Biasing your Rivera Amp
By Richard Johnson
Updated: October 18, 2003
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For reference, Willie Whitaker's (Lord Valve) "How to Bias Your Amplifier" and Rivera's "Biasing Instructions" is still on the website.
Let's get this part out of the way right now…
WARNING: Your amplifier is a dangerous place to poke around. It has lethal Voltages in it. If you don't know what you're doing, then take your amp to a tech. If you are game then give it a try. Just be very careful. At any rate I take no responsibility for the use of the information in this document. You are totally at your own risk.
Why Bias an Amp
Why bias an amp? The same reason you set the idle on a car - because your amp will sound and perform better. Plus with biasing you can change the way your amp reacts. You can set it to be really clean with a lot of headroom and dynamics, or set it to break up quickly with a more compressed sound. To not be over technical, biasing an amp sets the amount of idle current on the output tubes.
Methods of Biasing an Amp
There are several ways to Bias an amp. One is with a Test Oscillator and an Oscilloscope (or 'Scope). This is how most techs bias an amp, is the method recommended by most manufacturers. Another way to bias is called the Transformer Shunt Method. This is a great method because it only requires a Digital Voltage Multimeter (DVM) and some alligator clips. A third method is the Cathode Resistor Method. This usually involves modifying the amp and isn't very easy to use on a Rivera, since the power tube sockets are on a circuit board. However, there is an easier way to use the Cathode Resistor method, which is to use a couple Biasing tools on the market. One is called the Bias Probe which actually includes a built in meter (but only works with amps using AC heater voltage - so it one works on the M and S-series Rivera amp) and the other is the Bias Kit from Groove Tubes (my choice) which requires an additional DVM. The final method is to measure the Main idle current method, which is the method recommended by Rivera. We'll call this the Rivera Method. The Rivera method requires an AC Mains Current meter, but a DVM will work as well and since they are inexpensive, I'd recommend getting the DVM for this method.
I am not going to describe the Scope method because it requires expensive equipment, including a scope an oscillator and a dummy load in place of the speaker. I will describe the transformer shunt and how to use the Groove Tubes Bias Kit. I used to use the Transformer shunt method extensively, but ended up buying the Bias Kit, because I'm lazy. With the Bias Kit you don't have to remember where to clip the DVM's leads. This is nice if you own several different brands of amps. Both these methods work great but aren't quite as easy as the Rivera Method (Mains Current), so this is the method I'm going to describe.
First, get a matched pair or quad of output tubes if you are changing tubes. Rivera uses Svetlana brand tubes but I've found that JJ/Tesla and good NOS tubes work well too. (Hint - check out the page on output tubes…). Don't worry about getting a "Name" re-branded tube from one of the fancy resellers. You just need a decently matched set from a reputable seller. If your amp has Rivera tubes in it you can also just buy the same rated tubes - however, you still might want to re-bias to get just the sound you want.
You also need a DVM. You can get one from your local Radio Shack for under $50. A regular voltmeter isn't good enough, you need the digital variety.
You also need a jeweler's screwdriver, or better yet a small insulated screw driver.
Unplug your amp's power cord and footswitch. Also remove any guitar chords from the input jacks of the amp. Remove the amp's chassis from the case. On most Rivera's you can do this by removing the back panel and then unscrewing all of the screws on the top of the amp cabinet. Unplug the speaker and reverb wires (if the amp has reverb). I find that you can usually place the amp on top of the cabinet. Just don't rest the amp on the power tubes, but you can rest it on the transformers. Un-plug the speaker cable - this is important. The Rivera method is opposite of most in that it requires that the first speaker jack has no connection. If your amp has a Modern/Vintage (or Pentode/Triode) switch make sure it is in the Modern (Pentode) position.
WARNING - be very careful inside the amp. Do not poke around with your fingers. You can seriously hurt or even kill yourself by touching the wrong component.
With the amp open and the circuit board facing you, locate the bias pot. This is generally a small black plastic part with a white screwdriver adjustment in the middle. This is how you adjust the flow of current to your power tubes and "bias" the amp.
The Bias Pot looks something like the above.
Make sure the amp is turned off before starting.
Biasing to Rivera's Recommendations
Clip the negative (black) probe of the DVM to the chassis of the amp. The positive (Red) lead needs to be connected to pin 8 of one of the power tubes. These connections are not easy to clip to. Fortunately, you can clip the positive lead to a connection on the Speaker jack output board. Locate the Speaker one jack and directly behind it you will find a small black wire connected to the board. Connect your DVM positive lead there. Set the DVM to the Auto Voltage setting and turn it on.
Make sure there is nothing connected to the speakers jacks of your amp. Connect the power cord and turn on the power switch. Wait a minute and turn on the stand by switch. Let the amp warm up for a minute or two. Read the DVM meter. If it is fluctuating over a large range then wait a while longer until it settles down. Once it is stable or fluctuates only a little bit you are ready to adjust your amp.
A good place to start setting your bias is with the Rivera defaults. Use the following settings from the Rivera FAQ as a starting point:
R30 and Chubster 40 Models are set to 70 MADC Total for both tubes, or 35 MADC for each.
All of our 55 and 60 Watt Output Stages are set to 80-85 MADC for both tubes, or 40-42.5 MADC for each tube.
All of our 100 watt models are set for 160-170 MADC for all 4 tubes, or 40-42.5 MADC for each.
Locate the bias pot and adjust the pot until the DVM reads within the above values for your model of amp. Now you have your amp properly biased!
Additional Notes on Biasing
This section will discuss the merits of using different bias settings than the recommended ones. The only wrong settings are those at the extremes or those that cause the power tubes to glow red. If your tubes are glowing red (not orange) then you need to turn off the amp and lower the bias. If you don't lower the bias, the tubes will last for minutes instead of months.
Why would you want to set the bias to a different setting? The reason is to fine tune the amp to your style or needs. Biased to stock settings the Rivera amp will cover a lot of great sounds; I personally find them perfect for my needs.
But let's say you play blues and don't want the cleanest clean. You want a slightly gritty clean and you'd like the amp to break up earlier and have a hotter sound. You can bias the amp hotter. Then you might want to set the bias to 45-50 MADC for a 55/60 watt amp. Just make sure the tubes are not glowing red. Disconnect the meter, turn the amp off. Hook up the speaker and your guitar and play the amp. Is it too gritty or break up too soon? Back off the bias setting a little and try again.
Let's say you play steel in a country band and want the cleanest sound. You would then want to bias the amp colder. So your might bias that 55/60 watt amp to 35-40 MADC to increase the clean headroom.
Remember the best setting is the one that gets you the sound and feel you want.
Of course the Rivera website's FAQ has a lot of great information. Some good references on tube amps in general are "The Tube Amp Book by Aspen Pittman" and "A Desktop Reference of Hip Vintage Guitar Amps" by Gerald Weber. I learned a lot from these books and some of the contents of this article are derived from what I learned. Also if you know a good tech, buy him lunch and pick his brain (but don't be too much of a pest!), you'll learn a lot.