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fuse trouble

1/15/2004 2:42 AM
fuse trouble
im working on my 2nd amp-its a 100w -4 EL34 Marshall i finally powered it up.My mains fuse is blowing.It seems fine down the circuit until i hook up the second side of the choke.When i do,it pops the fuse.  
1.does it matter which way the choke is hooked up? getting 700 vdc at the output of the rectifiers--does this seem high?(trans is a Hammond 273bx).If it is whats the best way to lower the voltage?  
3.when i first clicked on the Standby,i heard a "crackling" sound near the filter caps and power trans--i very quickly shut it down.-now when i measure the first 50/50 cap with my cap meter, im getting a "dead" cap.However ,this cap is NOT in the circuit when the fuse is blowing now.  
This power section is exactly wired like a Marshall 2203. Any ideas ???
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1/15/2004 5:01 AM
It sounds like there must be a short downstream from the choke.  
At this point all I can say is that somethin' ain't right in there. You'll have to keep looking until you find it. I can't even count how many times I had a circuit wired "exactly like...") and I had something wrong. With the fuse blowing, it's a searching game. But keep looking, the problem is right in front of you.  
You'll want to disconnect the PT secondaries and check them to make sure everything is still working. The crackling sound is a bad thing. Not real specific I know. Was there any smoke?  
Recheck everything in the B+ rail. Caps in proper polarity, standby switch wiring, grounds, etc...  
Your voltage is WAAAYY too high. Change from a full wave bridge to a full wave center tapped rectifier arrangement. With your PT that should supply around 450v. A full wave ct arrangement is like the one in the Traynor schem I gave you or the Marshall schem that 6 10 gave you in your "Tranny" post.  
That PT is a little small. When you get up and running I would suggest running on two output tubes when you can get away with it.  
1/15/2004 5:25 AM

no smoke--just that sound.i shut down as fast as possible.but after testing,the first filter cap(50/50) is showing as bad on my cap. meter.but...its not in the circuit now and the fuse is frying---i will go through the amp tomorrow
1/15/2004 7:30 AM
Yeh, You gotta change to a FWct rectifier too. With the FWB your actually trading some current capacity for higher voltage. There's no possible way that PT can handle it. Recheck the bias supply. Make sure all your diodes and caps are in correct polarity.  
If DT says that tranny will work for your project, it probably will. He's worked alot with the guys at Hammond so he may know that it's underrated by enough to fuzz the margins on it's specs. He's probably even used that PT for similar projects. His semantics and claims have been dubious at times, but he makes pretty good amps and mod kits. And to my knowledge he's never sold a part that wouldn't at least work when he said it would.  
1/15/2004 9:28 AM

Thanks for the help chuck-(sorry for all the questions-i just want to get the amp right!)--so if i go with that FWct rect--do i need to stick with the cap/resistor values(before the fuse) like the traynor?-or can i still use the Marshall values?  
When i go to the FWct am i going to be ok then with the Hammond?Im going to run the 4- el34,s and i really dont want to "push" anything(the hammond..)
1/16/2004 2:52 AM

I went through the entire amp today--everything looked correct except one resistor--it was a 10k on the cathode of the phase inverter(should have been 1 meg......oops..)-i hope that was all that was causing the fuse to blow.
1/16/2004 6:50 PM
That shouldn't cause the fuse to blow. I would suspect an error in the B+ rail. Check for a short after the choke.  

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