New Amp Build Thread- Modulus Marshbar

[bfool]

I had the same thing with the picklist on the Marshbar kit. Same as yours I think they went with an older version as I was missing a couple things and had a couple left over components that weren't on the layout. It wasn't a big deal for me and no need for Modulus to ship one resistor halfway across the globe. I either already had stuff on hand or grabbed out of the Frankentone kit and will replace later on my next mouser order.

Cool - I’m in the UK so getting Modulus to post a resistor doesn’t seem unreasonable - but yes shipping one to the US is a bit excessive! As far as I can tell that 220k was the only error on the Frankentone list - I do have a couple of extra components but I think that is because they are made obsolete by the 65/68 mod I included…

 

[MadAsAHatter]

Sorry for the snag.. it will be interesting for a newb like me to find out what turned out to be the issue and how you figured it out.

Something is shorting B+ to ground and probably took out a rectifier diode in the process. That’s my assumption. Outside the obvious that the tubes are all in working order.

@glpg80 You were spot on about the B+ going to ground. Other than the HV fuse popping it didn't take anything else out.

I just got word back from the amp tech (Rob DeAntonio). Turns out the core culprit was an error on the layout. Rob said the layout showed an extra wire that was grounding out the B+ and audio signal. So I followed everything as the layout showed, but what was shown had an issue.

All that needed to be done was snip that ground wire out and replace the fuse that popped. Rob said while he was in there he slightly redid the general grounding so it would be quieter. The layout has everything being grounded near the input jacks. He split it so the preamp & PI side went to the input jack side and the power amp portion now grounds on the opposite side of the chassis.

I'm happy it was something simple, but don't feel too bad about missing what was going on. I don't have enough experience to have known that wire shouldn't have been there in the first place. At least I was able to make it as far as figuring out the problem was on the power amp side.

On a high note Rob gave me really positive feedback on my work. He said everything looked really tidy and all the solder joints were well done. He said if he had to get really nitpicky on critiquing my work; wires on the tube pins were fine, but could have been snipped a little more flush to be 100% they wouldn't make accidental contact with something.

Looks great... Does Modulus provide plenty of wire?

Sorry I missed this question from earlier in the thread. Yeah, Modulus provided plenty of wire.
When running the wires I try to leave them long enough to give enough room to trim back, but not so long to where I'm wasting a lot of it. Doing that left me with probably enough wire to use for a second build.



For anyone who may give this build a try...
1. I'm not sure which wire was drawn in error on the layout. I don't have anything in front of me right now. Once I get the amp back to compare I'll post it.
2. Rob noted the way the presence is designed, it has DC on the presence pot. He said Marshall did it this way on some amps back in the 70's and a few builders still do it too. He said when you turn the presence knob it may crackle a bit like when you have a dirty pot, but this is 100% normal. So nothing bad, just something to be aware of.

 

[jchrisf]

I'm glad you got it resolved @MadAsAHatter and it wasn't anything you did wrong. I'm looking forward to hearing it when you get it back. I've always been a little leery of Modulus and I think I will steer clear of them after reading of your troubles. Their amps look interesting but I've never heard much completely positive about them. In the end you'll have a great amp but it's been a little trouble getting there. With Mojotone, everything was exceptional for me... think I'll just stick with them.

 

[DanTravis62]

@glpg80 You were spot on about the B+ going to ground. Other than the HV fuse popping it didn't take anything else out.

I just got word back from the amp tech (Rob DeAntonio). Turns out the core culprit was an error on the layout. Rob said the layout showed an extra wire that was grounding out the B+ and audio signal. So I followed everything as the layout showed, but what was shown had an issue.

All that needed to be done was snip that ground wire out and replace the fuse that popped. Rob said while he was in there he slightly redid the general grounding so it would be quieter. The layout has everything being grounded near the input jacks. He split it so the preamp & PI side went to the input jack side and the power amp portion now grounds on the opposite side of the chassis.

I'm happy it was something simple, but don't feel too bad about missing what was going on. I don't have enough experience to have known that wire shouldn't have been there in the first place. At least I was able to make it as far as figuring out the problem was on the power amp side.

On a high note Rob gave me really positive feedback on my work. He said everything looked really tidy and all the solder joints were well done. He said if he had to get really nitpicky on critiquing my work; wires on the tube pins were fine, but could have been snipped a little more flush to be 100% they wouldn't make accidental contact with something.



Sorry I missed this question from earlier in the thread. Yeah, Modulus provided plenty of wire.
When running the wires I try to leave them long enough to give enough room to trim back, but not so long to where I'm wasting a lot of it. Doing that left me with probably enough wire to use for a second build.



For anyone who may give this build a try...
1. I'm not sure which wire was drawn in error on the layout. I don't have anything in front of me right now. Once I get the amp back to compare I'll post it.
2. Rob noted the way the presence is designed, it has DC on the presence pot. He said Marshall did it this way on some amps back in the 70's and a few builders still do it too. He said when you turn the presence knob it may crackle a bit like when you have a dirty pot, but this is 100% normal. So nothing bad, just something to be aware of.

Man that would drive me nuts that the diagram they provided was non-working

I'm so stoked this thing is up and running, that head shell is unbelievable aesthetically pleasing

Once you get it plugged in let's get some clips man!

 

[glpg80]

@glpg80 You were spot on about the B+ going to ground. Other than the HV fuse popping it didn't take anything else out.

I just got word back from the amp tech (Rob DeAntonio). Turns out the core culprit was an error on the layout. Rob said the layout showed an extra wire that was grounding out the B+ and audio signal. So I followed everything as the layout showed, but what was shown had an issue.

All that needed to be done was snip that ground wire out and replace the fuse that popped. Rob said while he was in there he slightly redid the general grounding so it would be quieter. The layout has everything being grounded near the input jacks. He split it so the preamp & PI side went to the input jack side and the power amp portion now grounds on the opposite side of the chassis.

I'm happy it was something simple, but don't feel too bad about missing what was going on. I don't have enough experience to have known that wire shouldn't have been there in the first place. At least I was able to make it as far as figuring out the problem was on the power amp side.

On a high note Rob gave me really positive feedback on my work. He said everything looked really tidy and all the solder joints were well done. He said if he had to get really nitpicky on critiquing my work; wires on the tube pins were fine, but could have been snipped a little more flush to be 100% they wouldn't make accidental contact with something.



Sorry I missed this question from earlier in the thread. Yeah, Modulus provided plenty of wire.
When running the wires I try to leave them long enough to give enough room to trim back, but not so long to where I'm wasting a lot of it. Doing that left me with probably enough wire to use for a second build.



For anyone who may give this build a try...
1. I'm not sure which wire was drawn in error on the layout. I don't have anything in front of me right now. Once I get the amp back to compare I'll post it.
2. Rob noted the way the presence is designed, it has DC on the presence pot. He said Marshall did it this way on some amps back in the 70's and a few builders still do it too. He said when you turn the presence knob it may crackle a bit like when you have a dirty pot, but this is 100% normal. So nothing bad, just something to be aware of.

Awesome that I nailed it and glad you got it figured out and working! Great job on the assembly!

In the future I can provide a helpful tip. Anytime you’ve completed a circuit and before turning it on, always, and I mean always, check with an ohm meter resistance from your source to ground. You don’t have to disconnect anything - just check it with a meter. You should always read open circuit or out of range.

If you knew this before turning it on you would have seen a near zero short or numbers changing oddly and immediately known something’s amiss. From there you start lifting B+ connections in sections to isolate exactly where the fault exists until you find it.

Glad nothing else was damaged in the process. I probably would have replaced all of the rectifier diodes even if they measured good with a meter myself - typically they dont like the flyback of the PT when a fuse lets go. I’m not criticizing or critiquing just stating my own opinion as to why.