Problem with a Vibroverb

[Marcfrom]

I will retrace the layout with multimeter.
Thanks for your help

Marc

[JazzGuitarGimp]

Do you have a scope? If so, have you scoped the output in the failure mode? Just thinking there may be an ultrasonic oscillation pulling the HT down. I know this doesn't add up with only 3mA of DC. Just a thought.

[Marcfrom]

I have no scope.
What can cause the ultrasonic oscillation ?

Do you think it's a good idea if I disconnect the ground wires of the filter caps board? I want to be sure the filter caps isn't the problem.

[JazzGuitarGimp]

I'm not sure what the question is. If you are suggesting taking the filter caps out of the circuit, and running the amp, then no, I wouldn't suggest that. But you could tack solder in a new set of filter caps, if you have them handy.

The oscillation (if there is one) could be caused by a number of variables. But I just remembered that you swapped in a different OT. Are you sure you've got the phasing of the OT correct, so that your negative feedback loop is indeed negative? You might try lifting one end of the 820R feedback resistor and see if that makes a difference.

[Marcfrom]

Yes I talking about the filter caps, ok I don't do that But i don't have another set for the moment.

I'm sure about the phasing OT, because i was working like this before.

For the lift you talking about the 470R resistor on the socket?

[Marcfrom]

Today i checked the wiring and trace it on the layout, i can say that'
s ok.

Now I suspect a bad Filter cap, I really don't see anything else.

Thus, I re-explain the problem, the amp work and the Voltage and current value of the 6L6GC is ok without the choke (wire instead).

But with the choke in place the current value is completely dropped.

Any suggestion ?

Thanks
Marc

[JazzGuitarGimp]

Yes I talking about the filter caps, ok I don't do that But i don't have another set for the moment.

I'm sure about the phasing OT, because i was working like this before.

For the lift you talking about the 470R resistor on the socket?

I was referring to the 820 ohm resistor from the speaker output to the lower phase inverter input, but if you are sure the OT phasing us correct, then there is no need to do this.

[Marcfrom]

Yes I'm sure about the OT phase.

Do you know how I can check the Filter caps without a scope?

[Firestorm]

Today i checked the wiring and trace it on the layout, i can say that'
s ok.

Now I suspect a bad Filter cap, I really don't see anything else.

Thus, I re-explain the problem, the amp work and the Voltage and current value of the 6L6GC is ok without the choke (wire instead).

But with the choke in place the current value is completely dropped.

Any suggestion ?

Thanks
Marc

When you replace the choke with a straight wire, you are connecting a third cap (the 22uF/500v screen filter) to the B+. Since that works, it implies a problem with the main filter. Are you certain the series caps in the "totem pole" are oriented correctly: anode to B+, cathode to anode, cathode to ground (with the balancing resistors straddling each cap) ? Are you certain the main filter has a good solid ground? A photo of the cap board and one of the power end of the chassis would help.

[Marcfrom]

This is the pics, I don't know if you can see it.

[img1536]http://nsm08.casimages.com/img/2013/09/08//13090810323113106011535110.jpg[/img]

[img1536]http://nsm08.casimages.com/img/2013/09/08//13090810332013106011535115.jpg[/img]

The wiring is exactly like this:

http://www.tubeampdoctor.com/images/Fil ... 050712.pdf

Some link of the resistors and also the link of the 3 x 22uF caps are under the board, but you can see it on the PDF.

You can see the white wire (ground of the 100uF, 70uF on the PDF) on the star ground. and the red on the main board.

[Firestorm]

The caps look correct (sorry for asking). I wonder about the integrity of the main star ground; that is a lot of terminal lugs on one bolt. It may be fine, but often when a working amp stops working, a ground has let go.

[Marcfrom]

The ground of the filter caps is ok. I will replace the two 100uF and see what's happen.

[Marcfrom]

Ok Guys, the problem is resolved!

I changed the two main 100uF caps of the Filter board and the amp work perfectly.

Thanks for all!

Marc