What To Use Under The Power Transistors?

mlucitt

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
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Jacksonville, FL
I have heard it called Wakefield Compound, zinc oxide, silicone paste, or Dow Corning 340. Others use plain mica or Silipads. What do you use and why???
 
Sil-pads...better heat conduction.....oh....and less mess. Get the gray ones that look like they are embossed...
 
Do you use a torque screwdriver with them? I know they are prone to tearing after about 10 inch-pounds.
 
Not those gray ones, pretty damn tough Wakefield 400's....
 
Did you get those at Mouser? The Wakefields I am looking at don't specify the T03 size??? Do you have a P/N handy?
 
In the Archives there should be a part number in the "parts" thread...
 
laatsch55 said:
In the Archives there should be a part number in the "parts" thread...
Click to expand...
Lee, I am too dumb to find the "Archives" so I am taking an action item: I will ask Joe to include the information on the sil-pads he uses on his next BOM update. In the meantime, he was gracious enough to offer me (16) of the little buggers for a fair price along with the (16) new MJ output transistors he is sending me for the PL400. It will soon be singing a different (WOAPL) tune...
 
I think I am a convert to Sil-pads. Cleaning the white stuff off the mica insulators and out of these heat sink holes and from the outside and inside of the amplifier chassis probably took me about six hours (somebody used WAY too much). That includes removal of the four drivers that were still installed, removal of the heatsinks, and deburring the holes (there were several burrs from who knows what?).

Cleaned Heat Sinks.jpg
 
Good job Mark. Yes the punch direction on those heatsinks often left a burr that was not timesaved off. Your countersinking will fix that.
 
What is the consensus of the group on these capacitors on the PL400 transistor wall? They are not shown in the schematics I have. My early PL400 (serial #2225) does not have them, so I don't know the values. Are they like the ferrite beads in the PL400 Series II, an undocumented factory modification? Do they help with EMI or blocking AC? Thanks to Don for use of this picture.

PL400 Transistor Wall.jpg
 
They are there to counteract the inductance of the wiring length in the runs that come from the bulk caps to the sockets. Also the bulk caps are not good with high frequency filtering whereas these caps are. I don’t see them on schematics and we’re not on every amp. If for the PL700, go to 250vdc. 100 will do for the PL400
 
mlucitt said:
What is the consensus of the group on these capacitors on the PL400 transistor wall? They are not shown in the schematics I have. My early PL400 (serial #2225) does not have them, so I don't know the values. Are they like the ferrite beads in the PL400 Series II, an undocumented factory modification? Do they help with EMI or blocking AC? Thanks to Don for use of this picture.

View attachment 35612
Click to expand...

The "bypass" capacitors are there to help eliminate high-frequency oscillations particularly when Phase Linear began using faster devices like the TP9054 (MJ15024's). These oscillations may be seen when testing the amplifier at 20kHz at rated output. (NOTE: You will need to use an output from a distortion analyzer into a scope to see this.) The ferrite beads are there to prevent parasitic oscillations. On fully comp amps, you may see an additional cap (for a total of (4) caps) and slightly different wiring of the caps. Likewise on fully comp amps you may see a total of (4) ferrite beads being used.

There were some service bulletins issued which describe the use of "slow-down" caps on the pre-drivers on the pc board also to help prevent oscillations. The 400 and 700/700B series 1 pc boards can tolerate no more than 33pF before slewing (at 20kHz) will occur. In which case the amp will not meet the THD spec at 20kHz. The series 2 PL-36 pc boards can tolerate up to 250pF. However, it is best to use the lowest value part in order to eliminate these oscillations. Arbitrarily installing values higher than needed can actually cause oscillations. As Dean use to tell me, "use the smallest band-aid possible".

Testing amplifiers at 20kHz at rated output both channels driven should always be performed to ensure that the amplifier meets factory specs! The duration of test should not be too long. Otherwise, you may run the risk of blowing the Zobel network (particularly when driving a 4 ohm load).

Ed
 
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