#102 Troubleshooting

[fred.violleau]

So I finished assembling an amp in between #102 and #183 with a D-lator inside.

So far I have identified that everything up to the send of the d-lator seems to work fine. I used this output to send the signal into another amp and everything works.I have a little static noise in the background which I may investigate, but I have a bigger fish to take care of for now.

My trouble seems to reside in the power amp section. I can play the amp and get some sound, but :
- overall volume seem low
- I get sort of a vibrating doubling an octave lower note ?
- and after 20ish seconds of playing, a huge 60 cycles hum comes upfront, not responding to any volume knobs. It lasts for a minute ish, and then stops (as if something was discharging), and the cycle repeats... I can still hear the sound of the amp through... but you get the picture ..

here is a youtube link so you can hear the amp behaves :


So far I have ohmed all the gnd and they are usually around .02 ohms, which seems to refer to a good connection.
The only one higher is the FX loop, probably because of the longer runs of cables I used to have all of these going back at the same point. Gnd reads 1.2 Ohms.

Here are a few pics of the build

PXL_20201113_233339291.jpg

PXL_20201113_233347969.jpg

PXL_20201113_233359198.jpg

I am a bit clueless as to where to look, any help is much appreciated !

Fred.]

[norburybrook]

are your OT primaries definitely the right way, that can sometimes make for strange tones rather than the expected loud squeal.

a friend of mine had a similar thing recently and it turned out to be a missing connection on the power board to one of the main filter caps, took 2 weeks to find it as it was underneath.


Sorry I can't be of any instant help.

M

[martin manning]

- and after 20ish seconds of playing, a huge 60 cycles hum comes upfront, not responding to any volume knobs.

That sounds like 120 Hz sawtooth from the power supply to me.

[PicknStrum]

Although I'm not as experienced at troubleshooting, one thing that may help you narrow down where your issue starts would be to either check with an oscilloscope or an audio probe. I'm not as experienced with the scope but an audio probe is fairly easy to build and use. Check this out:





I'll defer to others for more acute / experienced observations, as I've only got a couple amps under my belt; this is just where I'd start. Hope that helps.

[fred.violleau]

Pop quizz : what happens when you forget to connect the main filters caps ???
You get a huge 60 cycle hum that comes and go, with a sawtooth on every note!

PXL_20201114_183417954.jpg

To your point Marcus, I tried to have all connection visible on the board, but when I soldered the Power supply board, it was done in multiple sessions, and I thought "I will solder this later on ... " and I forgot.

One down, a few more to go!
On my debuging session, I unplugged the choke, and re-instated a 10W 510R resistor. It seemed to have a beneficial aspect on the buzz, but the amp does not seem to react the same, feels more rigid ?! i will put back the choke to hear the difference.

I will now proceed on to moving a few grounds to see if I can tame the remaining buzz/hum.

Fred.

[fred.violleau]

Hey picknstrum,

Thanks for the tip! I did use a probe on my first build as I was having multiple bad cables. I was using regular jack cable as isolated cable. Turns out the inner cable's insulator did not support that much heat. Same applies for the insulating thread that would sometimes burn and short the guitar signal. The probe was very helpfull debuging where the signal stopped and then investigate what was wrong !

I read a lot about probing with an osciloscope, especially for the noise on the power supply, but the scope I bought is cheap and can only go as far as 50v, so I cannot use it on that part of the amp...

I definitely need to strengthen my skills with that instrument!

Fred.

[norburybrook]

excellent the first time I've been right about something like this


Just goes to show shared knowledge is a powerful thing.


M

[PicknStrum]

Hey picknstrum,

Thanks for the tip! I did use a probe on my first build as I was having multiple bad cables. I was using regular jack cable as isolated cable. Turns out the inner cable's insulator did not support that much heat. Same applies for the insulating thread that would sometimes burn and short the guitar signal. The probe was very helpfull debuging where the signal stopped and then investigate what was wrong !

I read a lot about probing with an osciloscope, especially for the noise on the power supply, but the scope I bought is cheap and can only go as far as 50v, so I cannot use it on that part of the amp...

I definitely need to strengthen my skills with that instrument!

Fred.

I'm just glad you found the problem - and quickly!

[fred.violleau]

Almost done! biased the amp with EL34. Waiting for the faceplates, need to complete the footswitch. Spent the afternoon playing with the amp.

It is very punchy and dynamic, super responsive.

Thanks Jelle for providing part of the NOS components, and a huge shout out to Marcus, Martin, Ian, Tony and all the ampgarage community for providing schematics, incredible knowledge and guidance for an apprentice like me in achieving this piece of gear.

This build would not have been possible without you guys



Cheers!

Fred.

[ChopSauce]

Congrats!

...the scope I bought is cheap and can only go as far as 50v, so I cannot use it on that part of the amp...

Can't you use a 10x probe and check signals with voltages up to 500V?

[didit]

Hello -

That all sounds good. The amp, your demo noddling, and the way your debug story resolved.

Best .. Ian

[fred.violleau]

@chopSauce, I need to understand this a little title better, my electronic course date back to when I was 14 and I forgot most of it

@Ian, thanks for the help, much appreciated!

Here is a little tip of the hat to one of the greatest.
Love the clean sound on this amp, super responsive.



Again, thanks for everything.

Fred.