Blowing Fuses

[EETStudent]

In the schematic, locate the 2amp fuse off of the conveniance outlet. For some reason the fuse all of a sudden blew. I had blown the other fuse before, and that was due to the B+ rail shorted to ground. After much extensive looking and prodding, I have yet to find any shorts or any suspects for blowing this fuse. Given its location, what are some areas that might be causing it to blow?

[Don Moose]

So, assuming you don't actually mean the 3A fuse connected to the convenience outlet, the only suspicious looking thing I see is S1B - position 3. It took me several looks to convince myself that wasn't a dead short across the line in that position.

C26 could be at fault if it failed short.

Failing that, work your way through R.G.Keen's most excellent Tube Amp Debugging page. Step by step, checking even the obvious things (especially the obvious things).

If it WAS the 3-amp convenience outlet fuse, how did you know it let go (what were you doing with it)?

[EETStudent]

Ah figured it out. A bad, radioshack recitifer.

[Enzo]

The 2A fuse is the amp main fuse. The convenience outlet is irrelevant.

My first two suspects would be the power tubes. Also fuses can fail from age.

[Enzo]

Oops, simulposts.

[EETStudent]

lol thanks Enzo. While we are the subject of diodes, what are the differences between 1n4001 through 4007 diodes?

[Enzo]

May I suggest looking them up? Most part specs are readily available either online or in printed material. When I encounter a part I don't know, often as not if I just google the part number a data sheet pops up in the listings. For example, this is hte first listing on google for 1N4007:

http://www.tnw.utwente.nl/onderwijs_...ten/1n4007.pdf

The difference in the parts is the reverse voltage rating. 1N4001 is a 50v part, while 1N4007 is a 1000v part, and the others of the series fall in between. Otherwise they are the same, adn y0u can always use the higher voltage part in place of the lower voltage ones. SO I stock 1N4007 but not the 1N4001, since the 4007 will work everywhere the 4001 does plus higher voltage applications.

[EETStudent]

So if I used say a 1N4004 diode in place of a 1N4007 diode would I achieve lower DC voltage safely?

[Enzo]

It depends on where you use it. If you are working with a 9 volt battery circuit like in a distortion pedal, then it hardly matters if the diodes will take 400 or 1000 volts, they never see more than 9. On the other hand, if you have a 500v circuit, the 1N4004 at 400 volts would not be a safe choice, while the 1N4007 with 1000v rating would be fine.

And if you had a 400v circuit, theoreticaly the 400v part might work, but there is zero room for error. SO you'd use the 1000 volt part.

[EETStudent]

Reason I ask is im trying to reduce the overall B+ in my amp. Its sitting around 465, and the only way I knew is to introduce a 'sag' resistor. Ive got some 4005s and 6s, would those bring it down?

[Enzo]

But they are not resistors, each one would reduce the B+ by about half a volt. How many you plan to mount in there? 50?

However, even if your 1N400x was rinnung from one point to the next with only 40 volts difference, you still need to use parts that can accomodate the entire B+. Uncharged caps, for example, look like a ground for the first moment, so potentially the diode could have B+ on one end and zero volts in a discharged cap on the other.

4005 - 600v
4006 - 800v

They would work fine in any guitar amp application, at least with your B+ levels.

[Jag]

Like Enzo said, unless you use a whole swack of them, those diodes aren't going to do much to bring down B+. The voltage rating on those diodes is a maximum not how much they will drop. I wonder if you're confusing this with the Zener method of dropping B+?