Build Log - #102 - So it Begins

[xtian]

Yes, the resistor should prevent pops! My discharge resistor is just 470 OHMS and it does not pop. Use your multimeter to check out your discharge rig.

[PicknStrum]

Yes, the resistor should prevent pops! My discharge resistor is just 470 OHMS and it does not pop. Use your multimeter to check out your discharge rig.

Thanks for clarifying - yeah I did check my discharge rig with my meter. I'm seeing a solid 20k resistance from the resistor tip to the alligator clip - I wonder if my clip was moving around while I was discharging. I may have to go through and resolder it better...

[PicknStrum]

Okay - so I finished the startup procedure and I have a working amp! Sounds great!

Unfortunately the buzz came back however, it's very intermittent and I have yet to figure it out. Although it sounds more like a buzz than a hum, it does sound like it matches the frequency of my strat single coils which would lead me to believe it's 60 hertz. The buzz does not change with the master volume nor standby switch. The only thing that completely takes it out is turning the amp off. I can definitely tell it's over in the area of the PT, the filter board, or the 12v PT. Like I said, it is intermittent and doesn't always do it. I may try plugging it in somewhere else.

Pictures and a video of the buzz to come. Thanks for everyone's help so far!

EDIT: Upon listening more and trying to locate it, it actually sounds like it could possibly be near the IEC jack or the power tubes. It's hard to pinpoint.

[martin manning]

This is a hum coming from the chassis? Some power transformers will hum audibly if the coils and/or laminations are loose. You might check to see that the mounting bolts are holding the laminations together firmly, and also holding the transformer to the chassis. Don't over tighten them, of course.

[PicknStrum]

This is a hum coming from the chassis? Some power transformers will hum audibly if the coils and/or laminations are loose. You might check to see that the mounting bolts are holding the laminations together firmly, and also holding the transformer to the chassis. Don't over tighten them, of course.

Yes, it's inside the chassis; no hum or buzz out of the speaker at all (other than my single coils when not combined). I did tighten down the transformer to the chassis and the transformer itself seems pretty well tightened on its bolts. It might be one of those things I'm being extra sensitive about - I'm thinking if it's inside a cabinet, I don't think I'd be able to hear it. I do think it is the power transformer though. When I first turn it on there is nothing, and then as it heats up a bit (within maybe 3-5 seconds) I can start to hear just that small buzz and it almost kind of sounds a little like foil crinkling. Like I said, it's a very light sound so maybe I'm just being super sensitive to it.

I'm embarrassed to say I somehow burnt out my 12v transformer. While I was attempting to tighten the transformer bolts to my chassis, I somehow jumped one of the primary wires on my 12v PT to ground I think. I had gone through and discharged my capacitors but stupidly didn't pull the power cord out of the back. Not sure if there was still some voltage in the 12v FWB caps or what - I'll have to make sure those 50v caps are discharged too and obviously pay better attention - lesson learned Try not to be too harsh

A couple of confirmation questions to make sure that one of the following issues aren't my actual problem:

1. On the main PT I have one of the primary wires going to the IEC and the other going to the mains switch - it does not matter which, correct? The primary wires are the same either which way you wire them, correct?

2. My heaters don't have a CT so I made an artificial ground with two 120R's. I didn't run the 120R's from the same tube, however. Is that an issue. They are on adjacent tubes and each is going from ground to the opposite phase (if that makes sense). In other words, I know I wired them so they are each connected to a different phase, I'm just wondering if it's okay that I didn't put them right where my PT heater winding comes in.

3. I'm certain this is not it but it looks like the DIN has an extra lug, I assume to ground the DIN case. Where should I ground that extra lug / should it be grounded?

[ijedouglas]

1. On the main PT I have one of the primary wires going to the IEC and the other going to the mains switch - it does not matter which, correct? The primary wires are the same either which way you wire them, correct?

2. My heaters don't have a CT so I made an artificial ground with two 120R's. I didn't run the 120R's from the same tube, however. Is that an issue. They are on adjacent tubes and each is going from ground to the opposite phase (if that makes sense). In other words, I know I wired them so they are each connected to a different phase, I'm just wondering if it's okay that I didn't put them right where my PT heater winding comes in.

3. I'm certain this is not it but it looks like the DIN has an extra lug, I assume to ground the DIN case. Where should I ground that extra lug / should it be grounded?

1. Correct
2. Correct
3. Is that the lug in the center? I just ignore that lug without issue.

[PicknStrum]

1. On the main PT I have one of the primary wires going to the IEC and the other going to the mains switch - it does not matter which, correct? The primary wires are the same either which way you wire them, correct?

2. My heaters don't have a CT so I made an artificial ground with two 120R's. I didn't run the 120R's from the same tube, however. Is that an issue. They are on adjacent tubes and each is going from ground to the opposite phase (if that makes sense). In other words, I know I wired them so they are each connected to a different phase, I'm just wondering if it's okay that I didn't put them right where my PT heater winding comes in.

3. I'm certain this is not it but it looks like the DIN has an extra lug, I assume to ground the DIN case. Where should I ground that extra lug / should it be grounded?

1. Correct
2. Correct
3. Is that the lug in the center? I just ignore that lug without issue.

Cool - Thanks for clarifying! Sounds like I'm pretty much set on those. I did leave that extra DIN lug unconnected.

Think I'm going to shut it down for the night. I will be sure to get pictures up soon and get a video of the buzz I'm hearing.

One additional thing, it's subtle but I do notice that the buzz is a tad bit louder when turning the standby switch to "play." So with power on and set to standby, the buzz is present but fairly quiet. Then switching to play, the volume of the buzz is a hair louder.

I guess the only other thing I'm thinking off the top of my head is the IEC jack is not filtered as far as I'm aware. Possibly that could cause an issue? Although, like Martin pointed out, the buzz is definitely coming from the chassis but it sounds to me like it's an electrical buzz, not sure how to make sense of it...

More updates to come.

[xtian]

I can't see your rectifier. Do you have caps in parallel with each diode? Sometimes the caps themselves can make mechanical noise from vibration at 60Hz. Sounds like a fish tank pump buzzing.

[PicknStrum]

Alright so I've got more pictures for you guys and hope to get a video linked here for you as well. This will actually be the first video I've ever uploaded to youtube, so we'll see how that goes.

Xtian - I would say your description of the buzz sounding like a fish tank pump is very accurate (although I haven't had a fish tank in a while). My boards are laid out as according to Martin's drilling templates (which are awesome by the way!). I did try to get a couple close-ups of the rectifier boards (main and 12v) for you. The thing that is weird, and you'll hopefully hear this in the video, is that there is also a sound sort of like crinkling foil - not sure if that's the PT heating up or what.

If anyone wants any other close-ups or has questions, let me know. Tried to get tons of shots - there is one eyelet in the power filter board that is unconnected, I just routed the standby and B+ line to the same eyelet. Also, as artguitar noticed, there is a green ground wire at the top of the power filter board that goes through an eyelet underneath, so it's not attached on top but is under the board. I'm getting good continuity there as well. Here are the photos:

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IMG_4902.JPG

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IMG_4896.JPG

IMG_4895.JPG

[PicknStrum]

EDIT: Additional photos - these are the correct ones:

IMG_4890.JPG

IMG_4891.JPG

IMG_4892.JPG

IMG_4893.JPG

IMG_4895.JPG

[PicknStrum]

And here is the video - note that the amp is off to begin with, switched to on / standby at 3 seconds. Then switched to on / play at 20 seconds. I then reverse the order as I turn it off. Sorry for the poor quality!

[PicknStrum]

I can't see your rectifier. Do you have caps in parallel with each diode? Sometimes the caps themselves can make mechanical noise from vibration at 60Hz. Sounds like a fish tank pump buzzing.

As mentioned above, I would say this is a very good sound description - maybe you can tell from the video I posted. Any way to remedy this if it is those caps?

I would again add that the amp plays / sounds fantastic.

Couple additional questions:

1. In addition to this slightly obnoxious noise, I seem to be having an additional problem (and or brain fart of sorts). When checking the bias with RobRob's output transformer resistance method, I'm showing tube dissipation at 30mA. However, I did put in the 1ohm cathode drop resistor on each tube and those are only showing 14.6mA. I did verify that I put 1ohms in, so I'm not sure what I'm doing wrong here.

2. I don't seem to be getting a voltage drop b/w the OT CT and each respective plate pin. Is that normal? they are all reading about 436.

3. Shouldn't there be a small voltage difference between the plate and screen? I'm getting 436 on the screens as well...

[Richard1001]

I cant hear anything wrong. Transformers can buzz a little, and buzz louder when you switch on the high voltage. Did you leave the nuts on the transformer when you inslalled it? (The nuts that are on the bolts of the transformer need to stay on and sit between the transformer and chassis. You need additional nuts to bolt the transformer on to the chassis)

As for your voltage reading: i don't know how accurate your meter is, but to read the diffrence in voltage over the transformer you need to connect your meter accordingly. The positive lead to the centertap (B+) and the negative lead to the anode(s) of the tube(s) you want to measure. The meter will read more accurate that way on a low voltage setting.

[pompeiisneaks]

I definitely can only hear a very light background noise and the switches clicking. I hear no noise at all.

The only other area that 'might' be problematic is the OT leads going to the impedance selector switch are somewhat close to the OD tube. It might have been better to route them underneath the jacks and behind the PI tube instead, and that can induce hum into other signal path areas pretty easily...

Not sure it's related, though since you've mentioned it's in the power section, not the OT.

~Phil

[ijedouglas]

I cant hear anything wrong. Transformers can buzz a little, and buzz louder when you switch on the high voltage. Did you leave the nuts on the transformer when you inslalled it? (The nuts that are on the bolts of the transformer need to stay on and sit between the transformer and chassis. You need additional nuts to bolt the transformer on to the chassis)

As for your voltage reading: i don't know how accurate your meter is, but to read the diffrence in voltage over the transformer you need to connect your meter accordingly. The positive lead to the centertap (B+) and the negative lead to the anode(s) of the tube(s) you want to measure. The meter will read more accurate that way on a low voltage setting.

I always used to do that until a discussion with Patrick from Mercury. He said to remove those nuts and bolt the transformer directly to the chassis, getting the transformer as close to the chassis as possible. I've done it that way since (even converting 2 previous amps) and have not had any issue. I discard the transformer nuts and replace with keps nuts.