Just your run of the mill 60hz hum

[polytrigon]

So I assembled the S.E.X 2.1 about 10 years ago with my father. He was an electrical engineer in a past life and was very knowledgeable and skilled at getting it all in one piece.  Flash forward to now and i've finally gotten around to putting my phono back to use and I had forgotten that there was somewhat of a hum when i plugged in my apple hifi to the headphone jack.

Now as a disclaimer I am a junkyards wars kind of EE, I have very little knowledge but have soldered a ton of drones and other doodads so if you show me a wire diagram I'll pass out, but if you ask me to solder some bits together I'll do so without burning myself. What I'm trying to say is that I'm a noob so please feel free to talk to me like you're talking to a baby elephant. I do however have some experience troubleshooting so I'll just lay out the information at my fingertips.

I've since ordered the C4S kit, hoping that it further reduces the hum but in the meantime I wanted to see if this is just normal or if I've done something wrong.

Oh I also went through the forums and read a bunch of other threads that sound similar. I checked all the solder joints and use a chopstick to poke at everything to make sure that it wasn't a bad joint.

- phono turned off
- apple hifi plugged into headphone jack
-  tubes in
= 60hz hum (what sounds to me as the 60hz hum, I've listened to some youtube videos and it sounds similar)

Additional notes:

- if I adjust the volume the hum does not increase or decrease
- if I remove the tubes the hum goes away completely
- I had a handful of tubes and i went through different pairs and found one specific set that has significantly less hum
- There were tubes that I plugged in that once they warmed up had a VERY pronounced hum... when i turned on the amp i could hear the 60hz hum and then after about 10-20 seconds when the tubes warmed up there'd suddenly be a much louder hum.

So with this said does it sound like maybe I just have some bad tubes and need to get different ones / allow this "okay" set to break in?

Thank you in advance!
M

[Paul Joppa]

It seems to me I remember there was a grounding issue with the heater power ... I can't remember the details right now but maybe someone else will. Do you have the original 2011 manual, or the rev 1.1 dated June 15 2012?

I think it was heater-cathode leakage, because it varies from one tube to another. And it would be 60Hz.

[polytrigon]

So i purchased the kit in 2013 (wow time flies). I do not have the original manual anymore and can't confirm which version it was.

I did get the more recent manual however. Is there anything in the new manual that I can check?

Would love to know about this mystery fix!

Ty for the response

Edit:

Spoke with my Father about it and mentioned the symptoms. He thinks that the swapping of the tubes didn't "fix" the issue only masked it, stating that the tubes which sounded better prob weren't amplifying parts of the lower range where the hum is occurring.

Still trying to figure out if I implemented the v1 or v1.1 manual but so far it seems like I've most likely used the 2012 v1.1

[Paul Joppa]

Found it!

https://forum.bottlehead.com/index.php?topic=5316.msg51191#msg51191

Hope that helps  :^)

[polytrigon]

Thanks so much for digging this up. It looks like the 100r resistors in series is the fix I'm looking for.

I'll definitely give it a try.

Ty again!

edit:

showed the mod to the expert and he said that he wasn't convinced that the 2 resistors alone were sufficient and suggested adding 1mf capacitors in parallel to each resistor. I'm going to give that a try to see if it makes any progress. very hopeful!

[Paul Birkeland]

The resistors appeared for a few decades at the outputs of lots of vintage gear for the headphone jack.  Putting the capacitors across each resistor will create an equalizer of sorts that will tip up the treble and wouldn't be something I'd recommend.

[Paul Joppa]

The resistors create a grounded centertap on the 6vDC heater supply. The 10,000uF cap is sufficient filtering. The technical story is complicated; there are better ways but this is the simplest and has worked well for several people.

I believe PB is thinking of series resistors at the headphone jack, which were in earlier versions of the amp.  ;^)

[Paul Birkeland]

Ah, yeah, I was thinking about the resistors at the HP jack and scratching my head pretty hard!

[polytrigon]

Okay! So i think my issue might be in a different place? I added the 2 resistors and it made a tiny bit of a difference but was hoping for the stark terror of deep space. I still have a little bit of a hum. I'm going to wait for the C4S kit to arrive to see if that gets me the rest of the way there.

At this point it reads much the same way as the other post that PB referenced. The hum is there if I listen for it, if there's a lull in the music I can hear it but it's not as distracting as it was before. So if this is as good as it gets for me then I can live with that.

I appreciate all the feedback!

Ty PB and PJ for all the help.

[Paul Joppa]

There's another possibility, a ground interaction with the Apple (a product I don't remember knowing of - had to google it!). Have you tried using batteries in the iPod HiFi?

[polytrigon]

I get the same hum when i listen with my grados. So I think that rules out the apple hifi.

[Paul Joppa]

Well then, let us know what the C4S does.