"When I dime the amp to get some nice power-tube overdrive, the sound gets a little spiky instead of turning big and round."
For starters, let me say your column is the first piece I read when PG comes in the mail each month. It’s very informative and easy to understand.
My question is about my Carvin Nomad, which I love (even the Soak Channel, which some people knock). The only thing that doesn’t totally please me is that when I dime the amp to get some nice power-tube overdrive, the sound gets a little spiky instead of turning big and round. I assume this is because of the EL84s, and I’m wondering if larger power tubes would solve my problem. I’ve done some reading on tube amps and realize that this isn’t simply a matter of replacing one type of power tube with another. The tube sockets are different and I may need to upgrade the power transformer and output transformer. Am I heading in the right direction?
If so, could I get my 50 watts from two 6L6s with my current transformers? I’m also not sure if the amp’s wiring would allow me to install octal sockets in place of the 9-pin EL84 sockets. I work in a sheet-metal fabrication shop, so I could do any required changes to the chassis, and I’m confident I could do the wiring too, while being safe. Any ideas?
Thanks for reading—and for writing. I applaud the fact that you’re looking to improve the performance of your Nomad, but I’m not sure you realize what an undertaking this would be. I’ll discuss this and also offer a couple of options that would be easier to execute while still getting you closer to where you need to be.
Let’s start with your question about possible alternative output tubes. Switching from the stock EL84s to some other output tube would certainly yield a sonically different output stage, but I’m not 100-percent sure it would remedy your issue. That said, if I were to suggest a different tube type, I’d say the tube with the closest match to the primary impedance of the existing output transformer would be a 6V6. The problem here is that, as you said, you would need to switch from the 9-pin sockets to octal sockets. On the surface, you being a sheet metal guy, it may not be a problem for you to punch larger holes in the chassis and install the octal sockets. The wrench in the works here is that the 9-pin sockets are mounted on a circuit board, and you would need to start from scratch installing all the appropriate wiring and components necessary for the new sockets and tubes. Additionally, you would need to modify the bias supply to get the increased voltage required to bias the 6V6s. That’s not terribly difficult for a seasoned tech, but definitely something to consider.
You mentioned 6L6s as an alternative tube type. While the primary impedance of the output transformer in most amps with four EL84s would be close enough to work, it would not be as close of a match as four 6V6s. You also mentioned changing transformers. The mains (power) transformer should be fine, and we’ll address the possibility of replacing the output transformer in a moment. In order to alleviate all the chassis cutting and rewiring required for the above changes, let me suggest three simpler (and cheaper) alternatives you can try first.
1) Have the bias of the output tubes checked and set. [Warning: This requires working around high voltages, so the procedure is not for a novice.] Carvin recommends setting the bias for a reading of 85 mA across the Standby switch (in the off position). This seems a bit low for four EL84s, though. I’d suggest setting it up closer to the 100 mA range. This will warm up the output tubes a bit. To my ears, EL84s always sound better when they’re running a bit warm.
2) Experiment with different brands of output tubes. Each make has very different tonal and distortion characteristics, so try some Sovteks, some JJs—even some new-old-stock tubes, if you have a little extra money to spend—or anything else you can find. They will definitely make a difference. Personally, I’ve always liked standard Sovtek EL84s in the amps I build. Always use a matched set, though, and remember you will need to reset the bias when you change the tubes.
3) Install an upgraded output transformer. An amp’s output transformer makes a big difference in the sound. The stock transformer may do an adequate job, but at higher output levels— like those needed to push the output tubes to the point of global meltdown and fantastic tone—the transformer may be saturating. Replacing it with a better transformer may make a very noticeable difference.
If you choose option number 3, search around and see what you can find that’s recommended for using with four EL84s. A Vox AC30-type transformer will work, but you will be limited to only 16 Ω and 8 Ω output taps and you’ll need to cut a substantial hole in the chassis, because the transformer is a lay-down type. You may also have limited chassis real estate due to the size of the circuit boards in the amp. If you can’t find an appropriate stand-up type transformer, contact me and we’ll get you into one of the transformers I use in my 30-watt amps. That should work well for your project.
There you have it. Hopefully that’s all you’ll need to bring that Nomad home!
Jeff Bober is one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff recently launched EAST Amplification, and he can be reached at email@example.com.