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.

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.

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.

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.

ECC88 pinout 12AX7 pinout

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:

worn_el84

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:

EL84 vs. 6V6 vs. 5881

159 Comments

  1. glennc717 says:

    Hello and this is a first post for me. Greenboard purchased new, when the master is turned to less than 2 the frequency range dies off. I can’t run the amp at 2 because of the volume. I do not care about the distortion, it is the sounds, clean or dirty at less than 2. A separate issue is that I would like it to sound less boxy. The issue is that different guitars makes the volume/frequency range more apparent. My newest LP Traditional is the main culprit. 57 and a 57+, stock at the moment. I have been told about post phase inverters, attenuators along with much other information. I haven’t read anything on your site that I can find that in my understanding, might specifically address the volume issue. Above 2 it the bass and highs sound better. Well first off, it has been suggested that due to the age of the amplifier, a new set of tubes might help. I have seen that your favorites have changed over the thread. Can you suggest the make tubes that would most likely in your opinion give me a fix or a help with the low volume muddiness. Is it even possible?
    I am an amateur and am just really learning the adjustment range and effects of the controls. Now I am noticing the boxyness more IMHO. I have read that a speaker change might resolve that issue. So far the tubes and the speaker are affordable and doable. I was wondering also if the twin like stack modification might help to clear the bass sounds at low volumes or if this is more for louder volume levels. Thank you for any assistance!!
    Regards,
    Glenn

    • bill says:

      Do the basic mods and the audio-taper master volume. Your amp will sound much better at low volume and you’ll be able to control it much better.

      A post-phase inverter master volume or an attenuator probably won’t help. The brand of tubes has very little effect on the overall sound of the amp; tubes are never a cure for a tone deficiency. A speaker with extended bass response, like the Swamp Thang, might help a little, but the speaker can’t reproduce sounds that never get sent to it. And that’s the issue. The stock tone stack and coupling in the Blues Junior keeps the tone bottled up in the amp.

      • glennc717 says:

        Thank you Sir for taking the time to comment on my issue and share your vast knowledge. Luckily I am able to follow your explanation. I recall that the amp is probably around 20 years old, so I believe I should get new tubes in any case. In reference to the speaker, your wonderful list is basically over my head. I don’t necessarily want to get more bass, per se as better response. I don’t have a particular style, I guess you could say the if anything I want it to sound better than the old stock speaker. I also noted after writing the first post, that when I turn the volume up on the amp without it being plugged into the guitar, I get a pronounced hum. Variable with the volume. At the levels I’ve usually play at I haven’t noticed it, but think it probably should be addressed. What is your opinion? I am a bit concerned over my ability to do the modifications. I certainly will consider it once I replace the tubes and maybe do a speaker change which is reversible. It has been brought to my attention the powering a separate cabinet with better and or more speakers could improve the sound. Also another thought line is that the a boutique amp or a 1 watt commercial might better suit my needs leaving the fender stock and producing the volume/response I desire. It really oinks me that the loudness problem is so apparent on my newly purchased guitar, whereas on a 335 I have not noticed it, or on an LP Junior with P90′s. Must admit my ears aren’t what they used to be. Thank you again for your generosity.
        Once I replace the tubes I will check back in.
        Glenn

        • bschultzjames says:

          I really have had a great experience with my Vintage 30. It seems to be a well balanced tone for many applications. Try out different cabinets at your local guitar store, asking what speakers are in them — listen to as many samples as you can. Youtube may not be the best quality source for good samples, but there are a TON of videos out there of people who changed the speakers in their JR’s.

          If the amp is buzzing and you’re not comfy looking at it, I would take it to a good tech in your area. I’m deathly afraid of blowing myself up if there’s a problem with my amp and I don’t know where the source is so I take my JR to a very trusted friend in my area.

  2. glennc717 says:

    Hello again! I must say to my ears, the tubes have made a significant difference. Much more clear sound, IMHO. As you noted it did nothing for my low volume/muddy issue. I have been searching for sound clips of as many speakers as I can. I do happen to like the sound of the Swamp thing and the Cannabis Rex. Haven’t found an A/B of these yet. As a complete newbie, I was wondering your opinion on removing the speaker from the combo a either putting it in a separate enclosure or maybe a 2X12 with my original speaker and a swamp thing. I really have no idea of what effect these things will have. I have read that the combo speaker is not good for the tubes and that a closed back cabinet of quality will enhance the sound of a speaker. Any advice or opinions you can share are welcome. And yes I have not decided when or if I will do the basic mods as of yet…..
    Thank you!
    Glenn

  3. vincent21 says:

    Bill,
    Awesome site, thank you for the great value of information. I actually have a couple of questions.
    1. I love the full, bassy and responsive nature of the Fender Hot Rod Deluxe III but am in no need of a 40 watt amp. Would I be able to attain this effect if I install just the basic mod, or am I going to have to convert to run 6L6 tubes?
    2. If I must run 6L6, would my upgraded Mercury Magnetics OT suffice?
    3. What is your stance on installing a choke? Does this effect tone in any way?
    Lastly…
    4. In your experience, have you come across any particular instrument cable you find more suitable than others to use in conjunction with a Blues Jr?
    Thanks so much.

    • bill says:

      Try the basic mods and a presence control. The Mercury OT is OK, not as bassy as the TO20, absolutely not suitable for 6L6 conversion. A speaker like the Swamp Thang or Cannabis Rex, with good bass characteristics, will also help, but not as much as doing the basic mods.

      A choke doesn’t do a damn thing for tone. Or anything else on a small amp.

      As long as the cable is decent– hum-free, doesn’t roll of highs, not microphonic, they’re all the same.

  4. keef says:

    Bill,

    Have you tried the Russian mil surplus EL84 tubes out yet? There are hoards of these for sale on ebay and I was wondering if they are decent sounding replacements for the BJr. They are designated 6P14P. some have -EV, some have gold-platinum grids. most say that they are equivalent to 6BQ5 and EL84 tubes.

    Thanks again.

    • bill says:

      I haven’t tried them. Maybe someone else reading this will have an opinion.

      • bschultzjames says:

        I actually just swapped out a set of Russian mil surplus tubes. They came from a mig :) they were MEAN sounding and actually died pretty quickly. Their tone was fantastic and very easy to push into overdrive, but sadly they don’t seem to last very long. I’d invest in a solid set of tubes from some reputable dealers. Again, tubes are tubes and they really all do the same thing.

        Lately I’ve been in need of a cleaner and quieter amp as I play in smaller settings or mic my amp backstage for very very low stage noise. I changed the entire tube compliment for the Blues Jr low-gain option from eurotubes. I know Bill said that the ECC83s in v1 was too bright and jangly for him, but through my UK made v30 I have in the amp it sounds perfect for my needs. Very quiet setup and enough volume before distortion for me.

  5. Scott says:

    Going to try JJ EL84′s, the TAD 7025-WA in V1, and JJ ECC832 (12DW7) in V2 with the cathode follower mod.

    Any benefit/harm to using a Sovtek 12AX7-LPS in V3, or stick with the stock Sovtek WC?

  6. wksohn says:

    I’ve tested a MIM BJ which has JJ power amp tubes (the amp was re-biased) and TAD 7025 WA. Overall, since it has less Treble comparing to Sovtek 12AX7, it almost meets my preference. Just one thing is, I see the every single note sound is thin to me and the overdrive sound with my pedals is sharp or thin as well, which doesn’t mix the sound from every single strings. Any suggestion please? Do I need more gain TAD preamp tube? or do you think the bias setup is too low etc?

    • bill says:

      Thin, compared to other Blues Juniors, or just thin? The basic mods kit will fatten the sound up considerably. If this particular one is thinner than other Blues Juniors, it should be checked over by a tech. Tubes are unlikely to cure the problem.

  7. btiller says:

    Bill,
    I want to make a very quiet studio type Bjr for playing clean jazz guitar. I have made installed most of the mods you have including the different transformer, but still have substantial hum at low volumes. I am changing the speaker to a cannabis rex and would like to change the tubes in V1 and V3 as you described above to get more clean headroom. Do I have to change the bias again to a different voltage setting when I put in a 12AY7 in V1 and V3? You do not recommend a 12AU7 but you said it makes for a quiet amp – does it reduce the noise floor as well? I even put in the angled galvanized metal strap piece next to the transformer but still too noisy for me. Anyway, any suggestions you have for making the quietest Bjr ever would be appreciated.
    Thanks,
    Bruce

    • bill says:

      The Blues Junior does not have a lot of inherent hum. A stock one in proper shape should be quiet enough for recording. So you need to address the source of the hum.

      If you haven’t already done so, you should go through the procedures here: http://billmaudio.com/wp/?page_id=882

      Cure the hum problem first, then worry about gain. If not, the hum will always be there. Does the hum go away when you press the Fat switch? That’s a sure sign that you need to replace V1.

  8. mikehoju says:

    Hi, just joined the blues junior community and have found this page to be a really helpful education. i think i have problem though. I bought my amp second hand and have found a massive loss of sound when the volume is above 4 and the treble above 6. There is a strange pop when i turn up the treble past the half way point and even a small amount of distortion will cause a plugged in guitar sound to breakup and die . Doesn’t sound great. Is there something basic i am missing or should try or do i need to bring it to someone? Thanks, mike

  9. teza says:

    Hi Bill
    My blues jr is actually under mods here down in London, basic + presence + standby switch. I also order two EL84/JJ-TESLA. Should be a good update for a start. Thinking also putting a 5751 on v1 following your advice and Dave Hunter’s book “Tone Manual” very good book on tone.
    But first of all I would like to what v1 mean (I suppose first valve of three, but witch way..) and if I do so should I only change v1?
    the last update would be probably the speaker for a Eminence GB12/GB128 or the Harma G12 Bright Sapphire as blues is the music I love.
    Love to read your pages.
    Thanks for your help.
    Teza

  10. btiller says:

    Bill,
    Here’s what I have tried. Swapped all the V1, V2, and V3 tubes around. Removed each tube in turn (taking out V3 drops it way down as you stated it would). Turned the fat switch off and on. I even took the metal strip between the transformers that you recommended on and off. Not much difference in the basic hum for all cases. I recorded the hum and ran it through “transcribe” software that will produce a spectrum and came up with major peak frequencies at 123 Hz and 294 Hz. Does those frequencies tell you something? I would like some guidance on where to start digging. I have access to voltmeters and scopes but I’m not sure where to start. I feel certain there is a way to get this amp to be really quiet but need a direction to start investigating.
    Thanks for all your work on these cool amps!
    Bruce

    • btiller says:

      Bill,
      Today I wrapped the ribbon cables to V!, V2, V3 with aluminum foil and grounded them to the chassis. That reduced the noise audibly. Then I untangled the red and green power wires, twisted them together (green to green and red to red) and switched the plugs to reverse the polarity – that also helped quite a bit. So I am making some progress. I’m still interested in this idea of adding some DC to the heaters that you mentioned at the end of the “chasing hum” writeup but did not elaborate. Thanks!
      Bruce

      • bill says:

        Twisting all the wire pairs is fundamental. Your BJr should look like this inside (other than the octal conversion):

        It shouldn’t be necessary to shield all the ribbon cables; if shielding V3 ribbon helps the most, you probably have phase inverter oscillation and you’ve been hearing subharmonic hum all along. The 100pF cap included with the basic mods kit cures that.

    • bill says:

      120Hz indicates a power supply problem or a reason why common-mode cancellation isn’t working.

      The full-wave bridge takes the 60Hz AC and converts it to 120Hz DC pulses. Most of the pulsing is removed by the first filter cap, and what is left is a 2 volt “sawtooth” waveform on top of the 330V output from the first filter cap.

      The sawtooth waveform is present on the plates of both output tubes, but since they are 180 degrees out of phase with one another, the waveform cancels itself out. IF it does not cancel, there can be several causes:

      • Output tubes that are far out of match
      • Only one output tube working
      • Defective output transformer, not balanced internally
      • Bad filter cap, so the sawtooth is too large
      • Bad filtering, 120Hz sawtooth getting past the second filter cap, into the phase inverter
      • Subharmonic hum, caused by severe phase inverter oscillation. If this is happening, you will see a large AC voltage in addition to the DC voltage on the plate of one or both output tubes, but usually V4, even with the master volume off.

      An oscilloscope is a big help here, but you can also test for the last case with just a voltmeter.

  11. Alex_Under says:

    Hello Bill.
    From your review on each 12AX7 you’ve tested, It wasn’t obvious to me whether you were talking exclusively for replacements on the V1 only or you were putting the tubes above in all preamp places (V1,V2,V3), each brand/model at a time.
    The reason I’m asking is, I’m planning to buy a Balanced Sovtek LPS for Phase Inverter position V3 (as I’ve seen recommended in many forums/sites) but I was hoping to try some other tubes on the other preamp stages. Is the LPS good for all 3 positions? Can I mix the LPS in V3 with ,let’s say, a TAD in V1 and V2? Should I leave V2 with the OEM Sovteks and mess only with the V1 and V3?
    I’m aiming for early break up, don’t need much headroom in my 2 X 4 meter study, were I do my playing.
    P.S. The V4, V5 already have JJs.
    Thank you.
    Many greetings from Salonika, Greece

    • bill says:

      My tests were all in the V1 position. A balanced phase inverter is a nice idea, but mostly a waste, since none of the other components are balanced. While it is possible to find hand-selected tubes with higher gain, V1 is the only position where it will make a difference, and the difference will be small. If you want breakup tones at reasonable loudness, just turn up the volume and turn down the master volume. You will not have output tube distortion, but output tube distortion is vastly overrated anyway. If you need more distortion than the preamp (volume control) can supply, use a pedal.

      Remember that these tests were very close, critical listening. Someone else listening to you play would probably think that all the tubes sound the same.

      • Alex_Under says:

        To be truthful,higher gain is not the only objective here.
        I’m experiencing great rattling noise at certain notes, on higher volumes and was hopping to see improvement with a tube exchange.
        I’ve eliminated every other known rattling factor, tighten every screw on the amp, fastened firmly every loose cable with tire ups, but nothing helped. One clue I have is that the rattling appears only after the amp has warmed up fully, after 10 or so minutes of playing. So I think that there must be a preamp’s tube matter.
        All the tubes have been checked for looseness.

        • bill says:

          You should be able to detect a rattling tube by tapping it while you hold the other tubes. But it’s always a good idea to have a fresh pair of output tubes and one 12AX7 to keep you up and running.

  12. Unklerob says:

    Bill, I play with mostly with OD but only use effect pedals to achieve this. I do not care for the amps OD. What preamp tube would be best in V1, 12AX7, 5751 or 12AY7 ? Again I only get my OD from pedals and want the warmest cleam tone and OD tone possible.

  13. Slestak71 says:

    On the EH12AX7 I was hoping you could clarify a bit on one point. I love the description of the tone of it and want to try one. I have the cathode follower mod. I am planning on putting a 12dw7 in V2 either an EH or a JJ and a EH 12at7 in V3. If I put an EH in V1 do you think I would have hum issues?

    • bill says:

      EH 12AX7s have not had good lifespans for me, so I stopped using them. Current production may be better, but the ones I tried several years ago developed hum pretty quickly.

  14. Joey says:

    So if I have perfectly functioning tubes, no pops or discoloration on the tubes themselves, and all I’m looking for is a subtle change I will only have to worry about changing the first tube? Am I understanding this correctly? Based on your article it seems like the EH tube is what I’m looking for and I know you said that one shouldn’t be used in the second spot anyway.

    • bill says:

      V1 affects the tone the most. But let me stress again: Unless you’re replacing a worn-out tube with a new one, the changes are minimal, hard to hear. Tube-swapping is not a way to magically change the tone of your amp. EH tubes sound decent, but have not had the best longevity in my experience.

  15. MikeD says:

    Hi Bill,
    I’ve done the basic mods, TO20 output transformer, adjustable bias, and the filter caps. My one wish is that it would break up a little later. Thinking about putting in the Sovtek 5751 in the V1 slot. I have a Tung-Sol 12 AX7 in there now and it sounds pretty bright. I’m trying for a slightly warmer tone, but will this take away from the volume at all? Thanks!
    Mike

    • bill says:

      The volume will be the same with the 5751, and you will get a little more headroom. Try backing off on the mids control for more headroom, and also try upping the volume and backing off on the guitar to change the balance. And of course, more master and less volume generally gives you the most clean headroom.

      • MikeD says:

        Thanks! One more question. I had an outdoor gig last week and the amp was extremely week. I had it on 8 and could barely hear myself with the band. I can usually be fine at about 3. All the tubes seemed fine and it was fine when I got it home. There was nothing behind us for this one, but I’ve played plenty of outdoor gigs with the same setup and it was fine. It’s not the best outdoor amp as it is, but it really struggled here. I’m thinking it was a power source issue with the club. We were running a lot of extension cords and I was thinking it made it weaker. But no one else was affected like I was. Come to think of it, this has happened to me in 2 other clubs that I can remember, but they were inside. Is the Blues Jr more susceptible to power source issues than other amps? Is this something that the TP24 can help with? Thanks again.
        Mike

        • bill says:

          Nothing eats up an amp’s power like being outdoors. A single speaker in a small cab with only 15 watts is at a disadvantage. I don’t think it was the extension cords; I think the amp was running out of steam. If you can, try mic’ing it through the PA.

          The TP24 alone might help a little, but to make a real difference, you’d need to go all the way to 30 watts–power and output transformers, octal conversion, 6L6s.

  16. MikeD says:

    Hi Bill,
    I got a Jan Phillips 5751 for the V1 position and I’m going to try a sovtek 5751 to try in the V3 position. I’m leaving a 12ax7 in the V2 spot. Does the V3 drive the reverb? Just experimenting a bit. Your thoughts?

    • bill says:

      V3 is the phase inverter. It drives the output tubes. The reverb is solid state, no tubes involved. You’ll have a little more range of adjustment in your clean tone with that setup and the amp won’t break up as readily.

  17. MikeD says:

    Hi Bill,
    I put in the 5751 tubes in the V1 and V3 spots. I played a gig Monday night with them. They sounded good but I was having a problem with feedback with the volume anywhere past 6. It was a low feedback that went on even when I turned the volume all the way done on the guitar. It went on until I turned the volume down on the amp. I basically had to do this all night. Is this a sign of a defective tube? Any other reason this would be happening?

    • bill says:

      Yes, one of your tubes could have a loose element inside, and you get a sympathetic vibration from the speaker to the tube and back again. If you have some spare or old tubes, I’d swap them out. If it’s this kind of mechanical feedback, it will do it, as you said, even if you turn the guitar volume down.

      You may be able to hear which tube it is by tapping them or flicking the tip of the tube with your finger to vibrate it.

  18. Julian_Australia says:

    I want to warm up and darken the sound from my BJ. The video below suggests that TAD tubes sound “warmer” that JJs (in V4/V5?), but you Bill say that they tend to brighten up the sound. I will appreciate any comments from Bill or anyone who has used these tubes. I want to get some feedback before spending money.

    http://www.youtube.com/watch?v=VNK8zKdN1k0

  19. Almeida100 says:

    Hi
    I have a JOLIDA JD202A from late 1990’s and have roll the Chinese valves with JAN PHILLIPS 12AT7 (inverters), SOVTEK 12AX7LPS (drivers) and four 1999 SED Svetlana’s (“S” logo).
    With this tube set, the amp sounded better, but dynamics and middle/treble extension was a bit shy, and dynamically, this set made the amp still a bit slow.
    Than I read somewhere a review on the ERSE MPX series capacitors, and decided to change the original block capacitors for these ones.
    The “new” amp was completely different, having a stunning dynamics, pin-point imagery, with mids/treble on the bright side. Sometimes, depending on the record being played, that dynamics and bright treble seemed to my tastes somewhat aggressive. Bass was not too extended, but utterly tight, tuneful, making it very easy to follow bass lines.
    That said, I went for a change on the driver stage, and tried a 5751 tube, this time from JJ TESLA, to calm downs the things. First impressions are that bass is slightly less tight and tuneful, as well as imagery is slightly less focused (and these are, for sure, strong points of the SOVTEKs 12AX7LPS), and the above mentioned aggressiveness has gone (at least hearing the same records on same chain).
    One of these days I’ll try the SOVTEK 5751s, said to be constructed the same way as the LPS’s.

  20. joelshep3 says:

    Hey Bill,
    Us there any way to buy the metal “grill” (I dont know what else to call it) that protects the tubes like on the BJr III? I have a BJr II.
    Thanks!

  21. barfoden says:

    The Fender blues junior might be biased hot with certain EL84 tubes and it is crazy that a grid biased amp is not adjustable..

    I have a cathode biased amp.. It has two EL84 biased with a 91 ohm resistor and no bypass cap. I know cathode biased amps are biased hot to avoid crossover distortion because the bias voltage is not stable and will increase as you turn up the amp on a cathode biased amp.

    The amp has a true plate voltage (-cathode voltage) of 318V. The screen though sits much lower (283V) as the amp has a large 4.7K screen dropper. The other cathode biased guitar amps I have seen with two EL84 have resistors from 120 to 150 ohm. Mine seems way underbiased with a 91 ohm resistor (hot). But my amp also has a lower screen voltage, no cathode bypass cap on the EL84 and a 10K OT impedance instead of the typical 7-8K seen for push pull EL84 amps (Higher impedance increases screen current but lowers plate current current whereas lower impedance increases plate current and lower screen current).

    I measured the cathode voltage over the 91 ohm resistor,, 9.3V. Divided by the 91 ohm = 102mA combined cathode current, or 51mA or tube at idle. Assume that the screen current is 10% of plate current, you end up with around 46mA plate current at idle or 318 x 0,046 = 14.63 watt at idle.. (Think I have to measure that 91 ohm resistor for its true value)
    The stock tubes have not had more than 12 hours use on them and I think I am allready beginning to see a slight browning around the plate window.

    I sent a mail to the amp company about the bias and got this response
    “Yes, amplifier runs extremely hot, this is normal. The paint on the output tubes will discolor, that is normal”.

    I freaking do not worry about the paint.. I care about tubes wearing out after say ~100 hours of use.. Not just playing but also at idling as these cathode bias idle hot. Do not leave these kind of amps on when you are not playing them..

    One guy with the same amp said that his amp gained a lot of clean headroom when he changed the stock tubes to sovtek EL84M tubes. Those tube have a very different structure and can take higher plate dissipation (~14W) and slightly higher screen current (2.2W). A stock EL84 is rated at max 12W/2W dissipation.

    I tried a pair of slightly unmatched jj EL84 in the amp and the cathode jumped to 9.55V with a slightly lower plate volt (324V). Again if you assume a 10% screen current the plate idle dissipation is 14.83 watt.

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