ODS 124 PCB Build

[Richard1001]

On the phase inverter pins 3 and 8 should be tied together. Maybe i am missing something, but i can not see a connection between those pins on the tube socket. Only pin 8 seems to be connected.

[professormudd]

On the phase inverter pins 3 and 8 should be tied together. Maybe i am missing something, but i can not see a connection between those pins on the tube socket. Only pin 8 seems to be connected.

There is a wire connecting them it runs around the socket against the chassis. Hum exists with/without PI (or any other preamp tubes) in place.

[professormudd]

I believe you reported NO hum with the PI tube pulled, correct? So you can rule out induced hum from transformers, mismatched or bad power tubes tubes.

As a follow up, what happens if you ground the input to the PI tube? If that kills the hum, then the PI is also OK, and the hum is entering from further upstream.

There IS hum with the pi pulled. The hum exists even without any preamp tubes.

[professormudd]

When in doubt: http://www.geofex.com/ampdbug/ampdebug.htm

I've read that several times. And I'll keep reading it over and over again.

[professormudd]

I had really hoped for something here. I thought that maybe this chassis was coated or something preventing good ground contact. So I removed every ground terminal, sanded the chassis under it, and replaced it. I did the same to all 6 jacks. I also re-ran every ground wire.

Still hum. No change at all.

I just wish I could actually, literally see where the hum is being introduced.

Here is a recap of everything I have tried to solve this 120Hz hum (out of order):

Power tubes (3 different sets of matched quads).
Preamp tubes (with all preamp tubes out, hum remains)
Re-did power supply with all new caps/resistors.
Copious amount of time chopsticking. The hum never increases/decreases.
Re-did all ground points.
Re-positioned choke.
Re-did any/all "dull" solder joints.
Tried with/without relay power.
Re-did filament wiring.
Re-tensioned/cleaned all tube sockets.

Is it possible that there is something wrong with the output transformer itself?

[pompeiisneaks]

Have you tried checking/tightening the transformer bolts? Sometimes the laminations need tightening to reduce/eliminate hum. BUT that should be 60Hz hum not 120. Not sure other wise.

~Phil

[professormudd]

Have you tried checking/tightening the transformer bolts? Sometimes the laminations need tightening to reduce/eliminate hum. BUT that should be 60Hz hum not 120. Not sure other wise.

~Phil

I did tighten all bolts and I ensured the enclosure for the OT was good and tight.

Another thought. Marcus had mentioned my output transformer primaries might be reversed back when I had oscillations related to bad lead dress.

However I am curious if that could possibly be the cause of this hum after all? Is that something that could cause this?

[professormudd]

Have you tried checking/tightening the transformer bolts? Sometimes the laminations need tightening to reduce/eliminate hum. BUT that should be 60Hz hum not 120. Not sure other wise.

~Phil

I did tighten all bolts and I ensured the enclosure for the OT was good and tight.

Another thought. Marcus had mentioned my output transformer primaries might be reversed back when I had oscillations related to bad lead dress.

However I am curious if that could possibly be the cause of this hum after all? Is that something that could cause this?

Just confirmed. That was NOT the issue! I am awake now.

[jcsifu]

Have you tried checking/tightening the transformer bolts? Sometimes the laminations need tightening to reduce/eliminate hum. BUT that should be 60Hz hum not 120. Not sure other wise.

~Phil

I did tighten all bolts and I ensured the enclosure for the OT was good and tight.

Another thought. Marcus had mentioned my output transformer primaries might be reversed back when I had oscillations related to bad lead dress.

However I am curious if that could possibly be the cause of this hum after all? Is that something that could cause this?

Just confirmed. That was NOT the issue! I am awake now.

Ha! I laugh because I know what that sounds like when you aren't expecting it...

[norburybrook]

I did tighten all bolts and I ensured the enclosure for the OT was good and tight.

Another thought. Marcus had mentioned my output transformer primaries might be reversed back when I had oscillations related to bad lead dress.

However I am curious if that could possibly be the cause of this hum after all? Is that something that could cause this?

Just confirmed. That was NOT the issue! I am awake now.

Ha! I laugh because I know what that sounds like when you aren't expecting it...

LOL me too it's quite a shock the first time

M

[professormudd]

Ya know... I knew it could happen. But reading about it and hearing it are completely different things.

[norburybrook]

Ya know... I knew it could happen. But reading about it and hearing it are completely different things.

indeed, it can be quite dangerous....I've heard stories of people jumping and pulling the chassis on to their lap or floor etc casing damage.

After it happened to me I used to put ear plugs in when I switched a new amp on....... it scared the sh*t out of me the the first time it happened.

I think it's a right of passage now for an amp builder. like electric shocks!!

I've only ever had an AC electric shock from amp building, I forgot the amp was still plugged in even though it was switched of and got a healthy 240vAC when I accidently touched the live.....that hurt......I'll remember that


M

[jcsifu]

Ya know... I knew it could happen. But reading about it and hearing it are completely different things.

indeed, it can be quite dangerous....I've heard stories of people jumping and pulling the chassis on to their lap or floor etc casing damage.

After it happened to me I used to put ear plugs in when I switched a new amp on....... it scared the sh*t out of me the the first time it happened.

I think it's a right of passage now for an amp builder. like electric shocks!!

I've only ever had an AC electric shock from amp building, I forgot the amp was still plugged in even though it was switched of and got a healthy 240vAC when I accidently touched the live.....that hurt......I'll remember that


M

I was at a dinner party pre-Covid and the discussion was on home improvements when someone said "I wonder what it would feel like if I accidentally got shocked". Fast forward 5 minutes and everyone is asking me follow-up questions to the short seminar I just gave about the difference in AC and DC shocks and how they feel. I will agree with Marcus that it seems to be a right of passage.

[pompeiisneaks]

If you want to hear it on my video of the champ my wife and I built together, it goes OFF after 4:12 minutes in this video:



It scared the shit out of us both

~Phil

[professormudd]

If you want to hear it on my video of the champ my wife and I built together, it goes OFF after 4:12 minutes in this video

That was excellent. Thanks for that.

I am throwing Hail Marys at this so I am not sure the value of any of any of these "experiments" but at least I am ruling out causes

Anyway... Here are a couple of things I tried this morning that did not solve my issue. Logging here for posterity:

I added 100nf snubbers to the rectifier. No change.

I put a wirewound resistor in series with the choke it might help. It did not.

I moved the OT primary center tap to the other side of my standby switch to see if I would get hum when there was only power going the first couple of caps and the output transformer. It did not hum. I am not sure if this tells me anything useful, but I presume the hum is being introduced after B+2.

Continuing to look for layout issues, I am considering my B+4 and B+5 leads. When I did them, I ran wire snug against the board straight to the back back of the board, making a 90 degree turn towards their connections by preamp tubes. You can see the run in the attached picture. The wires are zip-tied together.

When mounted in the chassis, this seems to go directly above and parallel to the OT secondaries. Could this be just a bad B+ run? I thought I was following advice I got here, but I misinterpret instructions enough to double check. Is there a better path for these?

When I dismount the board from the stand-offs and tilt the board, it does not change anything about the hum, so I am hesitant jump to that conclusion.