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Input circuit for grounded grid amplifier with 2x 813
Hello,
I got a pretty old amplifier with 2x 813 tubes in grounded grid. I runs nicely with 700 to 850 watts from 80 to 10m. I already did some modifications (vacuum relays, fan control, 9 band operation (80-10m) and now I am looking for some ideas for an input circuit. I meassured the input impedance of the tubes with about 120 to 130 Ohms which gives a bad SWR on a 50 Ohm output stage of a modern transceiver. I also tried a 1:4 UNUN. Results where slightly better but the SWR is still to high. As I want to avoid to use of an external tuner I am looking for someone out there who has build an input circuit (e.g. 1 collins pi-filter for every band) and can give me some advice how to calculate or build. tnx vy73 Rene DF9GR |
Input circuit for grounded grid amplifier with 2x 813
On 22 Jul 2006 08:39:03 -0700, wrote:
Hello, I got a pretty old amplifier with 2x 813 tubes in grounded grid. I runs nicely with 700 to 850 watts from 80 to 10m. I already did some modifications (vacuum relays, fan control, 9 band operation (80-10m) and now I am looking for some ideas for an input circuit. I meassured the input impedance of the tubes with about 120 to 130 Ohms which gives a bad SWR on a 50 Ohm output stage of a modern transceiver. I also tried a 1:4 UNUN. Results where slightly better but the SWR is still to high. As I want to avoid to use of an external tuner I am looking for someone out there who has build an input circuit (e.g. 1 collins pi-filter for every band) and can give me some advice how to calculate or build. If resistive I always figured anything under 3:1 was good. I've fed a pair of 813s in GG at close to the legal limit with a fairly modern transceiver with good results, but I never checked to see what I had for an input impedance. I guess that was one of those cases where ignorance is bliss :-)) I didn't know it was high so I didn't worry about it. For input impedance matching, look at a schematic of the Henry 2K4. They use a separate matching network plugged into an octal socket for each band. The ratios are different, but the design could easily be modified and the layout is easy to experiment with. I don't have the manual handy, but as I recall it's just an adjustable transformer with a tap. I think you'll be able to find the whole manual at BAMA Good Luck es 73 Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com (Get valid address from tnx vy73 Rene DF9GR Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com |
Input circuit for grounded grid amplifier with 2x 813
Hi Bill,
thanks for the idea. I didn´t knew that there is something available. I will check if I have the space available but I am afraid I have not. vy73 Rene DF9GR Bill Turner a écrit : ORIGINAL MESSAGE: On 22 Jul 2006 08:39:03 -0700, wrote: Hello, I got a pretty old amplifier with 2x 813 tubes in grounded grid. I runs nicely with 700 to 850 watts from 80 to 10m. I already did some modifications (vacuum relays, fan control, 9 band operation (80-10m) and now I am looking for some ideas for an input circuit. I meassured the input impedance of the tubes with about 120 to 130 Ohms which gives a bad SWR on a 50 Ohm output stage of a modern transceiver. I also tried a 1:4 UNUN. Results where slightly better but the SWR is still to high. As I want to avoid to use of an external tuner I am looking for someone out there who has build an input circuit (e.g. 1 collins pi-filter for every band) and can give me some advice how to calculate or build. tnx vy73 Rene DF9GR ------------ REPLY SEPARATOR ------------ Try an LDG autotuner: http://www.ldgelectronics.com/at-100amp.html This particular model is made for homebrewers to include in their amps. Bill, W6WRT |
Input circuit for grounded grid amplifier with 2x 813
Hello Roger,
thanks for information. I found the schematics of the 2K3 at BAMA (no 2K4) and I am currently downloading it. I will have a look on their solution. vy73 Rene DF9GR Roger (K8RI) a écrit : On 22 Jul 2006 08:39:03 -0700, wrote: Hello, I got a pretty old amplifier with 2x 813 tubes in grounded grid. I runs nicely with 700 to 850 watts from 80 to 10m. I already did some modifications (vacuum relays, fan control, 9 band operation (80-10m) and now I am looking for some ideas for an input circuit. I meassured the input impedance of the tubes with about 120 to 130 Ohms which gives a bad SWR on a 50 Ohm output stage of a modern transceiver. I also tried a 1:4 UNUN. Results where slightly better but the SWR is still to high. As I want to avoid to use of an external tuner I am looking for someone out there who has build an input circuit (e.g. 1 collins pi-filter for every band) and can give me some advice how to calculate or build. If resistive I always figured anything under 3:1 was good. I've fed a pair of 813s in GG at close to the legal limit with a fairly modern transceiver with good results, but I never checked to see what I had for an input impedance. I guess that was one of those cases where ignorance is bliss :-)) I didn't know it was high so I didn't worry about it. For input impedance matching, look at a schematic of the Henry 2K4. They use a separate matching network plugged into an octal socket for each band. The ratios are different, but the design could easily be modified and the layout is easy to experiment with. I don't have the manual handy, but as I recall it's just an adjustable transformer with a tap. I think you'll be able to find the whole manual at BAMA Good Luck es 73 Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com (Get valid address from tnx vy73 Rene DF9GR Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com |
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