ODS 124 PCB Build

[Richard1001]

Only connecting the centertap to the B+ before the standby switch means there is no voltage on the screens. The powertubes will not work without screen voltage.

What you know now is that the powertubes have to work for the hum to be present.

I think a scope would help you find the problem.

[professormudd]

Only connecting the centertap to the B+ before the standby switch means there is no voltage on the screens. The powertubes will not work without screen voltage.

What you know now is that the powertubes have to work for the hum to be present.

I think a scope would help you find the problem.

I am willing to purchase a scope. I was looking at this Hantek DSO5072P after reading posts on the forum here.

Is that more than I need for hobbyist tube amp work?

[Richard1001]

As far as functionality this is more than enough.
The only drawback with these scope's is the low maximum input voltage (5V/div =50V) and maximum input RMS voltage (300V)

Make sure you buy a probe that has a switch to attenuate the input signal. (X1/X10)

[xtian]

Is that more than I need for hobbyist tube amp work?

Yes, that's my scope I've been using for the past few years. Very good.

[professormudd]

Received my scope today. I will try to get acquainted with the manual here and maybe check a couple of tutorial videos.

Do you have any recommendations for how to follow breadcrumbs with this thing?

[pompeiisneaks]

I usually inject a 400-500mV sine wave into the input, then ensure I can see it at the grid of the first stage. Then I check that it comes out the other side of the coupling cap for the next stage well amplified into the 'several volts' range but it still a nice clean sine wave etc. Just keep moving through the stages until you can find a spot where the signal suddenly doesn't look right, or drops in output level etc.

~Phil

[professormudd]

Kinda kicking myself. I did not acquire a signal generator.

[pompeiisneaks]

There are apps on the phone stores... pretty easy to get a mini to 1/4" and hook it into the input.

~Phil

[professormudd]

Thanks for the tip. I'll spend time on this and report back any findings. Hopefully I will glean what I need to put this thing to bed.

I suppose I was expecting that since I can hear the hum even without input, I might be able to see something. However, what you say makes sense because I can start with a knowingly clean signal and find where it starts to get wonky.

[professormudd]

Well, I was able to utilize this oscilloscope to point something out to me that should have been much more obvious a long time ago. I should not have needed a scope, but I absolute leveraged it to find my problem and I learned a ton in the process.

I found a lot of AC ripple on the phase inverter B+ supply. I glanced at my board, glanced at the layout, and damn near slapped myself. I had the first 2 20uf caps soldered to the wrong holes, so they weren't filtering jack squat.

Moved them to their proper holes on the board and the thing is dead silent.

I'm not proud of how long and how much time and money I spent to determine something that should have been obvious (mind you I even replaced those caps, and still didn't notice the error) but I'm proud that I actually found it.

Cheers, fellas. I'm gonna go hang out with my amp.

I sincerely appreciate how much help all y'all are willing to give strangers like me. Heck, 90% of the Google answers I found were different forum posts from the same few screen names as here. So really, thank you all.

[xtian]

Hooray!

Hey, why do all your F&T caps look crushed?

[professormudd]

Hooray!

Hey, why do all your F&T caps look crushed?

My bench light is very close and the glare is intense. They're just as cylindrical as the day I got them.

[pompeiisneaks]

Glad you found it lol, I've been there, done that, had a ground missing on one build etc.

~Phil

[talbany]

Here is a quick method to check your Power supply.



Gnd the DMM on the chassis
1.. Check for continuity on the ground side of each Electrolytic cap (I horn the DMM)
2 ..With the gnd still on the chassis go down the + side of each node on each cap and check the resistance to ground where you should have either a high Meg ohm reading or the value of you bleeder resistors Nothing lower..
3...From + side of each cap check continuity to there respective B+ supply rails. OPT primary/Screen Supply/PI/V2/V1 (again horn the DMM)

I generally run this test before I plug the amp in..If it's blowing fuses make sure you drain each cap completely before running this test..

Here you basically checked off any dead shorts (shorted rectifiers) Shorted Caps all ground connections and anything that might have been not wired correctly..The test takes About 3min to do..This has saved me a few fuses and maybe a few small fires..

(not sure this would have caught your mis-wire but thought I would mention it anyway)
That was fun Huh!
Tony

[professormudd]

That was fun Huh!

That's one of the F words I was saying. I kept hoping for an epiphany to point out some glaring obvious flaw, and when I finally got it I just shook my head and laugh/cried for a second.

Thanks for the method. I'll employ that when I build my 183