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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube amp fuse blow out

Hello all,

I have a question: In two of my amplifiers, I am experiencing an annoying problem. I have an idea what it is, but I am intersted in what other thoughts there might be.

The problem: fuse blow out with no apparent reason. No shorts, nothing!

Here are the tubes involved:

Sovtek 300Bs (2).

Sovtek 5U4.

My conclusion, since both these tubes have similar looking filaments, is that they draw an excessive amount of current on cold or relatively warm (recently turned off) startup. But... more than 1.5 amps at the primary??? I mean, they are about 2 amps at 5 volts. That translates to about 85 milliamps at 120 volts! Should I use larger fuses? I am currently using 1.5 amp slow blow fuse.

A friend of mine suggested thermistors at the two AC legs. This worked well for him.

Otherwise, the amps work very well for long periods of time.

Thanks much in advance!

Just guessing, Gabe,

my TTs halogen lamp bulb has about 0.6 Ohms cold and rises to 7.2 Ohm when heated up. Factor of 12 in resistance.

A DHT filament should not be that different from a light bulb filament.

A 5U4 draws 3.0 A, a 300B draws 1.2 A filament current. 4.2 A stationary together and who knows what at switch-on, 10 times of that maybe?
Could be that your trannie draws too much current on switch on. The varistor could be a good work-around indeed. And 2A-fuse is probably also a good idea if the filament trannie makes it.