Well, two 5-volt heater indirectly heated rectifiers come to mind immediately, the 5V4 and the GZ34 / 5AR4. There may be others. - Jim, KL7CC On 6/17/2013 8:53 AM, WA5CAB at cs.com wrote: > AFAIK, all of the 5.0 VAC heater 4-pin and octal base rectifiers have > directly heated cathodes. The reason (or one reason) is voltage rating.