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KRIS 600 SSB-New Project
- Thread starter Dmans
- Start date
The schematic is here: http://www.cbtricks.com/Amp/kris/600ssb/graphics/kris_600ssb_sch.jpg
The driver tube has only one pin connected to G1, it's pin 2.
73
The driver tube has only one pin connected to G1, it's pin 2.
73
Wrap up of the 600SSB
I finally got back to finish the Kris amp for my friend. The last episode had the driver tube turning "Cherry Red" when transmitting. To combat this situation, I installed a negative Grid bias circuit to the driver (and once again learned a heap! (Thanks Nomad!!)
So I learned that the control grid on the 6JG6A driver tube is only brought out to 1 pin on the tube socket.
I also learned that the 6KV6A does not work well as a driver tube in this chassis. I believe it is because the plate voltage on the driver tube in this amp is around 400vdc. My tube book shows maximum plate voltage for the 6JG6A at 770vdc and the 6KV6A at 900vdc. I know that the tube book is not considering these sweep tubes in operation in a splatter box RF amplifier but since the 6KV6A worked as a driver in another amplifier, I can only assume that it likes to see more than 400vdc on the plate.
First picture below shows where I picked up the AC voltage to rectify to my negative grid voltage. It seemed the simplest, shortest route so I took it. This source is the filament voltage for the driver.
The picture below shows the negative control grid bias circuit components. Components used were 2-1N4007 diodes, 1 1000Uf 25V electrolytic capacitor, 1-.01Uf 1kV ceramic capacitor and 1-1k ohm 5 watt resistor. All of this tucked pretty well in the space and tied to support parts that were already there.
(All of this came from nomadradio's help with my Hygain Afterburner 482 project!)
The picture below is after installation of the grid bias components showing the rectified negative voltage at the control grid pin (G1)
As I found out on the Afterburner 482 project, the addition of a control grid bias does change the input match. So without having all of the necessary tools at hand to measure what was needed to solve this issue, I proceeded to the "Trial and Error" approach. After making 3 different coils and swapping the trimmer cap, the issue was resolved. Prior to the control grid bias addition, the amp tuned in with 6 turns of 14ga solid wire 1/2" id x 1/2" long. (I had previously posted that I used 12ga wire. That info was not correct. It was and still is 14ga). The addition of the control grid bias required 8 turns of 14ga wire,1/2" id x 1" long and a 464 trimmer cap. Input match was brought down to 1.6:1.
While trying to solve the driver tube "Cherry Red" problem, I also decided to increase (or provide some) air flow to the driver. I removed the shield that is between the driver and final tubes and cut a "window" in the shield about 2 1/2" tall and as wide as the driver tube. I came across some aluminum perforated metal recently and cut a piece to install inside this window to preserve the "Faraday Shield" effect. A "smoke test" revealed that air is now drawn across the driver tube. (Not a "smoke test" in the burned component variety but a carefully controlled cigarette held in front of the driver tube. Pretty scientific right?
)
Below is the modified shield.
So the final result of this effort was tested behind my trusty 139XLR. It has a max modulated output on SSB of about 22 watts. With the Kris 600 in line, I put a 1Khz tone in the 139XLR and tuned the Kris to 325 watts peak on SSB. This is very similar to my previous results but the difference this time is that I added the tone for 5 minutes straight while keyed down and output power remained the same, the driver tube never changed color and the amp never got too hot (by "never got too hot" I mean I kept my hand on top of the case the entire 5 minutes and was never uncomfortable at all) The test was repeated on AM with the same results. The Amp was also keyed for 5 minutes straight in SSB with no modulation/tone and the results were that the driver tube remained "cool as a cucumber" with no plate color change! After all of this testing, the exhaust air from the new fan may have been 10 to 15 degrees above the ambient air at the time. (I don't have a "backwoods scientific" test for this!)
I also used this amp last night talking to my local friends and some DX on SSB from around 6pm to after midnight without any issues or loss of power
Since the goal was to keep the driver tube from turning cherry red and keep the power from dropping down under a "normal key down" period, I would call this a success!
Thanks for the help from the forum members and for the education I got from doing projects like this!!!
Negative control grid bias on a sweep tube amplifier is the only smart way to go!
(My opinion only. Your mileage may vary. No children or animals were hurt during the testing of this backwoods hypothesis. Don't step on downed power lines or take any wooden nickels!)
73's
David
I finally got back to finish the Kris amp for my friend. The last episode had the driver tube turning "Cherry Red" when transmitting. To combat this situation, I installed a negative Grid bias circuit to the driver (and once again learned a heap! (Thanks Nomad!!)
So I learned that the control grid on the 6JG6A driver tube is only brought out to 1 pin on the tube socket.
I also learned that the 6KV6A does not work well as a driver tube in this chassis. I believe it is because the plate voltage on the driver tube in this amp is around 400vdc. My tube book shows maximum plate voltage for the 6JG6A at 770vdc and the 6KV6A at 900vdc. I know that the tube book is not considering these sweep tubes in operation in a splatter box RF amplifier but since the 6KV6A worked as a driver in another amplifier, I can only assume that it likes to see more than 400vdc on the plate.
First picture below shows where I picked up the AC voltage to rectify to my negative grid voltage. It seemed the simplest, shortest route so I took it. This source is the filament voltage for the driver.
The picture below shows the negative control grid bias circuit components. Components used were 2-1N4007 diodes, 1 1000Uf 25V electrolytic capacitor, 1-.01Uf 1kV ceramic capacitor and 1-1k ohm 5 watt resistor. All of this tucked pretty well in the space and tied to support parts that were already there.
(All of this came from nomadradio's help with my Hygain Afterburner 482 project!)
The picture below is after installation of the grid bias components showing the rectified negative voltage at the control grid pin (G1)
As I found out on the Afterburner 482 project, the addition of a control grid bias does change the input match. So without having all of the necessary tools at hand to measure what was needed to solve this issue, I proceeded to the "Trial and Error" approach. After making 3 different coils and swapping the trimmer cap, the issue was resolved. Prior to the control grid bias addition, the amp tuned in with 6 turns of 14ga solid wire 1/2" id x 1/2" long. (I had previously posted that I used 12ga wire. That info was not correct. It was and still is 14ga). The addition of the control grid bias required 8 turns of 14ga wire,1/2" id x 1" long and a 464 trimmer cap. Input match was brought down to 1.6:1.
While trying to solve the driver tube "Cherry Red" problem, I also decided to increase (or provide some) air flow to the driver. I removed the shield that is between the driver and final tubes and cut a "window" in the shield about 2 1/2" tall and as wide as the driver tube. I came across some aluminum perforated metal recently and cut a piece to install inside this window to preserve the "Faraday Shield" effect. A "smoke test" revealed that air is now drawn across the driver tube. (Not a "smoke test" in the burned component variety but a carefully controlled cigarette held in front of the driver tube. Pretty scientific right?
)Below is the modified shield.
So the final result of this effort was tested behind my trusty 139XLR. It has a max modulated output on SSB of about 22 watts. With the Kris 600 in line, I put a 1Khz tone in the 139XLR and tuned the Kris to 325 watts peak on SSB. This is very similar to my previous results but the difference this time is that I added the tone for 5 minutes straight while keyed down and output power remained the same, the driver tube never changed color and the amp never got too hot (by "never got too hot" I mean I kept my hand on top of the case the entire 5 minutes and was never uncomfortable at all) The test was repeated on AM with the same results. The Amp was also keyed for 5 minutes straight in SSB with no modulation/tone and the results were that the driver tube remained "cool as a cucumber" with no plate color change! After all of this testing, the exhaust air from the new fan may have been 10 to 15 degrees above the ambient air at the time. (I don't have a "backwoods scientific" test for this!
)I also used this amp last night talking to my local friends and some DX on SSB from around 6pm to after midnight without any issues or loss of power
Since the goal was to keep the driver tube from turning cherry red and keep the power from dropping down under a "normal key down" period, I would call this a success!
Thanks for the help from the forum members and for the education I got from doing projects like this!!!
Negative control grid bias on a sweep tube amplifier is the only smart way to go!
(My opinion only. Your mileage may vary. No children or animals were hurt during the testing of this backwoods hypothesis. Don't step on downed power lines or take any wooden nickels!)
73's
David
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