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 use Electro Harmonix 12AX7s and JJ EL84s.
12AX7-compatible tubes
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.
12AX7-INCOMPATIBLE 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:
I just got a 5751 for the #1 tube and a low-noise 12AT7 for the phase inverter. Now, of course, with all the mods and running all 12AX7’s, it sounds like a Mini Marshall with really great crunch, spank and drive, and good clean. But when I change out the preamp tubes, it gets a really sweet tone approaching a tweed, great for classic country, early rock, and jazz gigs. The overdrive is a different character, frankly a little mushy compared to the 12AX7’s, but that can also be seasoned with a judicious use of the added presence control.
Just thought you’d like to know that with the mods, the amp gets really sensitive to good tubes and is able to be tailored to a player’s particular style.
I got this amp for $250 brand new some 15 years ago when a local music store discontinued its retail line, blew out its stock, and focused on sound reinforcement. And, of course, the mods are inexpensive and only take a little bit of time as well. So now I have one of those fancy boutique amps that advertises changing personality with which preamp tubes are used, at a mere fraction of the cost!
All the guys I jam with, in a variety of different venues and genres, all really compliment the amp, and think it cost way more than I have in it.
Hello,
I need to replace the tubes on my Blues Jr. Where do you suggest to purchase tubes from? Also, what is the difference between brands, i.e, between Electro-Harmonix and Tung-Sol/JJ’s and Tube Amp Doctor?
Thank you for your time,
-Matt.
The differences between tube manufacturers isn’t all that great. Many are made by the same manufacturer, although the internal details of some brands may differ slightly. But over time you develop some preferences. As I said in the article above, the Sovtek tubes that come with the Blues Junior are decent, hardworking tubes. I tend towards Electro Harmonix for 12AX7s and JJ for EL84s. Vendors such as Tube Depot and Mojo Musical Supply carry multiple brands, so you can pick and choose among brands and comparison shop. Eurotubes carries only JJ. If you’re interested in high-end new-old stock (NOS) tubes — typically made in the 1950s and 1960s when tubes ruled, check out KCA NOS Tubes. Antique Electronics is another vendor of new and NOS tubes.
Update: the 12DW7 came in. Along with the tube, I got a 47pf capacitor, and modded C2 to a pair of jumpers to a DPDT mini switch installed next to the input jack so I can have a switchable “bright and brighter” switch, classic Fender style, with a choice of the stock 100 pf or a total 147 pf.
With this configuration, and the preamp tubes going #1 JAN 5751, #2 NOS 12DW7, #3 EH 12AT7, the JJ EL84’s, and this bright switch in, I am getting a rich, silky, clean sound with just the right sparkle that is almost identical to a 1978 Fender Silverfaced Vibrolux I bought brand new and had to sell a few years ago to make a house payment when I was between jobs. And it can get LOUD clean, again, not quite as loud as a Twin, but right there with how my Vibrolux used to be. Bill, at this point you’ve got me really thinking about the push-pull clean boost, but give me some time to play this configuration for awhile.
I also use the EH 12AX7’s when crunching the amp in mini-Marshall mode.
Ei (no longer in business) EL84 and some NOS (Tungsram/Radio Shack) used pin 1 for grid connection. Pin 2 is normally the correct connection. This will leave you with NO negitive bias and very high output tube current/no sound. Jump pin 1&2 on each EL84 and you are back in operation. I have been an amp tech for 20/30 years and Bill’s mods have truly made my jr’s a topic of conversation among the area’s guitar players. Hope this helps if a person runs into this problem while trying different output tubes.
warmest regards
Rick
That’s good information, Rick. Thanks for passing it along.
Would someone have a picture of an EI(or other) tube with a jumper on between pins 1 and 2? I have a few EI EL84 that I’d like to use in my BJr but when I’ve tried to use them I get all sorts of crackling noises. I presume it’s because of the afore-mentioned pin connection differences.
Thanks!
I don’t have any pictures, but you’d need to solder a jumper from pin 1 to pin 2 for each of the tubes. I would put the jumper on the circuit board. There’s a little room between the socket pin and the hole, so an inverted U of insulated wire going from one to the other would probably be easiest. It wouldn’t affect non-EI or newer tubes.
I use a Blues Junior for my harmonicas. I did the tone mod on the mid pot and resistor change for the biasing.
I put JJ tubes in it across the board. I leave both volume control up really high and control the level with the volume control on my mics. (I use a Shure Green Bullet with built in volume control or a Harmonica Honker with built in volume control.)
This amp screams. The first time I sat in with a friends band and I kicked it into high (custom FAT foot switch with LED so I know when it is on.) it blew him away. He looked at me and said “do you have a license to drive that thing?”
It is a great amp and easy to tote to gigs.
Bill, a store that caters to harmonica players (Harp Depot) advertises the following….
“You can replace any one or all three of the 12AX7 tubes that come standard in the Fender Blues Junior amp with these tubes that are designed to reduce feedback and create an even warmer, richer, fatter sound. The “feedback threshold” will be greatly improved with these tubes, meaning that you’ll be able to turn your amp up louder before it’ll feedback. These tubes can be ordered individually, but this 3 tube upgrade kit includes one ECC81, one ECC832 and one ECC83S. You position them as follows: The tube closest to your power tubes is sitting in the Position V3. The middle tube is in V2, and the tube farthest away from the power tubes is in the V1 position. Place the ECC832 in V3, the ECC83S in V2 and the ECC81 in V1.” -Price $29.95
My questions are:
1) Will this truly allow more actual volume before feedback, or will it just allow the knob to point to a higher number while feeding back at the same volume?
2) It seems like ECC832 in V3 will mean a distorted sound all the time, with no flexibility to dial in a clean sound when desired. Is that true?
I appreciate you sharing so much of your knowledge freely.
As you suspect, the lower-gain tubes will simply allow you to turn the knobs up higher before experiencing feedback. The ECC81 (12AT7) in V1 will give you darker tone as well as less gain, which some harp players might consider an improvement. Then again, you could just turn down the treble control. The ECC832 (12DW7) in V3 is half 12AX7 and half 12AU7, so it sends an unbalanced signal to your output tubes. There’s more output stage distortion as a result because the waveform is asymmetrical. So you do in fact get more breakup at lower volume, or more accurately, a thicker tone.
There is no increase in actual volume before feedback. I think a graphic equalizer like the MXR M108 is much more effective at pulling out your main feedback frequency. With an equalizer, you can “ring out” the room when you set up, since you may encounter different feedback frequencies in different venues.
Has anybody tried a 7025 for V1?
There’s nothing particularly special about a 7025; it’s just a low-noise 12AX7. Since nobody makes tubes to the 7025 spec anymore, you’re talking about a NOS or used-old stock tube. Whether you can hear a difference and whether it’s worth the money is up to you. I just swap 12AX7s into V1 until I find the one that’s least microphonic and quietest in terms of hiss or hum.
So I’ve got a MusicMan 210-Sixty Five, and I’m was really pleased when I switched out the 12AX7 for a 5751. In addition to being cleaner, it sounds thicker, though I suppose that could have had something to do with the fact that I was retubing the amp because my power tubes were going bad. Oh, and kids, be really careful when biasing your amp. I panicked, and grabbed the chassis at one point, and I got two of the most interesting holes in my finger.
Anyway, I just completed all the BillM mods, and my bassist and I are quite impressed with the improvement, but I’m looking for as much clean volume as possible (before I get into . I did the cathode follower mod, so I’m looking at a 12DW7 in V2, and beyond that, my internal voodoo is compelling me to just go with 5751s in both V1 and V3, but I have some questions:
“The 5751 or 12AY7 [...] only delay the onset of distortion and limit the total amount of distortion.”
Does this mean that the 5751 will give me less volume at 10? If it decreases the volume of the amp, I may just go with 12AX7s, and refrain from dime-ing out my amp.
“the 12AY7 [...] can be used in the phase inverter (V3) position if you want less drive to your output tubes.”
Does reducing the gain at V3 have less of an effect on volume? And if so, does that mean that if I want lots of clean volume, I should do either a 5751 or 12AX7 in V1, and a 12AY7 in V3?
I’d also like to say to anyone reading this that Bill has provided me with amazing support. I don’t think I’ve waited more than 30 minutes for a reply back from him when I’ve emailed him. Maybe my timing is exceptional, but that’s great service. And the mods are well worth it, too.
Yes, lower-gain tubes will reduce the overall loudness of the amp. They expand the range of clean tones before breakup, and soften the edge of breakup onset, but ultimately, you’re pushing less power through the amp. So 12 and 12 is going to be quieter than with all 12AX7s.
V3 is less responsive to tube differences, but it is a gain stage, so it does affect overall output. Personally, I stick with 12AX7s and use the volume controls accordingly.
In my Blues Jr. I use two Sovtek EL84 (matched pair) for power tubes and three TungSol 12aX7 tubes for the preamp. In the power tube bias path I replaced the 22K resistor with a 27K resitor. That provided significant improvements.
A buddy recommended me to look at this site, brill post, fascinating read… keep up the good work!
I just replaced the tubes in the Junior. I must admit that i’m not a fan of the Junior’s distortion, i try to keep it as clean as possible and use pedals. SO i decided to try the 12AY7 in position one. There really is very little breakup with this tube. Because i’m in an apartment, i haven’t had a chance to crank this up yet. But from what i can hear now, it sounds pretty good for anyone that uses distortion pedals a lot. The only con i see so far is that i’ve lost a little attack. i’m finding it hard to play clean but get a little dirt when i pick hard. Maybe i just need to crank up the master a little more, but it’ll be a couple of days before i have a chance to do that. i have a 12AX7 in reserve in case i decide to switch it out. but if you’re looking for a lot of clean, this is a great tube.
i had a chance to crank her up and let her do her thing. i was wrong, the attack is still there. the addition of clean headroom was exactly what i was looking for. i just had to crank it further than normal to get some grit in the attack. sounded #$*!@&’ awesome! if it is true that you lose some overall volume with the 12AY7 he effect is negligible for me, cuz i mic it 90% of the time. i highly recommend this tube configuration.
I just picked up a BJr today at a guitar show. I had one about 7 years ago but in a moment of insanity, I sold it. This one sounds pretty decent. It doesn’t have any mods on it but seems to be later production (creme PCB). The previous owner changed the tubes – he put an old Mullard ECC83/12AX7 in V1, a Sylvania 12AX7 in V2, and a Groove Tubes 5751 in V3. The EL84s have no markings on them but look just like some Sovteks I have.
The question is on V3. I’m wondering if he thought he was putting the 5751 in V1. But then why would he put a NOS Mullard in V3? Assuming he knew what he was doing, what would be the benefit of the 5751 in the PI other than slightly lower gain into the output stage?
I’m probably going to replace it with a JJ ECC83 with matched sides anyway… It’s a uzzle though.
Conventional wisdom says put your best tube in the preamp position. I have no idea why someone would waste a Mullard 5751 in V3.
Matched phase inverter tubes are a waste of money. Even if it’s only a dollar.
Bill, since I want the cleanest sound but as loud as possible, am I correct to think that this config for the preamp is the best choice?
- V1 12AY7
- V2 12AX7
- V3 12AY7
Thanks!
Lower-gain tubes don’t make the amp cleaner. They extend the adjustment range on the controls before the amp breaks up. As a practical matter, the BJr puts out all 15 watts of clean power with a full lineup of 12AX7s with the master on 12 and the volume around 5. With lower-gain tubes, you’ll have to turn the volume up more, but the clean power will be the same.