Marshall 18 Watt Build blowing fuses

[SeaTea1967]

I finished a Marshall 18 Watt build this past weekend and have been playing the amp for a couple of days. Tonight I turned the power switch on and waited for the amp to warm up as usual. When I switched on the standby switch the fuse blew. I swapped out the fuse thinking it might have been damaged in shipping or something and the fuses now blow as soon as I turn the power on.

I've removed all the tubes from the amp and the fuse still blows as soon as the power is turned on, so it's not a shorted power or rectifier tube. Nothing I can find on troubleshooting has anything beyond "check the power and rectifier tubes". I'm beginning to fear it might be a blown transformer.

Any thoughts on how to track this problem down? I can't do much troubleshooting if I can't even turn the amp on.

[elronhoover]

Is it a ceriatone kit? Do you have the recto wired with the diodes and cap across 1 & 7?
If so, try floating (and insulating) the hv secondaries from the standoff. If it's still blowing, you may be correct..

You might want to check all of your filament socket wiring also, verify you dont have any solder whiskers etc. that may be shorting..

Dave

[SeaTea1967]

Is it a ceriatone kit?

Yes.

Do you have the recto wired with the diodes and cap across 1 & 7?

Yes, I followed the TMB layout almost exactly, except that I used a terminal strip to wire up the diodes so that they weren't directly on the tube socket.
http://www.facebook.com/photo.php?pid=5 ... id=8648581
http://www.facebook.com/photo.php?pid=5 ... id=8648581

If so, try floating (and insulating) the hv secondaries from the standoff.

I'm not quite sure what this means. Could you elaborate?

You might want to check all of your filament socket wiring also, verify you dont have any solder whiskers etc. that may be shorting..

Good advice and I'll do that, but it strikes me as odd that it was working fine for several days, then stopped working. One possibility I thought of last night was that I might have wired the 4/8/16 ohm selector backward and had it set to 4 ohm when I thought it was set to 16. The speaker is 16 ohm, so even if that were the case I wouldn't expect it to cause trouble... but I'm using a Magenetic Components http://store.triodestore.com/outrfor18wam.html OT and I don't know how sensitive they are.

Will check that wiring possibility tonight after work.

I measured the resistance between the OT line going to the filter cap and the lines going to the power tubes. In each case it was 1.8k ohms, which the ampage debugger says indicates the OT is fried.

If it is, I definitely want to figure out why it fried so that I can avoid it after installing a new one.

[elronhoover]

re. floating the hv secondaries, I meant to just unsolder them from the diode points and tape them or whatever to insulate individually, then see if the fuse blows. Eliminating the recto socket and those components from the equation.

You can do the same thing with the filament wiring strings too, just to eliminate them. A solder whisker etc. on a socket could be contacting/shorting intermittently.

Same goes for the indicator bulb (it's 6.3vac right?). Those have been known to cause problems too, so you could not only check that out, but also remove it from the sec. load completely and test.

At that point, you would have all the PT sec. wires up in the air and fully isolated, i.e. everything on the chassis is fully disconnected from the PT sec., as far as checking for blowing fuses. If you step thru it as above, you may be able to isolate a short/fault, provided the PT itself is not the cause. BTW, what kind of PT is it, mag comp as well?

RE. the impedance mismatch, running a 4 ohm OT sec. into a 16 ohm load, especially if you have it cranked up, could definitely cause you OT problems IMO. Lots of folks don't seem to have any issue with doing this, but again, just my opinion, it's not good to do. Just as an aside, check your power tube sockets and look for any possible arc traces between pins.

RE. the OT pri. R measurements, were the power tubes pulled, and the CT unsoldered from the filter cap when you measured? Seems like high R there.



Dave

[SeaTea1967]

re. floating the hv secondaries, I meant to just unsolder them from the diode points and tape them or whatever to insulate individually, then see if the fuse blows. Eliminating the recto socket and those components from the equation.

You can do the same thing with the filament wiring strings too, just to eliminate them. A solder whisker etc. on a socket could be contacting/shorting intermittently.

Same goes for the indicator bulb (it's 6.3vac right?). Those have been known to cause problems too, so you could not only check that out, but also remove it from the sec. load completely and test.

Got it. Seems sensible. If the OT is indeed blown, however, won't that still be a problem since it's in the circuit? I guess at that point it would, indeed, narrow it down to either the PT (internal short) or the OT.

BTW, what kind of PT is it, mag comp as well?

Yes, I bought the set.

RE. the impedance mismatch, running a 4 ohm OT sec. into a 16 ohm load, especially if you have it cranked up, could definitely cause you OT problems IMO.

Definitely was not cranked up. The thing is indoors and LOUD!

RE. the OT pri. R measurements, were the power tubes pulled, and the CT unsoldered from the filter cap when you measured?

No. I can try it again unsoldered to see if there's a difference.

Thanks for the tips! I think I'm going to have to buy a whole lot of fuses to figure this out.

[Omn]

I think I'm going to have to buy a whole lot of fuses to figure this out.

Buy slow blow fuses... In case the ones you have been using was the "faster" ones.

[SeaTea1967]

FIXED! Woo!

Yeah, disconnecting the diodes made it start working again, so I looked very closely and saw a small bit that was grounding a connection to chassis near the diodes. Cleaned everything up to get all connections away from the chassis and it's all working again. Must have shifted very slightly when I hit the "standby" switch originally.

Many, many thanks for your help!

[elronhoover]

FIXED! Woo!

Yeah, disconnecting the diodes made it start working again, so I looked very closely and saw a small bit that was grounding a connection to chassis near the diodes. Cleaned everything up to get all connections away from the chassis and it's all working again. Must have shifted very slightly when I hit the "standby" switch originally.

Many, many thanks for your help!

Excellent, go and rock that thang!

Did you get your imp. selector sorted out?

Dave

[Metal Handles]

Bump for reference.