18w TMB Road Block. No signal past V1 No input signal

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Olde303music
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18w TMB Road Block. No signal past V1 No input signal

Post by Olde303music »

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Greetings all this is my first post. I suppose pride has gotten in the way of posting before, but I’m completely baffled at this point with my current 18w project.

As simply as I can I’ll describe my issue.

This is my 8th Mojotone TMB and 12th or 13th amp Build. Currently I have my amp “finished” and did the required testing per Mojotones updated voltage reading diagram. I also follow the sets in Tube Depots test section as it is also helpful.

With all tubes in, recto installed and a speaker connected at 8ohms, I get hum, and increased hiss when turning up volume or gain, but plug in an instrument zero signal.

I know from experience that EVERY amp I’ve built, of the few of them EVERY one I’ve found silly mistakes by going through prior to any testing with the highlighter. Even after being “good to go” I’ve still found more.

I’ve been through this one to the point I’ve had to step back for the day.

Here is what I know. While probing with an audio probe, following a 400hz generated tone, I bumped pin 6 on V1 making a connection to pin 7 and presto there is the signal. I do the same on the other side of the tube for the TMB channel with signal inputted and again signal. However in all 3 jack, with signal applied, I can follow the tone to the input side of V1 but no tone coming out to the turret unless I jump to the adjacent pin. So odd and I’m sure I’m missing something simple.

The only thing different in my build from the Mojotone kit as provided, is that I used an octal recto socket instead of the EZ81 9 pin, I’ve done this mod on several other 18w builds with zero issues and I’ve verified its correctly installed.

I post a couple pics of where I left the Build for the night.
Hope you all see something I’ve missed or have some input, humbly appreciate all your knowledge.
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Daviedawg
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Daviedawg »

Welcome to the forum. Nothing wrong in looking for help. We all need it sometimes.
Based on what you describe, it seems that there is a fault at V1 only. So my checklist all of which you may well have looked at already testing after each action would be:

Examine the wiring on the valve base with a magnifier looking for whisker shorts, tapping with a non-conductive "chopstick" as well;
Swap the 12AX7 for a known working one;
Measure the voltages at each pin to ground looking for something off;
Rewire the valve base;
Swap the valve base for a known working one and rewire back to the board (to save a step while the iron is hot);
Remove and check the input jacks then rewire entirely to the valve base.

Meantime I will have another think and edit this if anything else comes to mind.

Dd
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geoff 1965
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by geoff 1965 »

hello,
nice build! good components and detail, the grounding wire across the back of the volume & tone pots looks to be soldered to the tip lug of the input jack,should'nt that go to the sleeve lug?
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Daviedawg »

I looked long and hard at the picture, in particular at the input sockets and decided they were wired correctly. I also looked at the layout that goes with the kit and again decided that they were wired right. But I find it hard unless I have them in my hand to fiddle with as it were. It is certainly a common recurring issue.

So maybe You need to have a close look at the operation of the jacks and the wiring first.

Dd

Edit. I just had a wider look at the layout. If you used that layout and once you have resolved the immediate issue, if you have noise issues there are improvements which could be made to the grounding in addition to the pot link grounding. But since you have built these before maybe you are sorted already.
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by JMPGuitars »

You might be right about the jacks, but it's worth checking for sure. Also verifying which side is bypassed if they're not standard Cliff jacks. It can be confusing to look at the jacks, especially sideways. ;)
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Olde303music
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Olde303music »

You guys are life savers. I absolutely don’t know how I missed that. But regardless. I knew I had been looking over this thing too many hours.

A side note; is buss wire on the pots not a good choice. Appreciate the compliment on the work.

Will be returning more.
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Olde303music »

I’m going to check the areas you suggested and will definitely post back, and I’ve always used this grounding arrangement. I would be greatfull for a less confusing jack and grounding arrangement.

Maybe I shouldn’t say this, but I assemble a lot of these kits for customers and anything to simplify and improve my build process is welcome.
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geoff 1965
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by geoff 1965 »

have a look at the downloads in the navigation section,there is a lot of layouts you can study for the jacks & different grounding "a whole subject" in itself!
also if you click on "forums" then "18watt technical discussion" there is a search box you can type into for anything,there is a lot of good info and links in the previous posts if you take the time to browse through.
so which octal rectifier do you use in place of the EZ81?
good luck
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by JMPGuitars »

Olde303music wrote:
Thu 10/31/19 4:57 pm
A side note; is buss wire on the pots not a good choice.
I can't stand it. ;) It's a ground loop nightmare, and doesn't help.

Check out this thread: viewtopic.php?f=4&t=25372

That gives some good info about best practices, and references this layout: files/JMPGuitars_18_Watt_Lite_2b_Layout.pdf

Lose the bus on the back of the pots, and follow those guidelines and you should see less work and less noise.

Thanks,
Josh
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Olde303music
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Olde303music »

I didn’t get an opportunity to mess with the amp, other than moving the ground for the pots, on he jack. However it made no difference.

Having looked at the information you guys shared, and seeing that the “high volume squealing” I’ve heard on other builds is a result of grounding, I now realize I absolutely have to utilize a more appropriate grounding system.

In relation to this current build, I have to assume grounding in some form another is my issue. I’m not sure why else when I put the input signal directly to the output of V1 tube, I get a complete signal to output.

Regardless, would anyone be kind enough to post pictures of their grounding sequence for me to look at, and also a clear example of their jack wiring?

There are sooooooo many different ways to do these builds, I find myself “tuning a cold carburetor” if you catch my drift. I truly feel that I’m just missing something that we’re any of you looking at my build you could say “hey dumb arse” “that goes there” :D

If someone could show me real pictures of a build similar to mine with the improved grounding and or a fool proof input jack configuration I’d be in your debt.

Since all 3 jacks are dead so to speak, I feel it’s either grounding or a problem at V1. A tone is making it to V1, or signal I should say. It’s not leaving V1

I don’t know honestly, and now I’m doubting my work.

Grateful for all your time fellas.
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by Daviedawg »

Keep it simple for now and keep the faith with your build.
Did you swap the 12ax7? Nice and easy elimination. Then look at the valve base and wiring.

Dd
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Re: 18w TMB Road Block. No signal past V1 No input signal

Post by JMPGuitars »

Try the tube swap/socket stuff as Dd suggested.

For grounding, follow the ground scheme in my layout. It's drawn to scale, and applies to almost any 18 watter. Read the thread I posted if you haven't yet, that gives additional thoughts on it.

If modifying the tube or socket doesn't work- print a fresh copy of the schematic AND layout that you used, and verify with a highlighter every single component and connection. Even if you already did this. Make sure the schematic and layout are consistent with each other. Start with the input jack and move through the circuit from there.
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