RCA is good.

1. Common 6x4 tube brands include American RCA, American GE, British MULLARD, British PHILIPS, Japanese SHARP, Japanese SANYO, Chinese original, etc. Among them, RCA's 6x4 tubes have the best quality and reliability. Because RCA is a pioneer in the American electronics industry, its 6x4 tubes have the best quality and reliability, and the price is relatively affordable. GE and MULLARD also have good quality, but are more expensive. The 6x4 tubes of SHARP and SANYO are also of good quality and relatively affordable, but they are not as reliable as those of RCA and GE.

2. China’s original 6x4 tubes are relatively cheap, but their quality and reliability are generally low. In short, if you require quality and reliability, then RCA's 6x4 tubes are the best choice.