Check the listing for details. OUTPUT TUBE Filament Voltage CONTROLLER 2A3 300b 801 TUBE. Listed at 8.47 USD. >>>>VACUUM TUBE PRECISE FILAMENT VOLTAGE REGULATOR USING THE SAME TRANSFORMER,NOW YOU CAN DESIGN BUILD YOUR S.E. OR PUSH PULL AND YOU CAN ROLL YOUR OTHER OUTPUT TUBES.******EXAMPLE,,,,A TRANNY WITH AN OUTPUT VOLTAGE OF 12 VAC CAN BE REGULATED AT ANY FILAMENT VOLTAGE FROM 2.5VAC TO 12VAC.PICTURE SHOWS TUBES LIKE 2A3,300B,811-10 etc. WITH ADJUSTED FILAMENT VOLTS.2000 WATTS MAXIMUM POWER,ITEM MEASURES 1 1/4 X 1 7/8 X 1 1/16 SURFACE CHASSIS MOUNT.SCREW IN CONNECTIONS, 2 INPUT & 2 OUTPUT.>>>>>TRANSFORMER SUGGESTION; YOU CAN USE ANTEKINC TOROIDAL TRANSFORMER MODELS WITH DUAL FILAMENT OUTPUT. 1) AS-1212,,Ebay item# 372195246427 2) AS-0509,,Ebay item# 253108637967>>>>Also you can use this item to make your own VARIAC (Schematic is available for paid buyers only). *****QUESTIONS ARE ALWAYS WELCOME AND PLEASE CHECK MY OTHER COOL AUDIO STUFF.