Fuse blowing on start up

[crappyjones123]

If the fuse doesn't blow with the tubes out it indicates a problem around or in the tube.  It could be a shorted tube or a problem with the plate or cathode circuit.

i am having the same issue as the op. checked my wiring 4 times and resoldered all wires to speedball boards twice. blew 4 fuses in the process. just tried turning on the amp without the tubes and left it on for 4 seconds. fuse blew again. will post pictures tomorrow but any specific areas i should look at? i had the amp working just fine without the speedball kit.

[Grainger49]

Blowing fuses without the tubes in can indicate that one or more of the diodes is reversed, one or more of the capacitors is reversed.  It could also indicate a short from a clipped lead landing in the wrong place.  Or none of the above.  But check these things first.  Best yet get someone else who hasn't been staring at it for days to check it for you.  Sometimes you just can't see the problem because you have been looking at it so long.

[crappyjones123]

well the amp was running fine earlier without the speedball kit so i dont think the diodes are the issue or the caps for that matter. will ask someone else to take a look at the wiring tomorrow.

here is what the big speed board looks like. i wonder if the 2 deviations i took are causing the fuse to blow. instead of the two wires connecting the B+ and G pads on top of the board, i wired them under the board and instead of using a jumper, i created a solder bridge between the two G pads on top of the board and the same for the B+ pads.

is that where im going wrong?

[crappyjones123]

for some reason i cant post the picture in the above picture...

here is a direct link.

http://img816.imageshack.us/img816/7381/p9100517.jpg

[Grainger49]

Since I don't own a Crack (Doc made me say that) much less the Speedball I can only conjecture from general electrical knowledge that you might remove the power from the Speedball and give it a try again.  It should hold.  If there is no power going to the Speedball and it blows the fuse, then there has been a problem created somewhere else.  But it shouldn't blow the fuse. 

If the fuse blows then the problem is very likely on the Speedball board.

[STURMJ]

What are the values of the resistors in the R1 position on the large board?  I cant tell by the pict. but on mine the R1 positions on the large board has a smaller resistor  (1/8? watt 31.6 x2). Parts may have changed, but that's what stood out for me.

[crappyjones123]

the resistors are 33ohms. the kit does come with 31.6ohm but long story short, i screwed something up and had to use the 33ohms. the 31.6 ohm resistors are otw. the only other thing i can think of i might have done incorrectly are the MJE350s on the smaller boards. i didnt get either with the metal backs (mine have all plastic enclosure) so i checked online to find out which the "back" side was. the way they are soldered in right now - the face that reads mje350 (and has 3 circles) faces away from the leds on both boards. did i get that backwards?

[InfernoSTi]

Here is the proper orientation for the MJE 350's without a metal plate.

The great parts famine of 2010 has forced us to look for substitute parts for many of the kits we manufacture. These substitute parts are of equal or better quality, but some look rather different than the original parts we used in producing the assembly manual. The latest case is one that will cause some confusion -

For the past 14 or 15 years we have used MJE 350 transistors that have a "front" black plastic side with the MJE 350 designation printed on it. The other, "back" side of the transistor is metal.
The current batch of Fairchild transistors we purchased do not have a metal side. One side is printed with the MJE 350 designation, and this corresponds to the "front" side of older transistors. It can also be identified by the three small dots imprinted in the plastic around the center hole. The "back" side, that corresponds to the metal side on the old transistors, has a more matte finish with no printing except for one small dot impression.

[STURMJ]

33 ohm should be close enough (4%) Everything looks good from the top side. I guess the best thing is to be sure that the wiring is correct.  Are you using .5A fuses or 1A fuses? I had this problem with a early edition of the crack (see my previous thread http://www.bottlehead.com/smf/index.php/topic,677.0.html ) maybe that's it?

[crappyjones123]

hmm, i grabbed a handful of 1/2A fast blowing fuses from radio shack. should i get 1/2A slow blow or 1A slow blow now? wonder if that is all that is the issue...

also, the orientation of mje350s is correct in my build in accordance with the post. rechecked all the wires and solder joints again. everything is going where it is supposed to go.

thanks again for all the help guys.

[STURMJ]

I would go 1A fast since .5 seems to be the intended fuse,  and it dose not seem to be quite enough (forgive me if I'm wrong). The 1A gives more room, but the fast blow will die faster than slow, if something is REALLY wrong, which has to be a good thing.

[Doc B.]

should be 1A slo blo

[crappyjones123]

running the amp with .5A slow blow. everything works and sounds great. dismantled and reassembled the whole thing 4 times for nothing >< hehe. the speedball kit really does help. moar impressions later :p need to spend the rest of the day listening to some toones. man this amp sounds great.