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mtheory

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mtheory last won the day on October 26 2010

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About mtheory

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  • Birthday 07/12/1961

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  1. The TP18 would be what you'd want. Turretboards.com has the Heyboer direct-fit replacement on sale ($75). You might also consider just buying a used Vjr head, considering what a replacement PT costs. I don't recall offhand what the specs are, but they should be printed on the label of the transformer.
  2. Fwiw, it's not altogether practical these days to measure gain by part number. Any given 100 lot of any given tubes can have a profoundly wide variance of measured gain. So much so, in fact, that it's not at all unlikely to find 12ax7s within the same 100 lot that measure no more gain than an at7 or less. Virtually none of today's built 12ax7's match gain on a level that was common in yesteryear's tubes. So, right off the bat, the highest gain 12ax7 in a given 100 lot sample is almost certain to be considerably less than what you'd have seen 30-40 years ago. Couple that with the fact that at least some of those 100 tubes are going to be so far below typical modern-era 12ax7 gain that you might as well call them a different tube altogether, and you can see how unrealistic it is to try to make arbitrary comparisons based upon part numbers. That percentage comparison was created under the premise that all tubes of a given part number stayed within a certain range of acceptable gain levels for that particular tube. This WAS the case, many years ago, but it not anymore. That said, tweed is correct in saying that ax7s are not the same as at7s (or au7s or ay7s, for that matter). While they will plug into the same socket and will work, they all are designed with different specs, and as such, will perform differently in audio circuits. Most notably is that they will have a narrower frequency response than a 12ax7, so if a given amp was designed with 12ax7 tubes and you swap in a 12at7 in the V1 slot, for instance, you're likely to hear a thinner overall tone than a healthy 12ax7 would produce. For some, that might be just what they're looking for, for others, not. As for the initial thread topic, all tubes are at least marginally microphonic, though tubes considered to be "healthy" would be so low in microphonics tendancies that you wouldn't hear any adverse effects. Some tubes go excessively microphonic over time, some are excessively microphonic the moment you plug them in, for whatever reason. Sometimes, it's the actual construction design itself that causes a propensity for excessive microphonics. At any rate, combo amps are always more succeptible to problems from excessively micrphonic tubes, because, as stated earlier, the vibrations from the speaker causes an oscillation in a tube that is already prone to and succeptible to oscillations.
  3. I must say that the descriptives don't match any stock Vjr I've ever heard. I find them to be mushy, dark, almost devoid of headroom at all, and pretty uninspiring, in stock form. Modified, that's a totally different story. My El34 conversion is one of my favorite amps, even if only pushing 7 watts. Before it became a turret board build, I went through every Marsally mod I could, including an OT upgrade. Everything I did dramatically improved the amp, but the rebuild to SE EL34 was spectacular. But, whatever works for the individual is what matters. If someone can get inspired by the stock amp, more power to them. Regardless of what I may think of the tone of the stock amp, they're still pretty cool little critters.
  4. I've not seen a schematic, nor have I tried one of these, but I have read mixed comments about it. The reverb is apparently pretty weak, based upon what I've read, tho I really have no use for amp reverb, personally. Truth be told, I think there are better amps available for about the same money these days.
  5. Almost certainly a bad cable on the board. Might be a good time to consider upgrading all pedal patch cables. George L makes a very reasonably priced cable that's very low in capacitance, although beware that the screw-on plugs DO come loose and cause problems from time to time. They may well be simple to repair, but it still sucks when it happens during a performance, and who's got the time to re-cut and repair ANY cable while on stage? Far better to eliminate the possibility of this type of flaky cable failure altogether, imo. If you use George L cable, you should either consider blue loctite on the ends or (better yet) solder real plugs, such as switchcraft, G&H, or Neutrik, onto the cable. It wouldn't be a terrible idea to do some research at lavacable.com before making a purchase decision, although keep in mind that lava is in business to sell cable, so while the capacitance ratings may be entirely objective, the review comments aren't necessarily so.
  6. I have to disagree with this a bit. Many amps will quite easily tolerate 100% mismatches, and some might actually even prefer it. It's something that requires a scope and signal generator to be absolutely certain of, but there's really no hard fast rule that mandates never, ever straying from manufacturer recommendations. If that were the case, we'd have to assume that every amp every made was made to utter perfection, and we all know that's not the case. Oftentimes, in fact, engineers make some rather boneheaded blunders in designing amps. I can think of many, many amps that were very poorly designed, yet widely sold. There's no particular reason to assume that part of the design failures along the way couldn't be impedance recommendations. After all, how many amps really come off the line biased properly? Btw, did you REALLY just call me an "idiot," because I happen to disagree with you? I have to say that I find that a tad offensive, and more than a tad uncalled for. You have a right to your opinion, even though it appears to be based in some level of ignorance. You do NOT have a right to call someone an "idiot," simply because you don't happen to be armed with the proper knowledge to agree with that person. Capice? As to the author's thread subject, fuse called it...wire them in parallel, install a good switchcraft jack, use good quality speaker wire, make your connections rock solid, and you'll never have to think twice about your 4 ohm extension cab.
  7. Any delay will do reverb-ish sounds, to a degree. Most digital reverbs tend to be a bit on the grainy side, if that matters. As far as reverb goes, it's pretty hard to beat tube driven springs, although they can be a bit noisy. Most digital pedal reverbs tend to be "grainy" sounding, although that's something that seems to be a subjective issue that some can't tolerate, while others don't even notice. That said, Boss, Behringer, TC, and Digitech are among probably a large number of major manufacturers that offer pedals with both reverb and delay.
  8. I can't recall offhand what that OE jack and daughter board looks like, nor can I recall what the purpose of there being a PCB there at all, but it's likely just something to solder the crap jack to. Regardless, there is a two conductor wire running to it from the main board. Just snip the the wires near the crap jack, strip them back, and solder to the new one. The shielded conductor is ground, so that goes to the sleeve terminal on the new jack. The other conductor goes to the tip. If you're unfamiliar with basic soldering technique, practice on something else first. Remember that you're heating the surface to be soldered, not the solder. In other words, lay the soldering iron tip against the surface of whatever you're going to solder for a second, then touch the end of the solder to that surface, not the iron tip. When the surface is hot enough to flow the solder, you're golden. The solder should flow across the area freely, and should appear shiny and smooth when cooled. As soon as you've got adequate solder flowed out, remove the iron, to avoid overheating and scorching/burning.
  9. The stock jacks are crapola anyway. Replace with switchcraft, and you're light years ahead. Bummer if Epiphone stopped making the Vjr head, although there are about a billion of them floating around by now.
  10. Warranty on such a cheap amp wouldn't be a valid justification for me keeping it a medicore amp, imo, but I can see where if you're nervous about digging into it yourself, it can make sense to let them warranty replace it. Sounds as though you've got that, so that's cool. The JC20 is a far different beast. It's significantly louder, more articulate, and punchy than the Vjr, for sure. In stock form, it's actually quite usable, although it can tend to be a tad harsh, with mucho mids and highs. It takes some effort to dial it in, and what you find acceptable at one volume will have a tendancy to sway off kilter with volume changes. It's rather finicky, imo, in that it can sound really great at one setting and venue, and not so much so in the next. I'm going to gut and rebuild mine as a Maz Jr. clone, when I have the time. In the meantime, I've made a few changes to it, to make it more usable for me, but I still find it a bit too finicky for my liking. You definitely want to upgrade the tubes in this one when you get it, btw. That's a good chunk of the harsh edge right there. One thing I've learned...don't expect any cheaply produced, mass-market amp to be "all that" out of the box. There are a LOT of compromises that go into making an amp that can be sold at astonishingly low price points, so that means there's a LOT of room for improvement. There's a reason why boutique hand builders get the kind of cash they get for their amps...they're entirely usable, and in fact, rather impressive, right out of the box, because they're making the effort to ensure that every amp that they sell is as good as it can possibly be. The mass producers simply cannot mass produce, sell at low price points, and make that same sort of commitment to quality and tone.
  11. Good luck with that, Bruce. Do you mind if I asked you why you chose to go to all this trouble to have this repaired to original form under warranty, rather than look at some of the rather impressive mods that are widely utilized? It would be very simple to replace that OE PCB with a rock solid turretboard, as well as a beefier output transformer, and you'd not only resolve your problems, but you'd end up with a far better sounding, far better-built amp as a result. Not only that, but if you trained yourself on the skills required to do the job yourself, you'd inevitably learn a whole lot of valuable, lifelong information/knowledge along the way.
  12. The PI should be very strong, high gain, and balanced. On a fixed bias amp (not sure whether or not this amp is), swapping this tube will change your output tube bias, which is probably why you're hearing it sound different with different PI tubes. If you swap this tube, the output tubes should be re-biased (assuming the amp is fixed, rather than cathode biased).
  13. Eminance isn't the only speaker maker with a 12" 4 ohm speaker. Try Weber and Jensen, for instance.
  14. That's correct. However, that specific load that makes the amp most happy isn't necessarily always what the manufacturer SAYS it is. Nor is that magic load necessarily the only load that the amp will run safely at. Many tube amps can safely run 100% mismatches without any detriment whatsoever. The problem is, you would never know that for certain without putting the amp on a scope to see what makes it the most happy, and monitoring the output transformer for excess heat. Heat kills amps, as you said. The question is, how do you KNOW FOR CERTAIN what sort of load is either too high or too low? You don't, unless you put the amp on a scope and try different options. You can assume that the manufacturer is publishing the best load for a given amp, but that assumption doesn't necessary mean that what they say is the absolute best load for the amp. It just means that this is the load that they feel will work well without causing problems. We can only hope that the design engineers had a clue about what they were doing, but, as is the case with the stock Vjr, for instance, it's clear that this isn't necessarily always the case. They hosed up some pretty basic things in that amp. Given how badly the designers screwed up those very simple things, why would we just assume that they got the output impedance correct, right? Again, how do we KNOW what the "correct impedance" is, without seeing amp performance on a scope? We really have no way of knowing whether or not the reason he liked the sound of that speaker had more to do with the impedance mismatch than the speaker make/model. I can attest to the fact that swapping output impedance loads, even to an alleged mismatch, can yield quite dramatic results. My point here isn't to be contentious or to suggest that people just march wildly off to try all sorts of load variations that may or may not be detrimental to their amp. My point is to stress that, without actually seeing a sine wave of an amp run through different tap and load options, you really don't know. The only thing you can really do is assume that the designer of the amp knew what he was doing (a pretty bold assumption, imo), and that there is simply no chance whatsoever that he's made an error.
  15. Bear in mind that it MIGHT be that the reason the 8 ohm speaker sounded better was because the map was happier with an 8 ohm load than the stock 16. In that case, adding a resistor to create a 16 ohm load might be counter-productive. Generally, an amp will be happiest with whatever load the manufacturer says it likes to see, but that's not always necessarily the case. Sometimes, mismatches work better. The only way to know for certain, however, is to throw the amp on a scope and try different loads to see which it likes best. Otherwise, you could just guess, and run 8, since it's not all that likely that a 100% mismatch is going to kill the amp anyway (though there's really no way of knowing without scoping it, so there is a risk of overheating and melting the output transformer windings that you have to be aware of...if you try it, keep an eye on the transformer for signs of overheating).
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