HEATING IN TRANSMITTER

[KB5MD]

I have noticed that when using my transmitter on 40 meters am, the tank coil in the pi-network gets really hot in one particular spot.  The remainder of the coil gets warm, but this spot gets so hot it will burn you and the area is only on one turn of the coil and is about 1" long. Also the plate choke developes a hot spot during the same time. The transmitter is an RCA bta 1r1 converted BC unit.  Any ideas?  All of the output network was replaced during the conversion to ham band use. I am using the standard pi-network for the final output.

[WBear2GCR]

Sounds like you've got a very high quality VHF parasitic.

There are two main ideas on plate supressors... Measures and another fellow who advocates making the supressors into parallel circuit highish Z circuits...

Otoh, and others probably have more experience here, the issue may be as simple as the values you've chosen for the Pi-net being resonant at some VHF freq. Maybe a grid dipper will show it.

Also make sure you've wired the finals with proper bypassing on the appropriate tube terminals.

          _-_-WBear2GCR

[N3DRB The Derb]

break up the tuned circuit by making one suppressor slightly different than h other one to it kills he offending tuned circuit. I had 4 coils around my 100 ohm rx on one tube and 5 around the other. I had a smoking hot parasitic that burned a hole through he metal plate of one of my 812's before I could cut the power - took all of 5 seconds. added a 1 coil turn to one suppressor and no more problems, ran it that way for years.

having said that, have you even got such things on a converted bcl rig? 40 is vhf to a bcl rig. freaky things can and do happen, as you can tell.

[flintstone mop]

Hello
Did you re wind the plate choke for the "expanded bands"? The original might not like 40M.
And again there might be parasitics. Some Bcast rigs have long leads in the final and exciter and 40M might not work too well in your RCA Tx.
80M might be their max band of freqs.
I'll keep checking back to read your results.
Fred

[Tom WA3KLR]

KB5MD,

That's a good observation you made that the coil is hot in one specific spot.  This may be explained by the basic theory of the pi network. 

The network does not convert directly from the plate impedance at one end (say 5000 Ohms) to 50 Ohms at the other.  There is a point a little ways in from the 50 Ohms end where the impedance is actually much lower than 50 Ohms.  So the pi network can be though of as 2 L networks that are connected together back to back at a common point.  This common point must be an impedance that is well below 50 Ohms, and depends on the circuit Q chosen for that pi design.

This meeting point for the 2 L networks, being well below 50 Ohms is the lowest impedance point in the inductor and is therefore the point of the highest current and therefore the highest loss.

Is the hot spot near the 50 Ohms end?  If so, I would say not to worry. 

[KB5MD]

Problem solved.  Wound parasitic chokes for the plate leads to the 4-400's.  The original transmitter doesn't have them.  3 turns of 1/2" copper strap 1/2" in diameter shunted by 120 ohm 2watt resistors.  No more heating on tank coil or plate choke.  Thanks for all the replies.  This board is great.

[W2VW]

Try removing some of the plate choke turns especially if this was not done for the conversion. If the plate choke is near resonance, the thing will trade energy with the plate blocking capacitor big time.
See if this cools the trouble first.
The Tank coil should not be heating up. Check values of C required for a given frequency against a chart of Pi net requirements using your plate voltage divided by (plate current X whatever the magic number is for class C). Charts are online and in older handbooks put out by unfoulable organizations.

EDIT:

Looks like you fixed it by adding parasitic supressors. I ASSumed this was already done during the conversion.
Great job.