Jet City Blues readers will know that I have been fiddling with my Fender Bassman LTD amp to improve the tone of the beast. Sometimes a power tube change can make a positive change in tone, as I discovered when I installed a pair of reissue Tung-Sol 5881s in my old Victoria Bassman clone.
The factory specifies 6L6 power tubes for the LTD. A friend recommended trying JJ Electronic 6L6GC tubes, so I ordered a pair from Eurotubes in Oregon.
The LTD is an adjustable bias amp. This means that the plate voltage delivered to the power tubes has to be adjusted to factory specs after installation. Now, if I took the amp to a Seattle shop to have this work done, I would have to wait six to eight weeks to get the amp back. Eurotubes has detailed biasing instructions both in writing and in video format on their web page, So I thought I would give it a try.
Well actually, I asked my girlfriend Lauri to give it a try, since she worked at Seattle's Bozotronics back in the day, and knows how to get around inside of a hot amplifier without getting electrocuted. She was coming over to my place to join us for dinner on Thanksgiving, so she brought her Fluke multi-meter with her and we learned how to bias tubes that day.
We took the upper back panel off of the cabinet, revealing the amplifier components. We located the bias trim pot and bias test point on the printed circuit board and Lauri set the meter to read millivolts. The amp has to be on (including the Standby switch) for a few minutes for the meter to read correctly. First, we measured the factory setting for the stock Groove Tubes 6L6s. To get this reading the multi-meter black probe touches the amp chassis and the red probe touches the bias test point. This setting was 59 millivolts. I played harp through the amp for a little bit to get some sort of tone baseline.
Eurotubes recommends that the JJ tubes be set somewhere between 60 and 90 millivolts. How to decide where to set it? Simple - play through the amp at different bias setting to see which one sounds the best to you. Turning the trim pot with a screwdriver either decreases (turn left) or increases (turn right) the tube plate voltage.
Surprisingly (to me, anyway), the JJ tubes did not sound as good in the amp as the stock Groove Tubes 6L6s did, no matter what the bias setting. The JJ tubes were too clean and subtracted from the essential Bassman funkiness. I am guessing that the JJ tubes would be a good choice for a guitar player that wants a good clean tone though.
So, we reinstalled the Groove Tubes 6L6s and tried various bias settings. The best sounding bias setting? 59 millivolts - right where we started! So, no more fooling around with power tubes for now. By the way, the Weber P10R speaker clones that I recently installed in the amp are really starting to blossom after a few gigs. They are a BIG improvement over the reissue Jensens installed at the factory.
My thanks to Lauri for her help with this project!
Please remember that guitar amps operate on dangerously high electrical voltages. If you feel uncomfortable performing this type of work on your amp, DO NOT DO IT! Take the amp to your local repair shop instead.
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