New build troubleshooting help please

[Matt72]

Well I finally got my express clone done...but it's DOA!

I'm more of an amp assembler than anything, so I'm pretty clueless about troubleshooting. So when I turn on the standby the fuse blows. Take out the tubes, fuse blows. I disconnected the filters and the fuse held with the standby 'ON'. When I reconnected the filters but left the 2 before the standby switch disconnected, the fuse held with standby 'ON', so it seems the problem shouild be in that area? But everything looks fine, nothing touching things they shouldn't.

Any suggestions at what to look at next? I don't see any shorts.

Everything looks wired up right, I've rechecked several times. This is a build with Ceriatone's chassis and board and Moose transformers.

One thing that might be weird, I get continuity to ground at the junction of the 27k and 15k resistors in the bias supply. Is that normal? From the schematic, it doesn't look like that should be?

Thanks a million!

[paddy]

Hi Matt,

The junction of the 47k and 15k bias supply resistors should NOT be at
ground potenetial.It is very important that you find out why this point
is at ground(incorrect wiring,shorted capacitor?)
With the junction of these resistors at ground,you will have no bias
to the output tubes and they will live a very short life.

good luck
paddy

[Matt72]

Aha! Thank you I'll check into that.

[joe6v6]

Matt - There will be 47k ohms + whatever your bias pot is set to at that junction to ground, probably 52-56K. Can you post a hi-res picture? You need to carfully go over your wiering & check with the layout, you have something grounded that shouldent be. Its good news that with the caps disconnected the fuse dosnt blow, that means your transformer isnt shorted.

JOE

[Matt72]

Thanks, I really appreciate the responses!

So it turns out that on eof the bias filter caps was bad, so that's where the ground connection was coming from there.

But it still blows the fuse when I switch off standby with those 2 filters before the standby connected. Everything under the PS board looks fine.

I'm off to Radio Shack to buy more fuses! (I really need the light-bulb set-up I guess).

I won't be able to post pics for a few days. My wife's out of town for the weekend and has the camera (which is why I'm able to work on the amp right here at the computer...she hates it when I do that! Lol).

[Matt72]

So, to help me narrow the search down, where approximately should this short to ground be located?

Fuse blows when:
STBY is turned on if the 2 filters before the STBY switch are cennected to power. The fuse is fine if the filters are connected and the STBY switch is off.

Where should this short be then?

Thanks again!

[Matt72]

Ok, my meter beeps (continuity) when I check the 300v PT 2ndaries to chassis. Same when I check the heaters on the power tubes. Is that because the center-taps are grounded?

Is that normal?

If not I have a feeling some wires are touching the chassis through the hole they enter (I had a heck of a time stuffing them all through there. There is a grommet, so it seems unlikely...

Also I wired in all new filters, so that's not it.

[Shermock]

Hello!

Have you checked those first 2 filters? Are they shorted to ground? It seems like you've narrowed it down to the fact that those 2 filters, when included in the circuit, short power to ground and blow the fuse. Unless you've checked that already...then disregard this post!

Shermock

[Matt72]

Thanks Sherm,
Yeah, I wired in ALL new filters and the dang fuse still popped.