Adrian, are you running the amp on 240v? You should be!! These older amps like the 922, SB-220, L4B, Swan Mark II those in particular with the CW/SSB switches, were originally designed as full power amps "at the time" which was 1KW DC input. Therefore those amps did not need plate transformers that are as heavy duty as those amps produced these days that would be considered "full power amps" 1500 watts output. Also with the newer power rule there is NO LONGER a need to have a CW/SSB switch on an amplifier. Those switches are only there to conform to legal issues at the time those amps were made. These days those switches are unnecessary and on the ones that do have them are no longer needed. These days these same amps can be pushed beyond the old power rule of 1KWDC input and approach the new power rule of 1500 pep output. The aforementioned amps will approach the newer power output level with a full 100 watts of drive. Of course this is pushing them a bit harder than intended but it is not a big issue when used for SSB mode with no heavy audio processing. Running an SB-220 between 1200-1400 pep output and likewise for the others mentioned is perfectly acceptable. They can handle it on a casual SSB basis. CW would be a different story of course. Put to obtain these numbers the amp would most definitely have to be operated on 240v. Once these amps are pushed to these levels the first thing that goes to hell in the hand basket is the HV stability. If the amp is set to 120v these 1200-1400 watt numbers would be nearly impossible to obtain, but with a decent 240v line with #12 wire this is no problem. I generally advise not to even bother with the CW/SSB switch unless you are just learning how to load up an amp. Use the CW mode to get the amp at resonance and then switch to SSB and tweak the settings. Once you know your preliminary settings you can go right to SSB mode and load the amp up with no problem and the need for the CW setting is moot. Another thing, too many hams are anal about grid current especially with the 3-500 tubes. The 3-500 does not have a wimpy grid, they are pretty robust and they will have no problem whatsoever handling 300ma on occasional SSB voice peaks. Don't forget, the 3-500 can operate at 125-150ma of grid current, key down 24/7 so a total of 300-350ma for a pair on SSB is no sweat. No need to baby these tubes in these amps. Henry ran these tubes to produce 2000w PEP in their amps so you can see these smaller amps will never approach that number. Don't be afraid of loading that amp up quickly with a 100 watt carrier for max output and over couple the load a bit and run SSB with it. No problem.