge quickly when not on I found as I checked it the time before when I re-biased and had the chassis fully out. But still...
You can see I have set the idle Wattage to 16/17 Watts and the idle current sits at 35 or 36mA with a plate voltage of 472V. The ultra linear Twin reverb is often said to exceed 500V plate voltage but I don't think that's correct really and 470V is well within the maximum plate voltage specification of a modern 6L6GC.
Quite quick and easy and only one calculation to make with this version of Eurotubes meter as plate voltage and dissipation in Watts alternates in the display. It's merely convenience though and other meters care just as good.
You don't even need a meter as you can calculate bias current from first principles too but that's much more technical.
To sum up then; if you have new valves to install and want to save money in the long run, invest in either a TAD meter or the Eurotubes one and a good quality digital multimeter rated for 600V and do the work yourself. It is a bit nerve wracking, especially for me having been zapped once but at least you would know how to do it and have confidence that your amplifier is sitting in a safe zone of performance!
Good luck!
