VTA ST120 - 50 Hz hum when placing 5814A / 12AU7 JAN Philips into V1 position

Good Evening,

Since taking delivery of a VTA ST-120  on 3/7/2021 (kit form), I have been trying out many combinations of 12AU7 equivalents in the V1 position, along with various  KT88 power tubes, with no problem until now.
The amplifier itself has never had troubles of any kind, except for two defective power tubes that flashed and blew, and which were immediately replaced.

Today I was experimenting with the 5814A / 12AU7 JAN Philips in the V1 position and had a problem:

Upon inserting the 5814A, turning on and setting bias to 0.5 volts as usual,there is a noticeable 60Hz hum from the speakers.
Replacing with the current 5814A, the 50Hz hum is no longer present.
This has never happened with any of the other 12AU7 substitutions I have tried in the past.
I believe there are no ground loops at play.

Why would this simple tube substitution cause a 50Hz hum in the speakers?
How does one diagnose this issue?

Current configuration is

New V1 Tube:

• 5814A / 12AU7 JAN Philips, NOS-5814A-JAN
  

Markings on box:
5960-00-262-0210
FSCM 82219 MFR/ PNJAN 5814A
1 EA ELECTRON TUBE
DLA900-86-C-0530
А 3/87

Images:

• https://i.servimg.com/u/f16/20/47/29/43/5814a-10.png
  
• https://i.servimg.com/u/f16/20/47/29/43/5814a-11.png
  

System: Music Server Streaming (FLAC) --> RaspberryPi --> shairport-sync --> HiFiBerry DAC2 HD --> RCA outputs --> VTA-ST120 --> Speakers (8 ohm)

Amplifier:

• Svetlana KT88  x 4 (power tubes)
  
• 1 x 5814A Siemens Halske (V1 position)
  
• 2 x RCA 12AU7A ECC82 Clear Top June 1966 Matched Pair in (L/R side positions)
  
• Klipsch Cornwall IV speakers
  

Many Thanks,
Alex Bramford

You have gotten a bad tube. Throw it away !

I had the same issue about 10 years ago with some JAN Phillips 12AT7 tubes in the V1 position. (I have an old VTA 120 with the high gain board). The V1 position is pretty unforgiving of noisy tubes. I was able to move my noisy 12AT7 tube to one of the side positions on the driver board and put a tested quiet mullard 12AT7 position in the center and the problem resolved. Eventually I replaced all of the driver tubes with tested quiet 12AT7's. I am currently running Tung Sols.

Does it happen if only one channel is connected to your pre? If you short both inputs is it quiet?