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    Power Transformer Tempeture Question

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    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Power Transformer Tempeture Question

    Post by tkemmett on Fri Aug 01, 2014 1:15 am

    Howdy All,
    The power transformer on my VTA 120 seems a bit hotter to the touch than in the past. It very well could be that it always ran this way and I just didn't notice. The output transformers run comfortably warm to the touch in comparison. Any thoughts or feedback would be most welcome.

    Gregg R.

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    Re: Power Transformer Tempeture Question

    Post by Gregg R. on Fri Aug 01, 2014 3:42 am

    Bob's post/photo of July 02, 2014 shows temperatures on his ST-120. You might want to use an inexpensive laser thermometer to compare your temps to Bob's.

    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Re: Power Transformer Tempeture Question

    Post by tkemmett on Fri Aug 01, 2014 6:14 am

    Thanks for the link...I have a temp meter/probe that did the job. My measurements were in the same range as indicated in Bob's posting. It is nice to know the amp isn't self roasting.
    TK

    Bob Latino
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    Re: Power Transformer Tempeture Question

    Post by Bob Latino on Fri Aug 01, 2014 8:20 am

    Hi TK,

    Almost every summer (June through August in the Northern hemisphere) people will notice more the heat given off from their tube amps. It's warmer in your house and any heat source will just add to the heat. I don't know about others on the forum but my wife and I rarely run the oven in our stove during the summer time. We tend to cook outside on a gas grille so that we won't add any extra heat to the house. If it is already warm in the house then it makes it harder for the tube amp to give off the heat and the amp WILL run somewhat warmer in the summer time. Below is the test from the other post I did with the infrared thermometer. These are all Fahrenheit temperatures ..

    Bob



    wildiowa

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    Join date : 2012-03-19

    Re: Power Transformer Tempeture Question

    Post by wildiowa on Fri Aug 01, 2014 9:54 am

    Regarding Bob's readings: I am curious as to why the back power tubes run so much hotter than the front on each channel...they both do the same amount of work, looks like plus about 100 degrees F on both sides...perhaps because the back tube does not have the ventilation and air flow that the front tube has, proximity to the trannys, etc. Any implications as to tube performance, life etc.?

    Bob Latino
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    Re: Power Transformer Tempeture Question

    Post by Bob Latino on Fri Aug 01, 2014 10:29 am

    wildiowa wrote:Regarding Bob's readings: I am curious as to why the back power tubes run so much hotter than the front on each channel...they both do the same amount of work, looks like plus about 100 degrees F on both sides...perhaps because the back tube does not have the ventilation and air flow that the front tube has, proximity to the trannys, etc. Any implications as to tube performance, life etc.?

    Yes - The rear output tubes are between the warm output transformers and the front tube. The front tubes have the front side available to room temperature air and run 100 degrees or so cooler. Whether this will result in shorter tube life for the rear tubes is hard to say ?

    Bob

    Jim McShane

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    Re: Power Transformer Tempeture Question

    Post by Jim McShane on Fri Aug 01, 2014 12:15 pm

    Bob,

    That is a GREAT bit of work, that photo is a wealth of information.

    Just a couple things to add...

    a. Even a cheapie power trafo is good for around 200 F, so the 120-125 F temps of Bob's QUALITY trafo is well within its capability. FYI, for most people around 125 -130 F is the point where they can touch something without being burned, but they can't stay in contact with it. That's about how the power trafo should feel. Remember the outside surface is where the heat from the core is dissipated to ambient.

    b. If you want swap the front and back tubes in each channel when you set/check the KT-120 bias (and clean the socket while you're in there). That will help to equalize the wear on them.

    Thanks again for the terrific work Bob!!

    deepee99

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    Re: Power Transformer Tempeture Question

    Post by deepee99 on Fri Aug 01, 2014 2:27 pm

    Bob Latino wrote:
    wildiowa wrote:Regarding Bob's readings: I am curious as to why the back power tubes run so much hotter than the front on each channel...they both do the same amount of work, looks like plus about 100 degrees F on both sides...perhaps because the back tube does not have the ventilation and air flow that the front tube has, proximity to the trannys, etc. Any implications as to tube performance, life etc.?

    Yes - The rear output tubes are between the warm output transformers and the front tube. The front tubes have the front side available to room temperature air and run 100 degrees or so cooler. Whether this will result in shorter tube life for the rear tubes is hard to say ?

    Bob

    Bob, there is (theoretically) a vacuum inside the output tubes so (theoretically) there can be no heat xfer possible, even radiant heat, even if the glass gets hot, can there?
    I fired up the M-125s last night in 90+ degree (F) to entertain some visiting royalty, the radiant heat from the xformers and the tubes required a cooling fan just to keep the room cool, but I don't think the tubes noticed. You can't pass heat through a vacuum, can you?
    Am I missing something here?

    deepee99

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    Re: Power Transformer Tempeture Question

    Post by deepee99 on Fri Aug 01, 2014 2:34 pm

    I mean, the sun can pass heat through a vacuum, but it has to hit the atmosphere to warm things up, right?

    Bob Latino
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    Re: Power Transformer Tempeture Question

    Post by Bob Latino on Fri Aug 01, 2014 3:20 pm

    David,

    Physics lesson from the retired high school Physics instructor. Heat can be transferred by conduction (two things touch), convection (heat transfer by a moving fluid like water, air etc.) or radiation (heat transfer by radiant energy given off by an object). Yes - the last one, radiation, does not create any heat until the waves actually strike an object and the last one does not require any molecules for the energy to be transferred. Yes - The sun radiating electromagnetic waves through space is a good example and you are correct in that these electromagnetic waves have to hit something to actually create heat.

    Bob

    sKiZo

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    Re: Power Transformer Tempeture Question

    Post by sKiZo on Fri Aug 01, 2014 5:51 pm

    Lots of little things you can do to keep the temp down.

    I don't use AC here, so heat was a concern. Good news is, my three step plan seems to be working quite well.

    - Mounted the amp up high - five feet off the floor - and it can see the sky, as they say.
    - Did the custom case that spreads the tubes out more and amplifies the chimney effect.
    - Amp is high enough that it gets good air circulation from a ceiling fan as well.

    The ST120 WILL pump out a lot of btu's but it (like many things) is all over my head ...

    scottlowe

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    Re: Power Transformer Tempeture Question

    Post by scottlowe on Fri Aug 01, 2014 6:06 pm

    I have always run two small[5"] fans on my st-70's and I do the same with the ST-120.Is that a good idea?I think it is a good idea.I am building a "box" for my 120 to sit in and it has three outlets of filtered air. 1 large on the transformer, and two small on the output tubes.I can always touch my tubes and transformers.I have fans in the new box design mounted in the room behind the equipt. so no fan noise

    sKiZo

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    Re: Power Transformer Tempeture Question

    Post by sKiZo on Fri Aug 01, 2014 9:29 pm

    Nothing wrong with fans if they help, other than the additional noise. Better or bigger the fans, the less noise. I prefer passive cooling if I can get away with it ... that's one of the primary reasons I went with the custom case ...

    (What was that?? Yet another excuse to show off?? Twist my arm!!! )  lol! 



    Anyway ... you can see what I mean when talking "chimney effect". The vent holes around the tubes suck air up from beneath the chassis plate when they heat ... the bottom is also well vented on tall feet to allow better flow. I also left a few extra mm's of gap between the driver tubes and chassis for the same reason. I expect the extra space behind the tubes also help to keep the temp down on the iron, but I don't have a thermometer to prove it. I do know I can stick a finger on the transformers after a hard session and it's not uncomfortable to the touch.

    If you're not up to replacing the chassis, next best bet is just to leave plenty of open air space around the equipment.

    corndog71

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    Re: Power Transformer Tempeture Question

    Post by corndog71 on Fri Aug 01, 2014 11:34 pm



    I'm with sKiZo when it comes to making your own chassis.  I used a 17"x17" aluminum Hammond box to spread everything out which helps keep everything cool.  I even spaced my output tubes 4" apart center to center as recommended by the old tube manufacturers.

    Did you know the output trannys don't actually produce heat?  After having my amp on all day I can put my hand on one and it 's no warmer than if the amp was off.  Even my chassis doesn't get warm except right next to the PT and the tubes.  Everything on the original chassis is so close together that they pick up the heat of the PT and tubes.

    deepee99

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    Re: Power Transformer Tempeture Question

    Post by deepee99 on Sat Aug 02, 2014 1:24 am

    Bob Latino wrote:David,

    Physics lesson from the retired high school Physics instructor. Heat can be transferred by conduction (two things touch), convection (heat transfer by a moving fluid like water, air etc.) or radiation (heat transfer by radiant energy given off by an object). Yes - the last one, radiation, does not create any heat until the waves actually strike an object and the last one does not require any molecules for the energy to be transferred. Yes - The sun radiating electromagnetic waves through space is a good example and you are correct in that these electromagnetic waves have to hit something to actually create heat.

    Bob

    Dear Mr. Peabody,
    Thank-you for the psychics lesson. I had forgotten about conductivity, or that heat rises (which would explain the astronomic quality of this forum).
    Using my own impeccable logic, informed by your superior academics, and with the aid of the remote control on the TV, I have procured a gadget to shrink-wrap the M-125s. This should prevent any extraneous heat from entering these devices. I also took advantage of their free offer (plus S&H) of a tin-foil hat, which I will carefully de-form to further protect the amplifiers from this ungodly hot WX we are experiencing here in the Bitterroots.
    I would ask only one other thing of this Forum, and that would be if sKIzo might provide instructions for shrink-wrapping a VTA amplifier, with perhaps suggestions as to perform such actions on a pre-amplifier.
    With all due respect,
    Sherman

    scottlowe

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    Join date : 2014-04-05

    Re: Power Transformer Tempeture Question

    Post by scottlowe on Fri Aug 08, 2014 1:11 pm

    Does a anyone think it would be good idea to have say 5 output tubes and rotate them once a week into different positions as per a schedule just like one would rotate a set of tires.I throw this " out there " so I don't end up with more " triplets " of power tubes .it just seems that there is often a tube that goes first and if i was to always have an extra in the rotation it would be nice .But then I bought my st-120 from another person that had built it and it was not correct for quite a while because I couldn't afford to get it fixed .I am using my new kt-120's but am saving for a set of kt-88's and I have 3 sets of three kt-88's that are basically useless to me. but again I dont know if thats typical of new tubes. I have an eico st-40 amp with all mullard 7591's [for eico]and the amp has a stamp that says march 1962 so I'll bet those are 52 years old,and this amp runs hot with the cage on.thank you for your help I do appreciate it.

    peterh

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    Re: Power Transformer Tempeture Question

    Post by peterh on Fri Aug 08, 2014 1:46 pm

    scottlowe wrote:Does a anyone think it would be good idea to have say 5 output tubes and rotate them once a week into different positions as per a schedule just like one would rotate a set of tires.I throw this " out there " so I don't end up with more " triplets " of power tubes .it just seems that there is often a tube that goes first and if i was to always have an extra in the rotation it would be nice .But then I bought my st-120 from another person that had built it and it was not correct for quite a while because I couldn't afford to get it fixed .I am using my new kt-120's but am saving for a set of kt-88's and I have 3 sets of three kt-88's that are basically useless to me. but again I dont know if thats typical of new tubes. I have an eico st-40 amp with all mullard 7591's [for eico]and the amp has a stamp that says march 1962 so I'll bet those are 52 years old,and this amp runs hot with the cage on.thank you for your help I do appreciate it.
    There is no point in rotating 5 tubes in a st-70/120. All that is needed is matched pairs.
    You have triplets, well if they are simular you can use 2 from each triplet as a pair.
    On the contrary it could be harmful to keep switching tubes, each removal might disturb loose
    particles ( that is harmless at the bottom of the tube but will do harm if falling across an active cathode). Every tube replacement _should_ be done with proper bias adjustment, but at the
    moment of power up there is a definite risk that bias is out-of-bounds and will cause damage.


    sKiZo

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    Re: Power Transformer Tempeture Question

    Post by sKiZo on Fri Aug 08, 2014 2:49 pm

    Not even sure that matched pairs are all that important as each tube on an ST120 can be adjusted separately for bias.

    Yes, there are other considerations, but then again, what passes for tube matching in a lot of cases is pretty basic.

    peterh

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    Re: Power Transformer Tempeture Question

    Post by peterh on Fri Aug 08, 2014 4:03 pm

    sKiZo wrote:Not even sure that matched pairs are all that important as each tube on an ST120 can be adjusted separately for bias.

    Yes, there are other considerations, but then again, what passes for tube matching in a lot of cases is pretty basic.
    Bias adjustment will have the tubes fit at one point on the IaVa curve. During usage other points
    "on the curve" will be visited and if the tubes don't behave alike then distortion will creep in.


    corndog71

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    Re: Power Transformer Tempeture Question

    Post by corndog71 on Fri Aug 08, 2014 4:17 pm

    While for most tube rollers the technical aspects often get overlooked because tubes are so forgiving.  The "bias" or technically, the idle current doesn't tell the whole story.  Here is an excerpt of what Roger Modjeski has written about it:

    "The Virtues of Power Tube Matching
    By Roger A. Modjeski

    My experience gained from testing thousands of EL-34's, 6550's, KT-88's, and other tubes has produced a wide bell curve of the two most important parameters, Bias and Transconductance. The center of this curve is the "bogey" value that the manufacturer is trying to hit, and the ends show the cutoff points of what he is willing to allow out of the factory. The tubes at the ends are still perfectly functional, but may not work well in all amplifiers, and will certainly not work well if mated with another from the opposite end of the curve.

    Some amplifiers do not have enough range on the bias pots to handle the range of tubes so that selection-must be made from a particular part of the curve to ever achieve bias.

    In the "good old days," I'm told, the spread of values was much less, allowing reasonable performance with random selection. Today, random selection is strongly not recommended-I'll tell you why.

    Looking at bias voltage in a typical batch of EL-34's from the best producer, it will typically range from -32 to -42 volts for 50mA of plate current. At a typical transconductance for 7000 umhos (7 mA/volt) the 10 volt difference will cause a 70 mA bias difference. Obviously, one tube will be on and the other cut off if they are biased with the same adjustment pot. I match the bias voltage to. 0.5 volts or better, and thus have only a 3.5 mA (0.3 x 7 mA/v) difference in plate currents, which is only 4% maximum at 50 mA.

    Now, those of you with individual bias adjustments are probably thinking that you've got it all handled because you can adjust for these different grid voltages. Well, you can-but in doing so, you will create a greater problem.

    Say you put a -32 volt tube in one socket of a push-pull pair and a -42v in the other and bias them so that their currents are equal. Unless you can also adjust the a.c. balance of the driver to provide 32v of drive to one and 42v to the other, you will have very unsymmetrical clipping and reduced power output.

    In most amplifiers with a.c. balance, the range of adjustment is nowhere near this, as that adjustment is meant to balance the driver and not current for widely unmatched output tubes."

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