Please Note the product description from K&K,
its explain perfectly the prob cold tubes suffer
when dont have the benefit from Soft Start:
Time Delay and Soft Start Kits:
It is often very useful to delay the application of the high voltage in a tube circuit until the tube filaments have had several seconds to warm up. This prevents the phenomenon known as “cathode stripping,” where the presence of full voltage to the tube’s electrodes before the cathode is at emitting temperature prematurely depletes the coating on the cathode that is responsible for the electron emission which, in turn, makes the tube work. In a choke loaded power supply design, delaying the application of high voltage until filament warm-up has been achieved also prevents the large over-voltage event that accompanies a sub-critical load for the choke.
In addition, if you find the use of a tube rectifier not to be to your sonic taste, but still desire the “soft start” action implicit in the rectifier filament warm-up, this action can be imitated using a low impedance solid state circuit that is virtually out of the circuit once the voltage has risen to its maximum.
There two kits that address these needs. After a fixed time delay the Time Delay Kit circuit closes a relay that can control the B+ voltage or even the B+ transformer mains voltage, in the case of a separate B+ supply. The time delay is set for 30 seconds by a supplied resistor, but can be changed over a wide range by changing the value of that resistor. The Soft Start Kits incorporate the same time delay circuit to switch the B+ voltage followed by a MOSFET circuit that ramps that voltage up over about 30 seconds, so full B+ is achieved after about a minute. The Soft Start Kit supports loads up to 200mA and the High Current Soft Start Kit supports loads up to 500mA.
The Time Delay Kit PC board also accepts a small relay that can be used to add a time delayed mute function to any preamp, so that at power up and for 30 seconds afterward the outputs are shorted by the relay contacts. Once the 30 seconds (or whatever time you want to use) has passed, the relay contacts open and the audio signal is unimpeded. Many tube preamps pass an “ugly noise” onto the amplifier during initial warm-up that this action prevents.
The same PC mount power relay is used in both circuits and is rated for a hefty 8 amps. These relays have been used for years in products switching B+ voltages as high as 550V with no problems. Either circuit can be powered by 6-12VAC or 7-25VDC and draws less than 200mA with the relay actuated.
PC board sizes:
Time Delay Kit – 1.9″ (4.8cm) x 1.9″ (4.8cm)
Soft Start Kit – 2.4″ (6.1cm) x 1.9″ (4.8cm)
High Current Soft Start Kit – 3.4″ (8.6cm) x 2,5″ (6.1cm)
Time Delay Kit $25
Soft Start Kit $35
High Current Soft Start Kit $48