I have a Kustom HV 100 hybrid tube amp. When I power it up, It will play normal for about 5 seconds and then the volume drops off. The preamp tube is only about 2 months old and this amp only gets played once a week. Any help would be greatly appreciated.

Several possibilities (i have experienced all):

1. 120V.AC line noise. Try moving to a new receptacle in a different room or breaker. Try a In line AC filter (like a Furman power filter/conditioner)
2. RF interference. Plug in a different core and guitar to isolate.
3. Guitar wiring. Bad ground internally or pot (volume) became ungrounded.
4. Defective amp preamp section. Pot or transistor.

On the back panel there is a section labelled Patch Bay
There are 2 small switches which assign each Power Amp to either Monitor or Main Output.
It would seem you have them both set the same at present. Set one to Monitor and one to Main, and use the corresponding Power Amp outputs direct to speakers.

Note you can send signal to external amps or powered speakers from the preamp outputs. This is really handy.
Download and print the manual from https://www.manualslib.com/manual/90516/Kustom-Kpm-8420t.html
and keep it with the mixer.

( Of course don't send speaker outputs to other amps or powered speakers )

cracked solder joints, how old is it, they have 5 year warranties..

Have you checked all the other amp tubes? You may have a bad 12AX7 or 12AT7 tube.

if you have replaced the preamp tube than try looking at the capictor. i had just bought one and i thought the tube was out turned out i was worng

Have you tried connecting a single speaker to each output and cutting the volume down to see if you have a load or impedance problem? The amp may be perfectly fine except it's trying to play to a 2 ohm or less load :-)

Carl

First thing is that if you bought MATCHED pairs of the EL34's a COMMON mistake is to install the pairs wrong. Correctely install a pair as the OUTSIDE tubes of the line and then the remaining pair as the middle two tubes of the line of tubes before biasing.

Funny thing, I just ran across this problem you have with a Bedrock brand amp that uses the same configuration as your V3 tube does. This is really a stupid configuration engineering wise as they tie a plate at high voltage to the grid of the second stage in the tube as a cathode follower, This means that the second section has the cathode at a reasonably high voltage relative to the heater with wide audio swings on it as well... Tubes are NOT designed to work that way and result in noisy behavior, One would have to select tubes that have very low heater to cathode leakage in the second section of the tube... What you are seeing is the result of very poor engineering practice. Marshall is probably testing tubes for the leakage and branding them as such...

I think that designers of this circuit have NOT had experience with tubes and have applied transistor type designing to this use... it is plain stupid and myself I would re-engineer the circuit if it was mine or build a tube tester capable of measuring the heater cathode properties to select a suitable tube to use.

Often this is a broken input connector or an electronic failure such as a tube or semiconductor device.

On some units a failed switch on a jack for direct input to the power amp can cause this. Oxidation at the contacts causes this problem.

IF this is a vacuum tube unit, these symptoms are that of a tube that the heater is intermittent, that is it cools off due to a break and then liights again.

Open the back and observe the heaters in the tubes. DO NOT be fooled by some that have TWO heaters in them. (use a mirror if necessary) A single one that comes and goes can cause the problem and the voiration can make it come and go. This behavior in a tube is NOT somthing readily spotted in a tube checker. The delay in fading and coming back would be the delay in warm up following a heater going out and cooling.

Are the tubes bad? Do they light up? Check the tubes and install replacements if necessary. If it's a preamp tube that went bad, just replace that one tube. If a power tube went bad then replace the set of power tubes. Be sure to get a matched set when replacing power tubes, and to bias them.