Troubleshooting help? mojo 5E3 buzz on low notes

[Ben]

Hi,

Sorry for all the posts and a new thread. I thought i had my problem solved but its returned and it doesnt relate at all to the title of my other thread, so hopefully its okay to start a new one with a more accurate title descritption.

I'm hoping you guys can offer some troubleshooting suggestions for my Mojo 5E3 tweed deluxe kit amp. I am new to this so bear with me if you will.

Heres the problem:
On low notes I get a low level buzzing, almost radio static like fuzz after the note. It will also make this noise at the tail end of chords that are held for a few seconds. The amp does not need to warm up to exhibit this behavior.
High notes are clear as a bell. The signal is strong and the amp sounds absolutely gorgeous except for this unwanted hissy buzz on low notes. Its not vibration in the room, its definetly coming from the amp. The amp seems well grounded and is silent with nothing plugged in. It is not noisey while playing except of course for the problem mentioned.

Heres what I have tried thus far:
Tapping on the tubes or chassis does not affect this behavior. Changing preamp tubes with a known working one did not affect this behavior. I have not yet tried swapping power or rectifier tubes as i do not have any to try. I have not yet tried it with another speaker but will do so tonight. I have tried it with another guitar and with another cable and there was no change in this behavior. I opened it up, drained the caps and inspected my soldering. I inspected the soldering on my tube sockets also and couldnt see anything amiss. nothing loks blown, there is no evidence of arcing or burning or anything obviously loose. B+ voltage was measured and tho i cannot remember the number my friend told me it was in the correct range.

Heres the history:
When I first built this kit I had wired the input jacks incorrectly and was getting loud self oscillating feedback. I am wondering if my speaker was overly stressed by this noise? I also had miss wired the speaker jack and I had inadvertantly plugged the speaker into the ext speaker jack while testing it with nothing in the main speaker jack. I am wondering if maybe i did something to the OT? The low notes and the nature of the buzzing makes me think it may be related to vibration.

If Ive left out any info or you need some pictures of anything lemme know.
IM looking for some rough ideas of what my problem might be, where to start looking, maybe a good troubleshooting strategy, what to try first, ect, any help at all would be svery much appreciated. Thanks so much!

-Ben

[MWJB]

Down and dirty OT test, measure DC resistance from each power tube plate wire (pin 3)to OT centre tap (rect pin 8). If you have a significant difference (>15%) in ohms it might indicate shorted windings. But, I doubt that the amp would sound "absolutely gorgeous" with a blown OT.

If you think the problem is related to vibration, try another speaker cab.

Yes pictures of everything please, circuit board, pot wiring & tube socket wiring. Sounds like a parasitic, it's possible that all components are fine, just an oversight in wiring/layout.

DC at 6V6 pins 3, 4, 8, preamps pins 1, 6, 3 & 8 would be good too.

[Ben]

Thanks! I'll get some pics and those measurements up tonight.
On the voltage measurements, Ive never actually taken measurements like this myself. Question: Amp is plugged in and turned on and I stick black probe to chassis and red probe to pins? sounds scarry.
If it is a wiring or layout issue...im gonna be looking hard at the speaker and input jacks as that is where i had much confusion.

Heres some pics of the amp in progress as i was building it, but as I said, hopefully better pics to come:




obviously not done yet in this shot, but hopefully gives you an idea of my lead dress?

here are the input jacks BEFORE i corrected my mistake. All thats change since this shot is that the "switch" lugs on input jacks 1 are now connected to the ground lugs on same jacks. The ground lugs on the second set of jacks are not connected as they are grounded to the chassis (maybe this is my problem?)

Note that 2 grounds from preamp section are grounded at input jack #1
Everything else (except pots and jacks)is grounded together under transformer bolt.
pots are grounded to pot backs, jacks are grounded by contact with chassis.
OT black wire goes to ground lug on speaker jack.

Okay, more and better photos of the finished amp tonight and hopefully those voltage measurements as well. Thanks so much!!
-Ben

[PRNDL]

Have you measured the bias of the output tubes?

[Ben]

Update:vibrations or speaker

I drove home on my lunch hour and had just enough time to try it with another cabinet and the speaker unplugged and it no longer made the noise.

So, there are a couple things I am thinking about.
-the speaker itself
-the wire and male plug from the speaker
-or vibrations from the speaker causing a tube, tube socket, the entire board, or some component to rattle and emit this radio static buzz.
(the nature of the sound points me towards the tubes, but i dont know what a vibrating board or capacitor would sound like...or even a vibrating tube for that matter)

I am back at work now on break. I have an epi valve jr. head at home , and that could be used to test the speaker tonight , but I am kind of suspecting it may be vibration of a tube or something else in the chassis.

Speaker wiring, was white to positive on speaker and tip of L shaped plug, black to negative and then soldered to plug housing back underside, electrical tape was used to keep the two connections from shorting in the plug housing. It was a very tight fitting and lame right angle plug. i should replace it with a decent straight one.

Heres some other things i found odd about the mojo kit. Maybe they are relevant to this problem.
-Nuts for "chassis straps" were provided...I had to google "chassis straps" to see what they were....but no chassis straps were provided with the kit. I searchd but couldnt figure out if chassis straps serve a purpose other than ornamental? so i have leftover nuts from that. do i need chassis straps for anything other than looks?
-the bolts provided which i assume were for the tube sockets, did not fit thru the tube socket mounting holes, no other screws were provided that were even close. I had to go to the hardware store and get smaller diameter screws and nuts and lock washers to replace them. They did not have kepps nuts at the hardware store like the ones provided with the kit so I went with locking washers. so i have leftover bolts and kepps nuts from that. Does it sem possible or lkely that the tube sockets could be rattling at certain frequencies because of this? I thought i screwed em down pretty tight.
-there were four bolts coming from the baffle for the speaker, but eight holes in the speaker..this I assume is normal?
-no hardware was provided for mounting the boards, no holes were drilled in the board or in the chassis for this purpose. another trip to the hardware store. Not being able to find self tapping screws there, I bought self drilling screws. Nevertheless they seemed to tap the pilot holes and the board seemed very secure in there. Maybe these arent secure enough and the board is vibrating? Maybe i need some white glue on the screws to stop the vibration as a friend had suggested i do befoer I bult the kit ?(it seemed snug enough, plus I forgot)
-I did not use anything for spacers between the two boards figuring the solder blobs and wires running thru the back of the component board would provide the spacing between the two boards. This seemed to work fine physically. but maybe I need spacers bwteen the two boards? again none were provided with the kit.
-no tube sheilds were provided with the kit (I bought some and they are coming in the mail)

the power and rectifier tubes have not been swapped out and tested so i suppose it's possible one is bad and vibrations at a certain frequency bring up the bad behavior and cause the noise?

anyway thanks again. I'll report back after testing the speaker with the epi valve jr. head tonight.

[Ben]

speaker was fine so I opened it up and tightened everything up.
I tightened the screws holding the board in and on the tube sockets and that seemed to do it cause its stopped making the noise for now. Thanks again for the help.

[Bruce / Mission Amps]

Heres some other things i found odd about the mojo kit. Maybe they are relevant to this problem.
-Nuts for "chassis straps" were provided...I had to google "chassis straps" to see what they were....but no chassis straps were provided with the kit. I searchd but couldnt figure out if chassis straps serve a purpose other than ornamental? so i have leftover nuts from that. do i need chassis straps for anything other than looks?

**Chassis straps are only found on the Fender blonde, brown, black face and silver face amps... well, most of the modern vinyl covered ones use the strap too. None are used on the tweed amps but the NUT in the bag might be the same nut that they use to mount the tweed chassis to the cabinet... if so, I think it is wrong because the Tweed amps used a 10-32 nut and bolt, the other amps used 8-32 screws with a KEP nut.

-the bolts provided which i assume were for the tube sockets, did not fit thru the tube socket mounting holes, no other screws were provided that were even close. I had to go to the hardware store and get smaller diameter screws and nuts and lock washers to replace them. They did not have kepps nuts at the hardware store like the ones provided with the kit so I went with locking washers. so i have leftover bolts and kepps nuts from that. Does it sem possible or lkely that the tube sockets could be rattling at certain frequencies because of this? I thought i screwed em down pretty tight.

** Yes they can buzz... this why I went to PEM nuts to hold all my tube sockets in place. Buzzing: So can the jewel in the pilot lamp holder.
With my chassis, I Just hold the socket above the chassis and slip the screw into the socket and then into the PEM nut pressed into the chassis.
The right machine screw and nut combo is a probably 10-44. However, all the old ones used a self tapping 6-32 machine screws... perhaps they are using a 6-32 machine screw instead... and old sockets had bigger holes to use bigger screws.

-there were four bolts coming from the baffle for the speaker, but eight holes in the speaker..this I assume is normal?

** It is normal for KC (cabinet shop) at MOJO... he simple won't install the other 4 studs. With four you can use just about any 12" speaker basket layout but with 8 studs only baskets with all 8 holes will work.

-no hardware was provided for mounting the boards, no holes were drilled in the board or in the chassis for this purpose. another trip to the hardware store. Not being able to find self tapping screws there, I bought self drilling screws. Nevertheless they seemed to tap the pilot holes and the board seemed very secure in there. Maybe these arent secure enough and the board is vibrating?

** That is pretty dumb but sheet metal screws is the way Fender always did it. Again.. another reason I use PEM nuts to hold my eyelet board into the chassis.

Maybe i need some white glue on the screws to stop the vibration as a friend had suggested i do befoer I bult the kit ?(it seemed snug enough, plus I forgot)

** I doubt it. Just make sure they are tight.

-I did not use anything for spacers between the two boards figuring the solder blobs and wires running thru the back of the component board would provide the spacing between the two boards. This seemed to work fine physically. but maybe I need spacers bwteen the two boards? again none were provided with the kit.

** No, you did it right.

-no tube sheilds were provided with the kit (I bought some and they are coming in the mail)

** Well, they're not really needed in these low gain amps. If you local noise is high, they will help a little.

the power and rectifier tubes have not been swapped out and tested so i suppose it's possible one is bad and vibrations at a certain frequency bring up the bad behavior and cause the noise?

** I bet it ends up being the speaker after your low freq oscillation outbursts.
Oops I see you found the buzz....

[thumbs]

On my amp I had a problem with vibration from the upper back panel. Have you tried playing through it with the back panels removed? I used some foam insulation tape (for doorways, I think) to quiet it down.