These are NOS GEC or Mullard branded TT-21 power tubes.TT stands for transmitting tube and these are Genuine KT 88's with the B+ pin eliminated in favor of a plate cap. This enables the tube to be run at significantly higher B+ voltages, as high 1200 volts (that provides additional power for transmitting use) To use,run a wire out from pin 3 from existing socket to the plate cap out side. You can also find octal adapters with plate caps on ebay. Get some actual genuine Mullard/Genalex power tubes while they last. We have about a dozen left.
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