Ceriatone ots, hrm.

[CHIP]

good! I think you have to start using your scope

do you have a signal generator that puts out a sine wave? (you can also use a PC sound out and some sw, I use SignalSuite on mac...)

send a 250-300hz 100mV sine wave to the input of the amp, then connect the scope to the preamp output

you should see your sine wave on the scope

let it run for the amount of time usually required to build up your problem while looking at the scope, how the wave changes?

If the sine wave changes after a certain amount of time, what does this change direct you to fix?

[67plexi]

Frankie. You need a function generator for your oscilloscope.
http://www.doctronics.co.uk/scope.htm

I hope this helps. Steve.

[LPSGME]

good! I think you have to start using your scope

do you have a signal generator that puts out a sine wave? (you can also use a PC sound out and some sw, I use SignalSuite on mac...)

send a 250-300hz 100mV sine wave to the input of the amp, then connect the scope to the preamp output

you should see your sine wave on the scope

let it run for the amount of time usually required to build up your problem while looking at the scope, how the wave changes?

If the sine wave changes after a certain amount of time, what does this change direct you to fix?

I just read all of Frankie's descriptions and he basically describes: A loss in volume, warmth, lows and highs, including a fuzzier OD - all while the PS voltages remained stable.

With a scope he can now view the PS and pretty much detect any unusual noise or ripple etc. After that, it's just a matter of comparing the signal at various stages (in and out) before and after. When the first point in the signal path with a change is observed, that will mostly likely denote a nearby bad part - and let's face it, there aren't that many.

This of course does not preclude a dual problem. Let's say he has 2 bad tubes that are each doing something odd. Replacing one at a time, then, won't necessarily uncover that there are 2 bad tubes etc. So he has to keep that in mind.

Frankie, don't forget your scope has both AC and DC coupling. You'll mostly want to use AC.

...at least that's what I would do.

[Frankie]

I found one issue now. The B+ 5, must have had a cold solder even if I changed that cap, it still had a cold solder, the amp sounds much better, but the problem with the amp fading and gets more narrow are still there, but now it happens during 1 minute of playing. The overall volume is a little louder, but still dropping. I wait 5 minute and it sounds full for a minute again. So something has changed after the B+5 resoldering. But still one issue left thou!!
I need to get into that scoope, and learn how to use it! :)

[Structo]

Yes, check each B+ node for AC content.

The DC there should be a flat line.

[CHIP]

Thanks for the explaination LPSGME

[Frankie]

Heaters 6,3V ac, so they are fine.

I presume you mean before/after and that they are correctly ground referenced and not floating... anyway can you check also that each leg measure 3.15v relative to preamp ground?

Each leg measure 3,1 volt ac on the heaters.
And 6,3v floating. :)

[llemtt]

When the first point in the signal path with a change is observed, that will mostly likely denote a nearby bad part

+1

just to go easy I'll check the clean output at preamp out first and then move to check at the top of volume pot

I suspect the problem will show up also on V1a stage alone

[llemtt]

Each leg measure 3,1 volt ac on the heaters.

ok this means that the heaters winding CT is working properly

I presume you have checked them at V1 socket!

anyway I'm not totally convinced that your PT isn't doing something strange...

another thing to check: your relays and relays supply -> supply should be stable without any "ripple" so you can check this with the scope

I once had a similar problem due to ripple on relays supply that was preventing good and stable contacts but actually I don't know how to relate it to "after 1 minute playing"

[Frankie]

Each leg measure 3,1 volt ac on the heaters.

ok this means that the heaters winding CT is working properly

I presume you have checked them at V1 socket!

anyway I'm not totally convinced that your PT isn't doing something strange...

another thing to check: your relays and relays supply -> supply should be stable without any "ripple" so you can check this with the scope

I once had a similar problem due to ripple on relays supply that was preventing good and stable contacts but actually I don't know how to relate it to "after 1 minute playing"

Sorry but I don`t know what "ripple" is, can you describe it to me, and how do I see that on the scope? I am totally new on this, sorry.

[Frankie]

Did measure all those grids again, found 3mv dc on the V1 b.
Can that cause the problem?

[gilgalad101]

Sorry but I don`t know what "ripple" is, can you describe it to me, and how do I see that on the scope? I am totally new on this, sorry.

The voltage that comes out of your wall is of course AC. The power supply of your amp rectifies that AC voltage with diodes and turns it into a DC voltage. This voltage isn't very smooth if you look at it right after the diodes; it contains a lot of ripple. The big capacitors (filter caps) in the power supply smooth out as much of the ripple as possible to try to get a nice, stable DC voltage. The first picture is similar to what you would see right after the diodes, while the second is similar to what you would see after the filter caps, albeit somewhat exaggerated. Hope this helps.

[img:330:394]http://www.eleinmec.com/figures/018_04.gif[/img]


[img:465:165]http://hyperphysics.phy-astr.gsu.edu/hb ... ipple3.gif[/img]

[llemtt]

Did measure all those grids again, found 3mv dc on the V1 b.
Can that cause the problem?

It can be a sign of leakage from tone stack caps or from the LNFB cap (47n) but also a tube that draws a little current from the grid can do that.

Exchange V1 tube with another one then measure again, if you have a different reading there's a good chance that it's caused by the tube.

Anyway do measure while changing tone settings -> do the reading change?

Measure resistance from V1b to ground there are also chances you have a bad solder on the path from grid to ground.

Also test again the "happens after 1 min playing" issue with radically different tone setting and with PAB on/off -> does it change something?

[Frankie]

Did measure all those grids again, found 3mv dc on the V1 b.
Can that cause the problem?

It can be a sign of leakage from tone stack caps or from the LNFB cap (47n) but also a tube that draws a little current from the grid can do that.

Exchange V1 tube with another one then measure again, if you have a different reading there's a good chance that it's caused by the tube.

Anyway do measure while changing tone settings -> do the reading change?

Measure resistance from V1b to ground there are also chances you have a bad solder on the path from grid to ground.

Also test again the "happens after 1 min playing" issue with radically different tone setting and with PAB on/off -> does it change something?

Changed the LNFB cap, changed the tubes in preamp V1, V3, Changed the first volum pot, Reads zero on grid now but the tone and problems are the same!! :( Changes are the same with the PAB on and off.

[llemtt]

Measure resistance from V1b grid to ground.

Then go with the scope!! Put sine signal (100mV) at input then check the signal with scope at top of volume pot.

But if you want to go on with substitutions next candidates are bass and mid caps...

Post also more pics -> we can't see all the pots & relays area.