Mesa MkIV - obsolete now LDRs banned?

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CacofonixCacofonix Frets: 356
Having done some looking about, I understand the MKIV uses LDRs in its switching matrix, which I find pretty neat, apart from the fact they have a finite lifespan.

So, my question is, cam they be fixed by using,say a relay instead? I am assumin that an LED is used to trigger the switching, so there will be two points for the LED and two more for the LDR. Is it the same for a relay?  There will then be popping on the circuit.  Is a miniature variac in existence or possible?


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Comments

  • ICBMICBM Frets: 71956
    It would be very difficult to replace them with relays - not only do you have problems with popping, the physical fit of them into something as complex as a MkIV is going to be very difficult… most of them are in 'stacks'.

    The two-channel Dual Rectifier (inc Trem-o-verb) has the same issue too, as do quite a few other Mesas of the time, although most have far fewer LDRs.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    If you are talking ORP12s you can still get them here it seems....

    Relays are non-starter IMHO if for no other reason than it being very difficult to keep clicks out of the audio in highish impedance circuits. 
    My choice would be JFETS and the J112 is THE device to go for in signal switching applications.

    As ever, look up Duggy Self's book!

    Dave.
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  • ICBMICBM Frets: 71956
    Yes, you can still get them - the RoHS regs only cover new equipment (I think). I have wondered whether I should stockpile a few hundred though, just in case!

    Mesa have switched (no pun intended ;) ) to relays now, but that involved a complete re-design of their amps, and there are problems with popping - which they use muting circuits to solve, but that has the disadvantage of causing an audible gap in the signal. They also use JFETs, which have proved less than perfectly reliable - possibly worse than LDRs, although I'm not sure.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • jpfampsjpfamps Frets: 2723
    edited February 2014
    RoHS only applies to new appliances.

    You are not obliged to use RoHS compliant parts when repairing equipment.

    Furthermore, there are severals exemption from RoHS, including military equipment, which I can't see changing anytime soon: if you want a capable military you will need some US made equipment and the US haven't adopted RoHS regs.

    I see no problem acquiring optocouplers for MESA repairs in the forseeable future.

    Optocoupler have the advantage of being able to handle high signal voltages, require little power to drive them, and have "soft" switching, so don't pop when used in series configuration. However, they are relatively expensive, have poor "offness", are noisy, and have a shorter life span than other switching methods.

    Relays require more power than optocouplers (they often require a dedicated supply), and can cause a pop when used as series switches. They can handle high signals though and are generally my first choice of switching element for valve amps.

    It's perfectly possible to employ relays to switch channels in even very high gain amps, with NO popping. I know I've done it.
    However you can't use series switching; you need to use shunt switching, and this means that you need to design the amp in such a way to accomodate this; retro fitting relays in place of optocouplers in your amp us unlikely to be a pop-free solution.

    JFETs have the advantage of being cheap, require little power to drive and have "soft" switching; however they have quite low voltage limits (30-40V is typical), which are easily exceeded in a valve amp.

    I expect that overvoltage is the likely cause of failure that ICBM sees in MESA amps (I've not seen this often, but then again I don't work on many MESAs). Having said that a(very cheap) zener diode across the JFET would protect it from this happening..........
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  • CacofonixCacofonix Frets: 356
    edited February 2014
    jpfamps said:
    RoHS only applies to new appliances.

    You are not obliged to use RoHS compliant parts when repairing equipment.

    Furthermore, there are severals exemption from RoHS, including military equipment, which I can't see changing anytime soon: if you want a capable military you will need some US made equipment and the US haven't adopted RoHS regs.

    I see no problem acquiring optocouplers for MESA repairs in the forseeable future.

    Optocoupler have the advantage of being able to handle high signal voltages, require little power to drive them, and have "soft" switching, so don't pop when used in series configuration. However, they are relatively expensive, have poor "offness", are noisy, and have a shorter life span than other switching methods.

    Relays require more power than optocouplers (they often require a dedicated supply), and can cause a pop when used as series switches. They can handle high signals though and are generally my first choice of switching element for valve amps.

    It's perfectly possible to employ relays to switch channels in even very high gain amps, with NO popping. I know I've done it.
    However you can't use series switching; you need to use shunt switching, and this means that you need to design the amp in such a way to accomodate this; retro fitting relays in place of optocouplers in your amp us unlikely to be a pop-free solution.

    JFETs have the advantage of being cheap, require little power to drive and have "soft" switching; however they have quite low voltage limits (30-40V is typical), which are easily exceeded in a valve amp.

    I expect that overvoltage is the likely cause of failure that ICBM sees in MESA amps (I've not seen this often, but then again I don't work on many MESAs). Having said that a(very cheap) zener diode across the JFET would protect it from this happening..........
    Shunt switching - using resistance to ground to effect the switching, rather than rerouting the signal itself?  Like the Peavey VT series?

    Does that have an effect on tone?

    Noted Mesas are complex animals, though I saw a MKIV schematic the other day, and apart from being on 3 pages of PDF, it wasn't as hair raising as anticipated.

    Are they still making LDRs then?  The reason I'm, interested is that if they degrade, do they degrade over time, or due to use.  i.e. is an NOS LDR still ok twenty years on?

    I heard that Mesa get their sound from driving the valves hard, and that they do need replacing more than, say, a Lazy J because of this.

    Thanks for all responses.

    Duggy Self:  as I'm in Yorkshire, ths is poignant in the context of amp repair.

    Edit:  He's trying to get rid of distortion.  I hope he's talking only about hifi amps.
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  • ecc83ecc83 Frets: 1589
    edited February 2014
    "I expect that overvoltage is the likely cause of failure that ICBM sees in MESA amps"

    Hah! I was going to ask ICBM "WTF are Mesa doing to their FETs? " From my experience, never a problem. Even the simplest of "Those Pedals" used two of them and the more complex amps over a dozen and I cannot recall a failure. The junction TRANSISTOR that was a driver to a FET gave a bit of bother in the very early HT-5s but that was soon sorted. 

    Dave.
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  • ICBMICBM Frets: 71956
    Cacofonix said:
    Are they still making LDRs then?  The reason I'm, interested is that if they degrade, do they degrade over time, or due to use.  i.e. is an NOS LDR still ok twenty years on?
    Use, and in particular heat, seems to be a major factor. The Trem-o-verb combo (valves mounted under the chassis) seems to be noticeably more prone to it than the Dual Rectifier head (valves above the chassis) despite being identical in every other way excluding the reverb and tremolo, although only one of the LDR failures I've seen has been in the reverb circuit.

    Cacofonix said:
    I heard that Mesa get their sound from driving the valves hard, and that they do need replacing more than, say, a Lazy J because of this.
    Not true. If anything the power valves are biased notoriously cold in most Mesas (apart from the EL84 models), and preamp valves don't really suffer from being driven hard in that sense. They do suffer from microphonics when used for high gain though, so a Mesa will likely show up a duff preamp valve quicker than a Lazy J.

    ecc83 said:
    Hah! I was going to ask ICBM "WTF are Mesa doing to their FETs? "
    I don't know, and to be fair the number of failures I've seen is small enough to be statistical scatter probably. The chap who used to do the Mesa UK distributor's in-house repair saw a fair few though if I remember right.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • jpfampsjpfamps Frets: 2723
    Cacofonix said:
    jpfamps said:
    RoHS only applies to new appliances.

    You are not obliged to use RoHS compliant parts when repairing equipment.

    Furthermore, there are severals exemption from RoHS, including military equipment, which I can't see changing anytime soon: if you want a capable military you will need some US made equipment and the US haven't adopted RoHS regs.

    I see no problem acquiring optocouplers for MESA repairs in the forseeable future.

    Optocoupler have the advantage of being able to handle high signal voltages, require little power to drive them, and have "soft" switching, so don't pop when used in series configuration. However, they are relatively expensive, have poor "offness", are noisy, and have a shorter life span than other switching methods.

    Relays require more power than optocouplers (they often require a dedicated supply), and can cause a pop when used as series switches. They can handle high signals though and are generally my first choice of switching element for valve amps.

    It's perfectly possible to employ relays to switch channels in even very high gain amps, with NO popping. I know I've done it.
    However you can't use series switching; you need to use shunt switching, and this means that you need to design the amp in such a way to accomodate this; retro fitting relays in place of optocouplers in your amp us unlikely to be a pop-free solution.

    JFETs have the advantage of being cheap, require little power to drive and have "soft" switching; however they have quite low voltage limits (30-40V is typical), which are easily exceeded in a valve amp.

    I expect that overvoltage is the likely cause of failure that ICBM sees in MESA amps (I've not seen this often, but then again I don't work on many MESAs). Having said that a(very cheap) zener diode across the JFET would protect it from this happening..........
    Shunt switching - using resistance to ground to effect the switching, rather than rerouting the signal itself?  Like the Peavey VT series?

    Does that have an effect on tone?

    Noted Mesas are complex animals, though I saw a MKIV schematic the other day, and apart from being on 3 pages of PDF, it wasn't as hair raising as anticipated.

    Are they still making LDRs then?  The reason I'm, interested is that if they degrade, do they degrade over time, or due to use.  i.e. is an NOS LDR still ok twenty years on?

    I heard that Mesa get their sound from driving the valves hard, and that they do need replacing more than, say, a Lazy J because of this.

    Thanks for all responses.

    Duggy Self:  as I'm in Yorkshire, ths is poignant in the context of amp repair.

    Edit:  He's trying to get rid of distortion.  I hope he's talking only about hifi amps.
    We've employed shunt switching to mute the channel/ features that are not selected.

    Theoretically there can be some effect on tone due to stray capacitance in the switching element, which is more of an issue in high impedance circuitry, however in my experience with relays, I've not been able to detect a problem.

    To employ shunt switching the amp topology needs to be optimized, for example it's easier to implement in a two channel amp if the channels are completely separate, which is the "no compromise" approach, but there more expemsive to implement.
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  • ecc83ecc83 Frets: 1589
    "Edit:  He's trying to get rid of distortion.  I hope he's talking only about hifi amps."
    Yes he is well, expensive mixers really. And yes, from the distortion point of view, massive overkill for gitamps  (LDRs are very non-linear!) but the variety and implementation of the various switching regimes is interesting and the comments about keeping clunks and clicks out moot for amps with a lot of gain and power behind them.

    Dave.
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