Contex 500 Tube Amp

[Shadetree Mechanic]

I bought a dozen 6KM6 tubes a while ago because they were cheap and they cross reference to the 6LQ6 and 6ME6 tubes. Would it be a bad idea to use one to replace the 6ME6 driver? I was going to order a 6ME6 tube but thought that it would be nice if I could use what I already had.

 

[Shadetree Mechanic]

Found the data sheets here:
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/typeindex.html

 

[nomadradio]

Physical size is a hint here.

The smaller tube is rated for 20 Watts on the plate, only 2/3 the 30-Watt rating of the 6ME6/6LQ6.

It should work, if you throttle down to about 2/3 of what the larger tubes can do.

73

 

[Shadetree Mechanic]

Thanks for the replies guys, I will try to pull the cover off this weekend.

 

[Jay Mojave]

Hello SM and All: Great thread on the Contex 500 linear amp. My Dad had one back in the later 1960's when I was a creepy teenager. I added a direct keying circuit to it so that my Johnson 350 SSB only radio would key the amp with out yelling into the mic. But because back then few people had cable TV which wasn't bothered by TVI, using the Contex 500 amp usually lite up the neighbor hood with interference. So it was sold.

Please let us know how you make out here on the Contex 500 amp.

Jay in the Great Mojave Desert

 

[Shadetree Mechanic]

plate choke is lookin a lil dark

Upon further inspection the plate choke appears to just be an insulator with a red wire running up the back. Unless I am looking at the wrong thing. Are the big black spots on the tube from over heating or is it the getter material?

 

[Shadetree Mechanic]

Physical size is a hint here.

The smaller tube is rated for 20 Watts on the plate, only 2/3 the 30-Watt rating of the 6ME6/6LQ6.

It should work, if you throttle down to about 2/3 of what the larger tubes can do.

73

Thanks for the confirmation, I put the 6KM6 in the driver socket and turned it on, tuned it up and the drive knob came right back to where it was before. I just happened to notice the final tubes appear to be mismatched different brand tubes. The one has big spots by the anode and I assume this means that it is on its way out?

 

[Shadetree Mechanic]

Hello SM and All: Great thread on the Contex 500 linear amp. My Dad had one back in the later 1960's when I was a creepy teenager. I added a direct keying circuit to it so that my Johnson 350 SSB only radio would key the amp with out yelling into the mic. But because back then few people had cable TV which wasn't bothered by TVI, using the Contex 500 amp usually lite up the neighbor hood with interference. So it was sold.

Please let us know how you make out here on the Contex 500 amp.

Jay in the Great Mojave Desert

Thanks for stopping by to check it out! This is really a neat amp and I have been learning a lot about it with many thanks to the knowledgeable people here willing to keep me pointed in the right direction. I plan on replacing the finals in the near future and would possibly convert to a different tube. I will be sure to post pictures along the way.
Chris

 

[kopcicle]

Actually the dark spot is a n artifact from tube production. After pulling a vacuum while heating the tube and sealing it off one more operation attempts to remove the remaining gasses and deposit them on the envelope . This is the "getter" that is is inductively heated as one of the last steps in tube manufacture.


Classic case of plate damage due to overheating.

https://www.worldwidedx.com/threads/hygain-afterburner-plus-482-grid-bias.228873/#post-623790

I'm sure Nomad or Dmans will help you perform this version of amplifier suicide prevention. I can also but I've been a little less reliable lately.

 

[Shadetree Mechanic]

I wanted to try 4 of the 6KM6 to see if the output would be any better. So I pulled the old tubes out and broke a resistor wire. My question is can I use metal oxide resistors like the blue ones that I see for the SB220? Or should I replace them with carbon composition? Is two watt ok or should it be bigger? I am going to re do all four of them. Also is the total length critical? I would like to make them about a quarter inch longer. Thanks in advance for any help!

 

[Jay Mojave]

Hello SM: Yes the metal oxide will work as a resistive load, and 2 watts should be fine. I would use the same or close length of wire for coil around the resistor.

I tested the metal oxide for 1 KW amp, that used 3ea 100 Ohm in parallel to get to 33 Ohms. The 33 Ohm metal oxide resistors showed a VSWR of 1.4 to 1 on a network analyzer which is close enough for me. You can also test metal oxide even carbon resistors by transmitting into then with low power and seeing what the VSWR is going to be. 100 Ohm resistor will see about 2 to 1 VSWR, 47 Ohm should be 1.0 to 1, and 33 Ohm 1.5 or ball park close.

Many years ago dear ol Dad got a new Contex 500 Amp, and took out everyones TV and toasters with interference. The Parasitic resistors and plate caps are somewhat fragile as you have shown. Good luck getting the Contex 500 ah hummin and a buzzin.

Jay in the Great Mojave Desert

 

[Shadetree Mechanic]

Hello SM: Yes the metal oxide will work as a resistive load, and 2 watts should be fine. I would use the same or close length of wire for coil around the resistor.

I tested the metal oxide for 1 KW amp, that used 3ea 100 Ohm in parallel to get to 33 Ohms. The 33 Ohm metal oxide resistors showed a VSWR of 1.4 to 1 on a network analyzer which is close enough for me. You can also test metal oxide even carbon resistors by transmitting into then with low power and seeing what the VSWR is going to be. 100 Ohm resistor will see about 2 to 1 VSWR, 47 Ohm should be 1.0 to 1, and 33 Ohm 1.5 or ball park close.

Many years ago dear ol Dad got a new Contex 500 Amp, and took out everyones TV and toasters with interference. The Parasitic resistors and plate caps are somewhat fragile as you have shown. Good luck getting the Contex 500 ah hummin and a buzzin.

Jay in the Great Mojave Desert

Thanks Jay! I will get something on order tonight. I will also be looking for a set of 6KG6 tubes as well.

 

[Shadetree Mechanic]

Ken's has a lot of good information.

https://kenselectronics.com/charles/linear.htm

Free Technical Repair Tip: Tube replacement notes: The 6KG6/EL509 tube still made in Europe & Russia can be used as a heavy duty replacement for a 6LQ6 & many other "sweep" tubes. A 6KG6 is about 1/2" taller than a 6LQ6 so you first need to see if you have room for a conversion. The socket pin connection change is easy: Pin 3 of the 6LQ6 was the cathode while pin 9 of a 6KG6 is the cathode; move the lead formerly on pin 3 to pin 9 (not used on the 6LQ6); add a jumper from pin 3 to pin 2 on the socket. You may need to squeeze or swap the plate cap to fit the 6KG6. The pins of a 6KG6 are a little larger than the pins of a 6LQ6; however--with a used socket--the pins just fit a little tighter in the socket! In some units it may be necessary to shorten the "pi-net" coil by 1 turn or add a 100 to 200pf Silver Mica 1000V capacitor in parallel across the antenna-load capacitor to bring the "tune" & "load" capacitors into the proper tune-up positions.

I can get the 40KG6 for about $25 each from RF parts. I could wire the heaters in parallel for 30v. Would this be enough voltage? I can get a different heater tranny if needed. They are about a half inch taller than 6LQ6 so this would bring it about an eighth of an inch from the top cover. Would the power try to jump to the cover if it was this close?

https://www.rfparts.com/40kg6a-west.html

 

[Dmans]

Shadetree Mechanic,
I cannot say if the plate caps would arc to the top cover or not but 1/8" would be too close for my comfort. Any way to lower the tube sockets in the chassis?

I also think 30 volts on a 40KG6A would be too little. 10% plus or minus wouldn't hurt I think but that is about 25% less than rated-and if your line voltage sagged to say 115V or less?????

Just my 2 cents.

73's
David

 

[Shadetree Mechanic]

Hey DMANS,
I will have to look at the sockets, that's a good idea. I might be able to lower the sockets 1/4 to 3/8 of an inch, it just might work. I would just have to figure out a good way to get 40v then. I could replace the heater transformer but I will have to see if anything else is using that power.

Here is one that is 36v and 2A, if it fit the hole in the chassis it would be a bonus. Two amps would be enough shouldn't it?

https://www.rfparts.com/p-8672.html