TROUBLESHOOTING HELP!!!

[skyyamps]

So, I have designed an amp that is similar to a JCM800. I have gotten it all wired up, turned on the power, the tubes heat up, and then as soon as the Standby switch is flipped, I immediately lose power and the 2A fuse it burned out!

I can't find the problem, so I am hoping you guys have some ideas!

I'm attaching the schematics.

Thanks!


PS The Presence Pot on the schematic isn't wired as shown, I just didn't want to dig through my libraries for a rheostat.

[collinsamps]

Your schematic shows a 500ma for the mains fuse, obviously not correct, and you say you are using a 2 amp which would be correct for a 2204.

If the amp is stable with stby on, and you are popping the mains fuse when it's flipped, there is an issues with current draw on the power transformer B+, which may be also tied to the bias tap as it only overcurrents when B+ is activated and bias is pulled from one leg of that even though the problem appears to be downstream of the stby switch.

Pull the power tubes and try it, if it still pops you know it's not a shorted tube. Then look at the rectifier diode arrangement, and make sure it's wired correctly. If that is not the culprit, look at the choke & filter cap wiring and make sure that they are - to ground etc. If the power supply rail checks out continuity wise, then look at your bias cap/diode/resistor/pot network is hooked up correctly(to include + to ground caps).

If you have to, disconnect things down stream and work backwards to the PT until you find it, but continuity checks through the B+ rail and bias supply should find it before having to do that.

The fix should be in one of those.

[skyyamps]

Thanks!

I will try those out and get back to you!

[labb]

Last time that happened to me I had a filter cap in backwards.

[John_P_WI]

Check your bias caps, + to ground, not indicated in the scheme.....

John

[M Fowler]

Reflow your solder joints on the AC wiring switch and standby switch, tighten your ground lug bolts and recheck the grounds solder.

[skyyamps]

Thanks! I figured it out! Two of the filter caps were turned around! I'm feeling like a moron a little bit! Still needs some work, but I'm sure I can figure out the rest now that it stays turned on.

[labb]

Would be a good idea to check the dropping resistors just before the caps. Also the caps that were in backwards..With the 500 ma fuse the resistors are probably ok but you could have cap damage.

[labb]

Just for info: there is a really good start up procedure and also some pretty good trouble shooting tips over at Paul Ruby Amps site. Worth the read I think.

[skyyamps]

Thanks for the tip! Great article!