SOVTEK Midget 50 Head blowing main fuse

[misterc57]

I had picked up this Midget 50 head with missing fuse holders. Last time it had been used by the previous owner it blew a fuse and he had lost the holders.

I replaced the fuse holders. Tested all the tubes (all good). Sprayed cleaner in all the input and output jacks and all the tube holders.

I placed a 4 AMP in the main power fuse holder and a .5 AMP in the other fuse holder.

The amp powered up. After giving it a couple minutes to warm up I flipped the standby switch on and the amp hummed for about 5 seconds and then blew the 4 AMP fuse.

The voltage selector is set at 120 volts. Options are 100, 120, 230, 240.

Any thoughts on where to go next and how to diagnose without blowing more fuses?

Thank you. MC

[Gtr_tech]

Its either pwr supply or pwr amp at fault. Remove the output tubes and see if it stays on with no fuse blow. If so...replace the pwr tubes. If not, there's an issue in the pwr supply with shorter diodes or cap(s).

[Gtr_tech]

Shorted that is....

[Enzo]

I bet it still blows that fuse with the power tubes removed. Disconnect the center tap of the output transformer. If it no longer blows fuses, that means either the transformer is shorted or the plate leads are. If the amp has flyback diodes, see if they are shorted. Just measure pin 3 to ground for resistance on each power tube socket.

Also look for signs of arcing.

[misterc57]

slow update

Per advice of a friend I am waiting to borrow a variac.

All below was done with the amp unplugged and no main fuse attached.

pin 3 of the power tube sockets measured against ground both started in the high mega ohms, like 9 M Ohms and slowly kept dropping over time, as low as 5 M Ohms when I finally removed the probes. This was measured with the power tubes removed.

I see no evidence of arcing at the power tube sockets.

I recently bought a capacitor tester (Tenma 72-8150) and tested all the caps.

The 4 larger 220uf caps all measured high, around 270-293uf. All the other caps measure close to their designation.

Here are some links to pictures...

http://www.noalibirocks.com/000/midget50h3.JPG

http://www.noalibirocks.com/000/midget50h4.JPG

http://www.noalibirocks.com/000/midget50h2.JPG

http://www.noalibirocks.com/000/midget50h1.JPG

[misterc57]

update

Here is what I have done so far...

Test # 1

These are my readings on the amp running the variac at 30 VAC and looking for roughly 25% readings...

Power tube sockets (no tubes installed)

2 & 7 heater, 1.76 VAC
3 50.4 VDC
4 51 VDC
5 -9.45

Power tube

2 & 7 heater, 1.76 VAC
3 51.6 VDC
4 51.1 VDC
5 -9.53

Test # 2

Replaced the 6L6 power tubes with another known good pair.

On the variac at 90 VAC with volume and preamp pot set to zero I hear a hum coming through the speakers.

Turning the variac up slowly to 100 VAC, the main fuse blows.

[Alex R]

I have twice come across intermittent shorts in the mains transformers of these amps. In both cases I could actually hear the short crackling away inside the transformer. So you might try pulling out all the tubes, bringing it up on the variac and listening.

Some of the windings look a bit odd but I have used standard Marshall transformers to replace the Sovtek original with just a few little circuit changes here and there, so if it is the transformer, ask for details.