Slow Build...

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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lavrgs
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Slow Build...

Post by lavrgs »

I'm mainly a lurker but now that winter is coming maybe it's time to get this build going...

I have had a Mojotone kit for several years and am having trouble getting through the build, mainly due to lacking motivation This is not my first build, I built an Allen Accomplice – that kit had excellent instructions and I was able to build, turn on and play but I really didn’t learn as much as I was hoping… still a new builder

The Mojotone challenges, for me , are many; the confusing layout and ultimately the style of amp is not the sound that I go for, I like clean amps...but this kit will get me closer to the original intent – learn how tube amps work, what parts of the circuit do etc. I just read about 6CM6 tubes and want to know if they would transform the amp to get cleaner tones.

I have the board built and the chassis populated, tube heaters wired. The next step is the control wiring… and as I watch videos on amp build it occurred to me that using all white wiring is not a great idea. I will go back and replace the fly leads on the board with appropriate colors (?) and remove the ground bus off the pots. My original intent was to build the kit as it was intended and modify it later, but now I’m thinking I just want to simplify it – not build the tremolo channel. Thoughts?

Mojotone recently updated their layout and I found the wiring on my input jacks was wrong so I updated the wiring. I am confident the board components are correct and in the proper location.

It seems that if I could find a layout for a TMB with no tremolo That might be my choice – I was planning on building the kit as it was intended then making changes but I think if I simplified the build I may just get ‘er done…
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Re: Slow Build...

Post by JMPGuitars »

Hi,

A mojotone kit is a great way to learn how to build amps...but mostly because their kits are not great, and people almost always spend time troubleshooting them. BUT eventually they come out nice, and that troubleshooting is where the real learning happens.

You can use all the same color wire, but I don't recommend it for a first or second build. If you're careful, it's fine tho.

There are PLENTY of options for you in our downloads section: viewforum.php?f=25

The amps are generally either TMB or Tremolo. Two channel TMBs or Superlite TMB are all great. Even simpler is the Lite 2b series, which is a fantastic sounding/fun amp.

However, if you've already populated their weird layout on the board, it might be more effort than it's worth to try a different build. You have to decide what you really want first, and then do that.

Thanks,
Josh
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lavrgs
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Re: Slow Build...

Post by lavrgs »

"However, if you've already populated their weird layout on the board, it might be more effort than it's worth to try a different build. You have to decide what you really want first, and then do that."

That is sage advice. I will stick with my original plan of building it as it was designed.
I just pulled out Robert Megantz's Design and Construction of Tube Guitar Amplifiers and it does seem to be a great resource.

I will update as I run into problems. I also realize that I truly am a lurker - this is post #2 8-(
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Re: Slow Build...

Post by Daviedawg »

Lurking is a long respected use of the knowledge here. But it is also nice to hear from you. Welcome.

Dd
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lavrgs
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Re: Slow Build...

Post by lavrgs »

I am at the point of connecting the fly leads from the board and I have a question about connecting the resistors on V4 and V5. There are two Mojotone layouts: a newer one that shows connecting the 100 ohm resistors across pins 1 & 9 and earlier one that shows the resistors connected to pin 9 and to the lead going to the board and the big cap. Is there an advantage to either one?
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Re: Slow Build...

Post by colossal »

lavrgs wrote:
Mon 12/30/19 4:43 pm
I am at the point of connecting the fly leads from the board and I have a question about connecting the resistors on V4 and V5. There are two Mojotone layouts: a newer one that shows connecting the 100 ohm resistors across pins 1 & 9 and earlier one that shows the resistors connected to pin 9 and to the lead going to the board and the big cap. Is there an advantage to either one?
The 100Rs are the screen grid resistors. Using pin 1 (unused) to bridge the resistor to the grid pin (9) is convenient as it locates the resistor right on the grid and the tube socket. You can see this approach in most any Fender schematic. This is the best practice from the technical perspective (minimizing oscillation by keeping resistance right on the pin). But 100R is a pretty small resistance and offers little protection to the tube. That resistor does effect the sound. You can take either approach in your build, however. Putting the resistors on turrets on the main board can be convenient if you need or want to change them out in the future. On the Lite IIb schematic, which is considered the foundational build for the 18W tone, 1k is often used for each power tube and will aid in keeping the screen voltages manageable with 2k2 as the screen B+ dropping resistor. The plate voltages should run 340-345VDC with the screens about 20-25VDC lower.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

I've gotten to the point where initial wiring is "done"...I've already found some mistakes and corrected them. I have a question about Cliff jacks - the tremolo jack is a six pin version when I input a sine wave into the jack I only see the signal one one of the pins furthest away from the input...is that normal? I see a lot of these jumpered is that why?
I now see that the pins for the tip are connected when there is no plug and open when there is...do the wires go to the connected side when there is no plug, making the tremolo work without a switch plugged in (?)
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Re: Slow Build...

Post by Daviedawg »

You can wire as you choose to. Mine is wired with the empty switched position to ground so that default situation is off and with plug in is switchable on or off.

Dd
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lavrgs
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Re: Slow Build...

Post by lavrgs »

Ah yes now I understand. I have it wired so the switch is required to turn it on. Mojotone updated their layout and I had it wired per the previous version. The schematic shows the switch open - implying it's required vs. wired so it's on.

Since it's taken me three years to get this far I want to learn how things work...my first build was an Allen Accomplice which I built in a month, turned on and have played for the last three years. The only thing I learned from that build was that I can follow instructions 9-) This build has made me aware of the function of various components - which was the original intent- but I still have a long way to go.

I have plans, and a kit, for build #3 but it's going to take some planning...more on that later
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Re: Slow Build...

Post by lavrgs »

I've gotten to a point where I was going to apply power...but since it's taken me a long time to get here I thought I'd try posting a few photos to see if there is obvious problems I should address.

I'm not sure the best way to post photos...any advice?
edit: this kinda works...
https://photos.app.goo.gl/1HxVUsqoKSYBAySE9

https://photos.google.com/photo/AF1QipP ... uelsYexAEx

https://photos.google.com/photo/AF1QipP ... ocuC_E3y-6

https://photos.google.com/photo/AF1QipP ... cLBVPMBYCY
Last edited by lavrgs on Wed 01/15/20 8:40 pm, edited 3 times in total.
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colossal
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Re: Slow Build...

Post by colossal »

Well, go through your Power Up procedure, fill out your Voltage Chart (and please post!) and we'll go from there. Just looking at that one picture in the link you posted, we should address your "lead dress" (neatness and routing of leads) and I definitely see some solder joints that are questionable (your cathode bias resistor and cap), but we can get to that later.

For now, ONE variable at a time.

1. Power up on a light bulb limiter or variac
2. Check your voltages
3. Post your results

Nice work!
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lavrgs
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Re: Slow Build...

Post by lavrgs »

Thanks I was concerned about the lead dress...it's a bit of a mess - I'll go look for a better example. Should power up with all tubes or just check the transformer first?
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Re: Slow Build...

Post by colossal »

lavrgs wrote:
Wed 01/15/20 7:47 pm
Thanks I was concerned about the lead dress...it's a bit of a mess - I'll go look for a better example. Should power up with all tubes or just check the transformer first?
Yeah, you can get to the lead dress later *. I would, however, consider reflowing your cathode bias turrets before starting up. The joints look dry. One thing I note is that you have used the PT mounting bolt as a ground point. That is generally not considered good practice. You have another ground point just adjacent. I would ground your PT centertap, heater centertap, main 32-32uF cap can ground, and cathode bias resistor/cap ground to that single point. Arrange the ground tabs in a little rosette around that bolt. I was not able to view your other photos. The link is asking me to sign into Google.

I would definitely power up without power or preamp tubes (you need the rectifier tube, of course). If you have a variac, pull the tubes and then power the amp up at a very low voltage and switch it out of standby (if you use one). Then raise it up and leave it for a bit, and repeat. This will form the electrolytics. If you pull the tubes and hit it with full mains voltage, the caps see the Secondary x 1.414 on startup. So for the first power up, I like to ramp the amp up slowly and ease it in. A light bulb current limiter is a useful tool if you don't have a variac.

On startup, once the amp is up to full mains voltage, I take a round of initial measurements with a multimeter. I start in AC mode, and measure the AC mains input voltage at the Power switch, then go the secondaries on the rectifier socket, then the heater filaments. Then I switch to DC and go through the amp. Things will look a little odd because there is no load on the amp and everything is railed at full B+. Look for any smoke. If the voltages are stable and no smoke, then install your tubes and repeat. This time, you'll want to record your voltages again on the 18W Voltage Chart and see how you did. If your amp squeals terribly once you flip it out of standby with the tubes in, then you need to swap your output transformer leads. We get a 50/50 shot at getting it right the first time (unless you keep building amps with the same OT and know what lead goes to which socket :lol: )

* later might mean sooner if you have oscillation or other problems.
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lavrgs
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Re: Slow Build...

Post by lavrgs »

Thanks for the great input. I'm going to go do some clean up. Let the fun begin... I'm going to reflow some solder and do one more check before flipping switches.
I'm excited to use a variac for the first time, that I've had for 30 years ... along with the DBT I built. :D
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Re: Slow Build...now working...spoke too soon

Post by lavrgs »

See*
I started the turn on process with a variac - determined my DBT was't working. I initially started with the rectifier tube installed and got it to light up. I took some voltage readings at the tube- pin 1 315.7 Pin7 315.7 and pin 3 127.5..probably 12.75 9-)
I decided to put all the tubes in and somehow blew a fuse - at this point i'm guessing my probing may have caused it to blow. I got some new fuses and put in an 800mA (slightly under sized) fuse in and turned it on...viola sound... plugged in a guitar and it sounds good to my untrained ear - I now hear a slight 60hz noise but nothing I would say needs fixin...I will check some voltages and post.

*OK as I was checking voltages I found a wire (going to pin 6 of V3 that wasn't connected - when I connected it I got a farting/beating sound...it may be motor boating (?)
(update) so I disconnected the and started checking voltage again...well when I was checking V4 pin 7 I shorted it to ground and it did not like that...now I have no sound, is it possible I fried my output transformer? (UPDATE) I'm back to okay - not sure why it was "sleeping"
Last edited by lavrgs on Fri 02/07/20 9:38 pm, edited 3 times in total.
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