Mojo 5E3 sound problems

[Rocketrob]

+1

I measure all the resistors too. The color bands don't always look obvius to me.

[Diablo]

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)

Same here, and I also check the caps too.

[Resonator Guy]

I will check resistors and caps before installing them now as well. I will read them more carefully too.

I have a bit more work to do on this. The second channel quit after installing the chassis in the cab and I hope that is just a bad connection that I can find easily. It was working on the bench.

There is some hum, not 60 cycle but higher pitched. After doing some more reading I figure I have too many ground connections. I want to try running the filter cap grounds, the ground for those artificial centre tap resistors for the Filament winding, and the cap off the ground switch to where I have the PT and the AC line grounded, and maybe leave the bypass caps grounded to the input jacks.

How does that seem? (I won't ask how it sounds )

Other than the hum, which I can't hear when I am playing, the amp is going to sound pretty cool to me!

[Mandopicker]

Definitely seperate the preamp cathode/bypass cap ground connections from the power and screen filter cap grounds.

Also check the connection at the volume controls for the intermitant channel. This design has interactive volume controls and share a connection wire. One of the connetions may not be soldered effectively. You muight try the chopstick test and poke at it while running to see if you can make the channel come back. (BE CAREFUL and keep your hands out of there.)

Drain the FILTER CAPS before doing any major work around those areas.

[Resonator Guy]

Well another disappointment. I installed a terminal strip to do my grounding and moved all the grounds except the preamp bypass caps to it. And nothing!!!
Then I noticed the preamp tubes and the first 6V6 weren't lit. A tap with a chopstick on the lower left corner of the tag board resulted in a horrible sound. So I shut it down, drained the caps and I think I will leave it for a couple of days before I desolder and pull out the tag board.
At least I enjoyed it for a couple of days, even with the hum.

[Mandopicker]

That's to bad...won't be the first time this happens...exactly the situation that leads to those long sleepless nights.

I would suggest making the amp work again before removing the tag board. Even so much as just putting it back to the way it was...even with hum...just to make sure you know it works. Something else must have come loose during the last operation. This alone is good reason to remove the board.

On the other hand, if you feel that a connection at the bottom of the board is suspect...try the continuity tester of your volt meter first to see if the things that should be connected are. (If you hear the "beep," then there is a connection.) Check the junction of the 2 x 220K resistors and ground. In a standard 5e3, this junction is often connected under the board to the preamp and screen filter supply ground junction. These 2 x 220K resistors, on the other end both connect to the power tubes to pin 5 via a 1.5K resistor.

Also...no light in the power tubes...check the filaments...are the heaters working for the preamp tubes? Yours are not wired like the old schematic attached because you use the artificial center tap (2 x 100 ohm.) Check each solder connection.

* The photo is of a Deluxe-style amp I built that uses 6L6's instead of 6V6's. Gets agout 35 watts or so. It is a very quiet amp. Not pretty at this stage so early in my building learning curve, but quiet. Note the grounds of the filter caps. Funny it was the second amp I ever built and it fired up perfect first try. It was only after I tried to make it pretty that a similar scenario to your recent issue occurred. Now with almost 30 amps and numerous repairs under my belt, I seem to be a bit more careful. (knocking wood...)

Have fun and good luck.

[Resonator Guy]

I realise now that I shouldn't have tried to move all the grounds at once. One at a time with tests in between would have been better. I hope to have a look at it today and do some testing but with spring coming here in the frozen north it may have to wait until tomorrow evening. I hope I don't have to take it all apart. Oh and I did put it all back the way it was and it still didn't work so I hope I just messed up something simple.

[Resonator Guy]

I spent quite a bit of time measuring grounds and resistors and caps yesterday, and put everything back the way it was. I then checked all the connections except those under the tag board and I have found my main problem!!!!. A bad solder that the push back wire was hiding from my failing vision. I fixed that and the amp worked again. Then I started moving one ground at a time to my new terminal and testing in between. It gets better every time. I still need to move the filter cap ground, but I stopped there last night as I have noticed a tendency to rush when I am nearly done and that's what causes problems for me. However the hum is greatly reduced so far. I was also reading about twisted pairs for heater filament wiring. I will go back and do that too! I thought that was done just to make things pretty and twisting them would make it harder for me to trace things. I will do some cautious removal and twisting. Then I have to track down an intermittent static problem. Again, probably a questionable solder joint.
Anyway, the tone is there and it sounds terrific. It is also way louder than I expected. I have a couple of vintage store bought amps with two 6V6s and this will blow them out the door. Actually louder than I wanted but I am sure that will be fine.
A couple more days of work and I trust I will have it licked. The next one I do will come with instructions.

[mbratch]

Congratulations RG! I'm going to start working on my kit this weekend.

If you have the chance, maybe you can post some pics?

[Mandopicker]

Way to go...I knew you'd get it.

For the static issue, assuming it is not a sypmtom of a troubled output tube that got stressed because of the original 100k ohm resistor mixup, I would get some real good single strand shielded cable for the volume control connection to the first preamp tube. Be sure to ground the shield at only one side (the volume control ground.)

Good move knowing when to say when.

It should be smooth sailing and tidying up from here.

Have fun and enjoy that amp. Once you start breaking it in (speaker and cabinet) it will sound incredible.

[Resonator Guy]

Mandopicker, here is the wiring diagram I used. When you say the wire from the volume control to the preamp tube, do you mean the wire from the middle lug of either volume to the coupling caps? I am willing to give that a shot although our town of 4000 souls doesn't have an electronics shop, I will find some good wire.
I had thought about tubes getting damaged earlier and that was why I was using a pair of newer replacable JJs. I put in a pair of older tubes and yes, the static got less, but even better, the remaining hum just disappeared. It is dead quiet now. Still very loud though! I am still saving my best NOS tubes for when I feel it is really done.
I will still twist the filament heater wires, but I don't know if I need to carry on with changing the grounding of the filter caps. I imagine it wouldn't hurt.

[Mandopicker]

After looking at the schematic you are using...it reminded me of a photo I found on the web...some time ago. I would like to give credit to whoever shot it, but I do not remember where I found it. (Anyway...thanks to whom it may concern)

Take a look.

Attach the shielded wire from the far right tab of the volume control on the right...connecting to pin 2 of the second tube. If it was me, I would use 2 more small pcs. of the same wire to connect up with the next volume and tone controls and ground them at the nearest point. Do you see how they share a yellow wire in the photo?

All that said...if it sounds great...with just the normal level of hum...and the static is gone...I wouldn't really change a thing. Though shielded wire never hurt if used in the right places.

[Resonator Guy]

Mandopicker, the hum is gone, but there is still some static.

I do see now the wiring change you mean and will try that as soon as I can. I am going to check all my grounds and solder connections now as well, but I am really pleased with the amp. ( My wife thinks it is too loud, so it must be okay)

I have a 12AT7 in the first position because the only 12AY7 I had turned out to be noisy when hitting low notes. I have a Mullard 12AY7 on the way so that may tame it down a bit.

Once the static is gone and the speaker has been used for a few gigs, I think I may sell a few other amps that I won't need (HA!). Thanks for your ongoing assistance.

[Bruce / Mission Amps]

After looking at the schematic you are using...it reminded me of a photo I found on the web...some time ago. I would like to give credit to whoever shot it, but I do not remember where I found it. (Anyway...thanks to whom it may concern)

Take a look.

It's one of my proprietary pictures. Mission Amps LLC.
I don't know where it was found for general viewing on the Net but it is one of about +80 photos I store on a secure server for use by my 5E3 kit customers.
I include that "secret" URL with my 5E3 kits so when needed, my builders can review with me photos of portions of the assembly, procedures, hints & kinks, tricks and other commonly found "troublesome novice builders" issues.

[Mandopicker]

My apologies Bruce. I agree with your ownership rights as well as the concept in general.

Do you know if there is anyway to remove the image? Perhaps with some moderator assistance it could be done.

Again my apologies.