Thanks for clearing that up, my mind is a little foggy.
Ah the archaic 15db rule, many others need updating as well.
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Thoughts on the BLF188XR
- Thread starter LeapFrog
- Start date
Ah the archaic 15db rule, many others need updating as well.
Nothing archaic about the 15 dB rule. It would have never come to be if there was not so much use, or abuse, of amplifiers on the 11m band. Since probably 99.999% of modern amateur transceivers more than easily meet the drive requirements of amateur amps there is really no reason to drop that rule. There is nothing from preventing someone from building an amp that requires much less than 50 watts drive to achieve 1500 watts output. Most commercial amps are grounded grid design to limit the gain. A conversion to grounded cathode design (grid driven) will result in MUCH higher gain from the same pair of tubes.
This shows typical operation of a single 3-500Z tube and the difference between being grid driven or cathode driven (grounded grid design).
Note how for the same plate voltage (3500) the power out is 920 watts with 81 watts of drive but 850 watts with only 22 watts of drive. This is approx. 10.5 dB gain and 15.8 dB gain respectively simply by changing the configuration from GG to grid driven.
Just curious, Capt. K... does this make things easier on the amp all around, or just on the tubes? And is there mod info available on how to do this?
73,
Brett
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It happens, mobile platforms suck imho.
Valves, before my time but they hold a lot in my heart! More to learn!
From what I gather one would change the physical position and supporting configuration of the tubes, I assume one would lose efficiency (draw more current) changing configuration.
Simple if you understand tubes, this is still new to me so im rambling on here sharing ideas (not knowledge). Im off to play with my "transistorized plastic".
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Fixed that quoting problem for you.
Not really any easier on the amp in any way at all but simply allows for more gain and thus lower drive levels for the same output. Grid driven amps tend to be a little fussier about being over driven as the grids are fragile and can be destroyed easily with improper operation depending on the tube type. The tube I have chosen for eventually building an amp with is the 3CX3000A7 which has a grid dissipation of 225 watts so it will be impossible to destroy it with my 100 watts of drive. Grid dissipation is not necessarily an indicator of max drive levels however and tubes with as little as 30 watts grid dissipation can be driven with much more power than that as long as the grid is not forced to handle any more than 30 watts of heat.