Motor Boat in Mojotone 18W Build, HELP!!

[roadshow]

Hello!!

I’m pretty new to the group, and after doing some research on this site last fall I took on my first 18W build. Hopefully the experience in this group can help me resolve a problem. Because I wanted to build my own 1958 similar to the Mojotone Front Mount British 18 Watt Cabinet, I built my own cabinet and ordered the 18W kit from Mojotone so that I could get the top mount faceplates. Aside from some errors with the schematic/wiring diagrams (pdf example included) they sent me and mechanical fit problems, I got the amp successfully built. Almost. I’m including pictures and a voltage chart.

Problem 1: I’ve got a real bad motor boat going on due to the tremolo channel ( I believe ), and on occasion a faint one in the normal channel. I've only found a couple of references to motor boating in this group from over 10 years ago and they didn't seem to apply/help.
• When I measure the voltage on V2-2,6,7 (phase inverter) the motor boating gets real quiet!
• When I measure the voltage on V3-1,2 (tremolo preamp) the motor boating stops!
• I lifted the wire bringing supply voltage to the tremolo preamp section at the 100k resistor and the motor boating stopped. So, I cannot use that channel, but can play through the normal channel fine.
• I compared the Mojo and RH circuits I’ve seen here and on Tube Depot, and other than some resistor/cap value differences, the circuit seems the same.

Problem 2: Sometimes it seems like the amp is barely working, and when I wiggle the preamp tube it comes to life. I’m guessing a bad socket? Hasn’t happened for a while…

With the tremolo circuit supply voltage disconnected and a guitar is plugged into the normal channel the amp sounds amazing and aside from the Tone Control sounding too heavy with bass I’m happy with it (I suppose some cap/resistor changes will help that). I’ve played through it for a couple of hours now ( I’ve just set the chassis on top of the cabinet for now)

So, if you can help me solve the motor boating I’d be grateful and thrilled. I’ve reviewed the wiring over and over and over, followed the tips I’ve found in this group such as grounding strategies and have tried to cover the bases.

After I resolve this I want to take a look at the tone control and perhaps add the PPIMV circuit, but first things first.

FWIW, last spring I built the JTM45 Metro kit I got from Valvestorm, and it worked like a champ right out of the chute.

[Daviedawg]

Welcome. For Problem 2 I would suggest tweaking the pins on the tube as first option for the best fit. Cleaning the socket pins as well with switch cleaning fluid.
The motorboating may be a lead dress issue causing cross feedback. Have you tried chopsticking the leads looking for a change? I suggest that because it is simple to try. Does anything change when you switch or short the trem? From what you say and your voltage chart I think something else is in play such as the trem oscillating wrongly. With that in mind, your voltages on the trem preamp are lower than I would expect. Mine for example are 189v on Pin 1 and 145v on Pin 6. The trem works reliably at those voltages. Do you get test voltages to aim for from Mojotone?
So if the simple answers do not bear fruit you should go back through the schematic starting from the trem input, marking it up as you go to check for errors in components as well as bad joints etc. It is easy to swap similar sounding value resistors for example. I know only too well.
Good luck. This amp is well worth the effort.

Dd

[JMPGuitars]

That's a nice looking build, I can tell you have good attention to detail.

Dd gave you some great advice and you should follow all of it.

There's only 1 fatal flaw I see in your build that will cause issues beyond Dd's advice, and it's something you need to consider. All of your solder joints are relying 100% on the solder connection, with no physical/mechanical connection support. If you have a weak solder connection on a socket (for example), and you wiggle the socket, the lack of the physical support can make the socket behave less consistently (which you're already seeing).

What's the solution? When you use a solder lug (sockets, terminals, etc...), the lead should be bent around the lug and pressed together prior to soldering. With turrets, you should use a combination of the outside being wrapped 3/4, and only minimal use of the inside of the turret.

DSC00446.jpg

That's a close-up of my 36 watt amp before it was done. You can see the wire leads wrapped on the solder lugs making a strong physical connection before I solder them. This isn't just important for the connection quality; if the connection overheats, the physical connection stops the wire from coming loose.

Thanks,
Josh

[roadshow]

Hey guys,

Thanks for all of the fast replies. I plan to make the changes you've suggested where possible and make them habits in the future.

While at the office today I did some surfing about motor boating and learned quite a bit ( I plan to post what I found in a separate post ).

But...

I kept thinking about what I had said and included in the voltage charts that when I measured the voltages at V3-1,2 that the motor boating stopped. I tried just holding the meter probe to it w/o the multimeter tonight and that was just as effective. SO, I touched just a wire to V3-2 and that worked, and it made me realize it was the small capacitance of my body that was doing the job. SO, I added a 20nF cap from V3-2 to ground as shown in the picture and the motor boating is STOPPED and the amp sounds fine!! There is some weirdness towards the end of rotation on the pots, but I can tell they aren't high quality and need to be replaced.

I think it may be well served to rebuild the turret board, replace the pots with good ones, and replace the tube sockets as well perhaps. So, time to collect parts and target a day for the end of the month or early February. I want to get this done before racing season starts (picture 2)

Again, thanks for the quick input, it sure helped and was encouraging!!

18W Added Cap.jpg

2017 05 27 GLD 1.jpg

[Daviedawg]

Well done finding the solution.

Nice pic. Good luck when the time comes.

Dd

[roadshow]

At the risk of sounding like I'm beating a dead horse which I am

As I kept reviewing schematics from this site, take a look at the Steve Luckey 10/13/2006 schematic, in particular the Cx and Cy caps in the tremolo circuit and the note that goes with it. EXACTLY what I stumbled on / figured out!!!

[Daviedawg]

You did well to figure it out, right enough.

I have not looked at that schematic before. But I did add an Rx on the normal tone as a mod after the build. Not sure where that came from. But once I had the trem working I left it alone. I manipulated the voltages on V3 until it was stable. My problem was not starting on demand but after a while as it were.

Dd