YAOB (yet another one-zero-two build)

[aflynt]

I've got it set at about 32ma @450VDC right now bias-wise. Lowering the bias didn't really seem to affect the hum, though. I've already got the artificial center tap thing going. Neither PT had filament center taps.

The only other thing I noticed is that the transformer hums pretty loudly just by itself, though I guess that's somewhat normal.

-Aaron

[aflynt]

I had no luck curing the hum last night. There's no way to mount the PT completely flush with the chassis sandwiched between the transformer and the bell. It's just too wide to fit. I moved all the wires around with a pen and there was no change in hum at all. I also tried insulating the mounting screws from the laminations with some plastic washers (thinking maybe there was a loop with the orange ground wire) and that did nothing either. I also tried spinning it around 180 degrees with no resulting reduction in hum.

The amp definitely didn't hum before the power transformer replacement. Here are the possible explanations as I see them in order of decreasing likelihood:

1. The Power Transformer just hums more than the old one either due to design or due to the in-optimal mounting scheme.

2. The higher voltages produced by the new power transformer are exposing a problem somewhere in the amp.

3. I inadvertently fried a component or messed up a solder joint causing the resulting hum.

The hum increases with preamp gain, IE: with all the amp controls off there is no hum. With only the master up there is no hum. On the clean channel, turning up the tone controls or volume increases the hum. The hum is loudest on overdrive also increasing with gain and volume.

I still haven't tried the hum balance pot. Any other ideas? Just buy a new PT?

-Aaron

[Structo]

I hate those kinds of problems.

Can you identify the frequency of the hum?

If it is 60Hz it is most likely in the heater supply.

If it is 120Hz, it is in the high voltage power supply filtering or lead dress.


Maybe this will give you some ideas.

http://www.geofex.com/ampdbug/ampdebug.htm

[aflynt]

Can you identify the frequency of the hum?

If it is 60Hz it is most likely in the heater supply.

If it is 120Hz, it is in the high voltage power supply filtering or lead dress

I'm not quite sure. It's not low. It's similar to single coil buzz. I'm wondering if I messed up something with the relay supply. I did have to remove the relay transformer and regulator to get the pt bell installed. Also, since the hum goes away when I turn down the preamp volumes it would have to be coming from something before that right?

Thanks for the help,
-Aaron

[tubedogsmith]

Try pulling preamp tubes one at a time starting with V1, at the point the noise stops you'll know the section where the problem is, based on your comment about turning down the volume.

[Structo]

A low A string on a bass is 55Hz to give you an idea of the frequency.

An octave above that would be 110Hz

A B played an octave above low A is 123.5Hz

[aflynt]

I'm not sure if it's 60 or 120 cycle hum. It sounds like it's somewhere between an A and Bb pitch-wise. Here's a recording of the hum:

http://soundcloud.com/aflynt/amp-hum

The hum goes down a lot but doesn't entirely disappear when I pull v1. Varying the clean volume doesn't really change the hum significantly. raising dirty gain or level does, though.

-Aaron

[martin manning]

Sounds like 60 Hz to me. Suspect somthing in the filament supply or it's ground reference, the 2x 100 ohm resistors or center tap. Remember use one or the other not both.

[B]

I unfortunately don't have a solution, but was wondering how high you had the master for this recording?

My build has the exact hum about halfway up on the master and increases from there, but I just assumed it was normal?

You may have turned me on to an issue with my amp that I didn't realize was an issue.

[aflynt]

I unfortunately don't have a solution, but was wondering how high you had the master for this recording?

My build has the exact hum about halfway up on the master and increases from there, but I just assumed it was normal?

You may have turned me on to an issue with my amp that I didn't realize was an issue.

Well... The master was about 6 1/2, but I had the D'Lator patched in too. I think the D'Lator return was about 6 or 7 too for the recording. It's there a little on the OD channel no matter how high the master is. It just gets louder as you turn it up. It doesn't really get annoying until the amp is super loud, though. I think it would be fine for a job, but for recording it wouldn't really be acceptable. I brought the amp to rehearsal last night and it wasn't really loud enough to be a problem.

-Aaron

[talbany]

Does the transformer hum or vibrate?
Tony

[aflynt]

Does the transformer hum or vibrate?
Tony

It does both. Even in standby.

-Aaron

[Structo]

Does it hum without the D'lator hooked up?

I had a nasty hum in the first D'lator I built. I had to cut one of the shields on a cable to stop it.

I rebuilt it a while ago and isolated the jacks, no hum.

Oh and by the way, I have a Magnetic Component PT and it has a bit of buzz to it.
Not enough for me to replace it but it's there.

[aflynt]

Does it hum without the D'lator hooked up?

I had a nasty hum in the first D'lator I built. I had to cut one of the shields on a cable to stop it.

I rebuilt it a while ago and isolated the jacks, no hum.

Oh and by the way, I have a Magnetic Component PT and it has a bit of buzz to it.
Not enough for me to replace it but it's there.

It hums with or without the D'Lator patched in.

-Aaron

[aflynt]

I'm wondering if elevating the heaters by attaching the artificial center tap resistors to a voltage divider off B+ with a cap to ground would help. Kind of like this drawing I found on here:

http://ampgarage.com/forum/download/file.php?id=17532

I think I might at least try wiring this up with some alligator clips tonight. Otherwise I'm thinking maybe PT #3 is in order. On that note, does anybody know where I can find some of those non-export Heyboer Twin / Showman Power Transformers? Mojotone appears to be out of them and only has the export version.

-Aaron