Greetings Khazm,
I'm going to assume that we are looking at a Kustom mod# 112 50Watt Defender amp here... so here goes my thoughts. This amp uses two EL34 power tubes (or you can replace them with 6L6 tubes and change the bias switch position accordingly, but you would have to invest in a pair of them at around $35.00 each.... but I'm thinking that you lost one of the preamp tubes which should be 12AX7 type. There are three of them in your amp, the one up next to the first EL34 is the phase inverter, and it should be fine. The two at the bottom are preamps #s 1 & 2. First I would try swapping those two preamp tubes and see if there is a change in the output, this might temporarily "cure " the problem, but I believe that only one half of the tube in preamp #2 position is used and if it is weak you might see some increase in the output. You might take them both out and run down to your local tube amp repair joint and have them tested (you used to be able to test tubes at Radio Shack until they quit using them in TV sets), and replace the defective one. Power tubes don't typically fail in a half way manner, and when they do the fuse most times will go with it. But it sounds like you lost one of the preamps and the 12AX7s are my suspect.
Part two I think is making a choice of what to put back in there.... Personally I tend to be fond of the Russian made tubes as they are good quality and durable. We here in the west kind of got out of the tube business when transistors became vogue-ish and the Soviets still use tubes in the flight computers on the MIG29! They know their tubes! So having gathered the wrath of the "buy American" crowd, I would look for Sovtek or Svetlana replacements. You might find GE or Sylvania tubes also and they are good, but tend to not be as forgiving to surges as the foreign counterparts.
Proceed with care as some things in there "bite".
Take Care;
Doug
"what is this quintessence of dust?" - Shakespeare
