My SEX is HOT

[wwmhf]

Hi,

I have noticed that my SEX amp is very HOT. Using my finger to check it, I found that the transformer in the middle is the hottest one so that I could not keep my finger there for 3 seconds. The aluminum board is also fairly warm. After running for a while, the amp can even raise the temperature in my small study with the door closed.

Is this normal for the SEX amp? Or is this the consequence of a certain mod for higher power? I bought the amp used, the previous owner said the amp could not produce enough power even after those mods. At this moment, I am not sure what exactly the mods are. I plan to use this amp exclusively with Sennheiser HD650 headphones.

Thanks

 

[Grainger49]

This has been reported and PJ, designer of the transformer, assures us that this is the proper operating point.  This is one of the reasons the old Jupiter bees wax caps are not suggested for use in the SEX amp.

Not to worry, it is normal!  

 

[Doc B]

The power transformer gets quite warm on a stock kit, it is designed to operate this way. And yes, tube amps will warm up a room. Unless we know what the modifications are we can't really say whether the amp is running harder than usual or not.

 

[wwmhf]

The very obvious mod I found was that the resistors connected to the phone jack were taken out, but I have put them back in the way according to the official plan. I think I just need to spend a little more time to check the amp against the official plan to see what other modifications were carried out.

The previous owner said that he had been using the amp to drive a pair of speakers and a powered sub. He connected the speakers in the usual way through the speaker terminals, but the powered sub was connected through the phone jack without those two resistors. It sounds like that the amp was working under a heavy load in this configuration. Could this be possibly to cause a permanent damage to the amp?
If so, which part is the most probable one to get damaged?


 

 

[Grainger49]

If he used the line level (preamp out) inputs to his sub the impedance would be high enough that the amplifier wouldn't know it was there.  I would read what he said to mean that. 

Do you have an infrared thermometer?  You could get a reading on the transformer end bell and post it.

 

[Paul Joppa]

The cheap test it to put a wet spot on the transformer end bell. I just lick my finger and touch it. If it doesn't boil, then it's below 100 degrees C - i.e. normal.

 

[wwmhf]

Paul, Do you really mean that the amp normal so long as the end ball does not reach 100 degree C? If so, then I should not worry about anything and I just have to get used to this hot amp. It becomes very hot, but not boiling-water hot!

Compared with the S.E.X amp, my other tube amp is very cool. It is barely warm after a long time usage. However, that is an OTL amp.

 

[Paul Joppa]

Yes, that's what I mean. In fact the SE amp's power transformer is not nearly 100C on the outside, but if it were that would be OK. The transformer's internal parts are rated for 155C.

Almost all OTL amps are operated in nearly Class B, so the power dissipated in normal operation is much less than at full power into a resistive load. Therefor they do tend to run cool when run with music - they can get quite hot on the test bench. SET amps are necessarily operated in Class A and will run hotter. Even transistor amps, if they are Class A, will dissipate a lot of heat.

In addition, many designers like to operate the components cool. This is in general good for the longevity of components, but it is far from the only important design parameter. For power transformers in particular, you can run a transformer cool by using fewer turns of thicker wire, resulting in low-resistance windings. But the magnetic field in the core will then be very high, resulting in transformer vibration, nonlinearities that contribute to "buzz", and large magnetic fields that will induce hum into the nearby circuits, I believe it is better to give the transformer some magnetic headroom as well as some temperature leeway, rather than concentrating on a single parameter. Engineering is all about compromises!

 

[tsingle999]

You know just a possible idea here. When my SEX was running hot it eventually started shorting out. And it was the zener diodes (edit: schottky - sorry thinking of other kits diodes). One fo the solder joints had become unsoldered. I think because the leads on the zeners are so thick and suck the soldering iron heat up it is harder to get a good joint there. After i resoldered it the amp ran much cooler.

 

[Paul Joppa]

Ooohhh - good one! That would do it all right. (You do mean the Schottky diodes, in back right behind the power transformer, right?)

 

[wwmhf]

The Class A property does provide an explanation of the warmness. For my amp, I will certainly check those solder joints. I am even considering resolder most of them in this summer. Hope nothing major will happen before that.

 

[Grainger49]

The heat also drives any moisture from the windings.  Well, it did with large rotating machines (motors with an armature diameter of 15 feet).

 

[Paul Joppa]

...Hope nothing major will happen before that.

The great thing is, even if it does, YOU CAN FIX IT! There's a whole community here who will help if you need it. You'd have more confidence if you had built it in the first place, but you'd be surprised how much you can do with a little help from your friends.  :^)

 

[wwmhf]

Your helps here and the structure of the S.E.X led me to buy this S.E.X amp. I have start to have fun with it. Many parts (Pot, capacitors, etc.) have been ordered. I plan to turn it into a kind of super headphone amp. Thanks a lot for now, and I may have to come back to ask for more helps.