Blown fuse after modest mod

[SpeedyDad]

I am finishing up my Crack, and as I had planned installed a choke and a 2.2uF cap in the filtering section of the PSU. When I had everything in place, I plugged the unit in but found the fuse blown. Here is what I did:

Because I replaced all of the screws and installed aluminum spacers for the choke I ran a complete resistance and voltage check before working on any of the parts. All was good and well within range just as before.

I then tried out the choke and the cap individually with alligator clips. I desoldered one end of the wirewound resistor that runs between 15U and 21U and plugged in the choke just to compare it to the resistor. The choke worked well, no issues. I went back and forth a few times to see if I can notice any differences. Not many, but modding is only partly about the sound.

I connected the resistor again and then bypassed the last filtering cap with the 2.2uF cap. Again, with alligator clips, no soldering. All good, no issues. Probably not much of a difference in sound but I really do like the yellow color of the cap.

Then I ran both the choke and the cap together, still no soldering. All good. Crack sounded great. I took that as a "go" and installed the choke on the spacers (choke is nicely grounded, I sanded the underside to the bare metal) and soldered in the cap. But when I plugged the unit back in, I did not hear anything coming through. Fuse was gone.

My theory: I plugged the unit in and out of the wall each time I made a change (more than 10 times), and that might have been too much, assuming that the choke despite a similar electrical resistance (it's 20 Ohms lower) might draw a little more current than the resistor. So, at one point, the fuse blew. I ordered fuses, and if I am not wrong, I could simply replace it with maybe a 1.5A or 2A fuse.

During my trial runs I noticed that when the tubes warm up, one channel leads and then the other comes up a few seconds later. I did not notice that before, might be unconnected. Otherwise, there was no smoke, no crackling, nothing.

Thoughts on how I should proceed?

[Paul Birkeland]

Where you have mounted the choke is not a particularly wise choice.  It blocks access to the 9 pin socket, prevents you from installing the Speedball, and raises the potential for a higher noise floor. 

[SpeedyDad]

I thought of all of that when I picked that spot. The Speedball also blocks access to the 9 pin socket, and I am not sure I want or need the Speedball at this point. If I get the Speedball, the choke probably goes anyway, and it's an easy uninstall. The noise floor I wasn't sure about either and just thought I try it out. A few people have the choke in that spot or area and didn't seem to have any problems. When I listened, the background was as black as without the choke. I also thought of installing the choke on the side of the plate but these holes were already there, I just had to widen them a tad.

[Paul Birkeland]

Hmm, I've never seen it mounted there, so far from the power supply.

While the front Speedball board does sit above the 9 pin socket, if you need to get to it you can just unscrew it and tilt it up.  The amp can even be run that way for diagnostic purposes. 

The presence of the choke shouldn't affect the fuse, but if anything on the 9 pin socket is touching the choke mounting hardware, you could create some shorts.

[SpeedyDad]

I am not sure where I saw it, please don't make me find that image. It might not come through in my photos but the choke sits on a metal plate which can be unscrewed very easily. It just takes a minute. However, if you recommend I move the choke closer to the transformer, I can do that.

Do you suggest I replace the fuse and try again? I saw that everything between 1 and 3 A is fine, should I go with something higher than 1A?

I did not see any sparks or any other evidence of the socket touching the choke--is it possible my repeated trials could have caused that?

Thanks a lot!

[Paul Birkeland]

Sure, a 1-3A fuse will do the job.
 
I'd move it back over the PT if at all possible.

[SpeedyDad]

All right. I'll be back when I have the new fuse. I so tried to avoid drilling more holes for the choke...

[Paul Birkeland]

You can get some 8-32 standoffs and use them in place of two of the transformer nuts, especially if you get steel standoffs. 

[Tom-s]

Correct me if i'm wrong. But you have a capacitor running from the rectifier bridge to B+? 

Edit: I'm not PB. But do NOT try again to power it on; without double checking this.

[Paul Birkeland]

Correct me if i'm wrong. But you have a capacitor running from the rectifier bridge to B+? 

It's the ground out of the bridge and the last B+ node in the power supply.  It's kosher. 

[SpeedyDad]

That's how all caps are running. I am bypassing the last cap in the power supply, meaning it is parallel to the last 220uF cap. One side goes to PSU ground, the other to B+. It looks a bit odd but if you follow the wires, that is how the cap is connected. I had the Crack running with this, and since the caps are all in a DC circuit they can't short anything. If I am not mistaken.

[Tom-s]

Took me some time to get the picture straight. Thanks for educating me!

[SpeedyDad]

I installed a new fuse (1.5A) with no issue. It did not blow again. Now that my build is pretty much complete (except for moving the choke), I found two minor issues that I would love to get some input on (if this is the wrong place, please let me know):

1) I can hear a slight 50Hz hum when I crank the volume up. I first thought that the choke, its position or even the cap somehow pick up some AC but after bringing everything back to stock (removed the stands, wires, everything) it was still there. It goes away completely when I plug in my CD player or when I simply short the inputs. Even if the volume is at 100%, no 50Hz buzzing. So, I assume that the issue is not the amp rather than something it picks up. Would shielding the input wires have any effect?

2) I feel there could be more gain when I use my iPhone as an input. At 100% (phone at 100% as well, no volume limitation) I am slightly above my usual, comfortable listening level but not a lot. When I play from CD, I cannot go all the way up. I wish I had a little more room when playing from my phone. Is that common?

Lastly, I am attaching a little image of my final build. It doesn't come through in all the technical questions and discussions, but this was a lot of fun, and I am really amazed by the support and help you provide. I spent a good amount of time one the Crack and really love how it turned out.

[Paul Birkeland]

1) I can hear a slight 50Hz hum when I crank the volume up. I first thought that the choke, its position or even the cap somehow pick up some AC but after bringing everything back to stock (removed the stands, wires, everything) it was still there. It goes away completely when I plug in my CD player or when I simply short the inputs. Even if the volume is at 100%, no 50Hz buzzing. So, I assume that the issue is not the amp rather than something it picks up. Would shielding the input wires have any effect?

Without shorting plugs or a component picked up, you're just amplifying ambient noise.  This is normal and does not require any remedy.

2) I feel there could be more gain when I use my iPhone as an input. At 100% (phone at 100% as well, no volume limitation) I am slightly above my usual, comfortable listening level but not a lot. When I play from CD, I cannot go all the way up. I wish I had a little more room when playing from my phone. Is that common?

Usually we hear the exact opposite, that the Crack has far too much gain.  An external DAC for your iPhone should give you more output.

Lastly, I am attaching a little image of my final build. It doesn't come through in all the technical questions and discussions, but this was a lot of fun, and I am really amazed by the support and help you provide. I spent a good amount of time one the Crack and really love how it turned out.

Those are some solid color choices.  We'll be here if you run into anything else.

-PB

[Deluk]

Is your iPhone one that has the output limited by default, "save your ears mode"? Might be able to bypass that somewhere in the settings