Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: macfadyb on July 28, 2024, 06:48:41 pm

Title: 6g15 Mojo Build Woes
Post by: macfadyb on July 28, 2024, 06:48:41 pm

After many fruitless hours of close inspection and hair-pulling despair I am here with the hopes that someone might be able to point me in the right direction to getting my Mojotone 6g15 spring reverb build working!

I’ve gone through the schematic and checked all the expected voltage points there, and compared them to my fluke meter readings with all tubes attached.

I have systematically checked every wire, every ground point, every component value 3 times, all parts are correct.

Build photos:  https://imgur.com/a/jSEMmmR (https://imgur.com/a/jSEMmmR)

Schematic (with voltage points), and a chart of my recorded voltages with all tubes in: https://imgur.com/a/ekh0nLo (https://imgur.com/a/ekh0nLo)

Any advice or suggestions would be greatly appreciated, thank you!

Title: Re: 6g15 Mojo Build Woes
Post by: AlNewman on July 28, 2024, 07:16:41 pm

Your 12At7 seems like it's drawing too much current.  With the amp off check resistance to ground from pin 3 and 8.  Also look under board for stray wires or solder blobs.

Title: Re: 6g15 Mojo Build Woes
Post by: SEL49 on July 28, 2024, 07:42:53 pm

Quote from: macfadyb on July 28, 2024, 06:48:41 pm

I have systematically checked every wire,

You missed one wire.

Title: Re: 6g15 Mojo Build Woes
Post by: tubeswell on July 28, 2024, 07:43:10 pm

Can you tell us what the problem is? (Saying it ‘Isn’t working’ is too generic)

Title: Re: 6g15 Mojo Build Woes
Post by: macfadyb on July 28, 2024, 07:56:06 pm

Thank you all for the quick replies, pin 3 and 8 to ground are both around 1.48k respectively, as for actual 'problem' the unit will turn on but no audio will pass, and as for the missing jumper it is actually soldered on the other side of the board!

Title: Re: 6g15 Mojo Build Woes
Post by: AlNewman on July 28, 2024, 08:06:26 pm

Maybe there's something shorted in the socket?
Could be a bad tube...?

Title: Re: 6g15 Mojo Build Woes
Post by: AlNewman on July 28, 2024, 08:12:57 pm

Try pulling the 12At7 and recheck voltages.

Title: Re: 6g15 Mojo Build Woes
Post by: macfadyb on July 28, 2024, 08:26:21 pm

Will try this tomorrow and report back, thanks guys!

Title: Re: 6g15 Mojo Build Woes
Post by: Willabe on July 28, 2024, 08:52:03 pm

Here's a great way to check a new build that's not working;

https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630

Title: Re: 6g15 Mojo Build Woes
Post by: SEL49 on July 28, 2024, 09:02:54 pm

Quote from: macfadyb on July 28, 2024, 07:56:06 pm

as for the missing jumper it is actually soldered on the other side of the board!

Really. I bet you will measure no continuity between the two eyelets that the jumper should be connected.

Title: Re: 6g15 Mojo Build Woes
Post by: AlNewman on July 28, 2024, 09:59:50 pm

Oh, this wire...
Yes that would do it...

Title: Re: 6g15 Mojo Build Woes
Post by: macfadyb on July 29, 2024, 06:53:49 pm

 :worthy1: :worthy1: :worthy1:

Indeed that jumper connection was no good!  Pulled everything out to access the bottom of that board and the jumper was not making proper contact with one of the eyelets.  Reflowed solder joint and all is well.

Thank you all so much for your thoughtful responses!

Title: Re: 6g15 Mojo Build Woes
Post by: HotBluePlates on July 29, 2024, 09:51:26 pm

Quote from: macfadyb on July 28, 2024, 06:48:41 pm

I have systematically checked every wire,

Quote from: SEL49 on July 28, 2024, 07:42:53 pm

You missed one wire.

That, sir, was a savage display of skill!