EtherealWidow
Well-known member
+1. I prefer to revert to the theory that amps run off of pixie dust. They were a lot easier to pretend to understand then.edward said:
12AT7 as Reverb Send and Return (V5)
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Mark 63 said:
But that's part of my question that's yet to be answered definitively by someone who really knows this stuff ...as opposed to us gear heads :lol:
"Rated" at higher amperage indicates a higher capacity, which is not the same as actual current draw. The latter is determined by the circuit design and given load under specific conditions (oversimplification, which is how my brain works
).There is, no doubt, lots I'm missing here, which is why I'd like an EE or Amp tech/designer to clear this specific point up. Unfortunately the paper in question, which seems on the face of it very credible, doesn't address the point with any specifics that leads me to conclude that the higher mA rating of an AT7 results in additional draw, let alone "excessive" draw that would lead to taxing the circuit, and by extension endangering the amp or us. Otherwise, why would the author seem to encourage such experimentation in the PI where he does mention the specifics of tube choice and their respective ratings (even though he mentiones the differential) yet ignores the potential hazard?
Again, not trying to be argumentative here as much as I would like definitive answers that go beyond inference of an article that, on the face of it, doesn't even address a concern that others have raised that was not even an explicit point from within the article. Given a definitive "no-no," I'll be the first to start replacing my at7s with 5751s. But until then, there still is no "proof" I can point to that warrants concern. But this layman in basic electronics is willing to learn!
Edward
In some circuits, you could have a catastrophic failure drawing 10x the current the circuit is designed for should you push the tube that hard. Will you? I don't know. Can you? I don't have the specifics for any particular circuit to say yes or no.
For me, it's not an option. I'm not saying it's law. But with some specifics, I could do the math and apply ohms law perhaps. I don't have the data though.
If you ears like it, cool. If they work, cool.
It's kinda like the guys that put a .600 lift cam with 320* of duration in a passenger car or stock truck. It sounds cool, but it actually works worse in most cases. That's where I'm coming from. YMMV.
So yeah, give me a schematic and data, and we can go all EE. But in general, YMMV.
Has anyone sent an amp in to Mesa for warranty with 12AT7 tubes in the 12ax7 slots? I'm curious what the position of Mesa would be. The fact that Mesa sells 12AT7 tubes is not an endorsement from Mesa to use 12AT7 tubes in a 12AX7 slot though.
And, most of the folks I see doing stuff like this never leave the bedroom and stretch the amps out, so there's no real truth as to success in those cases especially.
For me, it's not an option. I'm not saying it's law. But with some specifics, I could do the math and apply ohms law perhaps. I don't have the data though.
If you ears like it, cool. If they work, cool.
It's kinda like the guys that put a .600 lift cam with 320* of duration in a passenger car or stock truck. It sounds cool, but it actually works worse in most cases. That's where I'm coming from. YMMV.
So yeah, give me a schematic and data, and we can go all EE. But in general, YMMV.
Has anyone sent an amp in to Mesa for warranty with 12AT7 tubes in the 12ax7 slots? I'm curious what the position of Mesa would be. The fact that Mesa sells 12AT7 tubes is not an endorsement from Mesa to use 12AT7 tubes in a 12AX7 slot though.
And, most of the folks I see doing stuff like this never leave the bedroom and stretch the amps out, so there's no real truth as to success in those cases especially.
Hi FXR,
Glad you chimed back in ...I was kinda hoping you had the answer!
So in seems like you're saying YMMV, depending on a specific amp design? Is it the circuit's design that dictates how much the PI will draw? I am wondering if the higher mA spec on the AT7 is even a relevant point to an amp's design in the PI spot (I know it is in the reverb spot on Fenders, but the PI is a different animal altogether). Pehaps the current draw in the PI position doesn't even merit discussion as it's so low (below the threshold of an at7, that is) and we're all just pointing to a distinction without a net difference (in terms of mA actually passing through the tube). Again, I don't know, but sure would like to.
Your example of cam tuning (lift, duration, lobe centers, etc.) is not lost on me. It's all too easy for a numbnut to throw on hot cams and larger carbs (or throttle bodies) because these are bolt-on go-fast goodies ...completely misunderstanding what these components do and how they interact with the rest of the engine. So it must be clearly "faster" ...when in fact, it is not! Speed is measurable and not subjective, unlike tone. Changing PI tubes clearly creates tonal differences; but only one's ears and personal tonal preferences can decide on whether it is indeed "better." I hear you though: change for change's sake is not by definition an improvement. One must be honest about that.
FWIW, I have played out quite a bit with my DC5 with a NOS Mullard CV4024 (12AT7) in there, hmmm for a couple of years now, and no maladies. Not that this "proves" anything, of course, but only to say so far so good, with nary a hint of any operation differences ...only "better" tone! Who knows how other amps will behave. But again I submit that until someone says how (if at all) the spec difference creates actual risk, especially in light of others (like the author of the paper) endorsing such a change, it is likely we may be making something out of nothing.
Edward
Glad you chimed back in ...I was kinda hoping you had the answer!
So in seems like you're saying YMMV, depending on a specific amp design? Is it the circuit's design that dictates how much the PI will draw? I am wondering if the higher mA spec on the AT7 is even a relevant point to an amp's design in the PI spot (I know it is in the reverb spot on Fenders, but the PI is a different animal altogether). Pehaps the current draw in the PI position doesn't even merit discussion as it's so low (below the threshold of an at7, that is) and we're all just pointing to a distinction without a net difference (in terms of mA actually passing through the tube). Again, I don't know, but sure would like to.
Your example of cam tuning (lift, duration, lobe centers, etc.) is not lost on me. It's all too easy for a numbnut to throw on hot cams and larger carbs (or throttle bodies) because these are bolt-on go-fast goodies ...completely misunderstanding what these components do and how they interact with the rest of the engine. So it must be clearly "faster" ...when in fact, it is not! Speed is measurable and not subjective, unlike tone. Changing PI tubes clearly creates tonal differences; but only one's ears and personal tonal preferences can decide on whether it is indeed "better." I hear you though: change for change's sake is not by definition an improvement. One must be honest about that.
FWIW, I have played out quite a bit with my DC5 with a NOS Mullard CV4024 (12AT7) in there, hmmm for a couple of years now, and no maladies. Not that this "proves" anything, of course, but only to say so far so good, with nary a hint of any operation differences ...only "better" tone!
Who knows how other amps will behave. But again I submit that until someone says how (if at all) the spec difference creates actual risk, especially in light of others (like the author of the paper) endorsing such a change, it is likely we may be making something out of nothing.Edward
Yeah, it's something I am probably being too anal about and while it usually isn't a problem in practice, sort of knowing what the numbers could mean kinda makes me leery. All the folks that do it prove it can be done. So for me, if I go there, it would be a 5751.
The statement, 10x the current available to push the output tubes in the PI example just looms over me in thought. I'm not an amp builder expert with the answer handy for when it's bad and not bad. So I am staying on the cautious side with a warranty to keep in mind.
The statement, 10x the current available to push the output tubes in the PI example just looms over me in thought. I'm not an amp builder expert with the answer handy for when it's bad and not bad. So I am staying on the cautious side with a warranty to keep in mind.
Ok, I'll try and shed some light. I can see why people are cautious about this.
The circuit around the tube will largely dictate how much current will flow. At 12AT7 can deliver a lot more current than its rating, 30 or 40ma perhaps into a short circuit. It will not live long though in that situation. The point is that the tube doesn't care even that its limit is 10ma, the circuit needs to prevent it from destroying itself. Same story for a 12ax7.
In general the various small signal stages found in our audio amps that use 12a*7 tubes have a high value plate resistor. A cathode follower stage is a little different, but as far as I know there are none of those in the Express so I wont bother with those. Someone may want to correct or varify that.
The plate resistor is largely what sets the current draw. This resistor is usually around 100K Ohms. If plate voltage is 300V (it wont be more than that, but could be a lot less) and the tube is hard on then max current can only be 3ma regardless of what the tube is. That is .9 of a watt and not likely to damage anything.
Personally speaking, I have tried a 12at7 in most of the slots, but have not found a reason to leave one in place. From sparkly clean to scream the 12ax7 works best for me.
The circuit around the tube will largely dictate how much current will flow. At 12AT7 can deliver a lot more current than its rating, 30 or 40ma perhaps into a short circuit. It will not live long though in that situation. The point is that the tube doesn't care even that its limit is 10ma, the circuit needs to prevent it from destroying itself. Same story for a 12ax7.
In general the various small signal stages found in our audio amps that use 12a*7 tubes have a high value plate resistor. A cathode follower stage is a little different, but as far as I know there are none of those in the Express so I wont bother with those. Someone may want to correct or varify that.
The plate resistor is largely what sets the current draw. This resistor is usually around 100K Ohms. If plate voltage is 300V (it wont be more than that, but could be a lot less) and the tube is hard on then max current can only be 3ma regardless of what the tube is. That is .9 of a watt and not likely to damage anything.
Personally speaking, I have tried a 12at7 in most of the slots, but have not found a reason to leave one in place. From sparkly clean to scream the 12ax7 works best for me.
Ahhh, thanks for that JJ! Just what I was thinking: current draw determined by the circuit is below the threshold of danger (for the amp's circuit, assuming all is good). Just because a tube can flow more amperage doesn't mean it will. Kind of like dipping a straw into a large river vs a trickling stream: your straw determines the flow, not the size of the body of water.
OK, so plate resistor, and thereby resultant voltage ...makes sense to this pea brain. So with Mesas being fixed bias and non-cathode following (which I believe is all Mesas ...except maybe those that tout "Class A," which as we all know is "defined" rather loosely by some, and others have deemed "Boogie's class A" as not "real" class A, to which I'll bow out of that one... ), it seems perhaps we're on the right track here. At least for amps whose plate voltage is around that neighborhood and such. Makes me think (guess?) that those with adj-bias amps or those who have modded their Boogs with bias pots could easily/potentially be altering said voltage to where an AT7 may, indeed, make a marked difference in current draw to the OP section. Only your VOM could tell. BTW, where the heck is MonstaTone on this?? Hey Andy...how about a bone on this one?!!
Edward
OK, so plate resistor, and thereby resultant voltage ...makes sense to this pea brain. So with Mesas being fixed bias and non-cathode following (which I believe is all Mesas ...except maybe those that tout "Class A," which as we all know is "defined" rather loosely by some, and others have deemed "Boogie's class A" as not "real" class A, to which I'll bow out of that one... ), it seems perhaps we're on the right track here. At least for amps whose plate voltage is around that neighborhood and such. Makes me think (guess?) that those with adj-bias amps or those who have modded their Boogs with bias pots could easily/potentially be altering said voltage to where an AT7 may, indeed, make a marked difference in current draw to the OP section. Only your VOM could tell. BTW, where the heck is MonstaTone on this?? Hey Andy...how about a bone on this one?!!
Edward
The output stage and gain stages are quite different. The gain stages (the ones using the 12AX7) are not fixed bias, they are cathode biased and have a cathode resistor. The gain stages are class A and can not be class B or AB since they are all single ended. By single ended I mean that there is one single triode in each stage.
The output stage (EL84 tubes) is a push pull and when run in 30 watt mode and is biased such that it is in class AB. In 5 watt mode one tube is switched out turning it into a pure class A single ended config. From what I can tell the bias config also changes to a cathode bias mode with a catohde resistor switched in.
I just checked the transfer chars for both the 12ax7 and at7 tubes and the at7 requires a lower bias voltage for the same current, about -3V compared with -1.5V for the ax7. The operating point will change a little with a 12at7 as the circuit balances itself. PLate voltage will decrease a little and plate current will incease a little, probably only 20% or so. Changing the operating point is not really an issue, but it does have some effects on tone and overdrive.
Edward, your straw and river analagy is pretty good. The only way to get more water through the straw is to increase the pressure or get a bigger straw (lower value plate resistor). In electronics pressure can be thought of as voltage.
A cathode follower is not usually a gain stage, but is used to increase current drive and generally has a low output impedance. A cathodfe follower can be useful before the tone stack and to drive things like the effects loop. The Lonestar has a cathode follower to drive the effects so I am thinking the Express will be the same here. My previous comment about the Express not having any cathode follower stages is most likely in correct.
The output stage (EL84 tubes) is a push pull and when run in 30 watt mode and is biased such that it is in class AB. In 5 watt mode one tube is switched out turning it into a pure class A single ended config. From what I can tell the bias config also changes to a cathode bias mode with a catohde resistor switched in.
I just checked the transfer chars for both the 12ax7 and at7 tubes and the at7 requires a lower bias voltage for the same current, about -3V compared with -1.5V for the ax7. The operating point will change a little with a 12at7 as the circuit balances itself. PLate voltage will decrease a little and plate current will incease a little, probably only 20% or so. Changing the operating point is not really an issue, but it does have some effects on tone and overdrive.
Edward, your straw and river analagy is pretty good. The only way to get more water through the straw is to increase the pressure or get a bigger straw (lower value plate resistor). In electronics pressure can be thought of as voltage.
A cathode follower is not usually a gain stage, but is used to increase current drive and generally has a low output impedance. A cathodfe follower can be useful before the tone stack and to drive things like the effects loop. The Lonestar has a cathode follower to drive the effects so I am thinking the Express will be the same here. My previous comment about the Express not having any cathode follower stages is most likely in correct.
I have asked Mesa more then once and they say there are no cathode followers in the Express 5:25. I asked and stayed on the subject a bit while trying to spec out what tubes to use. \
IIRC, we were trying to sort out if spiral filaments were needed. Does that sound right for a cathode follower, needing spiral filaments or something?
IIRC, we were trying to sort out if spiral filaments were needed. Does that sound right for a cathode follower, needing spiral filaments or something?
Some of the new production tubes (such as the Tungsol) had or have an issue with their insulation between the filament and the cathode. The 12ax7 spec for max voltage between those points is only something like 100V. Depending what the cathode follower circuit is used for there can be a few hundred volts from cathode to the filament. The early Tungsol RI 12ax7s were known to fail if used like that. I don't know if spiral filament tubes have better insulation between cathode and filament, but that could easily be the difference.
I thought either I just got lucky with my Tungsol, or the Express 5:25 doesn't run high enough voltages around the PI to hurt the tube.
Thanks for the confirmation With Mesa that there are no cathode followers.
I thought either I just got lucky with my Tungsol, or the Express 5:25 doesn't run high enough voltages around the PI to hurt the tube.
Thanks for the confirmation With Mesa that there are no cathode followers.
edmiller9999
Well-known member
Great discussion, guys. I have a MK IV and the reverb is a bit hot. Would this "mod" apply to my amp, too? I think it'd be fine to make the change, but if you have any opinions they'd be much appreciated.
I'm not overly familiar with the Mk4, but assuming it does have a tube driven reverb then the lower gain tube will almost certainly work for you. I'd try a 5751 tube first up since it is closer in spec to the 12ax7 the the 12at7. The 5751 has a little more gain, than a 12at7, but still a lot less than the 12ax7.
Let us know how you go if you decide to try it.
Let us know how you go if you decide to try it.
fridlmhupa
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today the postman gave me a little packet
from TAD with a 5751 inside.
I like the reverb sound with this tube much more...great!
Is it possible to add a little volume pot adapter into the reverb send cable?
That would be pretty easy to build, and gives me the possibility to
dial in the right send level....and all that without swapping the original tubes
for a "wrong" type
from TAD with a 5751 inside.
I like the reverb sound with this tube much more...great!
Is it possible to add a little volume pot adapter into the reverb send cable?
That would be pretty easy to build, and gives me the possibility to
dial in the right send level....and all that without swapping the original tubes
for a "wrong" type
