jtm45 failing under heat?

[guitarric3]

Need some help with my metrojtm45. bought this amp already built, have retubed it twice and had the bias checked by professional... here is my problem. after about twenty minutes of use sound will get very "clipped" noisey, volume will drop, and dynamics are lost.... putting it back on standby for a few seconds will fix it temporarally...if amp is out of its cabinet and a fan is run over components, it is good to go all day long. only after it heats up to a certain temputure does the problem occur. ? i am at a loss.. like i said the tubes are brand new and the bias is set correctly, i assume one of the components or solder joints is failing under heat? has anyone heard of this? please help!
thanks rick

[spaceace76]

do you have pictures of the inside? carbon comp resistors are known to fail under heat. usually they can withstand the heat within an amp but sometimes a resistor can develop a temperature coefficient (positive or negative) meanings it's value will change depending on temperature.

sounds like you aren't much on the tech side so bring it to experienced tech and don't open the amp unless you know how to deal with the high voltages within.

[PCollen]

Need some help with my metrojtm45. bought this amp already built, have retubed it twice and had the bias checked by professional... here is my problem. after about twenty minutes of use sound will get very "clipped" noisey, volume will drop, and dynamics are lost.... putting it back on standby for a few seconds will fix it temporarally...if amp is out of its cabinet and a fan is run over components, it is good to go all day long. only after it heats up to a certain temputure does the problem occur. ? i am at a loss.. like i said the tubes are brand new and the bias is set correctly, i assume one of the components or solder joints is failing under heat? has anyone heard of this? please help!
thanks rick

I would want to measure your DC voltage supply nodes, and the DC voltages on the power tube pins, under 'failure conditions' and compare to those under 'normal conditions'. It might be that your tube rectifier is acting up.

[Roe]

sounds like a heat issue. apply heat gently to parts and listen