Sony Heat Pipe amp ta f55 runs hot

[Trying to listen]

Hi folks,

New member here.  I just "reacquired " a 1980 sony ta f55 amp.  I had one through the 80's but by 95 I had showed it enough disrespect that it gave up the ghost.

It sounds really good, and it is gratifying to know I can still remember what I heard all those years ago.  Back then I ran it through a pair of large advents but this time I am running it through a pair of rp150ms.

 

It sounds even better.  But it runs hot.  I have reset the DC bias to factory spec (22 mv) and have the DC balance/offset fluttering around 1 mv off of zero and it seems like it is running somewhat cooler, but certainly not room temperature even at low volumes.

 

This is a "heat pipe" amp, with an oil filled copper pipe strapped over the power transistors which are right on the main board running to what look like aluminium fins.

 

Does anyone know if these amps tend to run hotter than the more traditionally cooled amps of that era?  The thing sounds so good I would hate to have blow on me...

 

Any help or opinions would be appreciated.

[Panelhead]

  I had a 1980 vintage Sansui that ran hot. Think biasing well into Class A was popular. 

  How much current is running through the output stage?

[Trying to listen]

Thanks for your reply.

Well,  I would need help to understand how to measure that.  I have pretty rudimentary skills in this area.  I know how to set the Bias to spec and check the offset/balance readings tp ensure they are as close zero as I can get them.

But, as I mentioned, not sure how to measure current.

 

Thanks in advance for any information you can provide.

 

[henry4841]

Not really too sure at what temp they are suppose to run at idle after doing some googling but I would measure the temp with a temp gun and would not worry too much about it if it was 125 degrees or less at idle. What I was searching for was it run in class A and could not find any answer which leads me to conclude it was A/B. The heat tubes were there because the power transistors are on the board and not separate from it like most amplifiers. Selling point to impress without any measure of being better just because of being n board. It would not hurt anything to lower the bias to say 15 mv if the heat concerns you. All bias does if prevent power stage from going from A/B to just B. In other words keep it from having crossover distortion. As long as the bias idles the transistors above cutoff it really does not matter that much what it idles at. If you lower it too much you will hear the distortion which is very unlikely at 15 mv or even lower. 

[Panelhead]

    The 22 mV bias is measured the voltage drop across a resistor. If the voltage is measured from a test point to ground, the resistance can be measured with the amp unpowered. Test point to ground. 

  Divide the 0.022 volts by the resistance to get current. As Henry said, if it runs hot, the current can be brought down.

  But that 22 mV bias was chosen for a reason. 

[henry4841]

5 hours ago, Panelhead said:

As Henry said, if it runs hot, the current can be brought down.

  But that 22 mV bias was chosen for a reason. 

It was recommended for sure. By their very nature the hotter you run a transistor the better it is going to sound. You would need to look for crossover distortion on a scope to be certain if lowering some more would hurt but I from experience I would not be afraid to lower it to 15mv if I did not here any difference in sound without looking at a scope. At 15mv the transistor is not cutting completely off going into class B. The non working transistor of the sine wave would still be running some. Just guessing, not looking at the schematic or circuit, I would think you would have in the neighborhood of 20 or 25ma of current running in the transistor at 15mv bias. If it still sound fine and lowered the temp I would give it a try. He really needs to check the temp with a temp gun to determine what the temp really is. It might not be as hot as he thinks. I have a number of class A amplifiers running at 55 to 60 degrees Celsius and at 60 degrees you cannot touch the heatsinks for longer than a couple of seconds. That is what I call running hot. 

[Trying to listen]

Thanks to all for your information and help.

 

I went in to the amp another time after I let it run for about an hour.

 

Bias was steady at about 23.4 mv on one side and 22.8 on the other.  So I adjusted again and got each channel back down to 22 mv.   This time I left the multimeter on each test point for 15 minutes and made sure the reading was stable.

 

It seems that kind of patience was something I should have employed from the outset. 

 

I will try and get my hands on a temp gun.

 

Since then it has run warm but not hot over about 5 hours a day of listening (this is the unit I use in my office).  The sound still astounds me.  I have a number of units around the house and the only one that sounds better is a Scott 299b.  And even then the sound is totally different. 

 

The Scott sounds like I am in the middle of live performance most of the time where the Sony sounds like there is someone inside the amp with a slide rule making sure every sound is 100% accurate.

 

Again, appreciate all of the help.  Coupled with the little klipsches and the st j60 tuner I am a pretty happy listener.