A Sampling of 6L6s
With the increased popularity of the octal conversion, many players are going right past the 6V6 to a full, 30-watt 6L6 Blues Junior. One of my customers sent me his green board amp for the full conversion. He also sent a bunch of different 6L6s and invited me to roll them through his amp and tell him which I liked best.
Some of the tubes below are called 5581s, others are called 6L6s. A couple have both numbers on the same tube. Originally, the 5881 was the same as the 6L6GB, a 23-watt tube. Somewhere along the line, people began to consider the 5881 a 25-watt tube. The 6L6GC is a 30 watt tube. A pair can produce 50 to 60 watts when pushed to the limit. Calling a tube a 5881/6L6GC is just confusing. I didn’t attempt to test these tubes for maximum power because my testbed is designed to max out at 30 watts. It’s a club amp that can still be played in the bedroom, not a barely controllable monster like a Hot Rod Deville.
Here’s the testbed. Normally, my octal conversions are cream boards. This one is a green board Blues Junior with the TP24 power transformer, a TO26 output transformer, the unused portion of V2 converted to a cathode follower, and the usual tone stack mods, power supply stiffening, and a presence control. It has my bias board (mounted in the center) for extended adjustment range. The heaters have a positive DC offset of 27 volts to reduce the possibility of hum and to reduce the stress on the cathode follower. With the mods, its closest relative is probably a simplified Bassman circuit.
I managed the line voltage with an autotransformer to a standard 120V, which resulted in 375 plate volts and set the bias at 70 percent of rated output for each pair.
I listened for bass clarity. Bass should start with a clear, round tone, the fundamental. Tubes differ in how much harmonic content they add to the bass tone. You hear it as growl, more and more “rrrr” tone on top of the fundamental’s “ooo” tone. I listened for midrange clarity and note articulation. And, of course, for open, airy highs. Too much harmonic content can make highs sound harsh. I gave every set second and third listens. Here’s what I found:
These are the tubes I use and recommend for my conversions. I like JJ output tubes a lot– more than I like their preamp tubes. The JJ 6L6GC is bright, with lots of high-end clarity. The bass tends towards the growl–harmonic richness, not round lower end. The tone is assertive. Second/third listen: a bit louder than most of the others. These are straight-ahead tone-makers, good at cleans and nice grind when pushed. Once you get out of the low range, there’s not a lot of harmonic complexity.
This small-bottle tube has a shiny black plate and a big getter on the side of the glass. The bass is clear, deep and round, not quite as harmonic-rich as the JJs. The highs were sweet, with not a hint of harshness. They’re sensitive, responsive, with nice overtones as you get into grind/overdrive. The plates are among the largest, perhaps the largest of the lot. Big plates can be a rough indicator of bass performance–more surface area, more electrons moving effortlessly through the tube. My second listen confirmed the big bass tone, and an overall sound less bright than the Sovtek 5881/6L6WGC. Can a tube have too much bass? Not really, but you’d like these in an amp that’s a little bass-deficient.
Mesa 6L6GC (STR 420)
These Coke-bottle envelopes are old-school. They also have small, uniquely shaped plates. The tone is uncomplicated and the low end is round, but they’re sensitive to drive. With no other changes other than correct bias, they seem to want to grind more readily than the other tubes. The overdrive tones had nice harmonic richness, but the clean tones were flat, uninteresting. The back of your Blues Junior isn’t the optimal place to show off the fancy shape, but they do fit, barely, without touching.
I had two pair of this popular tube. Interestingly, they didn’t sound very similar.
Pair 1 was dark-sounding, boomy. Notes would bloom and the highs were muted. On subsequent test runs, I decided that they were mushy and inarticulate, especially in the bass. If I didn’t know they were new, I might have thought they were used, worn-out tubes.
The second pair was brighter, crisper. The bass had more of that harmonic content, a nice growl. Overall, they were very open-sounding. The 5881WXT has a different base, but internally, it’s the same tube as the 6L6WGC, below. They’re all the same Sovtek 6P3C-E, a tube originally designed for the Soviet military. You can see from the photo below that the plate structure is the same; only the bases are different.
So here we are, with the third set of identical tubes and a strikingly different tone. I heard clear bass with nice growl, smooth mids, and an open bright top end. Translation: Twang! They sounded very Fender-y, the best of the lot in that regard. Sovtek 6L6s have a reputation for consistency, but I sure didn’t find it in this little sample.
The Tung-Sol 5881 is a real 5881, a 23-watt tube, well regarded for its sweet, classic tone. Just to see what would happen, I biased it hot, as though it was a 30-watt 6L6GC. In the clean range, it held together, with abundant bass. When I turned up, it started to get mushy, not so pleasant. Biased like a 23-watt tube, there was noticeably more clarity, more sparkle. If 25 watts of total power is enough, the Tung-Sol is one sweet tube.
Here are the tubes, in the same order as above. I aligned their plates rather than bases, so you can see the comparative difference more easily.
You could be happy with most any of these tubes. A minor adjustment of the tone and volume controls will get the most out of any of them. I still like the JJs for consistency and their growly, assertive tone. I was pleasantly surprised by the small-bottle TAD, especially the big bottom end. The overall tone was very nice; I could be very happy with these. But if all Sovtek 6L6s sounded like that third, small-base pair, they would be the hands-down winner, the runaway best-seller for every lover of Fender tone. But if you buy them and they sound like the first pair, you’ll send me hate mail.