Anybody have any thermally conductive tape

[Chip Fetrow]

Hmmmm.

If it is Kapton, it is not a very good heat transmitter.  It is,  
however, a very good high Voltage insulator.

I have rebuilt a large number of plate blocker capacitors in grounded  
grid FM transmitters.  Mostly 20 or 25 kW.  The plate Voltage is  
generally between 10 to 15 kV.

The plate blocker wraps around the fins on the final tube.  The tube  
is at HV, and the other side is at or near ground because a short link  
couples the RF to the output.

Bad stuff happens.  The Teflon gets punched, then arcs happen more and  
more often.

One way to fix it is to remove the Teflon and wrap it in the opposite  
direction.  It is generally about seven wraps.  Taking a plate blocker  
apart is a bit of a chore.

I would screw it down to a board.  Put dry ice chips inside the  
capacitor (where the tube would live) and heat the outside with a  
propane torch.  Eventually, they become enough different in size that  
they could be pulled apart.  Why do this?  Because the replacement  
part costs many thousands of Dollars.

Replacing the Teflon with Kaptron is not difficult.  I don't recall  
exactly the number of wraps, but I believe it took fewer wraps to get  
the same Capacitance.  Different thicknesses would change the number  
of wraps.  You only need to get fairly close on Capacitance, because  
you can tune it out.  It is a bit of a "building a new transmitter  
problem" when you first start out, but if you know what you are  
doing....

However, Kapton has a MUCH higher Voltage rating, so it wouldn't fail.

Teflon would.  A hole would be punched, then it would arc frequently,  
causing a plate overload in the transmitter, which was hard to find.

Also, Teflon is white, but Kapton is purple.  It looks cool in the PA  
cabinet.

--chip

On Mar 24, 2013, at 2:42 PM, tacos-request at amrad.org wrote:

> Message: 5
> Date: Sun, 24 Mar 2013 17:45:33 +0000
> From: Phil <philmt59 at aol.com>
> To: Tacos AMRAD <tacos at amrad.org>
> Subject: Re: Anybody have any thermally conductive tape
>
> Mike
>
> The tape is 'Kapton'. It's not a great thermal conductor, but it is  
> VERY highly electrically insulating. Doug Gentges scored a bunch of  
> reels at Dayton a couple of years ago, and gave me one (thanks Doug,  
> it's been very useful). Frank may well have some.
>
> Phil M1GWZ
>
> On 24 Mar 2013, at 15:51, Michael Chisena wrote:
>
>> All,
>> Working on a car amp.
>> The transistors are isolated from the case by a yellow/gold tape.
>>
>> I believe it's a thermally conductive tape.
>> Found some on the web but it's expensive.
>>
>> Any AMRAD guys have some to sell or share?
>> Need about 30 inches for the amp.
>>
>> Thanks in advance.
>> Mike