I constructed a SS LM350 regulated DC PSU for the 300B in my 5842-300B amp. The 5V voltage was steady without load. But around 5 min after connected to the 300B, the 5V voltage started to fall to around 2.5V which caused one channel to go dead silence. The LM350 got hot too even it is heat-sinked.
Please give advice on this problem and on how to fix it.
I have now connected 5V AC the 300B. There is heavy hum noise esp. at my normal sitting position. Hum dies off at around half way between sitting position and the speakers. Any advice on fixing this hum noise?
Thanks a lot to you guys.
I followed Groverg's advice and bought a 100 ohm 25W hum pot from tube amp shop last night. I had rewired the amp and guess what? THE HUM WAS GONE!!! Very quiet indeed!!! I can now enjoy great sweet music. Thanks again Groverg.
Jim, thanks for all your advice on LM350. I guess I'd stay with the AC in the meantime. Probably try out the regulated DC again later when I'm in an adventure mood.
PS. I lives in HK and the hum pot I bought is from Japan. The shop owener said it had very good sound and most DIYers used it. It costed HK130 each or around USD19 each. IS THERE SUCH THING CALLED GOOD SOUND HUM POT?
Yes, there is such a thing. Consider that your music signal passes through this pot just as it passes through the cathode resistor and bypass cap. If the pot is noisy, tracks poorly or is cheaply made, it will have an effect on the sound and could be frustrating to use if it injects noise into the circuit or is difficult to tune properly. $19 seems a *bit* high to pay, but on the other hand 100 ohm pots are not easy to find at your local electronics shop, so you were fortunate to find some. Glad it did the trick!
It sounds like the LM is going into thermal shutdown, as others have written. A similar thing happened to me. I think the LM 350 has a drop-out voltage around 2.5V at 1.5 amps. So, any input voltage to the LM above 7.5V is more than you need, and has to be dissipated as heat. For example, if you are hitting the LM with 12V and the output is 5V you are dissipating 7V times 1.5A or 10.5 watts of heat. I fixed my problem by using a 294-1027-ND heatsink from Digi-Key, and by lowering the input voltage as close to 7.5V as I could get. The heat sink still gets hot, but the voltage reg. hangs in there. The heatsink is rated for 17W at 25 deg C ambient; it was about the biggest one I could find for a TO-220, so you might want to make sure you have room. One more thing - the heat sink is electrically "hot" too. It's connected to the VR's output and will have 5V on it, so don't short it to the chassis and make sure it's not touching anything (including yourself when you poke around inside the chassis). I also used heat sink compound to make sure I had a good heat transfer path. Finally, put this part of the circuit as far away from other heat sources as you can; the lower the ambient temp of the regulator the better. If you put it near a tube, for example, and the air surrounding it is at 50-60 degrees C, you won't be able to sink enough heat to make the thing work. I guess with VR's, as with life, you gotta be cool! :-)
The LM350 will shut down if it gets too hot, and it sounds like that's what is occurring. You need more heat sinking capability, how much depends on the dissipation of the regulator. What is the voltage into the regulator? If it's more than about 9 volts you will be dissipating a lot of heat (1.2 amps x 4 volts across the LM350 = almost 5 watts of heat.
You'd be amazed how large a heat sink you need to keep the regulators cool enough.
Jim's question should lead you to the answer..
What is the voltage into the regulator?
So you have to lower the voltage BEFORE it gets to the regulator. Either by getting a lower voltage transformer, or dropping the voltage by means of a voltage dropping resistor..
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