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Thread: Tubes that can handle 500VDC on screens, or tame the power supply??

  1. #36
    Old Timer J M Fahey's Avatar
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    In no particular order:
    * thanks for the Sociologycal/Anthropological data, something which always interests me.

    Among tons of other things, I have read quite a lot about Skid Row which is a unique phenomenon with its own (unwritten) laws, didn´t know the brand Thunderbird, but would have instantly recognized the generic "muskie" , as well as its users: "winos" and even their "starting a Corporation" (pooling coins and spare change so as to buy one or two bottles and then share them).
    Plus the infamous "flophouses" where just *imagining* the smell makes me retch.

    Oh well, poor people, I´m certain nobody drifts there on his own will

    EDIT: almost forgot: my own simplified explanation is this:

    * the basic tube is the triode.
    Plate attracts electrons with a strength proportional to its voltage.
    That makes electron flow (current) VERY dependant on plate voltage.
    Datasheets show that, curves are inclined, with current increasing/decreasing a lot depending on plate voltage.

    * personal explanation, others may differ: screen which is always at a certain high voltage, strongly attracts a lot of electrons.
    Most of them miss it because being a fine wire grid it occupies very little physical space ... so most electrons miss it and overshoot.
    Then it´s easy peasy for plate to capture them, net result is that current is much higher than in a similar triode, and quite independent of plate voltage, so lines are *almost* horizontal.

    * tying screen to plate kills its unique effects and turns it into little more than a plate extension.

    Under same conditions, a triode strapped pentode will provide less power because maximum available current is greatly reduced.

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    Last edited by J M Fahey; 05-12-2018 at 01:37 PM.
    Juan Manuel Fahey

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    Quote Originally Posted by Gtr0 View Post
    ... But what is the nature of the relationship between the screens and plate anyway? I *thought* the screens purpose is to better control the flow of electrons from the cathode?...
    The effect of the screen grid g2 is to 'de-couple' plate voltage from plate current.
    See chart at top of p6 http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf
    So, all else being equal, plate voltage can vary over a wide range without plate current being affected.

    Quote Originally Posted by Gtr0 View Post
    ... How is it then does an amp like the silver jubilee cut out the screen resistor connection.. effective turning the tube into?? A triode?
    ...
    Yes, the low power switch re-configures the power tubes as triodes; the current limiting resistors, eg R55 & R56 https://drtube.com/schematics/marshall/2554amp.gif are still in circuit.

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    Most of them miss it because being a fine wire grid it occupies very little physical space ... so most electrons miss it and overshoot.
    Then it´s easy peasy for plate to capture them, net result is that current is much higher than in a similar triode, and quite independent of plate voltage, so lines are *almost* horizontal.
    Hmmm... to better clarify what I meant (I was on the road during the last post, so it was by phone) and you may have answered is what is their relationship voltage wise as well as physically. So why is it that the screens would come from the next B+ node and see only a very tiny (usually only a handful of vdc at most) v drop. So in relation to the plates they should only be slightly less in vdc as to appear somewhat negative in relation to the plates and highly positive to the cathode?

    But to be completely honest, I can't see the picture here as to why that would make the amp so much louder... only unless the high voltage of plates AND screens would attract that much more electrons...


    Then, meanwhile back at the ranch...
    As if on cue, I had a screen resistor go up in flames some days ago. Bastard still read 1K after removing it though.

    Once I replaced it, and the tubes for well roundedness, it has been fine. But that voltage is still high, though I have yet to order a 10 watt resistor in the 180 ohm zone to bump the voltage down. My EL34 tube supply has been stretched beyond limits. Time to stock up again

    BUT I did order a new custom tranny, this time 345-0-345 instead of 360-0-360 - so that's at least a start.

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  4. #39
    Supporting Member Jazz P Bass's Avatar
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    I think in reality, the screen grid, being between the cathode and the plate, is the anode of the circuit.
    The electrons reach the plate because of the wide open spacing of the screen grid.
    By lowering the screen voltage you are throttling back the dissipation of the tube.

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    Supporting Member eschertron's Avatar
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    I like that explanation ^^^

    Think of the tube as a setting for a slasher movie.
    The electrons (having been boiled off the cathode) are simply milling about (space charge) as if they were teenagers with nothing to do. The high positive potential of the screen grid is calling "psst. Hey! C'mon over here!" so the potential exists for the electrons to travel to the screen. BUT WAIT! IT'S A TRAP! The wide spaces between the screen grid elements allow the electrons to whizz past and be collected by the "monster", a.k.a. the plate electrode.

    The reason it's a trap is because for large portions of the signal cycle, the plate is actually at a relatively low voltage compared to the screen. The plate voltage varies with the rate of electron collision, but the screen collects fewer electrons and doesn't vary significantly from the B+ voltage at its node. So the electrons are still highly attracted (you know how teenagers can be) to the screen/plate despite the - on average - lower plate voltage. They "see" the continuously high screen voltage.

    Triodes (and triode-strapped pentodes) do not exert the continuous higher attractive force, and so do not deliver the higher currents, because the electrons do "see" the - on average - lower plate voltage.

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    Last edited by eschertron; 05-15-2018 at 04:22 PM.
    If it still won't get loud enough, it's probably broken. - Steve Conner
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
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    Old Timer J M Fahey's Avatar
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    So do-nothing teenagers are attracted to Movie Theater by a bloodcurling Bela Lugosi movie ... and when there they are attracted by the *real* Monster.
    Yes, I think I get it


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    Juan Manuel Fahey

  7. #42
    Better Tone thru Mathematics bob p's Avatar
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    The Blob is a great analogy.

    But that trailer has been colorized. Oh, how I hate it when they do that to B&W movies.

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    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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    Quote Originally Posted by eschertron View Post
    The plate voltage varies with the rate of electron collision, but the screen collects fewer electrons and doesn't vary significantly from the B+ voltage at its node.
    BINGO - there's the line that triggered the lightbulb.

    Thank you!!

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    The Screen current is more closely resembles a hyperbolic Cotangent function ..... ie, the screen current will climb as the plate load swings more plate positive... and decreases to it's idle current of a few mA as the load line swing back down towards the idle state...then into cut-off....when the other side of the Push-Pull is already conducting within the overlap angle...
    Marshall amps from the early 70's run about 500V plate and 498V screen at idle .... 1K screen dropping resistors... choke is roughly 110 Ohms..
    With the 1.7K plate load and running the amp at full tilt with Siemens EL34's..... the power supply will sag to roughly 430 Plate and 400V on the screens... of course your mileage mat vary depending on the PT's regulation..
    The screen current for each EL34 at idle will be about 6mA ...the screen during clipping is up about 32mA RMS ...this includes the Duty Cycle since the screen does not conduct the full degrees....roughly 225 give or take degrees in Class AB1...
    The typical NOS EL34 had 8W MAX of Screen Dissipation... What size screen resistors are you using ??
    One time I had seen a Laney Super Group amp have similar issues where some of the El34's were blowing up with white internals... It wound up being internal arcing of the tube that damaged the tube.... It only happened on a certain speaker cabinet... So I measured the fly-back voltages up to 1.1kV ...this arced internal to the tube between pins 3 and pin 2 heater....since heaters were at ground potential ....

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    Quote Originally Posted by cerrem View Post
    The Screen current is more closely resembles a hyperbolic Cotangent function ..... ie, the screen current will climb as the plate load swings more plate positive... and decreases to it's idle current of a few mA as the load line swing back down towards the idle state...then into cut-off....when the other side of the Push-Pull is already conducting within the overlap angle...
    Marshall amps from the early 70's run about 500V plate and 498V screen at idle .... 1K screen dropping resistors... choke is roughly 110 Ohms..
    With the 1.7K plate load and running the amp at full tilt with Siemens EL34's..... the power supply will sag to roughly 430 Plate and 400V on the screens... of course your mileage mat vary depending on the PT's regulation..
    The screen current for each EL34 at idle will be about 6mA ...the screen during clipping is up about 32mA RMS ...this includes the Duty Cycle since the screen does not conduct the full degrees....roughly 225 give or take degrees in Class AB1...
    The typical NOS EL34 had 8W MAX of Screen Dissipation... What size screen resistors are you using ??
    One time I had seen a Laney Super Group amp have similar issues where some of the El34's were blowing up with white internals... It wound up being internal arcing of the tube that damaged the tube.... It only happened on a certain speaker cabinet... So I measured the fly-back voltages up to 1.1kV ...this arced internal to the tube between pins 3 and pin 2 heater....since heaters were at ground potential ....
    For all intents and purposes, it is basically a plexi/2203 output section, with one exception. 1k-5W dropping resistors, also choke is in the same area, 115 ohms if I recall correctly. However, the exception here is that I have a 2k plate load, not 1.7 like a typical 100 watt marshall.

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    Quote Originally Posted by Gtr0 View Post
    For all intents and purposes, it is basically a plexi/2203 output section, with one exception. 1k-5W dropping resistors, also choke is in the same area, 115 ohms if I recall correctly. However, the exception here is that I have a 2k plate load, not 1.7 like a typical 100 watt marshall.
    Having 2k for plate load is perfectly fine that should not be an issue...
    I would suggest confirming the transformer color codes and make sure the solder terminals on the ohm selector line up with what the selector is indicating....
    When the EL34 tubes fail, where is the crack located ??

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    Quote Originally Posted by cerrem View Post
    Having 2k for plate load is perfectly fine that should not be an issue...
    I would suggest confirming the transformer color codes and make sure the solder terminals on the ohm selector line up with what the selector is indicating....
    When the EL34 tubes fail, where is the crack located ??
    I have not ever been able to find a crack, see the following image...
    Click image for larger version. 

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    As you can see the tube has whited out, and there is a burn at the base... this is near pins 7 and 8. Heater to Cathode violation?? In another case, the tube was white and I was unable to find any cracks or burns... but when they go down there is usually an accompanying flashing inside the tube with it. So I am at a bit of a loss on this subject. I am trying to make sense of it, but taking into account the reputation of "modern production tubes" and the fact that my PT is grossly abusing some of those voltage maximums... and while I can not rule out the possibility of a mis-wiring, I have gone over wiring several times not to mention used the exact same scheme in several amps. However one boo-boo I have found is that I was using metal oxide screen droppers... I will be switching those to wirewound.

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  13. #48
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Gtr0 View Post
    As you can see the tube has whited out
    It's kaput, broke, busted. Doesn't need lightning flashes inside to prove it. Glass need not be cracked in any place you can see it. A popular leakage site is the "header" where metal pins protrude through the glass tube base. Each metal/glass seal must be perfect, and stay that way, or else pfffft (sound of a vacuum leak.) It happens, even to the best of 'em.

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    I thought the getter was for trapping residual gas inside the tube. Any possibility that an an internal tube element is melting down or overheating and releasing gases inside the tube? I've always heard that overheated plates release gas... why not other elements? There may not BE a crack in the tube...

    Either way, as Leo said, it's dead. If they all are failing the same way, either there's a run of bad tubes, or...
    Justin

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Gtr0 View Post
    My EL34 tube supply has been stretched beyond limits. Time to stock up again
    Quote Originally Posted by Justin Thomas View Post
    either there's a run of bad tubes, or...
    Justin may be onto something. If you've been experiencing tube failure on a regular basis and pulling from a "stash" of tubes I have two questions...

    Is this tube stash a collection of all the same brand/type/batch? In which case those tubes may be faulty or simply not be up to the conditions in your amp. And,..

    When you have a tube failure are you simply replacing THAT tube from your "stash"? This could also be significant because in a hundy W if the tubes at one or the other side of the push/pull aren't at least reasonably matched then the higher current tube will be doing most of the work. So if you had a tube failure, then just plugged in a tube and THAT tube draws less current than the other one on that side then that other tube may be the next to fail. Making the problem seem random in nature.

    I'm not saying "matched" tubes are all that important for most guitar amp circumstances. But when you start pushing higher voltage and have tubes sharing larger wattage duty, well then yes, you really should have tubes of reasonably similar current draw.

    I don't think you need to replace the PT or add a series resistor or otherwise mitigate high voltage. Plenty of old Marshalls and others are notorious for plate voltages over 500 and are working just fine. If you like the sound of the amp you should leave the circuit alone. If you're not married to the tone it has then sure, go ahead and change it for lower voltage and enjoy longer tube life and reliability. But I think the real problem may just be mediocre tubes, unmatched and in an amp with somewhat demanding operating conditions. This is actually the notion you opened the thread with and I think you had it right then.

    Ruby selected Shuguang EL34B tubes. Buy them in matched quads. It may seem expensive at first, but it costs less than new transformers and the time it takes to do other modifications and in the long run it's likely that the life of the tubes will more than prove to be a value.

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    Quote Originally Posted by Chuck H View Post
    Justin may be onto something. If you've been experiencing tube failure on a regular basis and pulling from a "stash" of tubes I have two questions...

    Is this tube stash a collection of all the same brand/type/batch? In which case those tubes may be faulty or simply not be up to the conditions in your amp. And,..

    When you have a tube failure are you simply replacing THAT tube from your "stash"? This could also be significant because in a hundy W if the tubes at one or the other side of the push/pull aren't at least reasonably matched then the higher current tube will be doing most of the work. So if you had a tube failure, then just plugged in a tube and THAT tube draws less current than the other one on that side then that other tube may be the next to fail. Making the problem seem random in nature.

    I'm not saying "matched" tubes are all that important for most guitar amp circumstances. But when you start pushing higher voltage and have tubes sharing larger wattage duty, well then yes, you really should have tubes of reasonably similar current draw.

    I don't think you need to replace the PT or add a series resistor or otherwise mitigate high voltage. Plenty of old Marshalls and others are notorious for plate voltages over 500 and are working just fine. If you like the sound of the amp you should leave the circuit alone. If you're not married to the tone it has then sure, go ahead and change it for lower voltage and enjoy longer tube life and reliability. But I think the real problem may just be mediocre tubes, unmatched and in an amp with somewhat demanding operating conditions. This is actually the notion you opened the thread with and I think you had it right then.

    Ruby selected Shuguang EL34B tubes. Buy them in matched quads. It may seem expensive at first, but it costs less than new transformers and the time it takes to do other modifications and in the long run it's likely that the life of the tubes will more than prove to be a value.
    No problem...I am used to paying 100 - 130 euros for a matched quad.... I have a quad of TAD EL34B-STR (Shuguang), and so far so good - They have been in a Marshal Plexi for the better part of a year now. So those are also great options.

    As for the current problems and replacing them... I was convinced it was a batch problem. So much so that I emailed my retailer and complained. Of two tubes that blew... rather fantastically.. they were within 30 days, so I got replacements that were matched for my previous orders... so they were drop in replaceable. Always checking the bias then as well if they fall within 4mA, then I do not worry any further and set bias according to the highest tube. However, having emailed my retailer and asking about this particular brand, they claimed that they were one of the more solid tubes - these were JJ E34L's - I have used them for well over a decade, religiously, without a single failure, until last year. Since the two sets were purchased a month or two apart, I am going to continue under the bad batch / pushing them toward their limits assumption.

    My latest tube fiasco (mentioned above), a Tung Sol EL34 that - after about of month of awesome performance - began red plating - I caught it and got the amp turned off within a few seconds... and the amp fired up the next day and ran fine for at least 30 minutes. On a whim I swapped in another set... fired up the amp and the same tube (socket) flashed... This is a recent revelation, but doesn't change the fact of getting bad tubes these days.

    So I have not tested any further with the Tung Sols... In fact I ripped the amp apart down to the chassis because it is a roughed out prototype anyway - rebuilding it now.

    Of all of the tubes I have... The Tung Sol EL34 was a "to die for" tone - for me. Certainly haven't tried all, or nearly enough, brands
    Tung Sol EL34
    JJ E34L
    Svetlana EL34 (came in my plexi, originals from the late 1990's)
    TAD EL34B-STR

    That's my list, in preferred order based on sound. The difference between TAD and Svets were small... then E34L has some kind of lower mid juicyness, then the Tung Sol was midrange madness - in a great and musical way. I was seriously disappointed to see the one side red plating.

    Of course It is allllll subjective... but even in 6L6GC/5881 area, I found the Tung Sols were way nicer sounding than other brands... again subjective.

    Anyway, again, thank you! I am going to continue on bad batches... see if I can get a super solid build together and further experiment from there.

    Cheers!

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    Quote Originally Posted by Gtr0 View Post
    ... but even in 6L6GC/5881 area, I found the Tung Sols were way nicer sounding than other brands... again subjective.
    Slightly off topic, but...
    I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

    Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

    Anyway, carry on.

    Justin

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.
    Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!

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    Now see more detail into the tube damage...
    Very convinced this is Fly-Back arc-over voltage...
    Seen this quite a number of times....
    On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
    Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
    Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
    On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
    There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
    One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
    Then never blew a tube again

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by cerrem View Post
    One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
    Then never blew a tube again
    Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

    If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.

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    When the arc occurs over the air space between tube socket pins... I have used black RTV between the pins with some success..

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    Quote Originally Posted by Justin Thomas View Post
    Slightly off topic, but...
    I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

    Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

    Anyway, carry on.

    Justin
    Quote Originally Posted by Leo_Gnardo View Post
    Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!
    Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....

    Quote Originally Posted by cerrem View Post
    Now see more detail into the tube damage...
    Very convinced this is Fly-Back arc-over voltage...
    Seen this quite a number of times....
    On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
    Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
    Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
    On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
    There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
    One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
    Then never blew a tube again
    Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor? I started another thread trying to resolve if I picked up some legit R3000 diodes... this should help to a degree as well right? At least in terms of fly back voltage.

    Quote Originally Posted by Leo_Gnardo View Post
    Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

    If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.
    As far as I can tell, there has not been any carbon scoring. BUT... as said I ripped the amp down to the chassis... rebuilding it now including new sockets. AND (@cerrem) while of course it is entirely possible that I had wired the ohm selector backwards, though I do not think I did, I have no way of knowing now that I tore it apart (though perhaps I have a photo and in such a case I am extremely curious)... in addition I am wiring impedance taps direct to their own/own pair of switchcraft jacks this time to eliminate the switcher.

    EDIT: Okay, I did confirm via photos that I did wire it correctly. HOWEVER... there was a time that I had one of my cabinets wired wrong and the amp was set for 16ohm and cab was lower. Its hard to remember if these things occurred at the same time... I wonder.....

    Of course I realize, as usual, 99% of the problem is the person sitting at the amp, user error.

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  23. #58
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    Quote Originally Posted by Justin Thomas View Post
    Slightly off topic, but...
    I really hope everyone knows that the TS "reissue" 5881s are NOT equivalent to a 6L6GC, but are a 19W tube equivalent to a 6L6GB... It's the Sovtek 5881WXT that is the "super tube" that's equivalent or better than a 6L6GC.

    Also, did TS ever make an EL34? Hard to "reissue" something that was never manufactured in the first place. Instead, now we have Mullard 6L6GCs and TS KT66s. Maybe I'm just being petty, but this kind of crap on the part of New Sensor (yeah,I'm calling them out) just really pisses me off, because of the confusion and muddying it causes.

    Anyway, carry on.

    Justin
    Quote Originally Posted by Leo_Gnardo View Post
    Spot on Justin. To the best of my knowledge there was never a real TungSol EL34. It's a marketing gimmick by New Sensor to sell the s.o.s. for an extra couple of bucks. You can call 'em out but they don't care, har har we get the money, ka-CHING!
    Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....

    Quote Originally Posted by cerrem View Post
    Now see more detail into the tube damage...
    Very convinced this is Fly-Back arc-over voltage...
    Seen this quite a number of times....
    On at least several of these situations..I first asked the customer, who are DIY, to double check the impedance selector...I re-ask a thousand more times and they always come back saying it's fine....
    Well the amp gets on my bench and I find the impedance sector wired up backwards or off by a few solder tabs..... main reason is that many times some people assume that the solder tab right behind the OHM number is the correct solder tab , when it is actually the opposite end .... or even worse are the Marshall selectors of the 70's with those "Window Selectors" which are more deceiving....
    Getting back on point, the amp in many cases has the 4 Ohm wire inadvertently going to the 16 Ohm tab on the selector, thus making for HUGE plate load which made for crazy flyback voltage ratio ....
    On strong transients with tight compliance speakers....the back EMF can by significant...this back emf generated from the speaker acting as a generator will get stepped up by the OT ratio... then this fly-back voltage is additive to the amps B+ voltage and any IN-Phase signals on the primary winding... Now you can get some pretty high voltage spikes in the kV range....
    There are several paths this high voltage spike could take to arc-over..... Tube sockets can break-down, Air between PINS 3 and 2 can break -down.... or tube internals can break down at the bass pins...
    One method I used to prevent the arc-over was to eliminate the ground potential all together.... Since the arcing was from pin 3 to 2 ,,,since PIN 2 is at Ground Potential from the Heater Center-Tap being grounded... I lifted the Heater Center-Tap off ground and floated it.... Used a LARGE film cap to provide the AC imbalance current to flow with a little impedance as possible...I might have paralleled several caps to get the HUM to acceptable level...
    Then never blew a tube again
    Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor? I started another thread trying to resolve if I picked up some legit R3000 diodes... this should help to a degree as well right? At least in terms of fly back voltage.

    Quote Originally Posted by Leo_Gnardo View Post
    Please forgive if I've mentioned previously in this thread, I know I have somewhere within the recent past, Ampeg's ground float method they used in many 60's guitar amps. Just like you, Ampeg placed a cap between filament winding center tap and ground. They usually employed a 0.1 uF 200V film cap. No harm in upping the voltage say to 600 or 630V to withstand almost any B+, and changing cap value to whatever minimizes hum. I have occasionally been able to use this technique to good effect. Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.

    If there's been any arcing, tube bases and sockets should be closely examined for carbonized tracks or traces and dealt with accordingly. Replacement is always a prime option. In some cases I've been able to "save" bases or sockets by grinding away the carbonized areas with a Dremel tool.
    As far as I can tell, there has not been any carbon scoring. BUT... as said I ripped the amp down to the chassis... rebuilding it now including new sockets. AND (@cerrem) while of course it is entirely possible that I had wired the ohm selector backwards, though I do not think I did, I have no way of knowing now that I tore it apart (though perhaps I have a photo and in such a case I am extremely curious)... in addition I am wiring impedance taps direct to their own/own pair of switchcraft jacks this time to eliminate the switcher.

    EDIT: Okay, I did confirm via photos that I did wire it correctly. HOWEVER... there was a time that I had one of my cabinets wired wrong and the amp was set for 16ohm and cab was lower. Its hard to remember if these things occurred at the same time... I wonder.....

    Of course I realize, as usual, 99% of the problem is the person sitting at the amp, user error.

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  24. #59
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    I don't deny New Sensor the right to make tons of different tube types. But you can't reissue something that never existed in the first place.

    My bigger point was to NOT simply think of the TS "reissue"5881s as equivalent to a 6L6GC. They never were. The 5881 is a completely different tube. Sure, it might work in the same sockets, but it still only dissipates 19W to the GC' s 30. Except for the Sovtek 5881WXT, which is as tough as they come.

    Justin

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    Quote Originally Posted by Leo_Gnardo View Post
    Having the filament floating on the cap eliminates it as a ground potential source for arcs from the nearby plate terminal as you said.
    I'm not convinced. An arc will still develop if the voltage is sufficient and then the capacitor actually intensifies the arc by causing an oscillatory circuit and can reach a much higher, self-sustaining voltage that will short out the cap unless it's one designed to stand tens of Kv. I've used 30Kv series capacitors in a successful ignition system for agricultural engines to intensify the spark during startup for low-flashpoint fuels. The capacitor increases the arc energy enormously by storing and releasing energy.

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  26. #61
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    Quote Originally Posted by Gtr0 View Post
    Not knowing if the US Tung Sol ever made an EL34, it doesn't really matter to me personally... there is no reason that the Russian Tung Sol (New Sensor) can't make a variety of tubes, new or reissue. But saying this I realize there is a HUGE layer of BS to what tube manufacturers - especially New Sensor, do. One thing that disturbs me is that in one thread, maybe in this one or another thread at a different forum, someone said that Tung Sols are basically made in DIY basements. Then of course I have to wonder how this person knew that and if it is a directly known fact or the result of hear-say. Well, either way....
    Much of New Sensor's so-called "variety" comes from applying different labels to the same tubes, making different claims for them and charging different prices. Some of them are OK tubes, others - well you better use them in low voltage applications or else. What "brands" are currently in the New Sensor family? Sovtek, Electro-Harmonix, Mullard, TungSol, GEC Gold Lion. Sound off readers if I left any out. Don't worry, NS's TungSols aren't hand made in some comrade's basement or garage, rather the same factory, same assembly line as all the rest in beautiful suburban Saratov out there in the steppes north of Georgia.

    Interesting... never heard of floating the CT. How would one go about calculating the required value of this capacitor
    I don't know of any formula but you could start with Ampeg's 0.1uF value, 600/630V. Add extra caps & see whether there's a reduction of hum.

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  27. #62
    Bent Member Chuck H's Avatar
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    Perhaps worth noting...

    IMHE (Not: In my "honest" experience as some think, but, In my "humble" experience", as it should be.) the problem is likely occurring at high frequency. Since the typical load for a guitar amp is VERY high impedance at higher frequencies (just look at the plots for this!) and the choke to the screen grids is always limited in henrys it's possible to have very high voltage spikes (and kick backs, thank you cerrem) that find their way through the screen circuit. At these higher frequency impedances it's often possible that the screen circuit represents a lower load than the plates!!! Bumping henries on the choke or adding resistance (in the form of a resistor) after the choke can work. As can bumping the individual screen grid resistor value. Other than that you'd need to mitigate grid drive to reduce spiking. I've dealt with this in builds a couple of times and this is the scenario I interpreted. All of this will change the tone of the amp. for better or worse I suppose since tone is subjective. But I think it's possible that this may be a circumstance to consider.

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    Quote Originally Posted by Justin Thomas View Post
    I don't deny New Sensor the right to make tons of different tube types. But you can't reissue something that never existed in the first place.
    Justin
    No doubt - I didn't mean at all for it to sound snarky.. in fact I appreciate any and all info on these types of things.

    AND AND AND... I JUST looked at my supplier and in fact they list it as a reissue - so yes, you are totally correct. I had paid no attention to that when I bought them.

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  29. #64
    Bent Member Chuck H's Avatar
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    And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.

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    Quote Originally Posted by Chuck H View Post
    And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.
    I have done consulting for New Sensor over the years.... mostly with pedals and transformer designs for their amps... I happen to own two Tektronix 570 curve tracers that I fully restored and calibrated .... I offered New Sensor a great deal ...I would draw up proper data sheets for their product line of vacuum tubes... providing I-V curves , Dynamic transfer curves, Grid 1 current and screen current curves, Ultra-Linear curves....plate curves at all various screen voltages, ie usable data sheets..... I have thermal imaging camera that can give accurate bulb temperatures at various dissipation, to ensure proper tube capabilities.... Well I got a big NO !!!! They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
    Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....

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    Getting back to the original subject at hand.... Last night I blew a power tube in my Ampeg SVT while playing a show..... First time in many many years this happened to me...
    The reason is because I put the SVT head on top of the speaker cabinet and I was playing loud and VERY DEEP frequencies !!!
    You can see from photo the crack on the tube ..most likely mechanical vibration caused the fracture of the glass, which compromised the vacuum which then provided the lightening show...
    I normally stack 2 or 3 SVT heads on a separate dolly and keep it to the side, so not to direct contact with the cabs....
    Last night I made exception due to not much room on the stage due to a drummer that takes up 80% of the stage area...
    I am thinking maybe the original poster to this thread maybe if he has has amplifier in contact to the speaker cabinet ???
    Click image for larger version. 

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    Last edited by cerrem; 06-11-2018 at 07:21 AM. Reason: misspelling

  32. #67
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    Quote Originally Posted by cerrem View Post
    They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
    Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....
    Agreed, that is revealing. And pathetic. I will continue to mostly avoid New Sensor's tube offerings.

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    I'll still buy their tubes, but only if they're branded "Sovtek." Cuz despite the "reissues" being unable to hang, that ol' 5881WXT keeps chugging away; anybody know the actual max voltage & plate dissipation ratings on those yet?

    Justin

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    Quote Originally Posted by Chuck H View Post
    And... Even if the "reissue" tube never was, that's not important to the matter at hand. If you like the tube and it's up to the challenge that's more important. Is it up to the challenge? Well the New Sensor data sheet for the el34b (Tung Sol "reissue") is vague, but seems to indicate that the tube is equal to other el34's. That doesn't mean it's safe to exceed the screen voltage as is typical of older amp designs. I don't even know that it's fair to say it's implied by the equivalent specs to older tubes. So there's THAT. My point is, exceeding the screen grid voltage on older el34 amps was common, but that may not mean that new el34's are up to the challenge. New Sensor is notorious for regurgitating info from old tube data sheets. In fact it appears their Tung sol 5881data sheet IS the Tung Sol data sheet with the vintage dates removed. It's also common to find some arbitrary criteria, like voltages for "typical operation", along with all associated performance data at those voltages, etc. that are spot on with vintage data sheets such it's clearly just copied info. It's for this reason that I don't trust modern data sheets that are even a little suspect of not being data acquired by testing the actual tube in question. Because WHY would a tube manufacturer publish copied data if they have actual info on the product they're selling?!? I think these designs are based on the tube models in question and then tossed at the public wall to see if they stick and it's only assumed, by virtue of design copy, that the old data sufficiently matches the new products performance. But are the materials, tolerances and expertise of the new manufacturers equal to that of the old? I do not think so. That basically makes the data sheets cartoons. That is, they look vaguely like real life, but you probably can't safely do some of that stuff.
    I have noticed their deceptive datasheets. As if to imply that the new tube *is* the old tube. Perhaps it relays a psychological message to some - one of comfort and security - to know your using solid old school tech, which in this case is a desirable thing.

    Quote Originally Posted by cerrem View Post
    I have done consulting for New Sensor over the years.... mostly with pedals and transformer designs for their amps... I happen to own two Tektronix 570 curve tracers that I fully restored and calibrated .... I offered New Sensor a great deal ...I would draw up proper data sheets for their product line of vacuum tubes... providing I-V curves , Dynamic transfer curves, Grid 1 current and screen current curves, Ultra-Linear curves....plate curves at all various screen voltages, ie usable data sheets..... I have thermal imaging camera that can give accurate bulb temperatures at various dissipation, to ensure proper tube capabilities.... Well I got a big NO !!!! They are not even remotely interested in having this done....they like their situation just as is ...they don't want to spend a penny...
    Whatever... I thinks it pathetic that they want to be a big player in tube distribution but show ZERO interest in providing proper data sheets.... I have seen their "In house" tube tester...looks like a 5th graders science project ....
    Wow. As I was reading I was hoping to see it develop the other way... Just reinforces my thoughts above.

    It's too bad because it seems EL34s, in general, are generally not reliable. I used EL34 in various amps for almost 2 decades and didn't have a single problem until the last few years. So either they are getting worse or I am pushing the boundries too much... likely a combination.

    A few days ago, I saw Dave Friedman say similar things (checked out Tone Talk on YouTube), that it is difficult to find a reliable EL34. But at the current his amps come with EH, but that could be old news by now.

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    Painting with a VERY broad brush, I trust 6L6 because they have been made since FOREVER (think WW2 ) by Russians and designed/tested/produced for *Military* equipment (don´t think Uncle Vania and his Magic Accordion´s amp were the main focus) ; and a very distant second by their disciples: the Communist Chinese, who were very close friends up to 1965 so must have gotten at least some Technology transfer.
    Of course Chinese worry more about cost and huge production than quality control but even so, they have been playing the game non only for long but continuously.

    Now EL34 went plain out of production by old game players, Czech/Slovak/Yugo makers were always "second source" at best, and Chinese/Russians never ever made them in the old times to begin with, so the EL34 pond is quite murky.

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