I recently fitted the power stage of my Fender Hot Rod Deluxe with a pair of Ruby 6L6GCMSTR tubes. I gave a full review of the Ruby tubes in an earlier blog post. What I want to discuss here is the bias setting for the power stage in a Fender Hot Rod Deluxe.
I did a lot of research before deciding to replace the Hot Rod’s factory-fitted Groove Tubes GT-6L6 C(HP) power tubes with a pair of Ruby 6L6s and learned much about 6L6 variants and their different power requirements along the way. It’s all a bit of a mine field. But thankfully help is available in the form of an old article by Justin Holton at The Unofficial Fender Hot Rod Deluxe Owner’s Guide.
When you replace the power tubes on a Fender Hot Rod you should always have the bias setting adjusted to ensure that your tubes are operating within safe limits and you’re getting the kind of tone that you want from your amp. Unless you know what you are doing, never attempt to adjust the bias on your amp yourself. Without a good working knowledge of electronics you could blow your amp, or your tubes, and possibly kill yourself in the process.
When you adjust the bias on your amp you are adjusting the level of current available to the power tubes. Setting the bias on your amp is like setting the idle on your car. The higher you set your bias, the hotter your tubes will run and the earlier your sound will break up. They will also burn out sooner. This may be just what you want. But never set the bias current so high as to exceed the plate dissipation limit set by the tube manufacturer at any portion of the tubes operation. Beware that different types of 6L6 tubes have different operating level ratings and recommendations. The lower you set your bias, the colder your tubes will run and the longer they will last. You’ll get more headroom and a cleaner tone. But set the bias too low and your tubes could sound flat and sterile. You don’t want your tubes running at one extreme or the other. So long as your bias is set within the safe operating limits defined by the manufacturer of your tubes, there is no single “correct” bias point. It’s all a matter of taste.
Trawl the web and you’ll find that recommendations for setting the bias level of 6L6GC tubes vary greatly. This isn’t a problem. You can run your bias at what ever millivolt (mV) level you want, so long as you set it within the safe operating limits of your tubes. The problem is that many sources cite Fender as recommending this, that or the other particular bias level. But take a close look at the schematics from the old Hot Rod Deluxe or the newer version III and you’ll see, in very small writing which you may need to magnify a few hundred percent, the following:
Set bias adjust pot R82 to obtain a reading of +60mV at TP30
On the electronics schematic, R82 is the bias adjustment wheel and TP30 is the location of the test point for your multimeter.
Fender has never recommended any setting other than 60mV. Now, if you know anything about amps and tubes, you’ll know that 60mV is a little on the cool side. Fender recommend 60mv to ensure your safety and tube longevity. But Hot Rods tend to leave the factory with their bias set somewhere between 60mV-70mV.
I run my Ruby 6L6GCMSTR power tubes at 68.3mV. I find that this setting offers maximum headroom, warmth, shimmer and chime on the clean channel. I’m not too interested in driving the tubes for early breakup. I tend to use my Suhr Riot Pedal for crunch and higher levels of distortion. 68mV works as the sweet spot for 6L6GCs and similar power tubes, but is unlikely to work so well for other types. And it may be way too hot for some power tube varieties.