Mojo 5E3 sound problems

[Resonator Guy]

Well, after many delays I finished my Mojo 5E3 kit and I am terribly disappointed.
All I get is a very weak distorted sound out of it. I have rechecked all my wiring and solder joins and everything "looks" okay. No visibly cooked resistors or anything like that. I don't have any sophisticated test equipment but do any of you folks have ideas on where and what I should start troubleshooting?
The tubes are all good, I tested them in other amps. The caps and resistors were all new with the kit as were the Power Transformer and Output Transformer. I don't know where to start looking. Any help would be appreciated.

[loudthud]

You really need to get a Digital Volt Meter. They can be found for less than the price of a 12AX7. They are invaluable for finding what's wrong.

Any splashes of solder floating around in the amp? Remove them! A short at the speaker jack is common as are miswired input jacks. Check for wires that are too long at pots and tube sockets that are touching the chassis.

Any splashes or balls of solder floating around in the amp? Remove them

The 12AX7 or 12AY7 has two halves. Make sure you see the orange glow of the heater on both sides. Check that pins 4 and 5 are connected together on the socket.

Other than that, try taking some pictures. Use a flash or lots of light.

[Resonator Guy]

I do have a digital multimeter, I just am not sure what to check. I will go through the amp again today and recheck the stuff you mentioned. I can't take any pics right now as my camera is away on a trip, but as soon as it gets back I will get some.

[Diablo]

Check the resistance in the circuit and compare it to the correct values according to the schematic. For example, if some point on the board has a ground wire soldered to it, you should be able to measure close to zero ohms between that point and the chassis. Do the same thing between other points in the circuit to test for proper resistance. You may find a poor solder joint or an incorrect resistor or a wiring mistake.....it happens.

Do all of this with the amp power off and unplugged, and the power capacitors drained of all voltage.

If everything looks OK, then you can power it up and check voltage at the tube pins and at various points in the circuit to see if the voltage match the schematic.

[Resonator Guy]

Diablo and Loudthud, I checked for loose solder and all the tube socket connections, as well as input and output jack connections and they all look okay. I then made sure the caps were drained and measured as suggested all the ground points. They are all 0.00 except for the filter caps which read 0.01 which I will touch up and re-measure later.
For the next step, I have never measured live voltages in an amp. If you could walk me throught that at some point, then I will give it a whirl. Thanks.

[Mandopicker]

I would carefully measure each resistor to make sure that the value is correct relative to the color bands. I have recieved incorrect resistors in the past because the part picker at Mojo (frankly other suppliers too) selected the incorrect resistor. Orange can look like brown, etc.

Also, pay careful attention to the power tube cathode resistor and capacitor. (Connected to pin 8 of the 6V6)

Observations...when amp is on...does the power tube look like it is red-plating? At this point, it is easy to find out. Just turn the amp on and watch for some time. Note: Not all tubes are equally liked by every amp...simply testing in another amp may not be enough to know that it works under similar load when in the 5F1 circuit. I would try another tube from a known working amp.

This is a very simple circuit with few chances for error if proper care is taken. Just work your way back through each connection and you will find the problem.

Rarely is it a huge issue...not likely defective transformers.

This is the time when you learn the most. Enjoy the troubleshooting. it will serve you well in the future.

Good luck.

[loudthud]

Check this thread.

http://music-electronics-forum.com/s...cks+resistance

Do the checks I outlined in post #14. That will make sure the guitar signal is getting to the first tube.

[Resonator Guy]

Thanks Loudthud, I did those checks and got absolutely nothing. Here is how I did it, so tell me if I did wrong. With power off and no cable I put the red probe from my meter on pin 2 (and then pin 7) of the first preamp tube. The black probe I put onto the chassis. I tried a number of multiples on the ohmmeter but didn't get any reading at all for either input. So that suggests no input is getting to the preamp, yet I do get very low volume distorted signal.??
And Mandopicker suggested I measure all the resistors. Some are fine within a small variance, however the 68k cathode bias resistors read as 34k. The pair of resistors coming off the pilot light should be 100k each and read as 50k each.
One 100k V1 plate resistor reads as 98k while the other (also 100k) reads as .1 on the same setting.
Same deal with the plate resistors for V2. One reads ok at 58k while the other reads .1 at the same setting.
For the 4.7k 5watt resistor, I can't get a reading at all.

Last thing, the power tubes are not red plating and I took a NOS pair of Delcos I am saving until I get this working and they worked just the same way. Lit up nice but only a weak very distorted output.
The preamp tubes are lighting up on both sides as they should.
That is all I have tested so far and I do appreciate all your advice.
What might I try next??

[dwhutchens]

Here's an idea. Find the wire that goes from the wiper of the tone pot to the grid of the second tube. Unsolder it from the tube. Take a spare 1/4" jack and a couple of aligator leads. Connect the tip to the grid and the ground to the chassis. Hook up your guitar. You should get a good clean tone, not really loud but clean. If so you need to concentrate on the preamp circuit. If not the its in the power amp. That test will basically split your amp into two halves and focus your troubleshooting into a smaller area.

Good luck,
David

[Mandopicker]

The pair of resistors coming off the pilot light should be 100k each and read as 50k each.

Resistor values can vary, but when they are 100 or 1000 x's the correct value...lots of bad things can happen depending upon where they are.

Are you sure these resistors measured 100K? Are they brown, black, yellow or are they brown, black, brown?

They are the artificial center tap for the Filament winding. They should 100 ohm (brown,black,brown) not 100k ohm (brown, black,yellow).

Also, some resistors can only properly be read when on end is lifted...for the sake of exploration...don't be afraid to lift one end and measure it. Doing so to find a solution can save you MANY a sleepless night.

Just talking through the design will help you solve it.

P.S.
Did you measure the cathode resistor? I think you received a 470ohm 2 watt, perhaps. This bias's the 6V6. Being that the amp is Cathode Biased, follow the wire from pin 8 of the 6V6 to the board to find that 470 ohm resistor in parallel with a cathode cap. (Value of 25uf x 25 or 50 volts.)

Continued good luck!

[tubeswell]

Read the Jack Darr book - really simple to read, includes chapter(s) on troubleshooting

http://www.pacificrecone.com/JackDarrBook.html

Firstly you need to check that you are getting a VAC supply from your PT secondaries (i.e.; the HT winding, the heater wining and the rectifier winding, and the bias winding/tap (if your amp is fixed bias)) with the amp switched on. Be careful taking these measurements.

Then you switch off, plug in the rectifier (if it is tube rectified), and switch back on and check you are getting a high VDC on your B+ line. If you aren't getting this, it could be one of your filter caps is shorted.

Then you switch off, plug in your other tubes, switch back on and check the idle plate, screen and cathode DC voltages of all the tubes. Have your DMM hooked up between the respective socket pins and the amp chassis - you are measuring the voltages from these pins to ground. I suggest you use 'gator clips to secure your DMM, and switch off between each reading. Make sure the clips don't short across to other pins.

Also check that your heaters are wired up properly and you are getting 6.3VAC across the respective filament pins on all the sockets.

Read the Jack Darr book, do the measurements and report back with all the DC voltages

[Diablo]

Thanks Tubeswell for the link to the Jack Darr book.
I'm going to print that out for study in my "library"....

[Resonator Guy]

It works!!!

Well, I have made some changes on my workbench. I have a much better pair of glasses down there now, and from now on, my charming and talented wife (who backs me up on lap steel) will be checking my resistor choices.

Mandopicker, that was an excellent call on those resistors. I had my wife, who sees better than me, check them with me and I had switched a pair of 100k resistors with a pair of 100 resistors. I don't think I can blame Mojo for that error.
I made the change and the amp just about blasted me out of the room on 2.
Everything seems to work but I want to go back and tidy up some of these changes as I did have it all looking very neat and tidy before this.
There is a little hum, but nothing too oppressive. I will try it in the cab with no fluorescent lights around before I decide if it is ground loop issues or what. The big thing is it works and sounds terrific!!!

Tubeswell, I do have that Jack Darr book. It was too heavy going when I got it, but I am going to do some checking with it now so I can gain a bit more capability.

Loudthud and Diablo gave me some good stuff to think about that will save me hassles in the future. DWHutchens, I didn't get to try your test but it is filed away for future use.

To everyone who provided such helpful comments, THANKS A BUNCH. This forum is a pleasure and there are a lot of knowledgeable folks out there who are willing to help.

I think I will go out and get a couple of premium beers for later today after I have tested in the cab with its new Cannabis Rex speaker.

I am sure I will be back with further questions in future.

[Mandopicker]

Way to go! The colors can be trouble...just measure twice and solder once. It has served me well when not entirely sure from visual inspection. Now, have fun make it all look pretty.

* Funny, before reading of your success, I was thinking about my suggestion and I actually mispoke about the cathode resistor value...For some reason I was thinking 5F1...in a 5E3 it would probably be a smaller value. Anyway the problems are all solved in the same way no matter the circuit and you did it.

Enjoy that new amp. I look forward to following your next build. Yes...there will be more.

[tubeswell]

FWIW I check all my resistors with a DMM before I install them, just to be completely sure. (I don't trust that colour system - some peep's colours are different from others)