Biasing a Bassman
Changing the tubes on a pawnshop Bassman 50 - which to get and how to bias?
I purchased at a pawnshop a Fender Bassman 50 head and cab for a really good price. It seems to still have the original tubes. They are GEs (I might be wrong about those being the original ones), and now I am thinking of changing the tubes. A couple of questions:
1) What tubes do you recommend? I have bought JJs for my Marshall head and like the turnout for a more metal type of amp. 2) Biasing... What is the plate current I should get from matched pairs? I am thinking of buying a tube bias amp meter, and learning how to do the biasing myself.
Thanks for writing such a cool column.
Happy New Year from Mexico,
Happy New Year from the United States to you. Nice little prize you picked up at the pawnshop. The Bassman 50s and Bassman 70s can be some pretty good-sounding amps. While they’re not quite their blonde or blackface predecessors, they certainly don’t command the same price either, which gives them a pretty good bang for the buck.
With regards to recommending output tubes for the amp, nowadays that’s pretty much a matter of tonal preference and expectations. Twenty years ago I’m sure plenty of us would’ve told you to run as fast as you could from the Chinese 6L6-style tubes, and that the Russian 6L6-style tubes were less than stellar performers—but a lot has changed. Due to the effort and diligence of a few people in the industry, reliable and good-sounding tubes are available from most tube manufacturers. If it were my amp and I were playing traditional rock and blues material, I’d install a tube that was developed to have more of a “black plate” type of tone. This might be the TAD 6L6WGC-STR, the Groove Tubes GT-6L6CHP, or the Electro- Harmonix 6L6EH. This should produce a tone and response that is warm in the mids with a nice harmonic content, a decent top and bottom end, and a bit of natural compression. If I were looking to have the amp perform with maximum clean power and headroom, I might choose a tube that more closely matches the tone and response of a Philips-style tube. This might be the Ruby 6L6GCMSTR, the Groove Tubes GT-6L6S or the JJ 6L6.
As to your next question, “What is the plate current I should get from matched pairs?” I can interpret this question two ways, and since I’m not exactly sure what your intention was, I’m going to answer both. The first deals with the purchase of tubes. Many different retailers sell their output tubes in matched pairs and provide you with a grading number or bias current measurement of the tube. Although these numbers may help avoid re-biasing your amp during your next tube change—if you were to re-order the same type of tube with the same bias rating from the same retailer—there is absolutely no correlation between one retailer’s numbers and another retailer’s numbers. Each distributor measures the bias of a tube with their own test setup using their own predetermined set of voltages. This allows them to measure each tube equally and in a repeatable manner for their own testing purposes. There is, however, no established industry standard for measuring these parameters. This means that a matched pair of tubes with a reading of 35mA from one distributor using a high-voltage setting of 450Vdc and a bias voltage of 40Vdc could bias differently in your amp than another distributor’s pair of 35mA tubes using a high-voltage setting of 425Vdc and a bias voltage of 42Vdc. Then there are the other distributors that assign not a milliamp number but a performance rating, which again has no correlation to others’ ratings or any type of milliamp rating. It’s very confusing, but that’s simply the way it is, so if your question was about what current value tube you should purchase for your amplifier, the short answer is that it’s impossible to say.
If your question regarding plate current was about the proper current setting for biasing output tubes in your amplifier, the answer is: it’s completely subjective. The final “correct” bias setting for your amplifier will be determined by many factors: which tubes, the way they interface with the impedance of the output transformer, the expected performance of the amp, etc. If I were servicing your amp and you had chosen tubes that would enable it to produce the maximum power and clean headroom, I would more than likely bias your amplifier in the 20–35mA range, based on the performance of the tube. Each manufacturer’s tube will achieve its best performance for this desired result at different bias current settings. If you’ve chosen a tube so that your amp will sound “warmer” and have better natural overdrive characteristics, I would probably bias the output tubes in the 30–40mA range. But again, this depends on the particular tube. As you can see, this is far from an exact science. It takes years of experience, multiple measuring devices (I use an oscilloscope as well as current-measuring devices) and a very good ear. Purchasing a bias meter will at least allow you to adjust the bias control of the amplifier so that it produces the optimized desired result for you while being able to measure the current and ensure that it is within acceptable operating parameters. That, along with your ear, will hopefully allow you to set the “proper” bias for your amplifier. Biasing, like life, is mostly subjective and relative. That’s my non-answer and I’m stickin’ to it!
Jeff Bober, Godfather of the low wattage amp revolution, co-founded and was the principal designer for Budda Amplification. He can be reached at email@example.com.