Weber 5F6A Noise

[bluesky636]

I recently completed a build of the Weber 5F6A. The build went great, with only a couple of problems, and the amp sounds Fantastic. Hiss levels are just slightly greater than the hiss from my Fender Blues Deluxe Reissue and hum is almost nonexistant. I have documented the entire build here:

http://www.fender.com/community/foru...b05ef786f167cf

I do have one bug that is bothering me.

I am getting a scratchy sound while the amp is sitting idle with everything plugged in but not being played. It is sort of a crackling/scratchy sound that is distinct from the normal background hiss. It also appears to get slightly worse as time goes on. You can't hear the noise while actually playing, though.

I do know that the Presence circuit design allows a DC path to ground through the pot which causes a scratchy noise when the Presence pot is turned. The idle noise sounds just like that. I have the parts on order to convert the old Presence pot circuit to that of the 59 Reissue Presence circuit which blocks the DC path to ground. You can see the difference in the two schematics.

https://taweber.powweb.com/store/5f6a_schem.jpg

http://ampwares.com/schematics/59_bassman_manual.pdf

I am thinking that this noise is also a DC noise (Maybe caused by power supply ripple?) and if changing the Presence circuit doesn't cure it, I'll check the coupling and bypass caps for any DC where it shouldn't be. Oh, and before anyone asks, I did swap each preamp tube with a second set of tubes with no difference. I don't have a spare set of power tubes (except those in my BDRI).

Does anyone have any other suggestions as to what the cause of the noise might be and how to troubleshoot it? Thanks.

[loudthud]

First question: Do the Volume controls or any of the tone controls affect the crackle? This will help localize the source to part of the circuit.

Second question: Does pulling any of the tubes make the crackle go away?

[Chuck H]

It's probably a noisy preamp tube. I had a few Tesla 12ax7's that were noisy just as you describe. Other than some wierd oscillation problem I can't think of much in that build that would cause the noise. Sometimes under rated carbon comp resistors will crackle and hiss. Moisture in transformers, fiber board and carbon comp resistors can also crackle and hiss. If your in a humid environment try taking a blowdrier to the board for awhile and see if it changes the noise at all. I would probably try different preamp tubes first.

EDIT: Myself and LT had a simulpost. I want to add +1 to pulling tubes to isolate the problem.

[bluesky636]

Yes.

Yes.

I have pulled tubes and swapped tubes for known good ones (all tubes are brand new or fully test NOS).

OK. I tapped every single component, solder joint, tube, wire, etc. that I could get to without taking the amp apart. Here is what I found:

The only point in the amp that reacted to my tapping were the input jacks. Particularly, it seems, the Bright input.





Side note: I just noticed that the schematic and the wiring diagram differ as far as which set of input jacks the 1M resistor is wired to. I wired mine per the schematic which appears to be correct as far as which jack is high level and which is the low level input.

Anyway, I found that tapping lightly on the tip terminal of both normal inputs did not result in any significant noise. However, tapping lightly on the tip terminal of the Bright input jacks, and in particular, the High level input, would result in a loud, crackling noise. Pushing on the tip terminal to cause it to break contact with the switch contact caused the same noise. Also, tapping on the 1M resistor on the high level Bright input would cause the same noise. It was interesting to note that once the noise started, tapping on the tip terminal of the high level Normal input would cause the noise to occur with that input. Tapping the Bright jack tip terminal or 1 M resistor would cause the noise to stop.

So, it would appear that I MIGHT have a bad 1M resistor on the Bright input, or the tip and switch terminals are not making good contact. I can squeeze the terminals together more tightly (easy to do) or change the 1M resistor. So before I do either (after dinner) which seems to be the most likely cause of the noise? I should note that the acrackling noise which gets worse if I tap on the Bright input tip terminalmp has been on for almost 2 hours and is making a very slight .

I'll be back after dinner.

[bluesky636]

It is not a noisy tube. That has been verified. Humidity is not an issue. See my response to LT.

[Chuck H]

Good troubleshooting. Replacing that resistor seems like the next move then. That resistor can get pretty damn hot soldered to the jack lugs with short lead on either end. Sometimes when I must solder components with short leads to terminals that sink a lot of heat I'll either pump up the heat on my iron so as to spend as little time melting the solder as needed, or I'll solder just one side and wait for the component to cool off completely before soldering the other side.

JM2C

[bluesky636]

Good troubleshooting. Replacing that resistor seems like the next move then. That resistor can get pretty damn hot soldered to the jack lugs with short lead on either end. Sometimes when I must solder components with short leads to terminals that sink a lot of heat I'll either pump up the heat on my iron so as to spend as little time melting the solder as needed, or I'll solder just one side and wait for the component to cool off completely before soldering the other side.

JM2C

I'm going to try and tighten the contact between the tip and switch terminals and reflow the solder on the resistors and wires on the switches. I was very careful soldering the resistors originally and the way I mounted them required almost 1" of lead on one side and about 1/2" on the other. Both leads were insulated also. If that doesn't work, I'll have to pick up some new resistors tomorrow.

[bluesky636]

Ok. I have reflowed the solder on all of the connections of all four input jacks. I have also tried to tighten the contact between the tip and switch terminals of all four jacks. I also redressd a couple of wires to get get them away from the soldered eyelets.

The amp has been sitting idling for about 2 hours now. There is still a very slight crackle/scratchy noise riding on the background hiss, but both the crackle and the hiss seem to be reduced and the crackle is not occurring with as great a frequency as it was before. I'm not sure what else I can do inside the amp to improve things or if I should just declare victory and live with the slight noise. The parts for the presence pot mod arrive next Tuesday and I will install them next weekend. I'll see if that has any impact on the noise and take a look around inside the amp again. I'm also going to plug the amp into a different outlet to see if possibly my power conditioner is causing the noise.

Any other suggestions?

[bluesky636]

Plugging directly into the wall outlet insead of my power conditioner made no difference.

[Chuck H]

Does poking the 1M resistor still incite the noise? If yes "I" would replace it. What's the wattage and voltage rating on your preamp plate load resistors??? If you don't know then perhaps a pic could help.

[bluesky636]

Does poking the 1M resistor still incite the noise? If yes "I" would replace it. What's the wattage and voltage rating on your preamp plate load resistors??? If you don't know then perhaps a pic could help.

Tapping the 1M resistor does not seem to elicite a noise as previously. The overall noise level has decreased noticably, but is still present.

All resistors supplied in the Weber kit are 1 watt carbon film, except for the power supply resistors which are 2 watt carbon film.

Are you asking what is the plate voltage on the preamp plates? I don't remember. I measured it initially, but did not write it down. I do remember that it was within the acceptable range for a 5F6A. I am planning to check the power tube bias (initially set to 38 mA cathode current at 432 VDC plate voltage on a pair of JJ 6L6GC tubes) when I replace the Presence pot. I could measure it then or if I have time do it this weekend if the information would be helpful.

[Chuck H]

I was just asking about the resistor specs. You answered. Unfortunately I have no new ideas about the noise if your sure it's not a preamp tube. BTW, just because you tried a different tube doesn't mean it's a quite one. And NOS is no reliable marker of relative goodness either. A known good tube is the ONLY way to be sure.

[frus]

also check both bright jacks. it's possible they are not shorting properly

[bluesky636]

I was just asking about the resistor specs. You answered. Unfortunately I have no new ideas about the noise if your sure it's not a preamp tube. BTW, just because you tried a different tube doesn't mean it's a quite one. And NOS is no reliable marker of relative goodness either. A known good tube is the ONLY way to be sure.

I tried 3 different NOS JAN GE 12AY7s as well a a NOS JAN 5751 in V1. All NOS tubes were purchased from https://www.kcanostubes.com/ and are fully tested and guaranteed. V2 and V3 are new JJ 12AX7s and I tried two differen tubes in each spot with no difference.

[bluesky636]

also check both bright jacks. it's possible they are not shorting properly

I did.