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Old 17th May 2020, 04:54 PM   #1
juststeve is offline juststeve  United Kingdom
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Default Power transformer very hot

Hammond 374BX with KT88.
The transformer is almost too hot to touch. Is there a starting point on what check to find out why?
Thanks
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Old 17th May 2020, 05:37 PM   #2
brl0301 is offline brl0301
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how long does it take for it to get that hot?
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Old 17th May 2020, 05:45 PM   #3
juststeve is offline juststeve  United Kingdom
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half an hour and it is hot.
Ive already killed an Edcor XPWR059 which had been running for a couple of years.
So a bit cautious with this one
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Old 17th May 2020, 06:20 PM   #4
debije is offline debije  Netherlands
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My 374BX is about 48°C after 3 hours playing and the amp has KT88 and GZ34 tube rectifier.
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Old 17th May 2020, 06:27 PM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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I usually rate my transformers at the amps VA times 1.5
So 100 watts amp would have 150VA transformer.
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Old 17th May 2020, 06:31 PM   #6
brl0301 is offline brl0301
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Quote:
Originally Posted by juststeve View Post
half an hour and it is hot.
Ive already killed an Edcor XPWR059 which had been running for a couple of years.
So a bit cautious with this one
On my TSE, the transformer will get too hot to leave my hand on it for more than a few seconds. It will usually take a few hours to get that hot though. What are you wall voltages at? I know mine typically run quite high.
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Old 18th May 2020, 09:19 AM   #7
juststeve is offline juststeve  United Kingdom
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It seems a few peoples are hot to touch. So possibly worrying unduly.
My wall power comes from a regenerator and set at 230v and the trans is wired at 240v as my normal wall power can creep up to 252v.
Must of been just unlucky with the last trans. As everything else is OK.
Thanks
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Old 18th May 2020, 09:31 AM   #8
JMFahey is online now JMFahey  Argentina
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Quote:
Originally Posted by juststeve View Post
Hammond 374BX with KT88.
The transformer is almost too hot to touch. Is there a starting point on what check to find out why?
Thanks
Please answer these 3 questions, otherwise itīs a guessing game.

What is your filament load in Amperes?

What is your +V in Volts and its DC load in milliAmperes?
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Old 18th May 2020, 11:07 AM   #9
Tubelab_com is offline Tubelab_com  United States
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The amp is a Tubelab SSE. The load on the 6.3V winding with a pair of KT88's and a 12AT7 is 3.5 A. The 5AR4 / GZ34 draws 2 amps from the 5 volt winding. The B+ with a 5AR4 is usually around 430 to 440 volts. The total current from the B+ will depend on the bias current, which depends on the cathode resistor chosen at build time. The usual bias current is 60 to 70 mA per tube plus about 18 mA for the 12AT7. It should be under 160 mA total.

The only real unknown is whether this amp is running like all the others, so the two important questions are what value cathode resistors are in the amp and how much voltage is across them. Knowing the exact B+ voltage could determine the exact load on the transformer.
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