Another ? for the guitar amp guru's

I'm working on this Fender Deluxe Reverb. Customer had it all f&%ked up. Blown power transformer. OT running 16 ohm marshall speaker. Everything disconnected inside, etc. Anyway, I have replaced the PT and rewired OT (it's a hammond) to 8 ohms fender speaker. All voltages look good. When speaker is hooked up, it gives off this sickening high pitch squealing along with a low pitch hum (even with all pots turned to zero). Feeding a signal thru lets the signal pass, but all that noise is there. The strange thing is that when I hook the speaker jack up to a 8 ohm power resistor, I can't see any noise on the scope. When the speaker is plugged in, noise is there. I switched speakers, and it's the same.

What the hell?

At first, I was going to tell you to reverse the secondary leads on the replacement OT, thinking you were getting positive feedback, but on second thought that would manifest itself whether the load was a speaker or a resistor.

Check for microphonic tubes, check the grounding and power supply connections, compare the lead dress to the Fender layout diagram, and generally look for other "mods" which shouldn't be there.

Blackface DR Layout

You didn't mention if yours is Black- or Silverface.

Silverface. New tubes. I am seeing ugly clipping of the wave even with the load resistor. (not the noise I was talking about, but I figure that may have something to do with it...I am seeing a whopping pile of gain in the last AT7 (v6) At grid pin 2 things look ok (?) no clipping. Plate out, huge gain, clipping at top of wave, all downhill from there... Where the layout tells me to have 77v at the cathode of that tube, I've got 60v. Is that the problem?

At first, I was going to tell you to reverse the secondary leads on the replacement OT, thinking you were getting positive feedback

Click to expand...

cut the feedback wire if the high pitch noise go off the problem is on the PRIMARY of the OT you have to interchange the primary wires to the plates of the output tubes.

If you can change the high pitch with the gain , high control the problem is interstage oscillation.

If the feedback is taken off the secondary, switching either the pri or sec will fix it. Should be color coded.

As a last resort, if everything is wired correctly and the inverter is right, a 1 in a million solution is to ground the speaker frame to the chassis. Don't ask me why. Ask Gerald.

The guru's get it again. The primaries of the OT were reversed. The sign I should have gone from (now that I know) was that the feedback wire was disconnected when I opened it up...

Thanks guys. :guinness: from me to you.

Joel

The guru's get it again. The primaries of the OT were reversed. The sign I should have gone from (now that I know) was that the feedback wire was disconnected when I opened it up...

Thanks guys. :guinness: from me to you.

Joel

The guru's get it again. The primaries of the OT were reversed. The sign I should have gone from (now that I know) was that the feedback wire was disconnected when I opened it up...

Thanks guys.
Joel

The guru's get it again. The primaries of the OT were reversed. The sign I should have gone from (now that I know) was that the feedback wire was disconnected when I opened it up...

Thanks guys.
Joel

The guru's get it again. The primaries of the OT were reversed. The sign I should have gone from (now that I know) was that the feedback wire was disconnected when I opened it up...

Thanks guys.
Joel

Hmmm......Got SQL errors and said could not post...I see it was lying to me.

If the feedback is taken off the secondary, switching either the pri or sec will fix it. Should be color coded.

Click to expand...

mmm that could work. but switching the secondary, you reverse one tap . but what happend to the 4, 8, 16 ohms.

mmm that could work. but switching the secondary, you reverse one tap . but what happend to the 4, 8, 16 ohms.

mmm that could work. but switching the secondary, you reverse one tap . but what happend to the 4, 8, 16 ohms.

I´m having problems with the database too.

Lots of feedback problems on the forum today...

/Anders