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SB-200 newbie question
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Bob Green
¿ªÔÆÌåÓýHi all ¨C I have what used to be a working SB-200 amp. It is not working well at all now. I really want to get it on the air for the CA QSO Party next weekend. I come to you as a last resort. I used to be able to get about 700 watts out on 20M. Now I¡¯m lucky if I can get 50-80 out, with 40-70 IN. The 572 plates get bright red-orange, but no (or nearly no) power out. Someone on the OTHER reflector suggested the 33 ohm grid resistors were open. They are not. One was about 33.9 ohms and the other was about 45 ohms, but definitely NOT open. In looking at AG6K¡¯s website, it shows the 33 ohm resistors in parallel with approx 900 pf directly to ground, not through the Heath bias circuitry. I have no problem connecting the amp like this, but will it really help? And how do you read grid current, since none would be flowing through the meter? I¡¯m kind of new at this, so if someone would be willing to call me instead of just jotting down some thoughts, I¡¯d really appreciate it. Like I said earlier, I want to get this beast ready for a contest next weekend. I have two new tubes, but would rather not use them unless it is really necessary. Thanks in advance for your help!? My phone is 909-866-7742. If you would rather, send me your phone number, and I¡¯ll pay for the call. I have unlimited long-distance service. 73, Bob Green W6BBL w6bbl@... All email scanned by the latest update of Norton Anti-Virus 2006 and is certified to be 100% virus-free.
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FRANCIS CARCIA
Check your plate blocking cap and pi - network components. Also TR RELAY contacts. Sounds like you have plenty or drive but output circuit may beon th ewrong frequency. Plate and load settings changes would be a clue. You could be tuning up on 10 meters. Blocking cap open would not couple rf to the tank.?wa1gfz
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Bob Green wrote:
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Bob Green
¿ªÔÆÌåÓýI forgot to add that I have just replaced the two blocking caps on the SB-200, and added a third doorknob cap on the 80M switch position in place of the big mica cap. I have not yet fired it up with the new changes. ? Does it sound like I¡¯m on the right track? 73, ? Bob Green W6BBL ? This e-mail was scanned by the latest update to Norton Anti-Virus 2006, and is guaranteed 100% virus free. From:
ham_amplifiers@... [mailto:ham_amplifiers@...] On Behalf Of FRANCIS CARCIA ? Check
your plate blocking cap and pi - network components. Also TR RELAY contacts.
Sounds like you have plenty or drive but output circuit may beon th ewrong
frequency. Plate and load settings changes would be a clue. You could be tuning
up on 10 meters. Blocking cap open would not couple rf to the tank.?wa1gfz
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This wouldnt be the core problem I guess but if a
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33 ohm resistor has changed to 45 ohms it needs to be replaced, I hope you have done it already. To me it sounds like the problem is on the output side, since you can drive "input power" , check from DC blocking cap and all the way to antenna connector. Dont forget the RFC and the power meter pickup. 73 Jim SM2EKM ---------------- Bob Green wrote: I used to be able to get about 700 watts out on 20M. Now Im lucky if I can get 50-80 out, with 40-70 IN. The 572 plates get bright red-orange, but no (or nearly no) power out. Someone on the OTHER reflector suggested the 33 ohm grid resistors were open. They are not. One was about 33.9 ohms and the other was about 45 ohms, but definitely NOT open. |
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On Oct 1, 2006, at 4:05 PM, Bob Green wrote:
Hi all ¨C45 ohms is 36% high. If the outside of the resistor is not burned, the resistor may have been damaged by an intermittent condition, In looking at AG6K¡¯s website, it shows the 33 ohm resistors inIt does not help stabilize the amp, it's a mod that obviates the cockamamie Heath T/R biasing scheme and allows one to do the needed high-speed switching mod. And how do you read grid current, since none would be flowingGrid current flows from the grid through the 33¦¸ resistor to gnd, from gnd through the grid current meter shunt, to the negative HV, the same way the SB-220 meters grid current. . ... ... cheers, Bob R L Measures, AG6K, 805.386.3734 r@..., , rlm@..., www.somis.org |
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Bob Green
OK, I've got the grids of the 572's grounded through 33 ohm resisters in
parallel with 940pf of capacitance. No smoke!!! Plates are NOT glowing!! I'm getting a reading of approx 80-90 ma of plate current as read on the meter. This looks normal, as far as I can remember. Things are looking up! I must be really dense, but I still don't know how to hook up the meter to read grid current though. Can anyone help, PLEASE! 73, Bob Green W6BBL w6bbl@... All email scanned by the latest update of Norton Anti-Virus 2006 and is certified to be 100% virus-free. |
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You can look here:
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73 Jim SM2EKM --------------------------------------------------------- Bob Green wrote: OK, I've got the grids of the 572's grounded through 33 ohm resisters in |
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On Oct 2, 2006, at 3:50 PM, Bob Green wrote:
OK, I've got the grids of the 572's grounded through 33 ohm resisters inBob -- Grid current eventually gets back to the negative HV. It flows through the 33-ohms, to gnd, and from gnd through the grid-I meter shunt-R to HV-neg. Typical values for the shunt-R are c. 1.0-ohm with a small, few k-ohm calibration resistor in series with the meter to make it read accurately. cheerz R L Measures, AG6K, 805.386.3734 r@..., , rlm@..., www.somis.org |