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

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

    Hi All.

    I have a high gain amp that puts out 516 VDC on the plates and 511 at the screens. It is an EL34 based amp so screen resistors are 1K 5W. I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white) and someone mentioned that running the bias at 70%, with current producton tubes and with these voltages, very well may be too much. I play a lot - probably some 30+ hours per week.

    So let us assume this is true for the moment, the only tubes I can even find that are close to accepting 510VDC on the screens are the TAD EL34B-STR (claiming 500 VDC on screens max). Shuguang I guess? I have a set of these in a plexi and I find that they sound fine.

    The transformer is custom made and similar to PT for a SLO100, so it's a 360-0-360.

    The question is should I have a new tranny made, something that will yield something a bit less like 340-0-340??? Or should I put a dropping resistor after the rectifier circuit? Or is there a set of tubes out there that will accept a bit of abuse in this area (by your experience)? Or even perhaps raising the screen resistors??

    I have used JJ E34L for the last decade and a half... But after popping two of them some months back, I bought a set of Tung-Sol EL34s... it was about a month going before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.

    My temperature probe says the tube's surface runs at about 280 degrees F. But damn does it get HOT in the room.

    Thanks for any input you may be able to offer.

    Thanks.

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    I know folks with vintage amps that run high plate/screen voltages that have been using the Shuguang's. You can buy them cheap from RS :
    https://uk.rs-online.com/web/c/semic...triode-valves/

    I've used them in the past in a Trace Elliot Speedtwin and that runs about 500-510. It also has a standard Marshall 100W replacement OT as the tech who repaired it is a hack. It really should have a 2k-2.2k primary for that kind voltage.
    Screen voltage rating do get vastly abused in most amps. Said speedtwin also runs 6L6's and they are only rated for 450V on the screens and they see 500! Same is true of the SLO.

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    What (loaded) Vac is the PT putting on the heaters? If there's some leaway there, a bucker may be a good option.
    Are there any higher voltage taps on the PT primary?

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    for the best reliability,i would regulate the screen voltage to 400V max,with a mosfet and zener diodes.
    EL34 tubes are very unpredictable as far as lifespan,Winged C were good but expensive,today the most reliable seems to be the EHx,but over 500V?

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    Senior Member Pedro Vecino's Avatar
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    Quote Originally Posted by Gtr0 View Post
    I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white)
    ...before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.
    So much repetition is suspicious. Do you use any type of spring retainer on them? Sometimes they do more damage than other things. When a tube turns white it means there is a crack

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    Bent Member Chuck H's Avatar
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    I built a custom order a few years ago that was high-ish voltage, but not as high as your rig. I blew up a bunch of tubes of different brands. Shuggies were the only ones to consistently survive and that's what the amp is using now. Unfortunately it's a combo amp and the Shuguang EL34's tend to be microphonic. I ended up buying three pair to get one that was good enough. If I had known I would have bought a sextet and selected the best two. I have the other four tubes in my stash marked "head only". They sound as good as any EL34 IMO.

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    JJ 6CA7 is rugged on even higher voltages than 510V plate and 500V screen.

    maybe the choke needs to be replaced with a 300-1000 ohm resistor ?? (tame the power supply idea)

    Also though, it seems like the tubes are not dying because they would short and blow a fuse, or just turn on but not work at all . if the tube is losing vacuum the tube is not failing because of high voltage IMO. It would be failing from........being banged around???

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    Rather than looking for Terminator approved tubes , which to boot are used beyond any manufacturer approved rating (they *should* know ) Iīd lower screen voltages to sensible levels.

    The simplest and cheapest solution is to downgrade the screen supply: drop the choke and use a plain resistor instead.
    Anything from 1k 5W or 10W and up, whateverīs needed so at idle they sit at no more than 450V, dropping more when amp is driven full blast ... which is good.

    Or you can go Hi Tech and use a MosFet regulator set to fixed 400V.

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    It is good to drop the screen supply as juan says but it will make it sound more mushy and undefined in pick attack. Which I would guess this player wants since he is playing an SLO clone.

    What would you do to re-tighten the tone? Adjust NFB resistor?

    I usually use a 20W 1K resistor for 140W amp.
    This is a nice one -- https://www.mouser.com/ProductDetail...NE5tU87GTlE%3d

    I thought I could be a kinda lazy and use 500 ohm 10W in series with 470 ohm 6.5W in series (no 20w resistors around to use). By 3rd practice the 6.5W almost fell out because it was getting so hot the solder was melting and the resistor almost becaame loose in chassis! This is with amp absolutely cranked so it's an extreme scenario but I like 20W just to be safe.

    if you look at 5150 and a bunch of other amps of similar 100W power rating that don't want to buy chokes, they usually use 400 or 500 ohm 10W resistor. Ampeg V4 is about 540V plate and 530V screen and they use 470 ohm 7W, which occasionally falls out of circuit from getting too hot if you crank the shit out of it.

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    Last edited by nsubulysses; 05-10-2018 at 05:45 PM.

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    Supporting Member Jazz P Bass's Avatar
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    The failed tubes turning 'white' is not indicative of overvoltage or a screen failure.

    'White' means loss of vacuum, plain & simple.

    You may very well have a mechanical/ shock issue.

    30 hours a week play time, yes.
    But is the amplifier being transported?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    But is the amplifier being transported?
    As in loaded onto a rolling cart while still piping hot and wheeled out into the cold night air through the back door of a bar, grating along over bumpy, busted up asphalt past sleeping bums and broken Thunderbird bottles then heaved into the back of a van sitting on the metal floor for the drive home on surface roads.?. I did that all the time when I was gigging. I never cracked a tube but I did get circuit faults on the boards of my Marshalls three times.

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    biasing a high gain type amp 70% might be a problem? Why everyone does this is beyond me.

    many a burn , melted 6550 in my marshall over the years.

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    Agree with dstrat, 70% at higher plate voltages is often a problem. Try lowering it until you have too much degradation of the tone. Bias as low (%) as you can get away with. The voltages will increase somewhat, but that is not as much a problem as higher dissipation.
    Now it seems your problem is a mechanical one (cracked glass/vacuum leak), but the excess heat may be a contributor.
    Nevetslab has posted here about needing to insulate the metal type 'top hat' retainers which were causing cracks in their SVT rental inventory. Could be a similar issue if you are using those type of retainers, like Pedro mentioned above.

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    The easiest way to lower the screen voltage is to add 5W zeners in series with each tube screen.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Chuck H View Post
    broken Thunderbird bottles
    Had to Google that.
    Wow!!!!

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    Thank you for the suggestions. Much appreciated.

    I had prepared a long reply to most comments and somehow the forum asked me to sign back in and only the quotes remained in my reply.... [breathe]. So I will try to just address the overall comments.

    Let me take a step back and give a better overall story of what's going on. I first started learning and buying parts from Metro amp back in 2004 to upgrade my then JMP mater volume. As time went I started replacing more and more, then started building whole amps. But this is where I have done the bulk of my learning. Perhaps some of the info from the old Metro forums wouldn't apply to high gain amp techniques. Such as biasing to 70%. And I admittedly say I have always pushed those boundaries until two days ago. Now I get the idea that it is not such a good idea. Especially with higher voltages such as what I am seeing here.

    Having been building amps on and off for 14 years, you do learn some things, but learning this way leaves HUGE holes in one's education. I am trying like hell to fill those gaps in but there is still some mystery to things that I am desperately trying to get a grip on (like impedance). The internet is so flooded with bad info (and good), it's tough to weed out. I have turned to books where possible... started with Merlin's books and next (?) who knows... I will cross that bridge when I get to it.

    This amp initially set out to be close to a SLO. Looking at every schematic I can get my hands on though, I took some ideas from the dual rectifier (LOVE the 680pF on the treble control for example). The idea was that I would build one out, tune it to my taste, and use that as a template for what in my mind is the ultimate amp. I have been working on this amp for the vast majority of the last year and this is the third time that the amp has been rebuilt, from the ground up. So it is not this particular iteration of the amp that is blowing all of the tubes. But this iteration was involved with the latest tube blunder. Having never ever blown a tube in 30 years until last year, I had to look at what is different in this amp from the previous amps I have had in the past.

    Of course there could be an error somewhere in my wiring, however I have been over it, literally, at least a dozen times, tracing through schematics and my own drawn up plans. Perhaps I could post the schem and someone could take a look to aviod any "user" errors.

    In addressing comments;
    I do not have any retainer clips or other mounting hardware for the tubes. Since I built this more or less as some kind of prototype I just left them out. And yes, I do gig the amp. Not a good idea to cart this thing around, but I had no other suitable (as in high gain) amp at the time for this particular brand of music. I have since finished another prototype of the amplifier while the circuit evolves... now I have two to cart around... I know it is a somewhat asinine thing to do. I am working toward correcting that, but for the moment it is what it is.

    I am interested in the whole choke/resistor thing. I thought I read, or saw, in one of Merlin's books where he had a dropping resistor placed before the choke. At least I think it was one of his books and I will have to reference that. BUT is that a possibility? To receive the best of both worlds? As far as I understand the choke is far better at smoothing than a resistor. So if I could drop the supply something like 40 volts (I know that's a lot), that would help tremendously, would it not? I can make up for it with lower resistors between filter stages.

    Until now, that my second amp is finished, I did NOT have a fuse in the DC side of the rectifiers in the latest version of the amp (I did in previous versions). I did this for two reasons, one was I began to model the filtering/power section from the Dual Rec where there are NO fuses after the main fuse... bad idea, I know. The other reason was that I planned to fuse up the ac secondary as per ValveWizard recommendations (I just hadn't gotten there until yesterday). My latest build I have secondary fusing on the AC side. Today I will add fuses on the DC side of the rectifiers AND a fuse on the heaters.

    Do I abuse the tubes in general? I suppose so. As I said I am pushing the boundary of 70% dissipation. Until a few days ago when I was coming to the conclusion that I should not do that, I was pushing my tubes to the 30 to 35 mA area. I suppose the 25 to 28 mA area would be better?

    I have three other amp builds - two marshall 50 watt style and one AB165'ish (or at least used to be) build. Beside that I have a 100 Marshall plexi re-issue and a JTM45 reissue. Oh yea, and a Fender Hot Rod Deluxe also. None of them (though I do not remember the plexi's readings) put out 515VDC on the plates of the tubes, or 510 on the screens. SO these are the reasons I arrived to the original post. In general I am worried about such high readings.

    Thank you, everyone, if I can learn one thing from posting then I am happy. I have definitly learned and drew better conclusions from your comments!!

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    Old Timer J M Fahey's Avatar
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    Blanket statements should have little disclaimer labels attached

    A choke is "better than a resistor" because for any given impedance it has less DC resistance than an equivalent resistor, so it has lower voltage drop.

    Since in this particular case we do need significant voltage drop, the opposite is truth .

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    I've also been using Shuguang EL34s in high voltage amps without any problems. My previous high-voltage tubes were late-90s Svetlana stock that's now too expensive to use. Shuguang tubes have thin envelopes compared with TADs, but in my post-mortem on both tubes I was unable to discern any difference in the spacers, electrode assembly, finish or construction. The TADs weigh a little more (I think about 6g heavier) due to the thicker envelope. TADS are less microphonic than factory-branded Shuguangs, possibly down to the glass.

    As a sidestep the JJ KT77 spec sheet rates it for high plate and screen voltage, though I have not personally used these.

    I agree with other observations that a white tube is a mechanical failure of the vacuum seal and is not usually symptomatic of high voltages. A tube that fails due to voltage issues will often short, arc internally or if you're lucky the screen will go open.

    For me, I use a MOSFET voltage reducer to tame high amp voltages. Just a few components that are hard-wired onto the MOSFET pins and bolted to the chassis. It's dead easy, super-reliable and easy to change the voltage if needed. I reduce the overall voltage rather than regulate the screens. See R.G.'s 'MOSFET follies' for details. Here's how it looks (not my picture). There's no free ride - the MOSFET still produces as much heat as a resistor or Zener. Just cheaper, easier and neat.

    Click image for larger version. 

Name:	b+ reducer.jpg 
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    Senior Member Pedro Vecino's Avatar
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    Thanks, Mick. Two questions: What is the reference of the mosfet? Does it need an insulating mica or can it be screwed directly onto the chassis?

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    Have a look at the bottom of the page.

    http://www.geofex.com/Article_Folder...osfetfolly.htm

    Although not sure have never tried one, but it should be isolated from the chassis and large enough heat sink to keep the smoke in.

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    Last edited by dstrat; 05-11-2018 at 11:03 AM.

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    Quote Originally Posted by J M Fahey View Post
    Blanket statements should have little disclaimer labels attached

    A choke is "better than a resistor" because for any given impedance it has less DC resistance than an equivalent resistor, so it has lower voltage drop.

    Since in this particular case we do need significant voltage drop, the opposite is truth .
    And right after rectifier output is the right place? or in exact replacement of the choke? (between B+1 and B+2) - Thanks

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    that would be the place.
    you asked "I suppose the 25 to 28 mA area would be better?"
    Yes it would , Tubes dont so much care what the voltage is , its the current thru the tube that makes it hot.
    with that kind of plate volts 60-65% bias would not be a bad idea.

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    If you're after a tight metal/high gain sound then I'd try out the mosfet regulated screen supply as was mentioned earlier. I'm going to try this in my next high power build. It's a pretty simple circuit that you can squeeze on a bit of tag board quite easily.
    Check this out: Amplified music : Guitar valve amp service

    regulated screens deets are towards the bottom of the page.

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    Although not sure have never tried one, but it should be isolated from the chassis and large enough heat sink to keep the smoke in.
    Unless the metal back of your MOSFET is other than the drain then you can bolt it to directly to the chassis which will be both your ground connection and heatsink.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Gtr0 View Post
    And right after rectifier output is the right place? or in exact replacement of the choke? (between B+1 and B+2) - Thanks
    In original choke place.
    I am dropping only screen voltage, which to boot is a way lighter and colder duty (just a few mA) than reducing the *full* power supply voltage, including power tubes, which means *a lot* more power dissipation.

    Just what was asked for in the original question; itīs assumed that plates by themselves are quite more robust and can take higher voltage without big trouble.

    Of course, if you want to reduce full supply voltage, be my guest

    EDIT:
    Tubes dont so much care what the voltage is , its the current thru the tube that makes it hot.
    Well, both do, to be more precise their product
    W=V*A

    EDIT:
    Unless the metal back of your MOSFET is other than the drain then you can bolt it to directly to the chassis which will be both your ground connection and heatsink.
    Please look at the picture shown above, that drain is hovering at around +400/500V above ground; I would very carefully insulate it from chassis/heatsink.

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    Last edited by J M Fahey; 05-11-2018 at 02:42 PM.
    Juan Manuel Fahey

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    Bent Member Chuck H's Avatar
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    A note about dropping the screens with a resistor instead of the choke. This will drop voltage on the PI and preamp tubes as well and THAT will likely change the tone of the amp more than the difference on the screens. The following HV rail resistor feeding the PI and preamp in the SLO schematic is a 10k/2W. You should be able to reduce this value to bring the downstream voltage up again. Just measure voltage on the downstream side of that 10k resistor before changing the choke to a resistor so that you have a target voltage.

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    Quote Originally Posted by Pedro Vecino View Post
    Thanks, Mick. Two questions: What is the reference of the mosfet? Does it need an insulating mica or can it be screwed directly onto the chassis?
    The last batch I had were STW9NK90Z - 8A/900v/TO247. I've stuck with that rating for quite a few years, though various different types, depending on what's cheapest or easily available. I've never had an amp back with a failed circuit and it's robust and reliable - fit and forget. I just use mica and heatsink grease. The rating is way over what's needed and generally a 75W device such as the IRF830 is fine (4.5A/500v). I like the larger packages because the greater surface area means the device runs much cooler on a steel chassis as a heatsink and there's no problem in finding space. The legs are also more robust and the spacing is ideal for mounting the other components.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Chuck H View Post
    A note about dropping the screens with a resistor instead of the choke. This will drop voltage on the PI and preamp tubes as well and THAT will likely change the tone of the amp more than the difference on the screens.
    Well. *that* is the point.
    If original preamp expected to be fed from around 400V and in this particular amp itīs fed from around 500V , in both cases from the screen supplying node, taming that extra high voltage to the expected one is the reasonable thing to do.
    The following HV rail resistor feeding the PI and preamp in the SLO schematic is a 10k/2W. You should be able to reduce this value to bring the downstream voltage up again.
    Why?
    You are now feeding that 10k resistor the proper voltage, why would you want to overvolt the preamp?
    Just measure voltage on the downstream side of that 10k resistor before changing the choke to a resistor so that you have a target voltage.
    I suggest you use the original SLO voltages as "targets".
    Why correct a wrong one (in this case a too high one) at the screens and then "uncorrect" it?

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    Please look at the picture shown above, that drain is hovering at around +400/500V above ground; I would very carefully insulate it from chassis/heatsink.
    It looks like it's not used as suggested in R.G.'s article (to CT or to bridge negative) but in series with the B+ supply.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    The last batch I had were STW9NK90Z - 8A/900v/TO247. I've stuck with that rating for quite a few years, though various different types, depending on what's cheapest or easily available. I've never had an amp back with a failed circuit and it's robust and reliable - fit and forget. I just use mica and heatsink grease. The rating is way over what's needed and generally a 75W device such as the IRF830 is fine (4.5A/500v). I like the larger packages because the greater surface area means the device runs much cooler on a steel chassis as a heatsink and there's no problem in finding space. The legs are also more robust and the spacing is ideal for mounting the other components.
    It may be fine, but in general the datasheet rated dissipation is the most misleading rating of them all.
    None of your fault of course, but "creative" Datasheet writing.

    In the Real World no semiconductor will ever be able to dissipate that much, and will sweat a lot to even approach *half* that, if at all.

    Practical example, and straight from IRF830 datasheet:
    Maximum Power Dissipation TC = 25 °C PD 74 W
    Looks good, huh? 74W!!!!!

    read again: to be able to dissipate that, the *case* , not the heatsink, needs to be held at steady 25C .... good luck with that.

    In the beginning I corrected my mental image to transistor, this one or any other one (as in *all* others), being rigidly mounted to, say, a 1 cubic meter block of Aluminum held at steady 25C , only guarantee for it to *stay* there at least for some time.

    Hardly a realistic heatsink but whatīs implied by the rating.

    Only later I noticed an even bigger flaw in that rating: holding that "infinite heatsink" (their words, not mine) at steady 25C is not enough, since we are ignoring the thermal resistance between case and heatsink .... conveniently ignored in most modern datasheets (it was present in old ones, such as old 2N3055 datasheets, those which came, ugh!! , *printed*!!!! and using, forgive my French: ink and paper !!!
    Ok Mom, Iīll wash my mouth with Laundry soap now
    Maybe that spoiled me because to this day I use that value to design.

    I am talking Rcs here:


    Searching a lot (since itīs not even mentioned any more) I found "typical" minimum values of 0.3 to 0.5 C/W , no mica, greased case to sink junction.

    Assuming 0.5C/W as a safe value (since datasheet shows none), 75W through that means a 35C thermal drop.

    Which also means that if case is to be held at 25C, and itīs mounted to a "best heatsink in the World" 2.5 Ton aluminum cube, said cube still needs to be held 35C below that, or minus 10C ... 10C below ice temperature.

    Donīt know you but I find that dissipation rating "slightly" unrealistic

    Ok, ok, what is realistic then?

    Just a practical example, using same IRF820.

    Suppose itīs in an SS environment (nearby HOT tubes will definitely worsen the situation) , inside a cabinet, inside temperature may reach 40C , maximum design junction temperature is 120C (considering absolute maximum is 150C and we want a little safety margin) and we use a reasonable 2C/W heatsink such as this one:

    then final junction to air thermal resistance is: 1.69 + 0.5 + 2 = 4.19 C/W
    (the 1.69 Rjc value is "shown but hidden" in the datasheet, as its inverse: "linear derating" parameter )

    So maximum *actual* dissipation is (120-40)/4.19=about 20W .... a far cry from big bold lettering "74 W"

    And that in a very conservative design: mild ambient temperature, no mica, large heat sink, etc.

    For poor MosFets inside a HOT Tube amp chassis, bolted to not-too-thick steel sheet and of course using mica or Silpad, using a larger TO247 one such as suggested by Mick Bailey is the way to go.

    And in any case I would search (by Finger-O-Meter at least) for the relatively coolest spot on that chassis to mount that MosFet.

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    Juan Manuel Fahey

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    Agreed, the power rating is a paper maximum. When you think though we're generally dropping say 30v to 40v at maybe 500mA peak, then 20W is about what the dissipation will be at full volume when the tubes are drawing maximum current. That's not factoring RMS. A TO220 gets hot in this application and needs careful consideration about mounting. It does actually work OK in practice, given that most amps aren't run to maximum output. It's not a device selection I personally like to use because of the issues discussed and when I've experimented with TO220 packages with an amp into a dummy load they get way hotter than a TO247 when pushed to maximum output. That's how I settled on that particular package even though people often use TO220s. I just don't like the hotspot they create.

    In reply to Gregg, The circuit works in the B+ line or CT as per R.G.s original article. I use either method depending on the PS configuration and convenience of access.

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    Better Tone thru Mathematics bob p's Avatar
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    I might be in the minority on this, but I don't think that clamping the screen sounds good, especially if you aren't also clamping B+. IMO you need to regulate both of them or neither of them to get the best sonic results. Clamping one while letting the other sag just doesn't work well.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Had to Google that.
    Some of the cultural references may not be recognizable outside of an urban slum in the USA, which is the marketing niche where Tbird became famous. It was a cheap screw-cap fortified wine. And when I say it was cheap, I mean really cheap. Like 60-cents for a bottle. It had a high alcohol content (like a Port) and a cheap price, which made it popular with skid row bums.

    I grew up in an urban area where Tbird was popular. When I was riding my bike as a kid I had to be careful about taking shortcuts through the alley as it almost always meant a flat tire, caused by the shattered remains of a Tbird bottle. Tbird was one of the famous "bum wines", which is probably why Chuck mentioned it in his post. It was so famous in the 'hood that people even wrote songs about it:



    Tbird came around when the wine purity laws in the USA were relaxed in the 50s to allow "flavored wines" and alcohol-fortified products to be sold as "wine." That allowed the cheap vineyards to dump their low grade products by blending them, and adding flavorings and distilled liquor to bring the alcohol content up to about 20%, like a Port. The combination of low price and high alcohol content made Tbird popular in urban areas. The bums liked it because it was a cheap drunk, and the poor people liked it because it was a cheap mixer. I remember going to the grocery store as a kid and seeing Tbird on the shelf with packages of lemon Kool-Aid next to it. And there was the catchy jingle on the radio:

    What's the Word? Thunderbird!
    How’s it sold? Good and cold.
    What’s the jive? Bird’s alive.
    What’s the price? Thirty twice.

    Mixing tbird with lemon Kool-Aid was pretty common in the neighborhood. It was always regarded as a cheap mixer. I have to confess, as a kid we used to sneak Tbird and Kool-Aid at a friend's house. We did the same thing with Mogen David and 7-up at another friend's house. Even if you cut them in half, you still had 10% alcohol content, and one glass was enough.

    Tbird was always regarded as a cheap wine, but in the 70s they tried to make the brand more upscale with the grapefruit juice "Shake 'em Up!" campaign in the commercial that Juan linked. I confess ... I've also had Tbird and grapefruit juice "shakers", but that was something like 50 years ago when I lived in the 'hood ...

    Gallo was one of the most successful companies at selling cheap wine. Some of their most successful brands from the 50s-70s were Thunderbird, Ripple and Boone's Farm. They all had a screw cap. When the market got tired of these products, the same low grade wines ended up being marketed in a more upscale fashion as carbonated "wine coolers" in the 80s. Same juice, different blend.

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    Last edited by bob p; 05-12-2018 at 03:13 AM.
    "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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    Bent Member Chuck H's Avatar
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    Juan, If he likes the way the amp sounds now, but just wants it to stop eating power tubes then maintaining the existing preamp voltages might suit the end result of the effort. Submitted for consideration only.

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    Pardon my ignorance on the topic... 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?

    How is it then does an amp like the silver jubilee cut out the screen resistor connection.. effective turning the tube into?? A triode?

    Also how are amp manufacturers getting away with such his voltages on the screens as well? Obviously the SLO and similar amp the Dual Rectifier??

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