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Thread: Dealing with High B+

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    Dealing with High B+

    I'd like to get opinions on how to deal with high B+

    The amp is a JTM50 with a plate voltage of 525vdc (loaded). It freaks me out a little that the first two filter nodes (F&T 50/50 500v) are operating over max voltage. The caps are on the cold side of the standby. I could have sworn that the last time I read the plate voltage on this amp (ten years ago?) it was 501vdc.

    Should I:

    Zener?
    Mount another cap can (totem pole) inside the chassis (I'm not punching another hole)?
    Dropping resistor?
    Other tricks?
    Wear safety gogles, say a prayer, and let it rip?


    I'd appreciate your feedback.

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    Better Tone thru Mathematics bob p's Avatar
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    Voltage doesn't climb for no good reason. I have a vintage Super Twin that's supposed to be at 500 but it's at 530 now for the same reason -- climbing wall supply voltages.

    I wouldn't let the caps continue to operate beyond their rated voltage. That's just begging for a bad outcome to occur. So I'd do something about it.

    Other options that you haven't listed would include:

    - external bucking transformer
    - voltage divider on the B+

    Personally, I'd try to avoid raising the impedance of the supply rail, so I'd bring the B+ down a bit with a bucking transformer and use a totem pole.

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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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    Old Timer Gtr_tech's Avatar
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    CE offers 525v "can" caps. They are 4 section so you'd end up paralleling them to get 60/40u sections. If it were me I'd use series caps with balancing resistors like 2 100u at 350v with 220k parallel resistors a'la what Fender did. That way you have a 700v range so powering up with no output tubes shouldn't exceed the voltage rating.

    I'm one of the few who actually prefers a high plate voltage. It makes the amp punchier with a faster transient response and lessens mush out on low notes.

    A customer of mine brought in an old plexi for rehab some time ago. IIRC it had a plate supply of 560v. This thing had so much balls it would just about knock the wind out of you. Scary loud too....

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    Senior Member booj's Avatar
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    You could put a resistor between the power supply diode bridge and the B+ supply. Sort of get an unintended rectifier tube emulator. 100 ohms would probably be more than you'd need.

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    Suggest that you take comparitive readings of line, heater and B+.
    An issue with very high VB+ like this is that EL34 may be pushed over their limiting value of plate dissipation.
    Pete

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    Senior Member kleuck's Avatar
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    Use a 25 volts Zener (5 watts or more)

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    'Use a 25 volts Zener (5 watts or more)'

    My feeling is that a 5 watt rating may not be sufficient; at 400V 3k5 load, this http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf suggests 267mA plate+screen current at max clean, so guess that may increase to 377 when overdriven; then it's being run at a higher voltage, so current draw likely to be even higher still, maybe close to the 500mA fuse rating.
    So a 15 or 20 watt rating may be beneficial for good service life.
    Pete

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    Old Timer defaced's Avatar
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    Last item on this page, B+ reducer. MOSFET amplified zener.
    http://www.geofex.com/Article_Folder...osfetfolly.htm

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    -Mike

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    Noodle of Reality Steve Conner's Avatar
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    I'd vote for the "bucking transformer" aka RG's Vintage Voltage adapter. Your line voltage may well have gone up that much in 10 years. Vintage Voltage Adapter

    Or if you like the sound of the increased plate voltage, do as suggested earlier, change the filter capacitors to a series arrangement of 350V caps.

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    Thanks everyone for the great input. I've not read a whole lot about bucking transformers on this forum, so that's a nice bit of knowledge to add to the toolbox. Something about it makes me contemplate getting old -- like high blood pressure meds, hip replacements, pacemakers, a foley bag. "The poor old girl needs a little help gettin up the stairs now." But I digress. Do people mount these inside a cabinet, or do they stand alone between amp and outlet (or wheel it around like an oxygen tank or IV)? A device like this seems most useful for vintage amps or ones you wouldn't want to mod. It's a bit more pricey, but replacing the PT on a homie brew makes more sense to me.

    I've always enjoyed the headroom and the whoop-ass that this amp delivers. I'm leaning toward beefing up the filter cap voltage capacity, but I'm wondering what the most non-invasive and economical way to do it is. The amp has three 50/50 500v cap cans (shared ground) and I have at least one extra lying around. Hmmm....

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by chinrest View Post
    Do people mount these inside a cabinet, or do they stand alone between amp and outlet ...?
    Yes and Yes. I have a bucking transformer that I've built into a junction box that can be used with many amps, and I've built them into an amp cabinet as well. I think it's handy to have one built into it's own box that can be used on a benchtop or in a home environment with many amps. but when it comes to dragging gear out to a gig, I think it's easier to build it into the amp head or combo cabinet, so that the amp can just be plugged into any available outlet and it will take care of the voltage conversion on it's own without having to drag around an extra box that might get forgotten. If you size the transformer for the current that's needed by the amp and nothing more, then they're small, they don't take up much room, and they'll fit into the amp.

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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 pdf64 View Post
    Suggest that you take comparitive readings of line, heater and B+.
    Quote Originally Posted by bob p View Post
    Voltage doesn't climb for no good reason...
    Personally, I'd try to avoid raising the impedance of the supply rail, so I'd bring the B+ down a bit with a bucking transformer and use a totem pole.

    It's starting to sink in... A combination of bucking and totem poles. Yeehaw! Thanks guys.

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    Senior Member kleuck's Avatar
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    Quote Originally Posted by pdf64 View Post
    'Use a 25 volts Zener (5 watts or more)' My feeling is that a 5 watt rating may not be sufficient; at 400V 3k5 load, this http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf suggests 267mA plate+screen current at max clean, so guess that may increase to 377 when overdriven; then it's being run at a higher voltage, so current draw likely to be even higher still, maybe close to the 500mA fuse rating. So a 15 or 20 watt rating may be beneficial for good service life. Pete
    Yep, don't know what i calculated, they can be used in serie too, say 4*6,2 volts/5Watts.

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    Quote Originally Posted by bob p View Post
    Yes and Yes. I have a bucking transformer that I've built into a junction box that can be used with many amps, and I've built them into an amp cabinet as well. I think it's handy to have one built into it's own box that can be used on a benchtop or in a home environment with many amps. but when it comes to dragging gear out to a gig, I think it's easier to build it into the amp head or combo cabinet, so that the amp can just be plugged into any available outlet and it will take care of the voltage conversion on it's own without having to drag around an extra box that might get forgotten. If you size the transformer for the current that's needed by the amp and nothing more, then they're small, they don't take up much room, and they'll fit into the amp.
    +1.

    Here's a build-in I made in the vein of the "Vintage Voltage Adapter" Note the sizes of the ACin and AC out sockets. That's a 4x4x3 Bud box as I remember.


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    Don't forget the joker g1's Avatar
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    Looks good R.G.! Any plans for a production stand alone unit? I think it would be a hit, especially with antique radio guys.
    High B+ usually gets immediate attention due to the immediate damage it can do, but overvoltage in general seems to get overlooked.
    Heater voltages up around 6.8V or more do increase noise and decrease lifespan of the tubes. And more volts in to the PT means more watts dissipated in the PT so it runs hotter. For the real vintage unobtainium PT's, this can be critical.
    And not just for the vintage stuff, I've seen modern amps that are running class A way too hot, with too high heater voltages and filter caps right at their voltage limit. Put them on the variac and dial up 115V, everything goes back to normal. The PT's in these cases are usually offshore and labelled 115V. Maybe they are told to build to that spec., I don't know, but they seem to believe we still run on 115V. (I've mentioned this before and I think someone on the forum twisted it around to "they're sending us amps with 100V transformers for Japan!!" )
    Anyway, I try to always check heater voltage as a way of "taking the amps temperature", and I think the bucking transformer is usually a better solution than dropping the B+, except when the B+ is the only thing high.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by g-one View Post
    Heater voltages up around 6.8V or more do increase noise and decrease lifespan of the tubes.
    Good point and all the more reason to choose the bucking xfmr. As others have mentioned, looks like your power company may have done you the "favor" of boosting your AC from say 117 to 123 VAC somewhere along the line, not unusual. Did you move to a new location since you last measured B+? AC power varies practically from one block to another. And of course sags somethin' awful in midsummer when everyone's got the air conditioner going full blast plus hot wires are less conductive. Something to look forward to. Just like every year. Keep cool!

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    Something I've read, but never done, is putting zeners in the primary to drop a minimal voltage. See;
    Antique Radio Forums ? View topic - Low cost alternative to a bucking transformer

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    Something I've read, but never done, is putting zeners in the primary to drop a minimal voltage.
    Yikes! Incoming power waveform is already hacked-up enough in many locations without sticking zeners in the AC line. I'd opt for "inrush limiter" NTC resistors instead. Many commercially made amps already have one (Fender, Peavey, old tube McIntosh) and I've seen a recent Peavey model that strings about 8 of 'em in series for a deliberate line voltage knockdown effect. I guess this last is to simulate the line voltage that was available from the TVA in the "good old days."

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    Quote Originally Posted by Leo_Gnardo View Post
    Did you move to a new location since you last measured B+?
    I've moved several times, and picked up a few volts along the way. I'm reading 121vac out of the wall now.

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    Quote Originally Posted by pdf64 View Post
    Suggest that you take comparitive readings of line, heater and B+.
    An issue with very high VB+ like this is that EL34 may be pushed over their limiting value of plate dissipation.
    Pete
    Stand back, I'm holding a calculator. I've taken some comparative readings and calculated my desired bucking. My current transformer is a 115v primary, 375-0-375 secondary.

    Current readings
    121vac primary
    416-0-416 secondary
    381vdc rectified (SS)
    533vdc B+ loaded
    Heater 7.0v

    -10v bucking transformer
    111vac primary
    381-0-381 secondary
    349vdc rectified
    489vdc B+ loaded
    Heater 6.4v


    It seems to work on paper, right? It also seems that this tranny's 115v primary spec might be a little wonky. If I bucked 6v ( to bring it down to the 115v primary), it looks like I'd get 391-0-391 secondaries, not 375. Hmmm.

    This is a great thread. Well done everyone.

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    Don't forget the joker g1's Avatar
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    The HV spec. (375V) will be at a specified amount of current for that winding, which may not be what the amp uses. Bias will affect it.
    What are those readings you have called vdc rectified? A vdc reading after the diodes is B+ isn't it?

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    Quote Originally Posted by g-one View Post
    The HV spec. (375V) will be at a specified amount of current for that winding, which may not be what the amp uses. Bias will affect it.
    What are those readings you have called vdc rectified? A vdc reading after the diodes is B+ isn't it?
    That makes sense. This tranny is beefy (200ma?). You are correct about the "rectified" voltage reading--in standby, directly after the diodes.

    I whipped up a ghetto bucking transformer rig, with a handy 12v transformer, fuse, strain relief, outlet, junction box. It certainly does the trick. I'm down to 470vdc B+ and the heaters are a little soft at 6.2vac. I may build a nice one, with a lighted switch and 6v switch in case I ever want to run it closer to meltdown. I figure a 6v buck will give me right around 500vdc on the plates. Those F&T cap cans will have to sweat a little.

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    Old Timer J M Fahey's Avatar
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    More than the HV voltage, I'm worried about the filament one and find it a better indicator.
    7 VAC? .... OUCH!!!
    6.4V ? better.
    Bucking is the way to go.
    Get ready for the 127V you'll have in a couple years and plan accordingly.

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    Juan Manuel Fahey

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by J M Fahey View Post
    More than the HV voltage, I'm worried about the filament one and find it a better indicator.
    7 VAC? .... OUCH!!!
    6.4V ? better.
    Bucking is the way to go.
    Get ready for the 127V you'll have in a couple years and plan accordingly.
    ghetto bucking transformer
    HA! Love it!

    100% in agreement with J M Fahey.

    Cue Elvis : "In the Ghetto"

    Juan, do you think those ever-helpful power companies will dial up our power to 127V? Hm, the more voltage they send us, the more power they charge us for, so...

    FWIW my college apartment had 128V in 1972. We complained that the light bulbs were going bad, fast. Cant' remember they ever fixed it. Also had mushrooms growing out of the carpet. Brand new building. Circus of mismanagement.

    "Old" Italian: geto = furnace. IOW the part of town where scrap collectors melted down scrap metal for re-use in the Middle Ages. Spelling changed as time went on.

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    Quote Originally Posted by Leo_Gnardo View Post
    Yikes! Incoming power waveform is already hacked-up enough in many locations without sticking zeners in the AC line. I'd opt for "inrush limiter" NTC resistors instead. Many commercially made amps already have one (Fender, Peavey, old tube McIntosh) and I've seen a recent Peavey model that strings about 8 of 'em in series for a deliberate line voltage knockdown effect. I guess this last is to simulate the line voltage that was available from the TVA in the "good old days."
    Pardon my ignorance.....
    Are you saying that this Zener Method will change, clip, distort the Sine Wave as it comes from the wall source.?
    That is to say...will this add "noise" to the wall source.?
    It seemed like such an easy solution.
    Thank You

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    Noodle of Reality Steve Conner's Avatar
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    If you want to knock 10V off with zeners, it'll take a 10 volt bite out of the zero crossing. I imagine the effect would be a buzz similar to interference from a lamp dimmer.

    The zeners will have to be bigger than you think to withstand the inrush current when the power switch is flipped. (FWIW: I believe the series string of NTCs inside new tube amps is for limiting inrush and giving the heaters a soft start, not knocking down the wall voltage.)

    So by the time you factor in the cost of a box and some plugs and sockets, it probably ends up costing the same as the transformer while running hotter, being less robust and possibly making your amp buzz. Go ahead and try it if you want, but I'd use the bucking transformer. A toroidal transformer might be worth considering as they are smaller and lighter than the EI type for a given rating.

    RG's site shows a "super Zener" made from a small Zener and a power MOSFET. This would probably be cheaper than an array of real Zener diodes of the same rating. It can be used on AC by connecting it to the + and - terminals of a bridge rectifier, and connecting the AC terminals of the rectifier in series with the line.

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    Normally, the secondary HT will only be conducting at the peaks of the mains voltage, so that winding isn't an issue per se.

    If the heaters are AC then there could be a slight amount of higher frequency noise that could couple through, but probably not an issue.

    There would also be some noise from residual magnetising current being started and stopped. I guess shunt capacitors on the secondaries would knock some of the edge off any very high frequency transients.

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    Quote Originally Posted by g-one View Post
    Looks good R.G.! Any plans for a production stand alone unit? I think it would be a hit, especially with antique radio guys.
    "Lil' Buckaroo"

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    Old Timer J M Fahey's Avatar
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    Juan, do you think those ever-helpful power companies will dial up our power to 127V?
    I don't "think", I'm *seeing* it happen.
    Problem is, power consumption keeps going up steadily , I laugh (sort of ) at the Government "worrying about a Green Planet" and forcing nasty CFLs on us, when filament bulbs are only a small fraction of household power consumption, and so on.
    Copper is becoming more expensive too, I guess soon we'll start seeing homes wired with aluminum , etc.
    So going up in voltage is the sensible solution.
    Problem is, instead of taking the bold step of going straight from 110V to 220V , they are creeping up bit by bit.
    Just look at *old* Gibson and Fender schematics: those from the 40's showed straight "110V" , the original Edison DC standard .
    He meant 100V actually , lamps *were* 100V , but the generators made 110V at the power plant so inefficient DC transmission guaranteed 100V for the furthest consumers, not many blocks away.
    The race started when AC power meant the 110V could actually reach everybody.
    I remember "115V" blackface Fender schematics, then "117V" (the UL 135W era), now 120V is being specified.

    I sell amps in Brazil, and they have a maddening dual voltage standard, depending on "which World Power" was running their economy.
    1/2 the Country runs on "110V", 1/2 in 220V, sometimes in the same city
    And their 110V now is becoming 127V everywhere.
    Old Giannini amps (Fender clones made in the 60's and 70's) die like flies .
    *All* modern amps have a front panel 110/220V switch.
    I had to add the same to mine, to be sold there.
    I already mentioned here that every year I go to their Expomúsica, the Brazilian NAMM, and last time a friend of mine was invited to play.
    He carried his trusty VOX VT100, plugged it into the wall .... and burnt it.
    *All* of São Paulo is 110V, but the "Expo Center", smack in the middle of the City .... is wired 220V.
    Worst thing is that they *should* have a BIG, red letters on a white background poster stating "YOU ARE ENTERING A 220V WIRED BUILDING"
    People told me he was not the only one, that droves of Musicians had the exact same problem.
    When he complained, they showed him a tiny homemade paper label glued to the wall socket, stating "220V"
    Oh well, I had to go buy a soldering iron and a cheap multimeter and repair his amp ... which uses a hard to find Sanyo or Sanken power module.
    Oh well.

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    Juan Manuel Fahey

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Copper is becoming more expensive too, I guess soon we'll start seeing homes wired with aluminum , etc.
    Tried that in the USA starting @ mid 60's. Wasn't working out so well (house fires) so the "fix" was splicing copper ends onto the aluminum. That didn't work either so by 70-71 it was back to copper 100%. My father was a house builder & pointed this out to me, just in case "in the future" aluminum was back in the picture, insist on copper for your house wiring.

    Thanks for all the good answers!

    If I'm not mistaken there's parts of Brazil on 50 as well as 60 Hz power. When I first did any work there 1982 I was warned some zones even had 25 Hz. Japan also has 50 and 60 Hz zones.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by trem View Post
    Are you saying that this Zener Method will change, clip, distort the Sine Wave as it comes from the wall source.? That is to say...will this add "noise" to the wall source.? It seemed like such an easy solution. Thank You
    Yes, as Steve Connor and others have noted the zener will take a nice square "bite" out of the AC waveform, and that will likely show up in the audio, and maybe send noise into other things on the AC line as well.

    Distorted AC power has been an issue with a couple items I've been asked to look at recently. A gentleman of "retirement age" who has taken up electronics as a hobby brought me 3 amps he modified - a Dynaco Stereo 70 and two Dynaco MkIII - he installed new drive boards but noticed the power transformers making a "grinding" noise when he fired up the amps to test. He has a 'scope so I asked him to put the probe on the AC line. He reported that it was anything but a sine wave. Very distorted. So it wasn't the mods. His power is dirty as could be, and bad enough to make the windings in his power transformers rattle audibly. So he will have to get a serious filter or even an AC power "regenerator."

    The NTC "inrush limiter" is intended (on amps that have one) to soften the startup voltage & help preserve filament life. After warmup the NTC reduces the line voltage that the amp sees by about 1 volt. That Peavey amp I mentioned (sorry can't remember the model) that had 8 or so of these definitely was trying to make a major reduction in line voltage. The NTC's were all on a separate mini PC board @ the size of a playing card. And got so hot the solder was melting and the NTC's were falling off the board. What a farce. You're better off with a bucking transformer or autoformer.

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    Last edited by Leo_Gnardo; 04-22-2013 at 04:58 PM.

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    Quote Originally Posted by Leo_Gnardo View Post
    You're better off with a bucking transformer or autoformer.
    There shouldn't really be any need for an isolating transformer here should there?
    Autotransformers are significantly smaller (like half the size) than an isolating transformer for the same VA rating.

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    Quote Originally Posted by Ted View Post
    There shouldn't really be any need for an isolating transformer here should there?
    Autotransformers are significantly smaller (like half the size) than an isolating transformer for the same VA rating.
    Correct, and the way chinrest & others have created a bucking unit essentially are autoformers also. In jolly old London you should be able to find a selection of 240/120V autoformers at several locations up on Edgeware Road a couple blocks north of the Marylebone station. In the good old days there were a couple of excellent surplus shops there but I think they're long gone by now. The modern electronics shops there will have them. Probably can be found in other parts of town too but Edgeware had a concentration of electronics shops last I looked. And treat yourself to an excellent fish & chips at the Sea Shell takeaway just 2 blocks or so north of the station and a block east of Edgeware. (Sitting down there will cost a fortune but the takeaway is much more affordable and has the best finch & chimps I ever found in London.)

    If you can find an autoformer with multiple taps you can fine-tune the voltages inside your amp for near-perfect operation.

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    Introducing the new
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    Why not just put a small, thick O-ring under each screw and suspend the sockets like a Reverb tank? You could even use just a small rectangle of metal stock on each side with 2 holes drilled in them and use the original mounting holes. It would look clean if mounted from the bottom of the chassis. Sounds like a lot less work if you have a drill press and a hacksaw.

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