DIY AA764 Champ Squeals Loudly

[TxTeleman]

Hi.

I inherited a DIY AA764 Champ that was already built, using point-to-point wiring (no tag board), and wasn't working. The wire management and soldering skills of the original build left a lot to be desired.

It would not power on. The power transformer wasn't grounded. I fixed that. Now it turns on.

The issue is that as soon as the tubes power up, ten seconds or so after turning on, I get a very loud high pitched squeal from the speaker that is unaffected by turning any of the volume or tone knobs.

I tapped on all three tubes, but that made no difference. All of the wiring appears to be in the right place, even though it's ugly.

Any suggestions? Thanks it advance.

[52 Bill]

Welcome to the place!

The amp is oscillating, you just need to find out where the problem is to fix it. If it is wired correctly per the schematic, the problem could be in how the wires are run from place to place, or it could be a part that is far enough off in value to cause the circuit to do this.

Have you tried moving wires or parts around (away from each other) to stop this? Have you tried different tubes?

[Chuck H]

If the primary or secondary of the OT is wired out of phase this will also make the NFB loop out of phase (a positive feedback loop). Try lifting one end of the 2.7k resistor connected to the output jack. If this stops the problem then you'll want to reverse the primary OR secondary OT leads, not both.

[TxTeleman]

Thanks for the replies.

This amp uses the TF-103-48 OT. I checked the wiring.

The BLU primary connects to pin 3 of the 6V6. The RED primary connects to the 20 mfd multi-section can and then to pin 8 of the 5Y3, along with the PT Yellow.

The OT secondary BLK connects to the output jack sleeve, and the GRN secondary connects to the output jack tip; also the 2.7K f/b resistor.

So it looks right to me, but I will do some testing as soon as I can get a chance.

Thanks again.

[Jag]

It might look right but that doesn't mean the transformer is wired following your expected color code. It could be something else but my first thought is flip the primaries as well.

[Jazz P Bass]

I am going to vote for flip the primary.
As Chuck has noted, if you disconnect the feedback & the noise stops, that is classic positive feedback.
(Think oscillator).
On a working amp if you lift the feedback R & the output increases, then the output transformer is wired correctly.(negative feedback was working)
If the signal output drops, then the OPT is wired incorrectly.(positive feedback was working)

[TxTeleman]

Y'all rock!

I lifted the 2.7K NFB resistor, and the howling oscillation went away. Aha! So, I reversed the OT primaries, and... voila, it works!

I even got the print for the TF-103-48H and it says right there... Power RED, Plate BLUE. Wrong. Oh, well.

Thanks so much!

[jighead81]

Hi, I'm a little confused. I'm putting this same OT into a vibro champ build I'm doing from triode electronics. I'm actually all done but was about to wire up the OT and realized I didn't have any idea where to connect the wires. So where should the blue, brown, and red wires go? I was thinking Blue: Plate, Red: Power, but now I'm not so sure.
Thanks in advance from a noob.

- Steve

[Jazz P Bass]

Sometimes the tranny manfacturers get the colors wrong.
You have a 50/50 chance.
Hook it up.
If it howls, it's incorrect. Reverse them.

[jighead81]

Well, I hooked it up the way Triode said too, then I got the piercing squeal, then I swapped the primary's and it works now. There was a brown wire there too that I just heatshrunk and tucked away.

Ok....it's working, but:

There was hardly any volume until I got around 8 or 9, then a sudden jump in volume. But it's still not very loud with no break up whatsoever when dimed. Any ideas there? Am I losing some signal to ground maybe?...I lifted the NFL wire and it got louder, but still not nearly as loud as it should be.

Also, the intensity doesn't work until it's all the way dimed. The speed works but it's soooooo fast even all the way down.

Well, those are my only problems. I'm just so happy I got any sound at all from this thing,... being my first build. If you guys have any ideas what I might have done wrong please let me know.

Thanks!

[Jazz P Bass]

That Brown wire tucked away.
What's up with that.
Sounds to me like they sent you a Push-Pull output transformer.

You really should go back & recheck your work on that tremelo circuit.
Something is not correct.

[jighead81]

That Brown wire tucked away.
What's up with that.
Sounds to me like they sent you a Push-Pull output transformer.

Here it is: Fender Single Ended Champ Style Upgrade 15W Output Transformer 4/8/16 Ohm

You really should go back & recheck your work on that tremelo circuit.
Something is not correct.

I have another set of eyes coming over tomorrow that are far more advanced than me so hopefully they'll see what I'm missing. Everything looks perfect to me, but there's gotta be some cold solder joint somewhere, or wrong wire somewhere.

[jighead81]

Don't know if this worked, but here's the transformer info: Fender Single Ended Champ Style Upgrade 15W Output Transformer 4/8/16 Ohm

[Jazz P Bass]

Sorry for the disinformation.
The Brown wire is a separate 5K tap on the winding.

[jighead81]

Update:
I re-flowed all the solder joints and now it's at full volume and sounds amazing....but.

The treble and bass aren't very effective, and the vibrato is still doing it's same crap.

The speed works fine, but the Intensity won't do anything til it's about 8 and it's just putt puttin like crazy.

and there's still a hum. I've moved the heaters all over the place with some chopsticks and still no luck. I think it might be the carbon comp 68K's on the inputs? I read those can be a culprit.

Thanks for all the help so far, I really appreciate it.