What About Tubes?
Can I change the tone of my Blues Junior with a different set of tubes?
In a word, no. At least not to a major degree. But some of my customers disagree–see below.
Tubes are not a mod. They’re like the tires on your car–they get you places and they wear out. Different tires may feel a little different, but mostly they’re round and black.
Each manufacturer’s tube has a characteristic sound, but overall the differences are slight–about one notch on your tone control. Of course, a new tube will always sound brighter than an old, worn-out one. All tubes lose brightness as they age.
Your Blues Junior has five tubes, V1 through V5, numbered right to left as you look at the back of the amp. V1 is the preamplifier. It has the most effect on tone. V2 re-amplifies or recovers the signal lost in the tone stack. Only half of V2 is used unless you have the cathode follower mod. It has a relatively minor effect on tone. V3 is the phase inverter. It makes mirror images of the signal and drives the output tubes, V4 and V5. It too is an amplification stage, and has a minor effect on tone.
V4 and V5 are the output tubes. They’re EL84s and they should be purchased as a matched pair. Your Blues Junior probably came with Sovtek tubes branded “Fender” or “Groove Tubes.” These are decent, hardworking tubes, but it seems like everyone loves to hate what comes stock. Now that Fender has bought out Groove Tubes, I don’t know what will happen to the brand.
When I replace tubes in Blues Juniors, I generally stick with the Groove Tubes 12AX7s and JJ EL84s, but I’ve tried some others that I like. See below.
Many other twin-triode tubes are pin-compatible with the 12AX7. Not all are suited for the job. The 12AX7 is designed for high gain–lots of amplification. That’s why it’s used in so many guitar amps. It’s easy to generate distortion.
The 5751 has virtually identical electronic specifications as the 12AX7, but it only has 80 percent of the gain. If your Blues Junior breaks up too readily and you need a bit more control over clean headroom, the 5751 is a good choice for V1. Many 5751s sound warmer or rounder than equivalent 12AX7s, so if that’s a tone you seek, by all means give them a try.
The 12AY7 is also very similar to the 12AX7, but has less than half the gain. It’s the preamp tube that Fender used in classic amplifiers such as the Tweed Deluxe, many Champs, and the revered 57 Twin. Does half the gain mean half the volume? No. A Blues Junior with a 12AY7 in V1 will produce just as loud a clean tone as one with a 12AX7, but at 7 on the volume knob, while the 12AX7 will produce maximum clean at about 4. After that, the 12AX7 starts producing serious amounts of distortion. With the 12AY7, the onset is much more gradual.
The 5751 or 12AY7 don’t give you any more clean headroom. They only delay the onset of distortion and limit the total amount of distortion.
12AX7: Clean range ends at 4
5751: Clean range ends at 5
12AY7: Clean range ends at 7
So the effect of these lower-gain tubes is to expand the clean headroom across more of the volume control’s range.
These measurements were made with the Fender-specified 1KHz input signal at 50mV, roughly equivalent to a fairly hot guitar being run wide open tone/vol. When the THD (total harmonic distortion) goes over 5 percent, you start hearing the distortion.
The 12AT7 is the wrong tube to use in a preamplifier stage, even though it’s pin-compatible. It has 60 percent of the gain of a 12AX7, but it’s designed with heavier plates and lower internal resistance to push large amounts of current. The lower internal resistance affects the frequency response of the preamp stage, dulling the highs. Some people like that, but it’s better to use the right tube and turn down the treble control!
The 12AU7 has even lower internal resistance than the 12AT7 and even less gain (20 percent of a 12AX7). So it makes for a very dark and quiet amp. The 12AU7 is designed to deliver buckets of current, not a beautiful preamplified signal.
So stick to the 12AX7, 5751, and 12AY7 for your preamp.
Putting a lower-gain tube into V2 is pretty much the same thing as turning down your master volume. So keep a 12AX7 in there and don’t worry about it.
Both the 12AY7 and 12AT7 can be used in the phase inverter (V3) position if you want less drive to your output tubes. The 12AT7 typically has a somewhat darker tone than the 12AY7, but the effect is subtle. Both tubes are darker than the stock 12AX7.
Harp (harmonica) players may get a kick out of using a 12DW7, also known as an ECC832, in V3. This is a hybrid tube that has the gain of a 12AX7 on one side and the gain of a 12AU7 on the other. It drives the output tubes unequally, which causes distortion pretty much all the time. This can be good for harp, because you generally want to drive the amp into distortion to get your blues tone, but don’t want to play loud enough to cause feedback into the microphone.
Some Experiments with Different 12AX7 Brands
I was testing a customer’s Blues Junior that I’d just finished modding and thought it sounded a little dull. It had the usual tone stack and coupling mods, plus the cathode follower, TO20 output transformer, and 6V6 conversion.
I was using the customer’s 12AX7s, so I switched to my bench tubes, which are fairly low-mileage Sovtek 12AX7WCs. The amp sounded better.
With good tubes in V2 and V3, I decided to mess around a little.
Now I’m not a tube-rolling kind of guy. As I said earlier, tubes are like tires: they’re round and they wear out. And most of the differences you can hear are about the same as one notch on one of the tone controls (usually the treble).
So why bother?
Because I had all these different, new 12AX7s on hand. (Don’t ask me how these things happen.) I adjusted the amp to just below the verge of breakup on the volume, set the master for just below ear ringing, set the tone controls for a nice Fender-y scoop. No reverb. There are subtle differences; here are my observations:
Sovtek 12AX7WC The workingman’s tube. Reliable, long-lasting, “standard” tone. There’s a reason why Fender stuffs these into just about everything, but the tone is very straightforward, little harmonic complexity.
Sovtek 12AX7WA The same overall tone as the WC, but slightly more gain, closer to breakup on full barre chords. A bit more complexity. Maybe a little more brightness. Maybe.
Tung-Sol 12AX7 Conventional wisdom says that longer-plate tubes have better lows. The Tung-Sol reissue disproves that. This is a noticeably bright tube with a lot of shiiinng!: shimmery high harmonics, high-end complexity. A little quieter than the Sovteks. I tried another copy of the tube and it sounded identical. The low filament-to-cathode voltage rating means that you should not use these in a cathode follower circuit.
Electro Harmonix 12AX7EH The EH has noticeably more bottom end than the Tung-Sol, more than the Sovteks, too. The top end is smooth and sweet. The plate stamping is identical to the Tung-Sol (they’re from the same company), but the mica and getter are different, and who knows about the grid and cathode structure. One thing I can say about the cathode is that these tubes tend to go hummy in circuits that have un-bypassed cathodes, like the second preamp stage of Blues Juniors. I would not use these in a cathode follower either, but they sure sound pretty. Like the Sovtek WA, this one wants to crunch early on. I don’t use them anymore because of the hum tendency.
JJ ECC83S Bright, aggressive, jangly. Just the thing to wake up a dull-sounding amp. On the other hand, my alarm clock is bright, aggressive, jangly. You can get these selected for high or low gain from some vendors, but the couple that I had on hand were on the quiet side. Overall, I think they’re a little harsh-sounding.
JJ ECC803S Some of the hi-fi guys think this is the second coming of the fabled Mullard and Telefunken tubes of yore. I couldn’t wait to get it out of the amp. Dull, dull, dull. Did I mention dull? Or should I add lifeless? Or that hackneyed phrase about a blanket over the amp? However, I did another modded amp recently, with 6L6s and the TP24/TO26 power and output transformer. Normally, this is a very bright setup. The customer had supplied me with three ECC803s, and the combination of bright output and mild preamp was just perfect. A tech friend in the UK says that the hi-fi guys like the 803 because it’s so neutral.
Tube Amp Doctor 7025WA Antique Electronics was touting these on their page, so I bought a couple with my last order. They’re supposed to be premium, selected tubes. And, yes, that seems to be the case. They sound great, like the Sovtek WA after it’s been ported and polished and running on aviation gas. Firm lows, nice complexity in the highs, balanced from bottom to top. This is a triple-mica tube with short plates, so it should be somewhat more rugged in a combo, less likely to go microphonic.
Sovtek 12AX7 LPS The “long plate spiral” has been around for years and it’s often overlooked. Don’t ask me why; it has the best bass of the bunch, sweet clear highs, and is maybe the loudest tube of the lot. But it also has a bit of headroom. The supposed downside of long-plate tubes is that they get beat up in combo amps and fail quickly. But you can always put a couple of damping rings on it. I have one of these in V1 of my old green board Blues Junior and my jazzer and acoustic-y customers gravitate to it like a comet falling into the sun.
I wish I had a Mullard reissue for comparison, but the last time I played with tubes, I thought it sounded a lot like the EH, warmer and rounder than the Tung-Sol, pretty close to the Sovtek LPS.
Remember, these observations are the result of obsessive, close listening. Each of these tubes is very workable, only a tone knob tweak away from perfection. But you can, if you want, use the tubes’ tone in V1 to jump-start the signal chain and get it headed in the right direction for your playing. And yeah, when you’re up and playing, nobody in the audience would hear a difference with any of these tubes.
At first glance, the ECC88, also known as the 6DJ8, looks like a good 12AX7 substitute. It’s not.
The ECC88′s pinout for the plates, grids, and cathodes is the same as a 12AX7. And it has this cool internal shield to separate the two halves. But in circuits intended for the 12AX7 family, the shield will be connected to one side of the filament line. Worse, the ECC88 has a maximum plate voltage of 130V (one datasheet cites only 90V). Your Blues Junior will probably toast it. Someone who tried it wrote to me–the tube evidently shorted out internally, which took out one or both cathode caps and destroyed the Fat circuit. He’s still assessing the damage.
The ECC88/6DJ8 is on the left. Note the shield attached to pin 9. The 12AX7 family uses the pinout on the right. The 12AX7 has a 12 volt filament, but 4 and 5 are usually connected together and the other side of the filament line is connected to pin 9, so the tube can run on 6.3V. When you plug an ECC88 into a 6.3 volt socket wired for a 12AX7, the tube won’t light because 4 and 5 are both connected to the same point. Pin 9, the hum shield, doesn’t connect to anything else. The guy who tried it said he heard a notiecable volume drop when he installed the ECC88; I’m surprised he heard anything at all. Maybe the grid fused to the plate and it just became a path to the next stage.
Some folks in the hifi community evidently rewire their amps so they can replace their 12AX7s with ECC88s. It has a reputation for quietness in the phono circuit. In guitar amps, it’s an “oh no” circuit.
What does a worn tube look like?
It looks like this:
Here are two Sovtek EL84s. The one on the top is lightly used. It came out of one of my personal amps, with adjustable bias, of course, set at a reasonable level.
The lower one came out of a customer amp. It sounded pretty dull and lifeless. You can see staining or shadows on the inside of the glass above the rectangular “windows” in the plate. This is mostly cathode material that got boiled off. Because of the higher bias and current flow, the temperature in the bottom tube was much higher over its lifespan and its lifespan was shortened. The staining is a visible sign of tube wear.
I’ve also been experimenting with different/more powerful output tubes by switching to octal sockets. Here are some observations: