A Marshall AFD Sqealing problem (video included)

[yalesmith]

I built a ceriatone AFD 50 and it is a great amp, more scooped than i would have imagined but it is squealing a bit when I turn up the gain or volume...take a look at the video and i am open to any suggestions

http://www.youtube.com/watch?v=0HMmGK1O ... e=youtu.be

Thanks

[roberto]

Something is creating positive feedback inside the amp.
It's your build or you receipt it already mounted?

[ampgeek]

Brand new build? Does the squeal change significantly when you adjust the presence pot?

That sounds a lot like crossing up the OT primary leads on the output tube plates.

Try lifting the NFB lead from the impedance selector. If that fixes it, you will need to switch the OT primary leads on the output tubes.

Good luck!
Dave O.

[roberto]

If that's the problem, just switch the outputs from the PI to power tubes, it's easier.

[yalesmith]

yes it is a brand new build and the presence pot really does not affect the squeal but rather acts like a second master volume, which is a bit annoying....as far the primary's on the ot being switch, i wish it was that easy but that's definitely not it.

here is the design if anyone is interested

http://www.ceriatone.com/images/layoutP ... 35_50W.jpg

[roberto]

The problem is not the layout, but how you built it.
We need picture of what you have done.

[husky]

The problem is not the layout, but how you built it.
We need picture of what you have done.

Agree, sounds like major ground issues as well
Need a gut shot to help you.

[yalesmith]

Here is are a few pics....and again thanks for the help!

[M Fowler]

Are the wipers of the each of those three trimmer pots wired to the 3rd leg?

In your amp the wipers looks like they are bare not connected to anything?

Mark

[husky]

Here is are a few pics....and again thanks for the help!

First thing I would do is correct the lead dress to the master volume
Those wires for that type of master are extremely sensitive and IMO should be shielded. I would actually disconnect the master completely (add the bias dividers) and try it without it.

[yalesmith]

Do you mean the PPIV master volume and when you mention "disconnect the master completely (add the bias dividers) and try it without it" - i am loss in what you mean.

thanks!

[Structo]

That end of coax going from the master to the return jack looks suspicious
at the jack. (M)

Be sure to trim the shield back from the center or it will short to ground.

Try swapping X and Y with each other.

Does it squeal inside a shielded head cab?

[Dai H.]

note that in the video the open side of the chassis is facing the spk. cab (i.e. right on top of it)

[ToneMerc]

Are the wipers of the each of those three trimmer pots wired to the 3rd leg?

In your amp the wipers looks like they are bare not connected to anything?

Mark

Ditto, those three 10k trimmer are wired as fixed resistors. The wipers should be connected so that they become variable.

TM

[Structo]

I think he has the wipers connected to the resistors and caps sitting behind the trimmers.