68 Fender Champ

[Doc Stitches]

As soon as amp is turned on ... there is arching ( bright red and sizzling sound ) and a little smoke coming out of the OT.

I rebuilt the original Power Filter can with new caps and the OT still arches and smokes a little.

How do I begin voltage tests ? Is the OT gone ? What usually causes this in a Champ ?

Thanks in advance

Doc

[Leo_Gnardo]

Just like the most recent Champ to cross my workbench. Well, this one just makes a loud TICK as I bring it up on the variac, or sounds like machine gun bursts if I plug it straight into 120V power. Yes the OT's gone, we're both best off replacing it. Sometimes just old age & breakdown of internal insulation gets 'em. Last year I had a customer who abused his Champ, ran it at 10 with an overdrive box dimed at the input. Sounded great, then it went silent.

All you can do is the usual. Old Champs need new filter caps. New output cathode resistor & bias cap, put some distance between 'em so the cap doesn't get roasted. Some finer points, you could up the value of the cathode R, also select a 6V6 that draws a minimum amount of current. If you can get the bias current near 40 mA I figure you're doing OK.

Some tricks I haven't tried yet, increasing the value of the first dropping resistor in the power supply would lower the screen grid voltage and that should make life easier for the output tube. Sticking a screen grid limiting resistor on, why not a control grid stopper too while we're at it. It seems Fender's frame of mind in engineering the Champ was "hey it's just a cheap amp intended for students, don't overdo it, by the time little guitar johnny blows it up he'll be either ready to get a bigger better amp or else he'll have quit trying." So it falls to us to improve them.

[Enzo]

The old Champ was a sturdy amp. It was a practice amp, not an overdrive tool for stage.

A smoking arcing transfomer is bad, fixing other stuff won't change that. You have a bad transformer. My first guess might be a shorted power tube melted it inside.

[g1]

Where are you seeing the arcing? Could be you have a bad socket in addition to the OT issues.

[Doc Stitches]

Leo Gnardo, Enzo g1 ... yeh, I figured the OT was bad ... the arching comes from the bottom of the OT

I've replaced the main filter caps and will make other cap and resistor changes.

Enzo ... The 5Y3 and 6V6 sockets are suspect .... tubes seem too loose

[Enzo]

Don't be fooled, what matters is the grip of each pin in the socket. The fact the tube can move around just means there is room for the pins to be sloppy. Does wiggling those tubes cause crackles and noises? If so, tighten or clean the pins, if not, the socket is doing its job.

[Doc Stitches]

Well said, Enzo ... as soon as the OT get here I'll " wiggle, tighten, and clean "

Don't be fooled, what matters is the grip of each pin in the socket. The fact the tube can move around just means there is room for the pins to be sloppy. Does wiggling those tubes cause crackles and noises? If so, tighten or clean the pins, if not, the socket is doing its job.

[Doc Stitches]

OK ... so here's what we have

Installed new OT ...

Champ is sounding great. Tube sockets checked out good and tight ...

Tubes still wiggle in sockets but no noise/shorting when tapped

Amp has a very high squeal when the volume is between 1 and 2 ... I'll try another preamp tube ... otherwise it is a VERY quiet amp.

[The Dude]

It's possible that squeal is because the OT primary is backwards and you have positive instead of negative feedback. If you discover it's a microphonic tube, fine. If not, try reversing the OT primary wires.

[tubeswell]

What The Dude said.

[Leo_Gnardo]

It's possible that squeal is because the OT primary is backwards and you have positive instead of negative feedback. - - - If not, try reversing the OT primary wires.

Just been thru it, and yes the OT was reverse polarity though I followed Fender's color code. (WHY do they do that???) Daring to be different, I reversed the secondary wires and all's good now.

[The Dude]

...... (WHY do they do that???)......

Good question...... Maybe a different vendor?.......Maybe built on a Friday?......Maybe a color blind Chinese girl? I've wondered that myself. I've learned never to trim the wires to length until I'm sure it's right. It can be a crapshoot.

[52 Bill]

Just been thru it, and yes the OT was reverse polarity though I followed Fender's color code. (WHY do they do that???) Daring to be different, I reversed the secondary wires and all's good now.

What? I thought that Doc Stitches was working on the amp.

[Doc Stitches]

Thanks Dude and Tubeswell ... OT Primary was backwards ... squeal gone at all volume levels

Amp is happy, again. I did hear one little put sound while playing for 5 minutes. Should I go ahead and change all caps ?

... I have already changed the power filters and the 2 original 50mf 25V caps.

[Leo_Gnardo]

What? I thought that Doc Stitches was working on the amp.

True 'nuff. And I was working on another with the same problems at the same time. Including a replacement OT with reversed color codes. Some kind of synchronicity. Cue the mystery music, wooooooooo ooooooooo.......