[Texasrocker]

Plate caps wont be found in vintage radio work, but can be found in Amplifiers indeed!
Take the expensive 8-pin octal type 6550, same as ( 5881) beam power amp tube, it can be replaced with a less expensive 6146 ( same socket) but some wiring mods are required at the sockets, and (add wire) to send plate connection up-top of chassis, to allow the 6146 plate connection thru the top plate cap connection of the tube. The 6146 type tubes are therefore a dangerous tube to work around, and require the heavy-duty type ceramic plate-cap connectors. The 6146 type tubes sound better than 6550s in the vintage ( all tube chassis) Ampeg SVT Bass Amps! Ive been playing thru these amps for yrs! The earlier 1970s model Ampeg SVTs used six 6146 tubes in push-pull,for 300 watts full clean pwr into 4 ohms. The later 70s model SVT amps used six 6550 tubes in push pull into 4 ohms. Both different amps chassis designs sound great, but the earlier 6146 design ruled for more overall power, tone, etc. Using matched sets ( sextets)are best for biasing these type amplifiers. The 6550s are getting more & more expensive! Therefore,tube conversions in these amps are worthwhile indeed.
To see a plate cap type tube basing diagram, see GE tube manual base # 7CK

The 6550 same as ( 5881) tube base # is: 7AC Same type socket connections as a regular ol 35L6, or 50L6 used in radio work.