TSE loses bias after about 2 minutes

[Pooch]

I've changed out coupling caps and tried different 300b, still cuts out after warmup. Both channels at the same time. Resets itself after cool down. Would it be Q1=2 or U2+3. Curious why it happens together.

 

[w5jag]

Thermal cutoff, and a return to normal operation after a cool down, suggests that the filament regulator is overheating.

 

[Pooch]

You are referring to U1? What could cause it to overheat in 2 min.? All the voltages check out, right up to the time it cuts out, the 5842s stay in check, B+ jumps to about 440V. I presume the B+ is jumping because the 300Bs are dropping out. Thanks for the help, this is my wife's amp and she's a little testy with me right now

 

[Diabolical Artificer]

"this is my wife's amp and she's a little testy with me right now" Run away!


Andy.

 

[w5jag]

You are referring to U1? What could cause it to overheat in 2 min.?

Poor / inadequate heat sinking.

Easy enough to verify if this is the cause, or not. Put a voltmeter on the regulator output and see if the voltage goes away when your amp cuts off.

 

[Pooch]

"this is my wife's amp and she's a little testy with me right now" Run away!


Andy.

HEHE...good idea...but I think she's faster than me.

 

[Pooch]

Poor / inadequate heat sinking.

Easy enough to verify if this is the cause, or not. Put a voltmeter on the regulator output and see if the voltage goes away when your amp cuts off.

Would this be enough heatsink. I'm not exactly sure where to measure the output?

 

[Tubelab_com]

99.99 percent chance it is the regulator overheating. It will be too hot to touch. Add some more metal to the heat sink and you should be OK.

 

[Pooch]

Having trouble downloading a picture, but what I used was 2 stock 1 1/2 in. double heatsinks stacked and then mounted a 2 story finned heatsink to the back of them. I'll look for something larger.

 

[Tubelab_com]

My last post was done while I was a passenger in a car going 70 MPH down I-95 on my phone. That's why it was rather short.

Your symptoms are exactly the same as a regulator shutting down. The filaments in the output tubes lose power causing the tube to stop conducting. This causes the plate voltage to rise.

Try pointing a small fan directly at the heat sink to see if it fixes the problem. If so make some improvements in the heat sink, or airflow around it.

My experience with 300B's showed that a single small heat sink could be adequate in some situations, while a large heat sink pirated off the CPU in an old PC was marginal. It all depends on the airflow around it.

The Sharp regulator used in the TSE has a thermal shutdown feature to protect it, but there is considerable variation between chips as to how hot they get before shutting off. Usually they are all too hot to touch when they shut off though.

 

[nigelwright7557]

Poor / inadequate heat sinking.

Easy enough to verify if this is the cause, or not. Put a voltmeter on the regulator output and see if the voltage goes away when your amp cuts off.

Or see if heaters are still glowing.

 

[Pooch]

Thanks George...and to others. I bought a new heatsink that should do the trick, and some thermal paste, too. I don't think I used any paste the first time around. I'll try a fan, too. I've got one of those Harbor freight thermal guns, I'll see what temp it shuts off at.

 

[nigelwright7557]

Thanks George...and to others. I bought a new heatsink that should do the trick, and some thermal paste, too.

Unless the valve is directly heated I have never found a need for dc heaters.
In the numerous designs I have built any problems with hum have always been other problems and not the heaters.

 

[Pooch]

Or see if heaters are still glowing.

I'll check that next time I fire it up. I do know the heat on the outside of the tube drops. Thanks.

 

[Tubelab_com]

Unless the valve is directly heated

The TSE is designed for one of three different tubes, all are DHT's. In this case 300B's which don't produce much filament glow, but it should be visible from the top in a darkened room.

 

[Pooch]

After adding the humongous heat sink from hell (about 3x4x1.5 in.), all is well. According to the heat gun it's running around 150 F. Thanks for all the help!