fender princeton 5f2-a trouble

[djsadowsk]

im in the process of building my first amp a fender 5f2-a and i'm having problems with it. when i turn it on it oscillates like crazy and i can't figure out why from some reading online i think it might be a problem with my wire dressing but i have no idea what to do to fix it. any help would be appreciated. here's some pics
http://i1331.photobucket.com/albums/w598/djsadowsk/image_zps317c9a31.jpg

[sluckey]

Disconnect one end of the black wire that connects from the speaker jack to the board. Does that stop the squeal/howl? If so, then swap the red and blue leads on your OT. Typically you would put the blue on the tube plate and the red on B+.

[djsadowsk]

no that doesn't stop it

[eleventeen]

Do you have any kind of voltmeter to make some measurements?

First thing I would do is to reverse the connections on either the primary or the secondary of the output transformer. (Either one, not both! Whichever is easier) Just make quicky tack-solder connections and try it out. The reason why I say this is because the amp has a negative feedback loop and if it is out of phase, that will turn into positive feedback. That's the definition of an oscillator. We ordinarily do not think this is an issue on a single-ended amp but there's no reason why it can not be. Now, even though your red wire and blue wire are themselves supposed to indicate proper phase because that's how it's supposed to be coming out of the transformer factory and thus my (out-of-phase) assessment is very unlikely...it HAS TO BE positively eliminated by performing this test. It is possible that your kit supplier bought a load of output trannies from some other factory and the factory got the phasing wrong. Very unlikey, but a possibility that MUST be eliminated for sanity to prevail. Did you get the connection right on the output speaker jack? (would produce the same out-of-phase effect) Switchcraft jacks like you have, very very often fool beginners into thinking that the connection tabs are precisely the opposite of what they really are. And probably, it's hard to see the output jack so it would easy to get this wrong.

Your lead dress is not exactly Nobel-prize grade, but it's hard to imagine a kit going so badly astray if you've wired it correctly. I've never built one, so maybe others will chime in.

The final possibility I can think of is that somewhere, you may very well have a ground connection that is dependent upon a mechanical ground connection to the painted (or anodized) chassis that isn't making it. I see you have (the can of) one pot apparently grounded. The other one? The other input jack? The speaker jack?

[sluckey]

Your chassis is laid out backwards! Why did you do that??? This creates lots of problems for wiring that will just naturally want to squeal. Also, as eleventeen suggested, you're not getting a good ground connection for your chassis connections. It's very important to scrape paint and expose bare metal for every one of your chassis connections, especially that terminal strip for the filter cap grounds.

Personally, I would scrap that chassis and start over with a proper/logical layout.

[eleventeen]

The other thing you might try is to move, using an insulated plastic stick or a chopstick or similar, one or more wires as best you can...focusing on wires that make connections near the input of the amp and wires that make connections to the volume/tone controls. "Lead dress". (Not that I see too many other wires that can be moved) And tap on your 12AX7...maybe it's hopelessly microphonic.

[Willabe]

It looks like you have the PT on the inside of the chassis? And you laid out the turret board so that the input components are right next
to the PT.

You also have all the B+ filter caps by the input jacks and preamp tube. To do this you have the raw unfiltered B+ dcv running all the way across the chassis past the preamp tube socket.

You also have the speaker jack right next to the preamp socket which puts the largest signal voltages right next to the smallest signal voltages.

The input jack wires are crossing all the way over the chassis to the PT. These wires are very sensitive to picking up stronger signals.

Over all you have the turret board laid out backwards plus you put the PT inside the chassis right next to the input components where they might be picking up the magnetic flux field from the PT and wires are crossing the chassis back and forth, past things, components, sockets, etc, that they should not.

Take a look at any Fender tweed Princeton or champ and you will see they keep all the big B+ dcv parts and acv wiring, switches on 1 side and have the low voltage/signal parts/wiring on the other far side away from the big B+ dcv/acv parts/wiring/PT.

What are the green and white wires that I circled in your photo?


        
                Brad      

Edit; I was typing while Sluckey posted.  

[eleventeen]

"Your chassis is laid out backwards!"

Aiiieeee!!! Good eyes, Steve!

What do you think, any chance of a rescue flipping the parts board around? Of course, any parts pictorial will become totally invalidated....but that would at least get the filter caps near the power tranny and the high-gain end near where it wants to be.

AND get your grounds proper = bare metal where they're made.

There's absolutely no question that you can't run your speaker connection 3/4" away from your input wires, nor can you criss-cross signal wires multiple times! And get the power tranny on the outside of the chassis.

[Willabe]

He also has the PT (lams) mounted inside the chassis.


             Brad     

[eleventeen]

Other than that....

[sluckey]

What do you think, any chance of a rescue flipping the parts board around?

I thought about that but simply rotating the board 180 degrees would still have problems. The best solution for the board would be to install the turrets on the bottom side then rotate and flip over. That would at least get the board oriented properly with the chassis layout. But there are still a lot of issues with the chassis and chassis mounted components.

So, I would just chalk it up as a learning experience, get a new chassis, and start over. Probably be quicker than chasing gremlins in that chassis. I'm not trying to sound harsh or be mean spirited about this, but really, this is his first build and djsadowsk needs to hear it.

[Willabe]

djsadowsk, we all make mistakes.

For what it's worth, for a 1st amp build it's not totally wrong. The biggest thing is you got the turret board layout with respect to the PT and sockets/jacks backwards and the PT has to go outside for heat and noise issues.

If you redo the build in a new chassis and flip/flop the layout to be like Fenders I bet it will fly/sound just fine.

Way worse builds have been posted here before. It's much harder to do a build where you start with a blank chassis.

Don't give up, you'll do much better when you redo it.


                Brad    


Edit; Again I was typing when Sluckey posted.  I wish I knew how to type instead of 1 finger hunt and peck.   

[sluckey]

What are the green and white wires that I circled in your photo?

That's the power cord.

[Willabe]

Ohhh, I see it now.


            Brad     

[eleventeen]

I am not understanding....should not the kit contain the properly punched chassis..?

Don't think this would work, huh?



I take your word for it, my man....I've never built any of these little dudes.

[Willabe]

Yeah that might work. But he still has to flip/flop the preamp socket with the speaker jack.

should not the kit contain the properly punched chassis..?

I don't think its a kit.


               Brad      

[sluckey]

Don't think this would work, huh?

Sure that would work. That's what I tried to explain above. Notice that the components must now be mounted on what used to be the underside of the board?

I do see a fairly simple (and cheap) solution now that you posted that reversed layout. It would involve pulling the board, stripping it, adding 3 turrets (only 1 if you put the 68Ks on the input jacks), and reloading the board IAW your reversed layout pic. Still gotta deal with the other issues, but at least it's only labor, no more money spent.

[eleventeen]

Aha, you're 100% right. Parts would be under the board with that reversed dwg.

[loogie]

Do what Sluckey says, start over.  You'll be glad you did.  This is your Mulligan.  Watch a video on how to build amps.  Its nutin' but a thing.

[djsadowsk]

wow i was worried that i wouldn't get a response, thanks everyone for all the help. to answer some questions or statements, this wasn't a kit, i made the chassis from a piece of sheet metal, I'm not exactly sure why everything is reversed, lol i must have bent the metal the wrong way after drilling the holes. i think ill try putting the lugs on the underside of the board and building it backwards as well as putting the transformer on the other side. i won't be able to work on it for a couple days but ill post updates as i work on it. thanks again for all the help