Slow Build...

[lavrgs]

Update; The amp is now working...but I still don't have the connection to V3 Pin6 because of the noise it was making when I connected it. I may go try to connect it again but I'd feel better if I knew what was happening - any thoughts?
UPDATE - as mentionedin previous post I am back to having sound
Still haven't taken all measurements...
V1
pin 1 -140.7
pin 3 - 1.10
pin 6 - 140
pin 8 - 1.11
V2
Pin 1 - 214.0
Pin 2 - 52.2
Pin3 - 82.5
Pin6 - 221.7
Pin7 - 54.9
Pin8 - 82.5
v3
Pin1 - 128.7
Pin3 - 1.33
Pin6 not taken
Pin 8 not taken
V4
Pin3 - 11.38
Pin7 -355.1
Pin9 - 336.4
V5
pin 3 - 11.44
that's all I got so far

[lavrgs]

Update; The amp is now working...but I still don't have the connection to V3 Pin6 because of the motorboating it caused when I connected it. I may go try to connect it again but I'd feel better if I knew what was happening - any thoughts?

According to the schematic the wire goes to the plate of V3 and is in the tremolo circuit. I have some ideas I may try - unsolder the foot switch jack leads - and see if there is a change...

[colossal]

Motorboating can occur when multiple gain stages share a common supply node. Fender is (in)famous for feeding sometimes four gain stages split between two channels from one power supply cap. Leo "got away" with it because these amps weren't high gain, but it is generally bad practice. Electrolytics were not cheap and plentiful back in the day and Fender pinched pennies. I don't know if that's the cause of the issue with your amp, however. In my 18W amps, I always decouple the preamp from the phase inverter by adding another R and C to the power supply. Separation of church and state.

[lavrgs]

In my 18W amps, I always decouple the preamp from the phase inverter by adding another R and C to the power supply. Separation of church and state.

I like the sound of that but I am a complete dwarf when it comes to the tech side - I'm just starting to have a basic understanding of schematics. I was able to find

PROBLEM.JPG

that the wire connected to pin6 of V3 causes my problem - only because after I "finished" the amp and started to check voltages I saw that I had forgotten to connect this wire and it was after I made the connection that the problem arose. The vibrato channel works well until I connect it. Since I'm not a vibrato fan I would just as soon leave it off but that would impede my education...

[colossal]

Yes I agree it's important to finish it as your real issue may not be what you think, so best to get it working and then pull the trem tube if you aren't going to use the channel.

I am confused by that layout picture though. Do you have a photo of your amp and a full layout drawing of what you are building?

[lavrgs]

I just checked the 0.47nf capacitor and I believe it is bad...is it possible this will cause the pulsing?
Here is a photo and the layout. There is a blue wire with a wire nut on it, just below the cap that I believe is bad, that I have disconnected from V3 pin6 - when I connect this wire I get a pulsing on the tremolo channel. With it disconnected the channel works.

[colossal]

Great, thanks for the photo and layout. I see what is going on now.

Did you lift that cap and check for leakage? Are you also getting good ground for your input jacks on that Trem channel? I know you hear us keep grumbling about that Mojo layout, but the shared ground bus soldered to the back of the pots is really a poor method. The bus should be suspended in air, parallel to the top of the pots with the various and sundry grounds tied to it, but there should be one lead that goes from the input jack that ties the whole bus to a single (preamp) ground point on the chassis.

Second, it looks like they are feeding the screens and the phase inverter off the screen node, another bad idea. And finally both channels have to share that single 16uF cap. After you verify if that cap is bad, you might try lifting the blue wire which is feeding B+ to the Normal channel, hook up the Trem channel, and then see if the Trem still motorboats.

Also, you may want to separate that grid lead from that anode lead.

grid wire.png

[lavrgs]

Did you lift that cap and check for leakage? I will wait for parts before lifting
Are you also getting good ground for your input jacks on that Trem channel? I will verify

Second, it looks like they are feeding the screens and the phase inverter off the screen node, another bad idea.
I'm going to have to ponder this - is there an example of a preferred layout?
And finally both channels have to share that single 16uF cap. Do other layouts use 2?
After you verify if that cap is bad, you might try lifting the blue wire which is feeding B+ to the Normal channel, hook up the Trem channel, and then see if the Trem still motorboats.
I'll give it a try
Also, you may want to separate that grid lead from that anode lead. Done

I'm drinking from a fire hose trying to get a better understanding of amp building best practices.
Is there a list of the MOJOTONE deficiencies and alternate options? I've gotten bits and pieces from this site but feel a little uneasy about having a complete understanding of the terminolgy.
The bottom line is that the amp is working and sounding good - just the tremolo is a problem

[lavrgs]

This is my daily dumb question; How do you measure the voltage on a rectifier tube? I burned a couple fuses and think it was my measurement technique. The voltage on pins 1 and 7 are AC - what do I ground to?
My voltages generally seem a bit high.
As a side comment I am very surprised how loud this amp is - compared to some thirty and fifty watters I have this hold its own

[colossal]

This is my daily dumb question; How do you measure the voltage on a rectifier tube? I burned a couple fuses and think it was my measurement technique. The voltage on pins 1 and 7 are AC - what do I ground to?
My voltages generally seem a bit high.
As a side comment I am very surprised how loud this amp is - compared to some thirty and fifty watters I have this hold its own

To measure the AC voltage on your rectifier, set your multimeter to AC Mode. Then connect your black lead (ground) to the chassis. With your right hand and your red probe, probe Pins 1 and 7. Keep your left hand either in your pocket or completely OUT of the way of the chassis and ground. That way if you take a shot, the path to ground is not through your heart. You should see 290VACrms or so (depending on your line voltage). Your voltage chart shows 311VACrms. Your line voltage is likely high. HINT: Measure your line voltage to see how much above 120VACrms it is. Looking at your EL84 plate and screen voltages, they are reasonable. I see you have two different values from Pin 3 on both tubes, but since they share one bias resistor (150R), I'm guessing you recorded those values at different times.

The measurements shown in the second 12AX7 position: if that is your phase inverter, your cathode voltage is quite high at 99V. It should be around 69V. With the cathode voltage that high, you may get some hum because you are very close to the heater-to-cathode datasheet limit for the tube.

[lavrgs]

Colossal I really appreciate your help.
Not sure why I was blowing fuses - I thought it may have been during the rectifier voltage check. I will give it another check - and thanks for confirming that what I was doing was correct.
My line voltage that I measure prior to taking the measurements was 124.2. How much variation should I expect on the voltages? For example my measurements on V1 were
pin1 140.7 (161.6) Pin 2 1.10 (1.33) Pin 6 140 (160.3) pin 8 1.11 (1.34) Should I have the volume up or down or check at a known condition?
I'll take another set of voltages when I remove the chassis from the case...probably in the next day or two.

[lavrgs]

I verified that testing the voltage on pin 7 of the rectifier blows the fuse. I will have to go figure out why.

[colossal]

I see in your posting of voltages above that you have a much larger range than you should have. The mains voltages for an area won't vary too much, IME anyway, so your plate and cathode voltages for an amp will be pretty consistent when everything is working as intended.

Blowing fuses is definitely indicative of a miswire or a blown cap or other issue. We have to go back to basics. Pull all of the tubes. Start at the very beginning by checking that your IEC power socket is wired correctly to the power switch. Check that the Load is your hot wire and your Neutral is 0 to a few mVAC. You need to be very systematic. Guys often jump all over in their excitement to get their amp running. Also, don't be in a hurry to change anything at first until you have verified the basic assumptions. Once you make sure AC power is getting into your amp correctly, then move on to the power transformer. Verify that each of the leads that should be grounded (e.g. the PT's secondary centertap, the heater filament string centertap) are grounded correctly and making both good mechanical and electrical contact. Energize the amp and then verify your floating AC secondary voltages coming from the power transformer to the rectifier tube socket (red leads tied to Pins 1 and 7). Start there and post!

Also make sure you have the correct fuses too....just saying...

[JMPGuitars]

Also make sure you have the correct fuses too....just saying...

This is a good point. Some of these kits use two fuses; one for mains and one for HT. The main fuse for an 18watter is usually 2A slow blow for US voltage. The HT fuse is usually 1/4 of that.

[lavrgs]

My Mojotone has one fuse - It was a 1 amp slo blow. I started with a lower value for start up purposes.
Any thoughts for troubleshooting the fact that when checking the voltage on the rectifier I blow the fuse? It seems I'm shorting to ground which makes me think I have a grounding problem. Been out for a while now I'm going to see what I can find out. I'm checking the rectifier voltage with all tubes installed should I remove all the other tubes?

[lavrgs]

I am trying to determine if my power transformer is ok - fingers crossed. Because I was blowing fuses when I tried to measure voltage I have removed all the tubes. I also found a problem with the heater wiring so I am taking a step back to figure out where I am. I'm
I need some advise for inspecting the pt. I have measure the resistance on all coils and I have no opens. I don't find any specs for the resistance but red to red 167.4 ohms blk to blk/wht 3.6 ohm and grn to grn .5 ohms.
My next step will be to disconnect all wires, add line voltage and check to see what I am getting. As you may recall I was blowing the fuse when checking the red wires - I have tried to find a short but it appears ok
UPDATE I tested the transformer and it works - whew so I'm going to redo the heater wiring and get back

[colossal]

My next step will be to disconnect all wires, add line voltage and check to see what I am getting. As you may recall I was blowing the fuse when checking the red wires - I have tried to find a short but it appears ok

This is what you want to do. Go back to basics. One variable at a time. Test each lead with respect to ground to verify that your PT is in good health.

[lavrgs]

UPDATED Voltage check - I no longer blow fuses when checking the pins on the rectifier. but pin 3 voltage was acting wonky, unstable, on the initial check. I let the amp sit for a while and now it's stable at 373 - it's not shown on the attached chart. I don't have a reading for V3 pin 6 because it's not connected.
After redoing heater wiring and disconnecting and rewiring the PT Still don't have tremolo because it's causing a ruckus... I once again lifted the wire going to V3 pin 6 and it's happier. I do have a lower volume on the tremolo channel vs. the normal channel and the amp does not seem to be as loud, after putting it back together. I haven't done any troubleshooting on that yet. I still haven't re-done my grounds to get them off the pots but I am happy with how quiet the amp is...

[colossal]

Your voltage chart is still confusing to me. Is the second line of measurements the phase inverter and the third your Trem channel? If V2 is your phase inverter, you have a very serious mismatch on Pins 1 and 6. These should be relatively close. My suspicion is that your Pin 6 measurement is erroneous as your Pins 3 & 8 (shared cathode) voltage is reasonable so Pin 6 should in reality be much closer to Pin 1's measurement of about 250V (still high, but due to your high mains voltage). The question of your EZ81 rectifier Pin 3 measurement being 36V is also wrong. 373V is high but reasonable. An unstable B+0 measurement sounds to me like you have an intermittent connection somewhere in your power supply; again, check each solder joint in the amp. The solder on a properly flowed joint should have a nice, shiny "mercurial" droplet look to it. It should not look dull and dark grey and be "collapsed" into the turret (a sign of overheating). JMPGuitars posted examples of properly and improperly soldered joints. Do not apply heat for long, you WILL damage parts. Run the iron hot, get in and get out.

Regardless, if you are getting an improvement in noise at idle, that's a step in the right direction. Sounds like you are making progress. Check out Explorerman's thread, if you haven't already, as he is dealing with similar problems in a Mojotone build.

[lavrgs]

As I wait for a couple parts I will go through the grounding scheme again. First goal is to remove the bus off the back of the pots - the plan is to use a copper buss wire for the preamp side.
Ground Confusion
My problem is that I am getting confused as I check where I have grounds and I am unsure if I should or shouldn't.
Pardon my first elementary knowledge on this but follow me through
Case 1 Input jacks - when I touch pin 2 & 7 on V1 they are grounded - I believe that is that correct - until the jack has a plug when they are infinite.
Case 2 Heater wire Pin 4,5 and 9 are grounded - technically there is 0.7 ohm resistance but I have my meter set to beep and it beeps. When I remove the center tap from ground I don't get a beep (infinite) - so I'm thinking it's ok.
Case 3 Speaker jacks - This one I don't understand...The speaker operates as expected but all 6 points of the connector are grounded until I disconnect the wire to the output connector and not have a plug in- I may be confused but I don't think there's a problem.

I have checked all the tube socket pins to make sure that only the heater pins are grounded.
I have checked the EL84 that I think is bad and the filaments are continuous and no other pins are shorted. I checked the EZ81 and it's the same filament pins are connected - that's all I can do at home.

When I get the new tubes I will do another voltage check to see if anything changes.