Stumped! Mojotone 18watt Tmb blowing fuses

[Martinoils]

Here’s some photos of my18 watt Tmb that I just finished. I have built one before this.
Problem is that once I install a power tube into either so socket it blows a fuse, well I am using a light bulb limiter so the light burns bright, so same thing
I can’t find anything wrong with the wiring but maybe a set of fresh eyes will.
I would like to post some voltages but should I take them without any tubes or with the rectifier or rectifier and preamp tubes? By the way I am sure that the tubes are good . I once had a similar problem before . Amp would blow a fuse on start up with the rectifier installed, took awhile to figure out that it was a bad socket, could I have the same problem here?
A bad output tube socket?
Thanks I’m advance for any help, much appreciated![/img][/img]

[Martinoils]

Here’s some photos of my18 watt Tmb that I just finished. I have built one before this.
Problem is that once I install a power tube into either so socket it blows a fuse, well I am using a light bulb limiter so the light burns bright, so same thing
I can’t find anything wrong with the wiring but maybe a set of fresh eyes will.
I would like to post some voltages but should I take them without any tubes or with the rectifier or rectifier and preamp tubes? By the way I am sure that the tubes are good . I once had a similar problem before . Amp would blow a fuse on start up with the rectifier installed, took awhile to figure out that it was a bad socket, could I have the same problem here?
A bad output tube socket?
Thanks I’m advance for any help, much appreciated![/img][/img]

[mac dillard]

try one power tube at a time and see if it just one tube. Then with power off, check each pin to ground, check the heater wiring too. Does it blow the fuse with no tubes installed just the El 81 running?

[Martinoils]

I tried different el84’s that I know are good first in one socket V4 then in V5 it blew in either one so it’s not a bad socket or tube . Fuse does not blow with rectifier and preamp tubes installed.

[mac dillard]

measure the resistance from pin three of the power tubes to ground.

[Martinoils]

So clip the black lead on my DMM to the chassis and the red tip to pin 3 of the power tube sockets ? Should the amp be on or does it matter?
Thanks

[Martinoils]

Hi again, ok 129.4 on both pins with the amp off

[mac dillard]

Does the fuse blow when on stand by?

[Martinoils]

It doesn’t blow while in stand by but as soon as I flip it out of standby it blows
Thanks for your help

[mac dillard]

That makes me think you have something going on around pin 2. Not sure how it could be the same with both tubes though...I'm not much help. Just working to get you to the old divide and conquer to see which area the problem is in.

[Martinoils]

I appreciate any help you can offer
I’ll check that area closely
Thanks again

[JMPGuitars]

I didn't look at all your photos, but this one stands out:

Screenshot 2023-07-23 232851.jpg

There's a lot of issues in the build, but from what I looked at, that looks the most likely cause.

I abhor Mojotone's builds. the threads in my signature if you want to improve some of their problems.

Best of luck.

Thanks,
Josh

[Martinoils]

It looks like it’s shorting out on the chassis but it actually has plenty of clearance. I’ll probably redo it once I get the amp going. I appreciate your expertise and any advice. For now I want to solve the fuse blowing problem first then I’ll see where I go from there. I have built this one before and it worked out well so I know that it is possible.
Can you suggest the best way to check voltages? Tubes in or out? Etc
Thanks again for your reply

[Daviedawg]

My routine for this would be to go through the heater circuit first then the power distribution looking for a wrong connection, a short to ground or a "whisker" on the valve bases. At the same time check the components for open circuits. A failed valve base is possible but less likely than a mistake or a bad component.

Dd

[Martinoils]

Thanks DD. Good advice. I am going to check components

[JMPGuitars]

It looks like it’s shorting out on the chassis but it actually has plenty of clearance. I’ll probably redo it once I get the amp going. I appreciate your expertise and any advice. For now I want to solve the fuse blowing problem first then I’ll see where I go from there. I have built this one before and it worked out well so I know that it is possible.
Can you suggest the best way to check voltages? Tubes in or out? Etc
Thanks again for your reply

Not the top resistor, the bottom resistor with the gold band pointing down doesn't look like it has any clearance.

Per Dd's suggestion, do a full highlighter test checking for component values, accurate connections, and any shorts.

[Bieworm]

Josh is right. It looks shorted to ground on that resistor. But pics can be deceiving. Nevertheless check all connections in the B+ line for shorts where there is no direct ground connection

[mac dillard]

If you are going to try to check voltages, check pin 2 of the power tubes for DC voltage. Can do this with or without the tubes installed.

[JMPGuitars]

You can't check voltages with a blown fuse.

[mac dillard]

he can check for DC at pin 2 without the power tubes installed. He has said that it does not blow fuse with just the preamp tubes installed. That pretty much rules out direct short in the power rail and the heater circuit. He measured 129 ohms pin three to ground so the cathode bias looks good. Makes me think that he has bypassed the caps before the grids letting high voltage DC into the grids. Just another place to check for the problem.