MojoTone Studio One Watt

[GAStan]

Finished my first amp, built from a kit.  MojoTone was very quick to ship via FedEx,  and quick to ship parts the kit was short on: just a nut for the power switch.  Nice quality components.  JJ tubes.

It works!  Sort of. 

The dirty channel sounds great, nice crunchy distortion at low volumes.

The clean channel can BARELY be heard with EVERY pot dimed.  I double and triple checked all solder joints and verified correct wiring and component installation.

Troubleshooting will begin when I get home from work.  Plans are to:
1. Look over and check everything again now that I've taken a break.

2.  Check voltages.  A, B & C looked good when I checked prior to tube installation but D was low and would drop when I put the meter on it.  Wasn't sure if not having a tube in would affect this or not.  Didn't check them after installing the tubes just started playing through it.  My 13 year old son was with me so we just enjoyed using the dirty channel.  D feeds the clean channel so I'm pretty sure this is the problem, but if it's there...

3.  Break out the O'scope, dummy load & Sig Gen and signal trace.

Does this sound like a good plan?  I'm open to any suggestions, ideas or criticism anyone has to offer.  I don't have any pics of the amp, I'll post some this evening if anyone wants me to.

Glenn

Edit: spelling/grammer

[tdvt]

From your description, it sound like you need to sort out the power rail issue, if your B+ node voltages are not correct. How low is low (on B+D)?

Didn't see voltages on the drawings. Maybe you can post what you are getting..?


It looks like a fun little amp & a nice project to share with your son.

[GAStan]

Thanks tdvt.

I wrote down the voltages but they're at home on the back of the wiring drawing.

A was 345 VDC referenced to chassis common.  I can't recall B or C.

D would start at something like 200volts then drop, like when you measure the voltage of a capacitor that has been charged and you discharge it with the meter you are measuring it with.  It would slowly charge back up when the meter was taken off.  I agree this needs to be checked and figured out first. 

I was using a Fluke 113 meter, a Fluke but not their best.  This particular meter checks both voltages and diodes on the same dial setting.  This meter has shown some odd behaviors so I do not trust it. Can't really complain as it was a freebie with another item purchased for my job.  I am going to borrow a Fluke 87 to use today to make sure it's not an issue with my test equipment.

I'm rambling...just thinking out loud.

Glenn

[GAStan]

Problem found.  I wired the switching relay incorrectly.  Had the dirty channel and final inputs crossed.

Pic with yellow and orange wire to small circuit card is incorrectly wired.  Pic with two orange wires to small circuit card is corrected wiring.

Sweet sounding amp.  Dead quiet with no input and all controls maxed.  Still very quiet with humbuckers plugged into it.  Lots of 60Hz hum with son's Squier Strat plugged in, as expected.  Now it just needs played to let tubes burn in and new speaker loosten up.

Glenn

[sluckey]

A mis-wired relay could certainly account for no output from the clean channel. But what about that odd behavior when trying to measure voltage at node D? Operator error? Red herring? Something else???

[GAStan]

I did check the voltages.  D was steady with the tubes installed.  Not sure what was going on with it before.  Voltages referenced to chassis common, with tubes installed.  Nothing plugged in to input.

A 310 VDC
B 286
C 258
D 240

Some pics of the assembled amp.  I am just going to mount it on a board, no cabinet as it won't travel much if at all.

Glenn

[acheld]

I was writing this as Sluckey was hitting the Post button --

A Fluke 113 is a very good instrument.   An 87V has more range and is more accurate, BUT no components in a guitar amp are so high precision that an 87V is needed over a 113, or whatever.  (I'm not knocking the 87V, which is both robust and incredibly accurate).

You stated that yours was exhibiting unusual behavior -- that calls for a new battery and a spanking!  Either or both may or may not fix the behavior . . .

[GAStan]

A mis-wired relay could certainly account for no output from the clean channel. But what about that odd behavior when trying to measure voltage at node D? Operator error? Red herring? Something else???

That's a GREAT question, wish I had the answer. 

Operator error isn't out of the question, I'm the first to admit.  It is a simple voltage measurement.  Had the negative lead clipped solidly to chassis common.  Powered up the amp. Took the measurements.  3 were good, then D was odd.  I went back and forth between A, B, C, and D several times and they were consistent in how each read.  I can't think of anything I did to have caused it but welcome critique.

D acted like a capacitor that would discharge through the meter then recharge when the meter was disconnected.  The longer the meter was disconnected the higher the voltage would climb.

Two things changed between the strange readings and the good reading.  Tubes were installed and reading was taken with a different meter, Fluke 87V.  Personally I suspect the Fluke 113 meter.  Does it bother me enough to spend time to prove it for certain?  Not yet.  I simply won't use that 113 meter any more. 

The amp was working and sounding GREAT for the few hours I played through it last night.  If it begins to malfunction I'll certainly begin with voltage readings...

[GAStan]

I was writing this as Sluckey was hitting the Post button --

A Fluke 113 is a very good instrument.   An 87V has more range and is more accurate, BUT no components in a guitar amp are so high precision that an 87V is needed over a 113, or whatever.  (I'm not knocking the 87V, which is both robust and incredibly accurate).

You stated that yours was exhibiting unusual behavior -- that calls for a new battery and a spanking!  Either or both may or may not fix the behavior . . .

GMTA, it did get a new battery.  Wife gets mad when I spank things...

It's odd behaviour: it will NOT read a 220K resistor. Clip it on the leads and the meter does nothing.  Clip another value resistor in parallel it correctly reads the other value as if the 220K wasn't with it.  Put two 220K resistors in parallel it acts as if nothing was connected.  All 220K resistors were verified good with another meter.

No, I don't drink or partake 420 or anything of the sort.  Nothing against it nor anyone who does, just putting that out there.

I've used Flukes my entire professional career and trust the brand a great deal.  But I also realize the occasional faulty product is gets past QC and can't rule out this particular meter.  I didn't mean to slam all Fluke 113 meters, just the one I got as a freebie.  One thing it doesn't do though is current.  For this reason alone I would not purchase it.

[PRR]

....I was using a Fluke 113 meter, a Fluke but not their best.  This particular meter checks both voltages and diodes on the same dial setting.....

No, not according to Fluke's QRG. Put the knob on the diode&capacitor icon.

Yes, in addition to checking diode health, it gives a number for diode Forward Voltage. 99% of modern diodes, this will be in reach of 0.6V, but will hint at stacked arrays and other oddballs.

[GAStan]

....I was using a Fluke 113 meter, a Fluke but not their best.  This particular meter checks both voltages and diodes on the same dial setting.....

No, not according to Fluke's QRG. Put the knob on the diode&capacitor icon.

Yes, in addition to checking diode health, it gives a number for diode Forward Voltage. 99% of modern diodes, this will be in reach of 0.6V, but will hint at stacked arrays and other oddballs.

According to the meter in front of me the DIODE setting is the same one for VOLTAGE and CONTINUITY.  The CAPACITOR and OHM setting is the other one.