Re: OT: modifying your amp for lower drive requirement
Posted by AD6XY on Dec 24, 2010; 10:03pm
URL: http://elecraft.85.s1.nabble.com/OT-modifying-your-amp-for-lower-drive-requirement-tp5854009p5865633.html
Luckily - most of the world is not like the US and gains of over 15dB are legal and common in non-us amplifiers. Modern FETs have much more gain than older devices and it would be quite normal to have 20dB gain from a stage - even after some feedback. There is an interesting design in the latest Dubus for a 1kW 2m amplifier that requires only 5W input. That is tetrode territory.
The popular THP 1.2KFX and 1.5KFX amplifiers use a 6dB input attenuator. Bypassing this allows you to drive them with less power under 25W, though it is not encouraged. The attenuator serves a couple of other purposes - it prevents serious over-drive with a 100W rig and it improves the input match.
Incidentally - someone said with a 3dB pad the VSWR can't be more than 3:1 - that is only true if the PA does not have a negative impedance or to put it another way, return gain. That can happen when an amplifier takes off.
Mike