First build troubleshooting

[9voltchicken]

Greetings! I am a luthier based out of Sacramento, CA, and have been building and repairing guitars for a number of years now. Many of my customers ask about tube amplifier repair since there is a shortage of technicians in my area, I thought it might be wise to expand my knowledge a bit and turn this newfound interest into another facet of my profession. I sure am having a hell of a lot of fun so far! I am fairly inexperienced when It comes to troubleshooting electronics outside of guitars and basses and it shows. I've done a fair bit of reading up on tube amp theory, watching Uncle Doug poke around in old amps on youtube, and I am looking at taking a course on circuit analysis to further clarify some of the more advanced topics.

I ordered an amp kit from Stewmac (actually a mojotone kit) and built the amp without much fuss other than two problems. The first problem was 60 cycle hum, which was resolved after chopsticking and replacing a bad lead dress wire. The main problem that has stumped me for weeks is lack of gain. There is no discernible difference between normal and tremolo channels. Hi/low inputs for the tremolo channel work just fine, but both inputs for the normal channel are identical.


Here's a link to more pictures of my build. Let me know what you think!

https://photos.app.goo.gl/r1GwRzaWQ93qmFBU7

[JMPGuitars]

Welcome to the site!

If you want to get serious about this, and it sounds like you do, start here:

Soldering technique: viewtopic.php?f=4&t=25396
Ground schemes: viewtopic.php?f=4&t=25372

Study the content of those well.

Back to your build:

2 of the most important tools in your arsenal are chopsticks, and highlighters. You already fixed one problem with the chopstick, now break out the highlighter.

Print a copy of the schematic and layout. With the amp off, verify the value of every component, and every wire connection. Highlight every part on both the layout and schematic, only after you're sure everything is what it's supposed to be.

Obviously start at the input jacks and see if anything jumps out at you. You may want to compare your schematic/layout to our set in the downloads section.

Thanks,
Josh

[geoff 1965]

hello,
good advice from Josh, your amp is based on the classic 18W marshall so both preamp stages are fairly clean "not cascaded" so you don't get the overdrive of a TMB or superlite preamp,most of the breakup comes from the power tubes.
there is a bit of misconception with the normal channel inputs,they work on a single/parallel basis not a high/low signal like the tremolo channel so there won't be a difference in volume between the two inputs but if you listen carefully you will notice one input sounds slightly "fatter" in tone.one jack sends signal thru one half of the preamp tube "single" and the other jack sends signal thru both halves of the preamp tube "parallel" and the normal channel does'nt have the 68K grid stoppers like the tremolo channel.
here's a diagram explaining how the high/low tremolo inputs work,

download.jpg

hope this helps and good luck

[colossal]

That schematic is a mess and should have been properly proofread before being unleashed upon a bunch of hobbyists.

Stewmac is showing 130k for the power tube bias resistor. That will make for a very quiet amp. Should be 130Ω and more like 150Ω if the secondary on the PT is correctly specified.

Screen resistors are shown at 100k. Should be 100Ω or 1k.

Also, the bias resistor appears to be a mere 2-3W Metal Oxide. That part will get quite hot. I would up that to at least 5W.

[colossal]

What's wrong with this picture?

stewmac.png

[crgfrench]

What's wrong with this picture?


stewmac.png

Today's schematic is brought to you by the letter "k".

[geoff 1965]

christ! well spotted colossal!
lets hope those values are type errors and not actual,looks like stewmac will be getting a lot of comeback's with this one!

[colossal]

Hmm, I may be missing something or maybe there is another document showing the bottom of the turretboard, but I can't see how the Normal Channel has any DC voltage on the plates and is operating at all? There should be a 100k (carbon comp) plate resistor connected to the junction of the 0.01 and 0.0047uF coupling caps serving V1ab. There is no visible connection to the power supply or that resistor in sight. Is it supposed to be at the end of that chain of feeds off that 16uF and a resistor hiding under the board? Is that one 16uF cap serving the whole preamp?

Layout.png

This is what an 18W board should look like.

1974x board.jpg

[9voltchicken]

Wow, Thank you for all the feedback! I've poured over the schematic and followed it to the T and couldn't find anything wrong but I'll try again with the highlighter method to triple check all of my connections.

Hmm, I may be missing something or maybe there is another document showing the bottom of the turretboard, but I can't see how the Normal Channel has any DC voltage on the plates and is operating at all?

Here's a view of the jumpers on the back of the turret board. The layout diagram from stewmac is a bit different, so I've found. I wish I found this site BEFORE I built this amplifier. I would have done things a bit differently.

[colossal]

Wow, Thank you for all the feedback! I've poured over the schematic and followed it to the T and couldn't find anything wrong but I'll try again with the highlighter method to triple check all of my connections.

Here's a view of the jumpers on the back of the turret board. The layout diagram from stewmac is a bit different, so I've found. I wish I found this site BEFORE I built this amplifier. I would have done things a bit differently.

Ok! That makes better sense! I did see that lone 100k over on the other side of the board but wasn't sure what it was attached to.

Well, you got the amp working and that's to be applauded . Now that it's done, you can always rip out the preamp board, and make your own . Check out Josh's and others' layouts on this site. Definitely stay up way too late and do some searches here.

These amps can be magical. They are wonderfully simple but not easy. There are subtleties to be observed: good soldering, good lead dress, good layout, and proper grounding. Once you dial them in, you can personalize them for your guitars and playing tastes as well.

[crgfrench]

It looks like your output tube bias resistor is a 3W ONE OHM resistor? It looks in the picture like Black-Brown-Black-Gold which would be 1 Ohm 5%. Maybe my screen is showing the wrong hue... It should be Brown-Orange Brown Gold which would be 130 Ohm 5%. As Colossal mentioned this should be at least 5W. I use the Ohmite B8J125E here which is 125 Ohm, 5%, 8 Watt.

[zuceno]

If you're learning to fix amps, there is a useful old book on troubleshooting and amp repair called The Guitar Amplifier Handbook, by Jack Darr. This is so popular that a reprint of the 4th edition is available.


If you dig around you might find a pdf version online - there was one somewhere.

[JMPGuitars]

If you're learning to fix amps, there is a useful old book on troubleshooting and amp repair called The Guitar Amplifier Handbook, by Jack Darr. This is so popular that a reprint of the 4th edition is available.


If you dig around you might find a pdf version online - there was one somewhere.

That book is really basic, and much better info is available on this site and our sister sites. I had that book on Kindle, and it was terrible. The print version was only a little better. IMO

[9voltchicken]

It looks like your output tube bias resistor is a 3W ONE OHM resistor? It looks in the picture like Black-Brown-Black-Gold which would be 1 Ohm 5%. Maybe my screen is showing the wrong hue... It should be Brown-Orange Brown Gold which would be 130 Ohm 5%. As Colossal mentioned this should be at least 5W. I use the Ohmite B8J125E here which is 125 Ohm, 5%, 8 Watt.

It is indeed a 130ohm 3w resistor. I'll bump it up to a 5w once I find the problem with my channels being too similar. I'll also look into performing recommended mods to get more out of this amp.

I've checked through 2/3rds of the circut and found one discrepancy with a resistor between the center lugs of the speed/intensity pots being a 1m instead of a 2m in the schematic, but I doubt that's a factor.

I will keep checking the circuit and will share my findings. Thanks again!

[geoff 1965]

here's a good builders guide with a lot of useful info for tweaking gain,troubleshooting etc

18wattBuildersGuide31.pdf

also use the searchbox in the forums 18W technical discussion,there's some good info in there.
good luck

[9voltchicken]

here's a good builders guide with a lot of useful info for tweaking gain,troubleshooting etc
also use the searchbox in the forums 18W technical discussion,there's some good info in there.
good luck

Thanks for the useful guide! I'll save it to my ever-growing folder of resources. I was lurking on the forums for a couple of weeks while I tried to figure this out. There is indeed a veritable trove of useful info on this site. I wish I had found it sooner.

Sch2.jpg

I spent most evenings this week going over the schematic twice and found more than a few discrepancies, but no errors. What is the next thing I can try?

[geoff 1965]

if you go on downloads,then technical info & troubleshooting,there is a voltage chart you can use and then post your results,it helps to see if you are in the "ballpark" with your voltages.double check which position your phase inverter tube is compared to the chart.

[JMPGuitars]

there is a bit of misconception with the normal channel inputs,they work on a single/parallel basis not a high/low signal like the tremolo channel so there won't be a difference in volume between the two inputs but if you listen carefully you will notice one input sounds slightly "fatter" in tone.one jack sends signal thru one half of the preamp tube "single" and the other jack sends signal thru both halves of the preamp tube "parallel" and the normal channel does'nt have the 68K grid stoppers like the tremolo channel.

This post from geoff is accurate. However, you can usually perceive some level of difference between single or parallel input. How much may depend on other factors including the tube. You may also find the difference more noticeable depending on the pickups being used.

Look closely at the schematic and you can see that the high input uses both halves of the input tube, and the "low" input uses only one half of the input tube.

Try a different tube in this position and see if it becomes more noticeable. Maybe try a db meter and see if that shows a difference in volume. Listen also for tonal changes and see if you perceive a difference.

Thanks,
Josh

[9voltchicken]

Here's what my voltages look like...
Voltage at the wall is 121v

[9voltchicken]

there is a bit of misconception with the normal channel inputs,they work on a single/parallel basis not a high/low signal like the tremolo channel so there won't be a difference in volume between the two inputs but if you listen carefully you will notice one input sounds slightly "fatter" in tone.one jack sends signal thru one half of the preamp tube "single" and the other jack sends signal thru both halves of the preamp tube "parallel" and the normal channel does'nt have the 68K grid stoppers like the tremolo channel.

This post from geoff is accurate. However, you can usually perceive some level of difference between single or parallel input. How much may depend on other factors including the tube. You may also find the difference more noticeable depending on the pickups being used.

Look closely at the schematic and you can see that the high input uses both halves of the input tube, and the "low" input uses only one half of the input tube.

Try a different tube in this position and see if it becomes more noticeable. Maybe try a db meter and see if that shows a difference in volume. Listen also for tonal changes and see if you perceive a difference.

Thanks,
Josh

I took the time today to do a little more testing. Found a notable difference between the high/low on both channels (less so with the normal channel, as described) but no perceivable difference between the tremolo and normal channels.

I'm honestly tempted to redo the whole preamp board and rebuild it as a TMB. I didn't know such a thing existed before this kit was ordered.