I will detail the sound of each European tube type below.USA 12AX7/ECC83: These tubes include those produced by RCA, GE, Sylvania, Tung-Sol, Raytheon, Ken-Rad and a few others.For example, you can find British Mullard ECC83s that are labeled Holland Amperex, and vis-versa. Britain' on the tube can actually be an Ei, RFT or Amperex. It gets even more confusing when you find European ECC83s with American logos such as RCA or GE.That's why it is important to understand tube construction, tube codes, and tube history, or at least purchase your tubes from someone who does.
Manufacturers of black plate 12AX7s included: RCA, Sylvania, Tung-Sol, Raytheon and Ken-Rad.
Some of these tubes were 'OEM, - Original Equipment Manufactured' for other companies.
This means that, for example, producers like RCA or Mullard manufactured tubes for other companies, and would put a different company logo on the tube.
The main sonic differences between Long Plates and Short Plates can be summed up in three words: '3-D,' 'Lively,' and 'Open'.
By and large, Long Plates have a bigger sound in guitar and audio amps than short plates (with exceptions.) However, Long Plates can sometimes be vulnerable to noise and microphonics.