ODS 124 PCB Build

[martin manning]

That last one is too far different to pass for matched.

[professormudd]

That last one is too far different to pass for matched.

Thanks. I better get a hold of Amplified Parts and see if they can rectify the issue.

What is the typical +/- acceptable tolerance?

[pompeiisneaks]

As I recall in the past someone somewhere mentioned 3-5mA is not a big deal, beyond that you're definitely falling out of matched range.

I don't recall if you said it before but does that mA rating follow the tube or the socket?

sometimes you have a specifically weird issue with the negative bias to that specific socket and it causes a slight difference.

~Phil

[professormudd]

I don't recall if you said it before but does that mA rating follow the tube or the socket?

I just did a test based on your post and it follows the tube. I got a return set up with amplified parts and a new set of apex matched JJs coming.

Now for more of my favorite pastime... Waiting on deliveries.

[professormudd]

Ugh...

New tubes, they all match nicely. 440 plate voltage, set bias to 37.5.

Still hum. Not as pronounced, but present.

I noticed it somewhat pulsing. However, I also noticed the lights very very slightly flickering in my garage at the same cadence; even without the amp plugged in. So I am pretty sure that's the cause of the pulse.

I decided to go ahead mount it in the head cab I got from Nik at Ceriatone to see how it looks.

I attached an audio recording of the hum with me sweeping the master volume. It sounds much more pronounced in the recording because I had the input gain on my Zoom H5 cranked and the mic right up against the speaker cloth.

[xtian]

Maybe you need to take your amp out of the house! I'm so f-ing bored, I hum at home, too.

[professormudd]

Maybe you need to take your amp out of the house! I'm so f-ing bored, I hum at home, too.

No kidding.

Anyway the pulsing goes away when the PI is pulled, as it does when I plug into the return directly. V4 is also a tad microphonic. ¯\_(ツ)_/¯

Would an oscilloscope help diagnose this? I've been looking for an excuse to buy one anyway.

[xtian]

Anyway the pulsing goes away when the PI is pulled, as it does when I plug into the return directly. V4 is also a tad microphonic.

Try another tube? Also, should we revisit the NFB loop? If you disconnect the NFB wire from the output jacks, how does the hum react?

Would an oscilloscope help diagnose this? I've been looking for an excuse to buy one anyway.

Probably not, honestly. But I can't stop you from buying a scope, and I think you should have one, and isn't that enough of an excuse?

[erwin_ve]

Still hum. Not as pronounced, but present.
stupidhum.WAV

Sounds like you got a Star Wars lightsaber: check for Sith!

[professormudd]

Try another tube? Also, should we revisit the NFB loop? If you disconnect the NFB wire from the output jacks, how does the hum react?

I had previously disconnected the nfb and did not notice any change. I switched each one of the preamp tubes out one by one and the pulsing persists. No change from chopsticking either.

[professormudd]

The pulsing is no longer present. No idea why. Not upset about it.

Aside from the hum, this amplifier sounds amazing. The clean tone is the best I have ever played through and has almost every characteristic I have been unable to express in words but wanted to achieve. The overdrive is much more aggressive than my Brown Note Blue Monkey (which I consider a very good thing). This thing can create quite a bit of feedback even at low volumes. I have not really opened it up too loud yet, but even at the good-neighbor volume I was playing at it screamed on demand. The tone switches are also have much more drastic impacts on tone than the Brown Note did, and I really appreciate having the R/J double as a PAB switch since I have not yet built a pedal.

I have thrown 2 different speaker cabinets at it so far; a 4x12 Mesa/Boogie full of Vintage 30s and a homemade open-back with an Eminence Swamp Thang. It seemed to get along with both just fine. It also seems to get sound out of my Telecaster than was missing in other amplifiers. It is weird, but the Telecaster pickups seem like they have higher output plugged into this amp than any other amp I have tried. However, every other guitar I have plugged in seems to also get a warm welcome.

The only thing wrong is that hum, which I only notice when there is no other background noise and my ears are level with the speaker. Still, I want to get to the bottom of it, but I think I am going to let this thing live in its head cabinet for a couple of days so I can put some miles on it.

I fully intend on recording some samples of this thing when time and background noise permits (I have an 8-month old that runs the house and my schedule).

[professormudd]

Took it back out. I redid the filament wiring because I didn't like the green/white and I accidentally burned some of the insulation at some point, so I cleaned it up. My only regret is the twist changing direction in the middle.

I also experimented with various capacitor combinations for power supply reservoir and filter caps with no change in the hum. I am still happy with the F&Ts as opposed to my original power supply. This one looks a whole lot neater.

One day I will make this thing silent. I've heard about something called a "hum balance pot" to inject a cancelling noise to kill hum. Is that an option here?

[xtian]

I've heard about something called a "hum balance pot" to inject a cancelling noise to kill hum. Is that an option here?

"Inject a cancelling noise" - no, that's not its function. The hum balance pot takes the place of the two 100R "artificial center tap" resistors for the heater winding, and allows you to "balance" those two resistances for least noise.

Here's a quick test: Turn amp on and let it warm up. Listen for the hum. Now, while listening, flip the power switch to OFF. Does the hum stop immediately, or does it fade out slowly? If it fades out slowly, heaters are not the problem.

[professormudd]

It fades out slowly. Thanks for helping me avoid that rabbit hole.

[xtian]

OK, Prof, see how you have a single buss bar that runs the full length of the pots? I've learned the that preamp stages and tone stack should be grounded at the input jack, while Presence control and Master volume should be grounded with the PI or reservoir cap. I suggest you cut that ground buss in half and try moving the Presence/MV ground elsewhere.