Mojotone 18w TMB build - no sound (rookie builder)

[geoff 1965]

hello,
the trinity TMB does'nt have a master volume it has hi/lo inputs so you can't use that as a reference,also the richie layout is different it does'nt have the 68K resistor on the TMB input.
i think i've found the problem here,the mojotone build guide says to wire the green wire from pin 2 to the rear left lug of the TMB input jack but it should go to the front left lug. have a look at this 2015 mojo layout.

18-Watt-TMB-WD.gif

[Returnsnull]

Thanks, Geoff. That layout matches what I ended up going with.

[geoff 1965]

good! i can see why you struggled with the jack wiring,apart from the error in the build guide the illustrated inputs on the latest layout don't show a clear wiring scheme like the older layout.

[JMPGuitars]

the richie layout is different it does'nt have the 68K resistor on the TMB input.

That's true, the layout doesn't have the 68K grid stopper resistor on the TMB input. Do you know what that resistor does?

[Returnsnull]

I initially wired mine up without the 68k resistor and had a lot of radio noise being picked up by the amp. Reinstalled it to see if it helped and the radio noise disappeared. Found this article (http://www.regiscoyne.com/tech/68kinput/) that seems to indicate that is its purpose as well.

[Returnsnull]

Although, I suppose one could argue a better grounding scheme would do the same thing.

[geoff 1965]

yeah the main purpose of the 68K's on the normal channel is to attenuate the signal between hi & lo although they will prevent interference it's a low gain preamp. the TMB channel is cascaded so a lot higher gain which is more prone to interference/oscillation so the 68k here is to prevent that rather than attenuate.
how are your voltages now,have they settled?

[JMPGuitars]

Correct. The grid stopper is to prevent unwanted radio noise. So it's theoretically optional if no noise is present (but recommended to have some value in there). 68K may be much higher than needed, and can negatively impact tone. Using between 10K to 33K is more ideal when possible.

The normal channel is dual-purpose, as Geoff mentioned. They attenuate signal, and prevent unwanted noise.

Ground scheme helps a lot with noise, but not usually the same type of noise...unless your ground scheme includes wires that are too long or poorly dressed.

Follow-up question: where do you think the best location is for the grid stopper resistors?

Thanks,
Josh

[Returnsnull]

I know that I saw other layouts that had the resistors at the tube end instead of the input end. This makes much more sense to me.

If the wire is, basically, an antenna that picks up noise - it would make sense to filter at the *end* of that chain rather than the beginning of it before it has picked up the majority of noise.

I was very impressed with the customer service at Mojotone and the way that the kit arrived, packaging, etc... but the more I learn about this, the less fond I am of their chosen layout (mistakes aside).

That being said, I love that the point-to-point construction is so easy to follow / alter as necessary so I can start here and then work my way into a more functional layout if necessary. Not ideal, but possible!

[JMPGuitars]

Correct! As close to the tube as possible is the best placement of the grid stopper. It still works at the jack, but is not ideal.

I like some of mojotone's products, like their cabinets. But I'm not a fan of their 18watt layouts (and all the errors in them) for obvious reasons.

Anyway, how's your amp? Is it all set? Do you have final voltages yet?

[Returnsnull]

Anyway, how's your amp? Is it all set? Do you have final voltages yet?

I... think so? I'm almost scared to say it, but I just remeasured all the voltages and I think we're good voltage wise!

The only issue I currently have is a very high-pitched whistle on the TMB channel when both gain and master volume are high (treble or mids generally need to be above halfway also).

This was a much worse noise, but I noticed when chopsticking that touching the wire going to v2 pin 7 (or even the chassis) would silence the noise. I noticed that there was a bit of a long tail on the wire going to v2 pin 7 and resoldered it shorter. That made a significant difference.

I'm thinking that shielded wire for the input jacks, the wire going to pin 7, and also that gain pot lugs would significantly help this issue. Definitely open to suggestions though - I can also record an audio clip later if it would help to hear it.

[JMPGuitars]

Shielded wire is recommended for the input wires, and also sometimes the gain knob. Make sure to only connect one end of the shielding to ground.

If that doesn't help, we may need to discuss improving your ground scheme a bit. For one thing, ground points should have a direct connection to the chassis with no other purpose.

[geoff 1965]

your voltages look good now, just check the lead dress with the preamp tubes,ideally your grid wires "green" should'nt touch or be close to the plate wires "blue" and go to the tube sockets from the turret board without laying against the chassis. i notice on your pics that the grid wire going to pin 7 passes over the blue B+ wire underneath so make sure they are'nt touching.
seeing as the squeal is on the TMB channel you could easily start there with a shielded cable grounded at the input jack and place the 68K directly to pin 2 of the tube socket like Josh mentioned earlier.
good luck

[Returnsnull]

Thanks everyone! I appreciate everyone's help so much. I've learned so much throughout this process with everyone's help and I now have a (although still a work in progress) great sounding amp instead of an expensive paperweight.

I'm going to take a look into adding shielded wire (as soon as I can get some) and also improving the lead dress as discussed.

Bonus question: Does anyone know of a brick-and-mortar store that might sell shielded wire? I'm in Houston, so I have access to most retailers.

I know one (amazing) place across town that most likely has it, but that's one hell of a drive. I'll be in that area Monday though, so I'll pick it up then if I can't find some sooner.

Thanks again!

[JMPGuitars]

Bonus question: Does anyone know of a brick-and-mortar store that might sell shielded wire? I'm in Houston, so I have access to most retailers.

I know one (amazing) place across town that most likely has it, but that's one hell of a drive. I'll be in that area Monday though, so I'll pick it up then if I can't find some sooner.

Thanks again!

Not all shielded wire is created equally. Whatever you get needs to be good quality mil-spec cable, and if you're lucky, pre tinned. Look for 20AWG. I usually use silver-plated PTFE, but the silver plate isn't necessary.

[geoff 1965]

Josh is right! you've done all the hard work so you can't go to a "builders yard" for your wire! that's like buying a Ferrari and running it on paraffin!!