Blowing fuse at startup

[dujuarez]

I finished building the AB763 single channel and had it working fine. I noticed that when I increase the reverb pot it started getting low feedback sound. In the forum some had suggested to run the shielded cable from the reverb pot to the reverb tube. I did this and it didn't change. So I thought just put it back and live with it. This is when I started getting blown fuses on start up. I unsoldered the wires going to the pots and check underneath the board, I did find a exposed burnt wire. The wire running from the intensity pot to the center leg of the bias pot. I replaced it and put insulating fiberboard on all three boards.(main board, filter board and solid state rectifier board)
I need help as to what procedures I can follow to try and determine where the short is.




Thanks a lot.

[dpm309]

You need to build a light limiter to use to find the cause of the short. You can find information on how to build and use one on this forum.  Plug amp into the light limiter and turn it on without any tubes installed.  If the light glows bright, you still have a short somewhere in the power supply, OT or PT. Try disconnecting these one at a time to see if one of these is the culprit. If the light limiter does not glow bright, try adding one tube at a time. I have always used a Light Limiter on repairs and new builds and it does a great job finding out where the short is.

Good Luck

[shooter]

determine where the short is.

I'm guessing after the repairs fuse still blows?

powered down and discharged, pull the power tubes, do an ohm check of each power-rail tap, including the bias tap.  There should be many thousand ohms, the caps will start charging as you do this, if there is no dead short.  If you have a tap that only yields a few ohms and no charging, that's probably a good place to start.

[PRR]

Welcome.

> burnt wire. The wire running from the intensity pot to the center leg of the bias pot.

That will turn the power tubes FULL ON about 10 seconds after switch-on (or instantly if you warm up on standby and then switch to Play). Yes, this can blow fuses.

Is it still blowing after your repairs?

[dujuarez]

Yes, still blowing fuses after repairs. Following dpm309 advise about the current limiter. I have one, just never knew how to use it to troubleshoot. I have removed tubes and still getting bright light from limiter. Will disconnect output transformer and test. And do the same with the power transformer.

[kagliostro]

Did you pull also the rectifier ?

Franco

[dujuarez]

Sorry, I haven't answered anyone with what is going on. I went to try procedure using the current limiter and the bulb blew. I hope it's not related to the amp. I usually use a 100 watt or 200 watt bulb, but it's getting hard to find them. Everything is LED or some type of low wattage. Now that I brought that up, can I use anytime of bulb as long as it draws enough wattage? And what wattage would that be?


Thanks.

[jjasilli]

Suggestions:


1.  Get a circuit breaker (or a few of them for likely values) for amps under test.  It's frustrating to blow fuses; you can blow them faster than you can buy them.


2.  When a few likely diagnostic approaches don't work, use a Flowchart.  E.g.: http://geofex.com/ > Tube Amp Debug Page > Pops Fuses; Tom Mitchell's "How to Service Your Own Tube Amp" has great Flowcharts.


3.  Always suspect tubes first.

[PRR]

> Everything is LED or some type of low wattage.

It MUST be incandescent. ("Halogen" is also incandescent.)

Here in Maine USA I can readily buy plain 150 Watt incandescents. (They evade the rules on =<100W lamps.)

I can also buy a "90W Halogen- replaces 100W" bulb. I pay more, but it is also a good porch-light, and saves 10% electric in that application.

Over in the Work Light aisle I can get a worklight with a 150W halogen. (Beware: they go to over 500W!) The replacement tubes are affordable-- good because they don't last long.

[dujuarez]

I'm hoping someone will see my reply since I haven't been online with the holidays, etc. But here is what I have done. Upon using the current limiter and not disconnecting anything in the circuit, I get brightly glowing bulb. But I noticed that the indicator lamp doesn't come on. I then disconnected the output transformer and still got the brightly glowing bulb and no indicator light showing. Next I disconnected the 2 red secondaries and tried the limiter and didn't get glowing bulb, but the indicator lamp started working. (I forgot to mention that I'm using solid state rectification) Am I right to believe that the problem is somewhere after the 2 red secondaries and is in someway related to the heater wires?

[sluckey]

You're half right. The problem is somewhere after the red secondary wires (shorted diodes, shorted filter cap, short on the B+ line, choke, etc.), but it's not likely related to the heaters. It is probably related to the work you did with shielded wires to the reverb. It may be something that happened accidentally while doing that work.

[dujuarez]

sluckey,


What procedure should I take from here? I can go over the wiring again, which I did again before starting the current limiter procedure. I'm not sure how to check for shorted diodes. I can also go over the work I did with the shielded wires on the reverb.

[sluckey]

Most digital meters have a diode check function. The procedure can vary slightly with different meters. Read your manual. If it doesn't make sense to you just replace the diodes. They're not worth 50 cents of worry.

If diodes are not bad then measure resistance from your B+ line to chassis. Should be fairly high.

Your tubes do light up with the red wires disconnected, right?

[dujuarez]

Sluckey,


Yes all tubes light up. Do I measure the B+ as i have it now disconnected? Or do I reconnect to solid state rectifier?

[dujuarez]

Measuring without the B+ connected to solid state rectify, I have 40.6 ohms on one line and 43.7 ohms on other line.

[sluckey]

Leave the red leads disconnected while checking diodes and B+ rail for shorts. I'm asking you to measure RESISTANCE, not voltage. Turn the power off. Unplug the AC cord. ETC.

Measuring without the B+ connected to solid state rectify, I have 40.6 ohms on one line and 43.7 ohms on other line.

This sounds like you are measuring the resistance of the transformer red wires??? That's not what I mean.

[John]

    Sluckey said:

It is probably related to the work you did with shielded wires to the reverb. It may be something that happened accidentally while doing that work.

I have overheated that shielded cable more than once, thereby making contact between the shield and the conductor wire inside. Snip off the braided shield where you have it connected to ground and see if that stops the problem. Or if it's easy, unsolder just in case that's not the problem.