Tube question....

[RSDx]

Just wondering.... I have 2 Epi VJr's, a v1 combo and a v3 head'n'cab. Decided to play around with 12AT7 and 12AU7 preamp tubes, and found that the 12AT7 is horribly microphonic in the combo, but works perfectly fine in the head. Am assuming it's the amps and not the tube, then???..... I kind of like the T tube - a bit lower in volume maxed, I guess, but sounds way better/cleaner, but still breaks up maxed - heck, once you get past 57/10 or so w/ the 12AX7, it just gets muddy anyway, even if it is "louder" ... it ain't particularly usable volume, unless that's the tone you want, I guess. The U tube I like better than the X, but not all that much (but, in all fairness, one is a EH, and one a JJ, so ... results may vary between manufacturers as well, I'd imagine). Guess I answered my own question, but it was weird - I tried the T first in the combo and it was horribly microphonic... decided to give it a try in the head, and it was fine....strange. Go figure. The v3 head'n'cab w/ the 12AT7 and a single coil guitar sounds sweet...at any volume....for my uses, anyways

[Guest Farnsbarns Wunterslausche]

Just wondering.... I have 2 Epi VJr's, a v1 combo and a v3 head'n'cab. Decided to play around with 12AT7 and 12AU7 preamp tubes, and found that the 12AT7 is horribly microphonic in the combo, but works perfectly fine in the head. Am assuming it's the amps and not the tube, then???..... I kind of like the T tube - a bit lower in volume maxed, I guess, but sounds way better/cleaner, but still breaks up maxed - heck, once you get past 57/10 or so w/ the 12AX7, it just gets muddy anyway, even if it is "louder" ... it ain't particularly usable volume, unless that's the tone you want, I guess. The U tube I like better than the X, but not all that much (but, in all fairness, one is a EH, and one a JJ, so ... results may vary between manufacturers as well, I'd imagine). Guess I answered my own question, but it was weird - I tried the T first in the combo and it was horribly microphonic... decided to give it a try in the head, and it was fine....strange. Go figure. The v3 head'n'cab w/ the 12AT7 and a single coil guitar sounds sweet...at any volume....for my uses, anyways

 

Combos are torture boxes for tubes. A tube that has gone microphonic in a combo will often be fine in a head. Not for long mind, it's on it's way out, full stop.

[L5Larry]

Different types of amps, Class A, Class B, Class A/B, etc., use tubes in a technically different manner. Not all pins (or elements) of each tube is use in every type of amp. So the same tube may have a very different performance in two different class amps.

 

As to the combo question, the main reason for combos eating tubes is vibration. In a combo, the tubes take the full and constant brunt of the vibration of the speakers. In a "piggyback", the amplifier, tubes and all, are somewhat insulated (and isolated) from the full vibration of the speaker cabinet. Over time this makes a huge difference in tube life and performance.

 

Back in my 70's R&R days when we were using HUGE tube amp setups, we wouldn't put the heads on the speaker stacks, but on fold-up type suitcase stands (stolen from motels of course). This isolated the amp from the 200 watts of vibration, AND made it easier to reach for setup and adjustments.

[tweed2]

As L5Larry said, vibration is most likely the culprit. There are various ways to fight this. I use large rubber feet on my head cabinets and small rubber isolaters where the chassis mounts to the head cabinet. There are aftermarket tube dampers that help a little, but have a tendency to make the tube heat up a little more, so in my opinion, are counterproductive. I'm not familiar with the chassis orientation between the two amps in question, but amps that have tubes that hang down are more prone to these issues (heat and vibration) than chassis that position the tubes "up". Easier in the long run to just buy another tube (and maybe a backup, for the future).

[RSDx]

As L5Larry said, vibration is most likely the culprit. There are various ways to fight this. I use large rubber feet on my head cabinets and small rubber isolaters where the chassis mounts to the head cabinet. There are aftermarket tube dampers that help a little, but have a tendency to make the tube heat up a little more, so in my opinion, are counterproductive. I'm not familiar with the chassis orientation between the two amps in question, but amps that have tubes that hang down are more prone to these issues (heat and vibration) than chassis that position the tubes "up". Easier in the long run to just buy another tube (and maybe a backup, for the future).

Actually, it was microphonic at even the very lowest of volumes, not enough to induce any kind of vibrations, really (basically microphonic whenever turned on...). can crank the head and it's fine. I may just keep the combo with the 12AX7, though, as, with it's smaller speaker, dirtier tube, it gets dirty sounding at lower volmes, which is ok, really, when I don't feel like really cranking the head loud...

[tweed2]

Hmmm.. I'm not familiar with the two chassis you're talking about, but my only other thought would be the other one that L5 brought up. Although the 12AT7 is a similar tube to the 12AX7, it not only has a lower gain value, it has a higher transconductance to it. So there may not be anything wrong with the tube, it's probably how the tube is used in the two different circuits. If you get the chance, try a 5751 tube instead. They provide a reduced gain (70% instead of the 12AT7s 60%) but are configured more like the usual 12AX7. I love what it does in my Champ/DeLuxe clone. You may find that it's more compatible with the Epi circuits. The 12AT7 and 12AV7 seem to have the most trouble playing well in most amps.

[mtheory]

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.

[tweed2]

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.

 

Eloquently put, my friend. I have an aquaintance that I've purchased tubes from for several years now. He sells "high end" and hard-to-find tubes (and makes a damn good living at it) to those that purchase 4 and 5 figure tube stereo systems and knows about as much about tubes as anybody. Unlike the good old tubes of yesterday, that were built to high standards and maybe had a cull rate of 1 in 100 more, today's tubes are mass produced and as many as 50% are pitched due to being junk. The business model being cut costs and quality and you can afford to throw more away, and still make a profit! I would purchase a 50 year old tube, with less than half it's life left over the best "new" tube any day. I'm a firm believer that good tubes allow your amp to be the best it can be. Unfortunately, the good tubes are slowly disappearing.

[Fadedepi]

The 5751 is a great tube replacement for a 12AX7. Less gain (70%) and noise. IMO the best choice for any comb or amp head. Try one!

 

Faded.....