It specifies (4) 6H30 and (4) 6Z4 in the manual. The 6Z4 are Chinese made tubes with very few substitutes. They are pretty inexpensive. I was experiencing some tube noise in mine and changed them all out. I used Electro Harmonix gold pin 6H30s, so the complete tube change out was less than $200. I can't say if NOS 6H30s would be an improvement over the EH, but I am satisfied with the result.