Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Curranm on December 07, 2017, 06:27:45 am
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I have a home brew amp with a classic tone power transformer. the 40-18024
http://www.classictone.net/40-18024.pdf
i am running four 12AX7's, and two KT88's my plate to cathode voltage is 482vDC and I have the bias set at 51mA. the transformer is getting hot after about 30 minutes. it passes the 10 second rule(finger on the laminates for 10 secs with out pulling my hand back) BUT it is still pretty darn hot(I run the amp for hours on end in my recording studio). this is the second version of this amp I have built with the exact same layout/components/tubes/transformers. the other amp does the same thing. HOW worries should I be? or should I even care. I understand power transfoerms should warm up and can get hot BUT how hot is TOO hot?
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I've seen PTs that run cool and I've seen PTs that run very hot. Depends on a lot of variables. I bet the '88s are hotter. You can always add a fan if it will make you feel better.
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how hot is TOO hot?
Causes of tranny failure are here: https://www.google.com/search?q=transformer+lamination+failure&oq=transformer+lamination+failure&aqs=chrome..69i57.6135j1j8&sourceid=chrome&ie=UTF-8
Seems the main problem is lamination/insulation failure from overheating. This is unlikely to be a real problem. Vintage amps, particularly bargain types, often used undersized trannies to save money; they ran very hot. Note that power tubes are also very hot, and do not pass the "10 second rule".
Anyway, you likely do not have an operational problem. If you want cooler operation, another option besides a fan is to remove the PT's bell housing (if it has one).
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thanks for the info! I just worry, my idea behind the 100 watt PT from classic tone was to keep it from ever getting hot, maybe its just in the design of the PT I guess
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so another question aboot mA on the HT of the power transformer and what it relates to. I use two types of PT for my amp builds the classic tone 40-18023(my personal favorite) and the 40-18095(supposedly a cooler running PT) the mA on the HT says 150mA. what does this pertain to for the PT use? I have 5a on the heaters so it looks like I could run (2) KT88's (1.6mA+1.6mA) and (5-6) 12AX7's @.3mA each for a total of 5A. is this correct? OR would I burn up my PT? i am developing an un-healthy love affair with the KT88
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the mA on the HT says 150mA. what does this pertain to for the PT use?
That's how much current can be drawn through the secondary HT windings of the PT, per manufacturer's specs. The PT is supplying all the tubes with voltage & current. But the preamp tubes draw only about 1 - 2mA ea.; so they can be disregarded for this calculation. Look at the tube charts to see how much current is consumed by all the power tubes, at the column of a close B+ supply voltage. Add the plate + screen current together. Do this at idle & also under full signal condition. A good PT can handle up to 50% over-current. That can contribute to voltage sag; and of course more heat. Whether or not to have sag is a subjective design issue.
I have 5a on the heaters so it looks like I could run (2) KT88's (1.6mA+1.6mA) and (5-6) 12AX7's @.3mA each for a total of 5A. is this correct? Yes.
i am developing an un-healthy love affair with the KT88. The KT-88 deserves all your love. :icon_biggrin:
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an un-healthy love affair with the KT88
many of us have that problem, I prefer mine SE, that way I don't have to learn French :icon_biggrin: