Finished my kit! Exciting. But a troubleshooting question....

[lmalinofsky]

It was with great pleasure I soldered the last capacitor and put down my soldering iron. I immediately began checking resistance values and DAMN!  They were **all** well within allowable limits.  (I think it was just a quirk of my meter that the values that were starred (*) and supposed to "settle" at certain readings showed as zeros.  When something is erratic or out-of-range that's sort of what my meter does...)

I plugged in the tubes, connected the power cord, and with a lot of trepidation flipped the "on" switch.  The room was too bright for me to see the tubes glow, but I took my first voltage reading, the "HV+" pads, and got 390.  Really close!

Now I was starting to feel smug and went to measure the voltage at Terminal 1-- it came to 414v, which 1s 18% high, but I figured I was OK.

Then--- without warning--- a loud "snap" sounded from somewhere on the board, kind of like the sound a cap gun makes.  I immediately shut down the power and inspected everything but couldn't see any charring or damage.  Hoping it was a one-time thing, I turned on the power switch again and took a reading at Terminal 2-- 248v, very good.

Then--- you guessed it--- SNAP!  Again I turned off the power at once, but saw nothing amiss anywhere.  Just for the h*** of it I went back and re-soldered every joint that even SUGGESTED it might be questionable.  I turned on the amp again, and after about twenty seconds--- SNAP!

So the status seems to be that I can turn on the amp and it will run silently for about 20 to 30 seconds before the loud SNAP!  I figured it had to be arcing of some kind so I checked everywhere anything looked too close to something else.  I saw no black marks that might suggest arcing anywhere on the aluminum board or on any of the wires or components.

I tried again, and --- 20 seconds --- SNAP!

Now, I'm kind of mystified and I wonder whether anyone has run into this, or if not, if anyone has an idea about what could go SNAP! in this way.  I've kinda been thinking I'll try it at night to see if I can spot an arc (and make sure the tubes are glowing).  (They didn't seem to be throwing significant heat or making any electrical hums at all...)

But... any ideas anyone?

Many thanks,

Leon Malinofsky

[Doc B.]

Please don't turn it on again until we sort it out. These circuits are amazingly resilient, but this amp is trying very hard to tell you that something isn't right.

The 20-30 second delay means that the problem is possibly related in some way to the 12AT7 input tube - which takes about that long for the heater to warm up. I know this sounds mundane, but the first thing to do is carefully inspect all the wiring connections and also to check the polarity of the electrolytic capacitors. Also please note one correction to the manual that has been posted here -

http://www.bottlehead.com/smf/index.php/topic,1492.0.html

If nothing turns up after checking those things, let us know and we'll come up with the next step to check out.

[lmalinofsky]

Thanks greatly for the focus on the heater tube.  I'll dissect its connections carefully before re-powering.

BTW, I notice the parts list for the kit says the fuse is a 1.5A "slo-blo" (which I infer is a time-delay, vs. a fast-acting, fuse).  In trying to remove the fuse for inspection I somehow broke it (pulled the metal cap off the end) and noticed the series "GMA" stamped on it.  At the local Radio Shack, they told me the GMA series is a fast-acting fuse, so now I'm trying to figure out which replacement to buy.  A local electronics wholesaler in my area has both the time-delay and the fast acting, so I guess I have my choice..... or if you tell me the fast-acting fuse is OK, then that I *can* get at Radio Shack.

Interestingly, during the SNAP! events I wrote about, at no time did the (fast-acting?) fuse I was using blow.  I guess that makes sense if you tell me something is slowly heating up or charging, slowly enough that the fuse doesn't blow, but pops on reaching a certain temperature or voltage.

Also interestingly, a 1 amp fast-acting fuse I located and tried blew immediately.   Must be an initial surge draw, because the amp certainly had no load.  Oh well.

So let me know what fuse to use, and I will report back after analyzing the heater tube wiring....

Many thanks,

Leon Malinofsky

PS I have to say building the kit was a good deal of fun, and the manual was excellent.

[lmalinofsky]

Doc,

I am happy to report back that I solved the problem (with your help pointing me to the 12AT7 tube).  I noticed that when the circuit board that is fitted over the underside of it was screwed down, the extra pressure from the wires caused two of the tube contacts to short.  My solution was twofold:  I created more separation between the tube pin contacts and decided to leave the circuit board unattached by screws (the wires are stiff enough to hold it in place just fine).

I still wonder about the correct fuse to use, but I am thinking a slo-blo would be better, since the circuit seems to draw enough of a surge on startup to blow the fast-acting ones.

I am most grateful for your attention to my problem and excellent advice which permitted me to solve it so rapidly.  Now that I have listened to the amp on my Klipschorns, I will be posting my (wildly positive) impressions of its sound in a new topic in this section of the Forum.

Incidentally, I noticed your write-up about the power cord kits, and I actually think I can imagine why the amps sound better when one is in use.  I think the smoothing effect obtained from the twisting and braiding makes the sine wave more perfect  (if you could magnify a part of it 100 times and look at it, the scope lines would be smoother or more perfectly regular).  I know we tend to think when we see a sine wave on the scope it *is* perfect as a sine wave, but we

[Doc B.]

GMA does indeed denote fast blo. Sounds like we may have gotten the wrong fuses in. You might try a slo blo of the same value.

[Paul Joppa]

Another note on fuses ...

Most amps (all of ours) have capacitor-input filters on the high voltage power supply. That lead to a current surge as the caps are first charged up, which has the potential to blow fast-acting fuses more easily. So it is often the case that a fast-blow fuse will need to have a higher current rating than a slow-blow in the same amp.

Fast-blow fuses are much cheaper than slow-blow, but the slow-blow allows a lower rating and some (modest) degree of increased safety.

Fuses are far from precision products, so one batch of fast-blow fuses may work fine in a particular amp, while another batch or brand might fail every time. We have in fact had this happen, where our testing led to a particular fuse, but customers who had to replace the fuse would have their fuses blow. This leads to some very frustrating troubleshooting, since you are then looking for a problem that is not actually there!

Finally, another odd note: fuses are made from different alloys, depending on the fuse rating. Different alloys have different thermal mass and hence don't take the same time to blow. In some cases, a larger fuse will blow on inrush surges when a smaller fuse will not. Since different manufacturers may switch alloys at a different point, you can see the potential for confusion. This is much more a problem with fast-blow fuses; slow-blow have at least some requirement to control the thermal mass to produce the right timing.