MojoTone 5E3 Fender Tweed Deluxe - HUM

[Janglin_Jack]

I helped my buddy build a 5E3 Fender Tweed Deluxe and I let him do all the soldering, etc. Got the amp working, but there is a deep hum. I took the amp from him and checked it over. I decided to make a new hand wired board with all new components. I wired it in and there is still a deep hum when the volume(s) are turned up past 2. The hum gets louder as the amp is turned up. Any ideas on what I should check?? I am at my wits end with this project.

Mike

[Roe]

check grounding and filter caps

[robert]

And don't forget to check the heater wiring (2x 100R or CT to ground)

Regards

Robert

[Janglin_Jack]

I am going to start by re-soldering the output jack. It looks a little sloppy and I am going to move the ground wire. I saw a couple other things so far, that I am not sure if they matter or not, so maybe these are something to change.

First, There is a wire going from the Rectifier Tube to the standby switch. A couple diagrams I have seen, show it going from Pin 8 of the Recto to the Standby Switch. Can this lead go from Pin 2 or does it have to be pin 8?

Second, I see two green wires, (Filament wires) going from the PT to pins 2 and 7 on the Power Tubes and then to pins 4 and 9 to the preamp tubes. In one of the diagrams, I see the wires go from the PT to the Pilot Light and THEN to the 6V6 P2/7. Ours has the wires going from the PT to P2/7 on the 6V6 and then on to the other tubes. We have two leads coming off the 6V6 P2/7 and twisted to the Pilot Light. Will this matter??

I guess there is a third thing, we didn't have a 12AY7, so we are using a 12AX7 in V1

I will keep checking.

Mike

[Janglin_Jack]

I re-did the output jacks and moved the ground wire and it still had hum. I changed preamp tubes, (tried a 12AU7 and 12AT7 and swapped the other 12AX7 and both 6V6). It just felt like the issue was at the tone/volume controls. I checked the 1M pots and they seem OK. But I checked the cap on the tone control and it reads 473J, (.047uf??). I think the schematic calls for a 472J (.0047uf???) Can someone confirm if this is correct, if so, I will get a new cap tomorrow and give that a try. Sometimes it is the simplest things, but I didn't check that value as it was the cap that came with the kit. Anyhow, I hope this is it!!

Mike

[Roe]

i dont think that cap will generate hum

[Janglin_Jack]

i dont think that cap will generate hum

This sounds discouraging, as I was really hoping this could be it. Do you think using a .047uf vs .0047uf cap there would put too much bass into the circuit and cause it to be unstable? I am still going to change and cap and see, but I was really hoping this could be it.

Mike

[Janglin_Jack]

And don't forget to check the heater wiring (2x 100R or CT to ground)

Regards

Robert

Robert,

I just figured out what you were saying here. Our transformer does not have a center tap. I guess I need two 100ohm resistors to create a "center tap". I found this on the web.

If you do not have a heater center tap on your power transformer, you must run two 100 ohm 1/2 watt resistors to ground to create an artificial center tap. If you do not have a center tap, you will get 120 cycle hum. Each 100 ohm resistor is soldered to one of the heater wires. The other ends of the 100 ohm resistors are twisted together and then soldered to ground.
If your power transformer has a center tap wire, solder that to ground. Most power transformer heater center tap wires are green with a yellow stripe. This sounds like my culpret. Deep hum - 120 cycle hum.

I haven't seen this in any diagrams, (I guess most kits have transformers with CT wires).

Mike

[Janglin_Jack]

Ugggggg!!! I put the two green wires 3.15 each from the PT to the light, then I ran twisted wires from the lamp to pins 2 and 7 of the first 6v6 tube. I also ran a 100 ohm resistor from each leg of the lamp and twisted the other ends together and then ran those to ground. I hums so loud now, I have to turn it off immediately. I clipped the resistors and I am back to the original hum. What am I missing here?

I remember once I did a Marshall build and I took the amp off standby and got this massive squeal. I reversed the leads coming off the power tubes to the 220K resistors on the board and problem solved.

Would the potentially be a phase issue by adding the center tap? I am wondering if I should try reversing the leads coming off the 6V6 tubes, (pin 6) to the board?? Anyone??

Mike

[Janglin_Jack]

I rewired the PT, Recto and Power Tubes, so now the whole amp has been re-done. It still has hum, but it is tolerable. So my question now is, with the lack at a CT for the 6.3 filaments, should I put in the 100 ohm resistors on the first 6V6 tube? I already tried putting them in at the lamp, but the amp went nuts with hum. I am afraid it will do the same thing if I put them in at the 6V6 tube.

The hum now is deep with the volume rolled down, and then it gets quieter as the volume is turned up. Then the hum gets louder, but higher pitched as the volume nears max.

I am going to add shielded wire for the input jacks and I am thinking about changing the first two filter caps from 16uf to 22uf.

Thanks,
Mike

[Janglin_Jack]

I was checking out the amp and noticed while on standby, it hummed quite a bit. When I pull the speaker jack out, it hums much less. Does that mean there might be an issue with my output jacks?? Checking the connections there.

Mike

[demonufo]

I was checking out the amp and noticed while on standby, it hummed quite a bit. When I pull the speaker jack out, it hums much less. Does that mean there might be an issue with my output jacks?? Checking the connections there.

Mike

That's probably just noise induced into the secondary windings of the OT, by radiation from the PT. As soon as you energise the amp this will disappear as you are then introducing a signal to the primaries of the OT. The signal then generated from the primaries to the secondaries may well overpower this hum somewhat when you do switch standby to the on position.

Are perhaps your OT and PT too close together and facing the same direction I wonder? If so, switch one of the transformers around 90 degrees in either direction.

Any pictures of said amp?

[Janglin_Jack]

I was checking out the amp and noticed while on standby, it hummed quite a bit. When I pull the speaker jack out, it hums much less. Does that mean there might be an issue with my output jacks?? Checking the connections there.

Mike

That's probably just noise induced into the secondary windings of the OT, by radiation from the PT. As soon as you energise the amp this will disappear as you are then introducing a signal to the primaries of the OT. The signal then generated from the primaries to the secondaries may well overpower this hum somewhat when you do switch standby to the on position.

Are perhaps your OT and PT too close together and facing the same direction I wonder? If so, switch one of the transformers around 90 degrees in either direction.

Any pictures of said amp?

Changed the jacks to Marshall Cliff style and hum is still the same. Here are a few pics, let me know if you need more/different pics.

[Janglin_Jack]

As I noted previously, I have now rewired the whole amp. The hum is exactly the same. As a test I snipped the input jack wires at the resitors on the board, just to see if that eliminted the jacks as a source of noise. Amp hums the same.

I've tried various tubes, (preamp and power) no change. I DO NOT have a new recto tube, can that cause this type of hum? The only other thing is transformer layout. I used the stock mounting holes from Mojotone, so the bracket for the OT goes front to back, with the transformer winds exposed on either side. One of these sides is next to the PT. Should the OT bracket mount in the other direction so the bracket is perpendicular to the PT??

The other thing I might try is the 100ohm resitors from pins 2 and 7 going to pin 8 on the first 6V6 tube. I am reluctant to try this, as when I did the resistors coming off the lamp, ends twisted together and grounded, it hummed so loud I had to immediately turn the amp off.

Any suggestions? I am really stuck.

Mike

[Janglin_Jack]

I decided to test some voltages in the amp to see what readings I got. In all my builds, I never really tested them, as I never knew how to do it properly. First I took out all the tubes and with the power switch off I read 120v AC at the power switch/fuse holder. I turned the amp on and the lamp turns on. I checked pins 2 and 8 at the recto tube and got like 2v on pin 2 and .5v on pin 8?? Shouldn't that be 5v?? This was measured from pin to ground. I then checked pins 4 and 6 and measure around 400v, (seems high??) When I put the recto tube in and measured at pin 8, it crept up to 500v. When I put in the preamp tubes and measure pin 8 of recto again, I got 360/370v, (normal??).

I check pin 1 and 6 on V1 and got 120v, I checked pin 1 on V2 and got 140v and pin 6 was 160v., (seems about right??)

Now I am guessing those are more or less right, but what seemed very odd to me, were the heaters. I was expecting to measure 3.15v from pins 2 and 7 on the 6V6 sockets and got 2.2v, (same with pins 4/5 and 9 on preamp tubes). Is that about right, or is this a sign that some voltage is leaking and the source of my hum????