Debugging with a scope?

[Icetech]

Hi all, i have a scope (national,vp5321a), a sine generator. I'm trying to find a fuzziness in my amp that happens when i first strike a note then within a second or 2 sorta fades away with a crackling, kinda what a discharging cap would sound like i think..

Anyway.. main question i have is, I don't feel like going deaf doing this... and i don't have a hotplate or dummy load, is it safe to remove the power tubes? or does this still stress the OT?

Also, recommend the best places in the amp to try to locate a prob like this?


Thanks guys:)

[Icetech]

AHHH!!! finally.. found the problem.. (thank god for scopes)..

anyway.. tracked the problem to the NFB.. (well near the nfb, but took a guess) anyway.. remove the NFB, sound is crystal clear.. no fuzz... put NFB on, fuzz.. Has anyone heard of this problem before? The amp doesn't sound half bad without nfb really.. but would rather have it:)

[Normster]

That IS strange. Are the feedback resistors the correct values? Connected to the correct impedance tap?

[Darkbluemurder]

You may have an oscillation caused by running the feedback wire too close to the power tube input grid wires. Try to pull the fb wire away from those as far as possible.

Good luck!
Stephan

[Icetech]

Ahhh dark, thanks much... i will try that when i get home tonight, thinking about it, i believe the NFB is running right on top of one of the power tube input wires.. makes sense:)

[d2camero]

Another thing **might** be that the OT primary leads might be backwards. BUT, check this out with someone who really knows before trying it.

d1

[MarkB]

Another thing **might** be that the OT primary leads might be backwards. BUT, check this out with someone who really knows before trying it.

d1

If the primary leads are backwards, the amp would scream - you know it when you hear it.

[Chris333]

Had the same problem. A piece of shielded cable from the jack to the board cleaned it up nicely.

[Icetech]

Chris, you are talking bout the NFB wire? And yeah.. silly me has the wire running Tucked between caps in the PI... i will try that tonight... i am so happy to finally find the prob.. the amp has sat for like 6 months cause i got tired of messing with it.. but everything comes around eventually:)

[Darkbluemurder]

Another thing **might** be that the OT primary leads might be backwards. BUT, check this out with someone who really knows before trying it.

d1

If the primary leads are backwards, the amp would scream - you know it when you hear it.

Normally yes but not necessarily so - my Bassman hummed loud when I hooked an 8 Ohm cabinet to it. With a 4 Ohm cabinet it was mostly OK but not always. It never screamed so I was not really pointed at this. Only when I read that the AB 165 Bassman needs to have the primary OT wires reversed I did that and the problem was solved for good.
But from Icetech's description this does not appear to be his problem.

Icetech, try to isolate the NFB wire from the PI output coupling caps and grid wires to the power tubes. The best thing is to run it beneath the speaker jacks along the chassis and in a right angle in a straight line to the board where the NFB series resistor is mounted. If that doesn't get it then try a shielded cable mounted in the same position. If that still doesn't get it you can at least be sure that you have taken care of this.

And finally please let us know whether this approach worked!

Good luck!

[Icetech]

No luck yet.. tried rerouting the wires.. tried shielded, tried changing the nfb resistor value 4.7k.. 2.2k.. 10k.. all the same, well same problem.. varying degrees..

I am kinda stumped now..

The amp is based off norm's amp here

https://tubeamparchive.com/viewtopic.php?t=892

Also built in a bassman, the problem goes away with the NFB unsoldered, BUT the clean channel isn't too clean for long and the amp is kinda over the top without the nfb.. i dunno what to try next:)

[skyboltone]

Ice:
This is desperate and not scientific at all but try ungrounding the secondary of the OT with an isolated jack or just hanging loose for trial with the speaker hooked up and the rerouted/shielded NFB wire attached to the OT tap.

Worth a try. Maybe we got a leaky OT.

Dan

[Icetech]

Sky, not really following that, unsolder the OT ground going to the jacks, and solder the NFB wire to that? and what would i be looking for in that situation, sorry but im not a pro like alot of the guys here:)

BTW, a pic is worth 1000 words.. figured i would show what the scope is showing..

Here is a pic of v4 pin7 with the signal level just below where it gets fizzy.

[img:640:480]http://mywebpages.comcast.net/roadrebel/v47b.jpg[/img]

And here is v4 ping 7 just above the signal level that it goes fizzy..

[img:640:480]http://mywebpages.comcast.net/roadrebel/v47a.jpg[/img]

v4 pin 2 has a totally clean signal through the whole range..

[skyboltone]

I would just try to isolate the secondary of the OT from ground. Generally, the speaker jack has the ring grounded with the pin ungrounded. What I'm wondering is if you've got an insulation failure in the secondary (8 ohm) winding that allows the feedback portion of the signal to get dirty at higher levels.

A speaker output jack with no ground connection would be a good test with everything wired as normal.

Dan

[Darkbluemurder]

Just for clarification purposes: V4 is one of the power tubes, right?