One watt, two channel new build (clean channel issue resolved)

[JPK]

Hi All. I have so far created two threads about questions I had with this build. It's a Mojotone Studio One kit. Why I built this kit is because I prefer high preamp gain and the cranked output section sound at lower volume. I decided to just start a build thread since I seem to have a lot of questions (some may be kind of dumb). I'm attaching the schematic and wiring diagram for reference.


Resolved issue 1: They did not actually ground the isolated input jack to the chassis with a wire. It's grounded via all the pots which are not isolated from the chassis (thank you  for pointing this out member pdf64). I'll build the kit as the diagram shows but I may experiment with adding chassis ground near the input jack with a wire, and possibly a cap as well.


Resolved issue 2: After soldering the heater circuit the tube sockets were so tight I couldn't get the tubes in. Resolved by using an old tube to work them loose (thank you member Slucky). I'll be careful with my soldering method going forward.


New issue 3: It's not obvious to me the polarity of this Nichicon electrolytic cap. I'll post some pics in the next post.

[JPK]

I see a B on one side and B- on the bottom of the other after using a magnifying glass searching for a polarity marking. Would you agree that B is plus and B- is negative? I don't to ruin this cap putting it in backwards. This is a 2.2uf electrolytic cap on the cathode of V3B. I used a black Sharpie to mark the top of the cap negative so I don't need a magnifying glass when I install it....if that is the negative terminal.


Thanks

[pdf64]

I wonder whether you may be trying to discern meaning where there is none? The cap may be non-polarised.
With polarised radial ecaps, the longer lead often indicates +ve, and the stripe running down the side of the can -ve.

[sluckey]

I see a BP on one side of the cap. Maybe that's the polarity indicator.

This google search may help.

[JPK]

I wonder whether you may be trying to discern meaning where there is none? The cap may be non-polarised.
With polarised radial ecaps, the longer lead often indicates +ve, and the stripe running down the side of the can -ve.

The schematic shoes polarity. It's the cathode cap on V3B. Also it shows it as electrolytic on the parts list which I thought were always polarized. The other polarized caps are very clearly marked. Not this one.

[PRR]

It is a "BP", bi-polar, non-polarized electrolytic. (Made as two foils in one can.) It can go either way.

It "can" replace a polar e-cap. It costs more. This pill-size cap, the added cost over a one-way cap is trivial.

If this goes under V3b, 100V is about 98V more than you could need. Again the added cost may be moot.

[JPK]

Thanks, you guys were right (of course). It's BP for Bi-Polar. I learned something. I thought all electrolytic cap's were polarized. Link is below. Also data sheet is attached. Also image of top of datasheet.


https://www.digikey.com/en/products/detail/nichicon/UVP2A2R2MED/2539607
.

[JPK]

Well I hosed this one up. The first two amps I built powered up and worked right away with only normal tweaks needed. This one works ok on clean channel till you crank it up and has some scratchy nasty break up. The lead channel doesn't work at all. Clean sounds really good at low volume. They did a nice job on the tone. The V2 and V3 voltages are totally off. Not sure what I did. I'll look at it closer and do more checking tomorrow.


I attached the wiring diagram with my voltages in green. The red and black are what they are supposed to be. Also pic of final build. The schematic is in my opening post.

[sluckey]

Since V2 and V3 cathode voltages are impossibly high, I'm gonna guess that they have no (or very poor) ground connection. Probably a bad wire between the board and your ground buss. See pic.

BTW, that ground buss on the pots does not look soldered very well. Several solder joints just look like they are merely "stuck" to the back of the pots and could be easily flicked off with a jeweler's screwdriver or solder pick.

[JPK]

Thanks Slucky, I'll take a close look at that.


Edit: Well Slucky you found the exact bad solder joint that was causing the bad V2/V3 ground problem....from a picture (and a few voltages). Not the best picture either. The lead channel immediately began working after fixed that bad solder joint. Now both channels sound great except the clean channel gets scratchy at full volume. While re-checking my voltages I found a loose resistor solder joint on the B+ C node so I'm going to go ever the entire amp for bad solder joints. But at the volume I would normally play, the clean channel sounds fantastic. The amp is dead silent when you stop playing. It's a pretty cool sounding amp. Better than the AX84 High Octain that it will be replacing. I just never bonded with that amp. I'll keep posting progress and maybe some sound clips (but my playing, ugh).


Updated voltages below.

[JPK]

Just wondering if anybody else has problems getting solder to flow on the a JJ can capacitor like the one in the link. On all the other stuff it flows pretty well. On the tabs of this cap it's really a pain. Temperature is set to 650F. It's a Xytronic LF-3200 which is 120 watts. Is flux needed? Solder joints in the picture are pretty pathetic. A few were loose. Keep heating it and it just doesn't flow onto the tabs.

https://tubedepot.com/products/jj-can-capacitor-40uf-x-20uf-x-20uf-x-20uf-500v?utm_source=google&utm_medium=cpc&adpos=&scid=scplpCP-JJ-40-20X3-500V&sc_intid=CP-JJ-40-20X3-500V

[sluckey]

I've only used one of those. Don't recall it being difficult to solder. I do use good quality 63/37 rosin core solder and I also use liquid or paste flux as needed.

     http://sluckeyamps.com/smoky/3.jpg

[JPK]

I'm using this stuff:


https://www.digikey.com/en/products/detail/kester-solder/24-6337-9713/6029


I should have used flux on the cap terminals but I'm all out and need to order some new flux. The last I had was a Kester flux pen but the top popped off and spilled all over during this build. What a mess. What do you use?

Also check out my updated voltages. The V3 pin 8 is now too low (74 and supposed to be 174). The lead channel is now really quiet while the clean is full volume (still crackling at over half to full volume). It's weird because the lead channel was full volume before and went low after fixing that ground you pointed out. The problem swapped channels sort of. Any idea how I can get that voltage higher (supposed to be 174V)? I might run a separate ground directly from the filter cap to the bottom of that 100k R on the V3 cathode. If it's a better ground the current will be higher which should drop more voltage across the cathode resistor, right? Is that 4th stage just to supply more current for the tone stack? No voltage gain? There's no bypass cap on it (see link below, it's a lot to take in in 15 minutes). PS: I went through the entire amp to re-heat questionable joints with no joy. 


DC coupled cathode follower:
http://www.valvewizard.co.uk/dccf.html



 

[Esquirefreak]

Maybe try some careful sanding on the capacitor tabs to break the oxide? I know from brazing and oxy/fuel welding that clean metal is crucial for the solder to stick.

/Max

[JPK]

Good news. Adding a ground wire from the filter cap ground to the V3 cathode follower ground brought full volume back to the lead channel (now the cathode is 124v instead of 75v). Both channels are full volume. No hum or any noise. The lead channel is perfect now at cranked volumes. Sounds great. The clean channel below 1/2 volume sounds great but some crackling remains at 1/2 volume and up while strumming hard. I probably have some more ground issues. I may re-do all my ground wires. I may also run the other two pre-amp grounds back to the filter cap for more of a star ground. Before this added ground wire I re-heated all the ground solder joints enough to make them flow nicely. I'm not sure why they would not be conducting that well. But I am making progress. I ordered some new Kester flux to re-do the filter cap + connections.

[JPK]

Maybe try some careful sanding on the capacitor tabs to break the oxide? I know from brazing and oxy/fuel welding that clean metal is crucial for the solder to stick.

/Max

I know you said "careful sanding" but I'm always leery of sanding an electrical connection. I'm afraid I might remove some tin/gold/silver coating or whatever they coat it with. Maybe cleaning it really well with alcohol and using flux would have been a good idea. Maybe I'm too paranoid. 

[JPK]

Sound clips to demonstrate the amp sound, obviously not my playing capability. Also just used a phone camera. Other than the issue on the clean channel I really like the sound of the amp. I'm sure I'll get it ironed out (see previous posts). To my ears it really does both clean and dirty very well. And having foot switchable channels is pretty cool. The AX84.com High Octane amp I built doesn't sound nearly as good.


Clean channel using a Tele with Fender Nocaster custom shop pickups in the neck position. You can hear the break up and crackling at greater than 3/4 volume.

eature=share

Clean channel with the same Tele/Nocaster pickups using the bridge position.

eature=share


Lead channel using a Hamer Studio with a JB bridge. No issues at all (well except the playing).

eature=share
.

[sluckey]

I suggest drilling a hole in the chassis near the input jack. Mount a ground lug with screw and kep nut. Run a short wire from that lug to your pot buss. This will provide a secure preamp ground point and not rely on the questionable ground provided by soldering the buss to the back of those pots.

You can see my rosin paste flux in the white can in this pic. I don't see my liquid solder but it's in a small squeeze bottle with a long needle nozzle for  applying a small drop where needed. Notice the #0000 steel wool and sandpaper that are used as soldering aids too. And I have an assortment of small wire brushes that occasionally come in handy when soldering.

[Esquirefreak]

I would guess its plated copper. As long as it's not aluminum or stainless (which is highly unlikely) there's no danger in removing some of the plating. With some pots and alike you have to scratch the surface a little to make it sticky.

The flux has a much better chance to stick if the material is roughed up a little and base metal is exposed. The solder then protect the base metal/soldered surface from oxygen if done properly.

Also high heat helps get it done quickly, rather than heating up and possibly damaging components.

/Max

[JPK]

Thanks for all the tips on prepping the components for soldering. I did actually clean up the backs of pots and buss wire with sand paper and alcohol. I should have done the filter cap connection tabs too. There were some resistors and caps that had pretty corroded leads as well. I cleaned up on a few bad ones not all. Another thing I did was used the tops of the turret holes for the resistor and capacitor leads, I soldered most of the wires and jumpers to the sides of the turrets using half loop hooks. Isn't this ok to do? Especially for the grounds?

Slucky I will do what you suggested with the input jack (material was just ordered). The way Mojo did the grounding on this amp is not my favorite. I know I did things wrong but their grounding scheme has something to be desired. The amp is quiet but having a ground to chassis right near the input jack is pretty basic knowledge. I was not a fan of it before I started the build as my opening post points out. So I agree with not using the pot mounting to chassis for a chassis ground to the input jack.

Oh yeah after getting the clean channel sorted out I'll buying a head cab. I'll also buy an empty 2 x 12 closed bottom and buy some speakers for it. Maybe a Greenback and a V30. That's going to be a tough choice with all the nice speakers out there.

[scstill]

BTW, that ground buss on the pots does not look soldered very well. Several solder joints just look like they are merely "stuck" to the back of the pots and could be easily flicked off with a jeweler's screwdriver or solder pick.

I read on Fralin pickup site that there should only be one ground connection, otherwise there will be ground loops (https://www.fralinpickups.com/2018/11/12/understanding-guitar-grounding/). So why in amp building do we often see a ground buss between pots that are also connected to the chassis. Is this good practice or a possible source of noise, unless of coarse the pots are lifted from the chassis.

[shooter]

we often see a ground buss between pots that are also connected to the chassis.

the pot housing is isolated from the POT guts, so the chassis and back of POTS are basically chassis, adding a wire should still keep it at chassis potential, except when it's not soldered well, hence a common star point as Sluckey shows

[Esquirefreak]

A good thing to do whenever one is soldering wires/leads to a turret or a bus wire, is to wrap it (at the very least) a little more than halfway around, then pinch the component lead with pliers so it stays in place. That's what's called mechanical connection.

Solder isn't suppose to hold things in place like glue, but rather protect the surface from the atmosphere and air.

/Max

[JPK]

BTW, that ground buss on the pots does not look soldered very well. Several solder joints just look like they are merely "stuck" to the back of the pots and could be easily flicked off with a jeweler's screwdriver or solder pick.

I read on Fralin pickup site that there should only be one ground connection, otherwise there will be ground loops (https://www.fralinpickups.com/2018/11/12/understanding-guitar-grounding/). So why in amp building do we often see a ground buss between pots that are also connected to the chassis. Is this good practice or a possible source of noise, unless of coarse the pots are lifted from the chassis.

Yep Randall Aiken also likes that grounding system. I'm a big fan too. My first amp used that grounding system and it had none of these grounding issues I'm having on this one. It was my first build. Each section of the amp had its own ground wire back to the star center (and that pot bodies were not bussed together). As Randall mentions it can make sort of a rats nest with all those ground wires (making a long straight bus on the turret board is cleaner, did on my second amp, no issues). If you look at my other thread you'll see I was questioning Mojotone tech support on this grounding right from the start. Namely the input jack. But I wanted to build it as they designed it, and see how it worked. Now that it's done I'm going to try some things that have worked for many people. Like a separate chassis ground for the input jack as close as possible (per Slucky's post above).


If I did this amp again I would skip the buss wire on the pots and just use one continuous 20 gauge bare bus wire on the turret board. No cuts, just wrap it around each ground turret that it needs to contact. I would ground one end of it closest to the output stage to the filter cap ground (start center). I'd do the input jack as a separate chassis ground as mentioned above. 


https://www.aikenamps.com/index.php/grounding
.

[JPK]

A good thing to do whenever one is soldering wires/leads to a turret or a bus wire, is to wrap it (at the very least) a little more than halfway around, then pinch the component lead with pliers so it stays in place. That's what's called mechanical connection.

Solder isn't suppose to hold things in place like glue, but rather protect the surface from the atmosphere and air.

/Max

I tried my best to do that but could not always get the needle nose in there to curl it around tight. For the wires to the tubes I had to use scotch tape to pull on the wire while I soldered the turret side. Tricky.

[shooter]

could not always get the needle nose in there to curl it around tight

start by passing about an inch or more through the hole, use round-nosed needle-nosed pliers and sorta thread like you would in sewing.
continually rotate the wire in a circle, a good stitch with have 2 full wraps
have the rest of your needle-nosed ready, use as needed.  Don't feel shy filing the needles to fit

[PRR]

....leery of sanding an electrical connection. I'm afraid I might remove some tin/gold/silver coating or whatever...

You won't have gold or silver on any commodity R/L/C part. At best they "tin" it with the cheapest tin/lead they can find. When fresh this is very solderable. If it has been warehoused downwind from the sewage farm (actually most urban atmospheres) the surface is contaminated. Remove the surface gently but thoroughly, then "tin" it with flux and solder before it oxidizes again.

[shooter]

some of the "hard to solder" metals seem to have a sheen of oil, degrease that and things go better, don't degrease the pot guts though

[JPK]

some of the "hard to solder" metals seem to have a sheen of oil, degrease that and things go better, don't degrease the pot guts though

All my pots had a sticky goo on the outer shell. I was sure to clean that with alcohol. Then I sanded them and cleaned with alcohol again. They were clean. The cap terminals were not taking solder at all. I should have scrubbed those really good. Lots of lessons here. I enjoy it.


Do you have a pic of the previous post were you were describing the buss wire threading? I was having troubles seeing it.

[sluckey]

I float the buss behind the pots. Here's one example...

     http://sluckeyamps.com/supro/s-10.jpg

Plenty more examples on my website.

[JPK]

I float the buss behind the pots. Here's one example...

     http://sluckeyamps.com/supro/s-10.jpg

Plenty more examples on my website.

Very cool. Never saw that before. I saved your website. I could get lost in there for hours. 

[shooter]

 
time well spent

[JPK]

Can anyone explain the output stage of this amp? I realize it's a low power tube (12BH7) with this 1 watt amp but it looks like push-pull. Doesn't push-pull need a phase inverter? There is none. Both grids get the same waveform. The OT looks like it's for push-pull.


The first schematic is the amp I built in this thread. The second one is the AX84.com October Club for comparison which is a push pull amp with a phase inverter.

[sluckey]

Both grids get the same waveform.

No they don't. Look closely. The grid of the bottom tube is grounded. The input signal is applied to the grid of the top tube and is inverted at the plate. That same signal appears on its cathode and also appears at the cathode of the bottom tube but there will be no inversion at the bottom plate. Now you have your out of phase signals to apply to the push/pull transformer. It's magic!

[PRR]

> It's magic!

Peeking behind the curtain: it is the "Long Tail" phase splitter, but with a very short tail and a very heavy load for a small twin-triode. It is significantly unbalanced. That may be A Good Thing.

[JPK]

I have a lot of studying to do. I'll come back to this when I can ask more intelligent enough questions about it. Right now I have at least 3 or 4 "whys" lined up after reading both of your replies. Thanks for the awesome info as always.

[JPK]

I suggest drilling a hole in the chassis near the input jack. Mount a ground lug with screw and kep nut. Run a short wire from that lug to your pot buss. This will provide a secure preamp ground point and not rely on the questionable ground provided by soldering the buss to the back of those pots.

-I just made the above change and the crackling issue on the clean channel at 1/2 volume and higher is still there. No improvement.
-I added another ground wire from the clean channel pre-amp turret ground to the filter cap ground. No improvement.
-Just by a fluke I discovered that the problem went away when I drove the amp input with a Boss GT-001 effects processor (using the amp's clean channel). Not surprised of course. It seems the clean channel is having issues with signal strength.


I'm not sure what more to do. I went over the entire amp again re-heating/re-flowing and questionable solder joints. Maybe I should check everything in the clean channel signal path again for soldering issues. Is there anything else I could do?


Thanks

[JPK]

Anybody? See previous post. I attached the test voltages I'm getting (green text). All are very close to the expected test voltages (black text). But is the 1.02V I'm getting at the first cathode of the clean channel too low? Supposed to be 1.26V according to Mojotone. See big red arrow on attached. Not sure if I swapped out with a higher resistance if that would bring the bias voltage up? I've tried everything. I even swapped out the first tube with a 12AX7. Just seemed louder. Everything works fine except that crackling on clean channel at over 1/2 volume.


Thanks

[Esquirefreak]

A 12AX7 has more gain than a 12AY7, so it it's perfectly normal that it got louder when you tried that tube.

As for the voltages, I doubt that .2 volts makes for a drastic change.

/Max

[JPK]

A 12AX7 has more gain than a 12AY7, so it it's perfectly normal that it got louder when you tried that tube.

As for the voltages, I doubt that .2 volts makes for a drastic change.

/Max

The tube swap was only to test for a bad tube. It was worth a shot but that wasn't it. Thanks for your feedback on the cathode voltage. I have run out of ideas. The amp clean voicing is beautiful, just would like to get rid of the crackle at high volumes.

[JPK]

Well looks like I found it. A bad/damaged output tube socket. Wiggling the tube around with a chop stick temporarily fixes the problem. So I'll swap it out with a brand new socket. It was weird because all the solder joints were solid when you pushed each one with a chop stick, no crackle. They looked nice and shiny, like good solder joints. Then move the entire tube and crackle crackle. I could move the tube into a position and get full volume with no crackle. But if you moved it, it reverted back to crackle.   

[sluckey]

A little squirt of contact cleaner or deoxit will possibly fix that.

[JPK]

A little squirt of contact cleaner or deoxit will possibly fix that.

That was a lot easier than replacing a socket, thanks! Unfortunately it didn't fix it. I just happened to read a thread about Deoxit D5 recently and picked up a can. So I squirted a bit in the socket and ran the tube into the socket a few times wiping off any dirt. Did that a couple times. Cleaned off the tube pins too. Used a can of compressed air to blow through the socket to dry it off. I was able to get the crackling to stop by only by moving the tube around to a perfect point. Then I can play away and it doesn't do it. If I mess with the tube position again, it will return. Bizarre. Maybe it's a bad tube? I don't have any spare 12BH7's.


Edit: I had some pretty tight sockets and the tubes were hard to get out the first time. I wonder if I damaged the tube pins yanking on them. That or maybe too much heat on the sockets when I soldered. Not sure. Could be the socket or tube.

[Esquirefreak]

Any luck using a spring loaded tube shield to hold the tube in place?

/Max

[JPK]

Any luck using a spring loaded tube shield to hold the tube in place?

/Max

You know I thought of that too but this tube gets kind of hot being used as a power tube (the preamp tubes do not). Do you think there is there an issue with heat with the shield or does it actually help dissipate the heat like a heat sink? The shields I have at home are too short, I just tried them on. I couldn't push them down all the way since the 12BH7 is so tall. I tried two different sized shields. Mojotone didn't send a shield for this one. It's a great idea though. The spring would probably keep the tube nice and snug and centered. I went to check the web to see they have taller ones that fit these and found the below. It does say it's for the 12BH7 but not sure if my application is higher heat than normal preamp duty.

https://www.amplifiedparts.com/products/tube-shield-9-pin-aluminum-extra-long


One of my Celestion Cream Backs arrived today (a G12M-65). I have another one coming in a couple of days but a G12H-75. The combo in a 2 x 12 should sound killer with this amp. I found some YouTube videos comparing them and decided I wanted one of each. The M is a little more vintage with great woody mid-range. The H has a little more high end and tighter bass.

[JPK]

Here's my budget 2 x 12 bottom. $140 shipped (without speakers). The Celestion Creambacks were not budget but that's ok. They sound awesome. Well not the right one as it's on back order. I plan on getting a head cab but that will be on next months budget.

[sluckey]

I like the looks just as is.

[JPK]

You know, I may just leave it. Hate to cover up all that cool stuff on top.

[JPK]

Ok I got the amp back from my amp tech friend last night. He said when he plugged in his guitar, he knew immediately what it was: Blocking distortion. He said it wasn't my wiring but rather Mojotone's design that was the issue. So he added a 220k ohm grid stopper on the V4A output stage (see attached). Now it has full volume on clean and the amp sounds great. He suggested to experiment with changing the .022uf capacitors to 0.002uf on the lead channel first two stage plate caps (to tighten up base? can't remember). Also adding 100p next to the .022 (see attachments for schematic red marks). My head was full of information when I left.


Just for fun he tested a bunch of different preamp tubes in the clean channel first stage (it came stock with a 12AY7). We tried a ECC803S and the 5751. The 5751 at full clean volume sounded like one of of the best over driven rock tones that I've ever heard. I'm definitely buying one of these. It's a totally different sound than the 12AX7 or 12AY7. Meaner and darker.


He also gave me plenty of tips for building future amps. He does service on vintage guitar and studio gear so he has seen where they fail and knows all the tricks to building a solid amp. Many of these tips you senior guys have mentioned on this forum (repeatedly no doubt).

[JPK]

And here's the final product. My 2nd Celestion Cream back finally arrived last night and is installed in my 2 x 12 cab (G12M and G12H). The sound from this head and bottom is the best I have ever owned. The low end in high gain is just huge and tight. Dime the volume and gain and it just makes you smile. Awesome tone. Clean is nice and chimey then breaks up nicely at full volume. Can't wait to get some time on it and break in those new speakers. Celestion is not cheap but I'm really glad I splurged. So worth it. Now onto my 5E3 project. Parts ordered. Build thread coming. 
 
PS: Thanks to all that helped with build issues. Especially Sluckey for finding the first bad solder joint.