AB763 twin smoke

[Sonofastu]

Hello good folks!  I recently acquired a twin. I pulled two power tubes, pulled V1, put a 12ax7 in the PI, and disconnected one speaker. I was using the amp with my Rhodes and it sounds/ed great!  This past Friday I had a jam session. The power was out for hours but I have a house generator. I know the power from them is not the cleanest, the B3 wouldn’t play in tune. Anyway, at some point the magic smoke appeared. Yesterday I pulled the chassis and had a quick look. I don’t see any visible damage with the exception of a cracked dual capacitor, which I believe to be the V1,2 cathode bypass cap?  I pulled the power tubes and turned on the amp and standby. If it is the cap, could I have caused the failure based on my description?  Also, I found what appears to be a mod that I’d like to understand. There are 150 ohm resistors from pin 8 of the 6L6’s to ground, along with a wire that runs to a board with 3 caps in series. I’ll post a pic. Many thanks for any suggestion!

[Latole]

With 150 ohms to 6L6 pin 8 is it The Twin ?
Report amp model

With a "'new " used amp always check if fuse is the right one
Some generator may send to high voltage to amp more  the 125 volts

[Sonofastu]

The pics...after struggling to email them and save them...stupid iPhones!

[HotBluePlates]

...  Also, I found what appears to be a mod that I’d like to understand. There are 150 ohm resistors from pin 8 of the 6L6’s to ground, along with a wire that runs to a board with 3 caps in series. ...

That's not a "mod" but an AB568 Twin Reverb.  Those resistors & white Mallory caps are 100% factory-original.

... the magic smoke appeared. ... cracked dual capacitor ...

I usually think of burned resistors or transformers when I think of "magic smoke."

Even if the dual-cap is a dead-short, the 100kΩ 1w plate loads for the affected stages will dissipate 365v²/100kΩ = ~1.3 watts.  Not wonderful, but I bet they will handle that without much complaint.

You'll need to go through the amp & make some d.c. volt measurements to see what's getting powered & what isn't.

Are the filter caps the same original ones in brown cardboard, like the bias cap?  If yes, that's not a good thing.  It's also a bad idea to run a vintage amp on a generator, as those thing often have beyond-crappy regulation of their voltage output.

[Latole]

 "If it is the cap, could I have caused the failure based on my description?  Also, I found what appears to be a mod that I’d like to understand. There are 150 ohm resistors from pin 8 of the 6L6’s to ground, along with a wire that runs to a board with 3 caps in series."
-Sonofastu

Failure ( smoke ) by the cap ?  not in my opinion. Hot Blue Plate is right ; smoke are from transformer or resistor and a AB568 circuit


150 ohms look original from Fender like all parts I can see on picture.

 

[Sonofastu]

Thanks for the info!  Good to know the model. The tube chart inside the amp listed AB763, novice mistake I suppose!  I’ve seen folks reference a “dog house”. Is this the metal box on the underside of the chassis?  If so, is this where I’d look for the “brown cardboard caps (filter)”? 

[HotBluePlates]

...  I’ve seen folks reference a “dog house”. Is this the metal box on the underside of the chassis?  If so, is this where I’d look for the “brown cardboard caps (filter)”?

Yes and Yes. 

[Latole]

...." The tube chart inside the amp listed AB763..."

"Old" Silverface over stock cab use by CBS for the newer amp.

[Raybob]

... This past Friday I had a jam session. The power was out for hours but I have a house generator. I know the power from them is not the cleanest, the B3 wouldn’t play in tune. ...

Check the choke.  Few years ago, I played outdoor gig on generator, same one they used for refrigerators.  My old Fender BMR survived but keyboard player had something blow up.  Lead singer has a Fender clone I had built.  The clone did the magic smoke.  Turned out the choke had burned open or short, forget which.

[Sonofastu]

Many thanks folks!  Looking forward to getting to the bottom of this and learning something along the way. I’ll check a couple things and post as I go along. I’m extremely grateful for the knowledge base and won’t take it for granted.

[Sonofastu]

Pulled the dog house and I spoze these would be the original filter caps?  I have to admit, they do smell a bit pungent...as in similar to the smell of the smoke that escaped but they don’t visibly look damaged.

[SILVERGUN]

https://el34world.com/schematics.htm#Servicing%20fender%20amps

[Sonofastu]

Thanks for the link!

[2deaf]

Gotta love the doubled-up 4.7K's in place of a 2.2K.

[HotBluePlates]

Pulled the dog house and I spoze these would be the original filter caps?  ...

Yes, those are original.  They show date codes from 1967 and 1968.

[pdf64]

...This past Friday I had a jam session. The power was out for hours but I have a house generator. I know the power from them is not the cleanest, the B3 wouldn’t play in tune. Anyway, at some point the magic smoke appeared...

Generators are great for emergencies, eg pumping out floodwater, keeping power to fridges, freezers and essential medical equipment. But as you note, their electrical output isn't as well controlled as a regular wall outlet. Electronics can be fussy about such thing. Hence to avoid a similar recurrence of magic smoke release in the future, I suggest not to use it to power non essential electronic equipment.
So if it happens again, maybe cancel or try an acoustic session 

[Sonofastu]

The plan moving forward:

Okay Gents, or Folks rather, I believe after reviewing the links sent to me by Silvergun, I have a slightly "less than basic" understanding of a plan moving forward  .  Is is reasonable for me to assume I should just start replacing things to get the amp in to a better testing condition than trying to diagnose whats currently wrong with it?  If so, let me know if my plan makes sense.

First, replace the bias cap, filter caps and resistors in the dog house. 
Second, replace the seven electrolytic caps in the pre-amp (are these all cathode bypass caps?  Forgive my ignorance).
Next, replace the 470 ohm resistors on the back of the power tube sockets with 3W metal film (what are these called?).

Is this a good spot to test?

Things I think I'll still need to do:

Rewire the heaters so the wires don't cross over (I traced them and they're not very consistent).
Replace the Plate load resistors with metal film.
Rewire the balance pot to bias pot.

Did I miss anything?

If I make it this far and all is well in the world, are there any "need to have" mods I should do to this AB568 twin?

Much appreciation and many thanks!

[acheld]

Why are the heaters going to be rewired?  Maybe I missed something.

I think you need to diagnose the problem before you start replacing parts.

While it is true that many members of this forum would want to replace the electrolytic caps on a 50 year old amp, it is also true (IMO only) that this will not likely solve your problem.  In addition, it would not be uncommon to introduce new issues . . .

Your chasis pictures suggest that you have an unmolested old amp.  Nice!

[sluckey]

Why are the heaters going to be rewired?  Maybe I missed something.

I think you need to diagnose the problem before you start replacing parts.

+1

[Sonofastu]

The heater wires cross pins. In the link/s that I read through the recommendation was to rewrite them so they stay consistent, as in pin two goes to pin two of the next preamp tube instead of crossing over to pin 7 (if I’m recalling the correct pin, don’t have the schematic in front of me at the moment).

[pdf64]

That really is not worth dicking around with.
Did the amp have an obvious 60Hz hum issue before? Can any of these sources guarantee, demonstrate or cite evidence of a quantifiable, measurable improvement brought about by such rework? I suspect not.
If the wiring had been burnt, eg by a smoked screen grid resistor, then rework may be worth considering. But otherwise, nah.

[jim]

First...the ominous acrid smell of burning electronics is different from the mild smell of a smoke from a split electrolytic cap but they can occur together.  Look at your filter caps first for swollen split caps--In an old amp they are suspect.
They are in the small screw down box on the top side of the chassis.  There are resistors in there as well--look for burned ones.   If nothing flip the chassis over and with the amp unplugged stick your nose in there and smell the PT and OT for burn.  With magnifying loops or glass examine each resistor, cap, eyelet connection and tube pin for scorching.
Sometimes there is shiney melted flux that may still be wet.  Find a clue.  If there was smoke then something burned.  Jim

[sluckey]

You need to give us some symptoms. How is the amp behaving? Is it totally dead? Sounds crappy? This works but this doesn't work? Those kind of symptoms. So far all we know is you have seen some smoke and have a cracked dual bypass cap. You haven't even told us how much smoke.

And while the doghouse is open measure the voltage on the positive lead of all those caps. Post your measurements.

[pdf64]

When there’s a fault suspected, it’s best to power up via a light bulb limiter. If one’s not available, once the readily available parts are in hand, they’re quick to rig up.
Time spent sorting that out would be far better spent, than it would reworking the heater circuit polarity.

[HotBluePlates]

The heater wires cross pins. ... the recommendation was to rewrite them so they stay consistent, as in pin two goes to pin two of the next preamp tube ...

That doesn't matter; it's an old myth.

If you have a lightbulb limiter as pdf64 suggested, start there (to identify major short-circuit).  If not, measure d.c. voltages in at least the filter cap + terminals, if not all tube sockets.  See what's dead, what works first.

...  Is is reasonable for me to assume I should just start replacing things  ...

No.

If your car wasn't running properly, would you be okay with the mechanic jumping straight to "Install New Engine"?

[Sonofastu]

“No.

If your car wasn't running properly, would you be okay with the mechanic jumping straight to "Install New Engine"?”

Ha!  I spoze not but I guess using that analogy, I thought replacing the filter caps was like putting new brakes on a non-running car...as in “no it won’t get the engine running but while it’s on the lift...?

Good to know about the heater wire myth, that was info I got from the link sent to me from this site. Page 4, 2nd paragraph

https://el34world.com/charts/fenderservice4.hmm

I can’t say there was any hum issue to speak of anyway. Maybe some hiss.

I don’t currently have a lightbulb limiter. I will build one for sure! 

Sluckey- Once it started smoking I turned it off and pulled the chassis, so I can’t really say how it sounds. I Turned power and standby on with the power tubes pulled and there wasn’t any fire so I guess it’s safe to read some voltages!  I will get some of the info requested.
 
Besides the positive side of the filter caps, what other measurements would you suggest?
Many thanks again folks!

[Sonofastu]

On the positive side of the 70uF caps I measure 222V. On the three 20uF caps I measure 468, 445, and 378V.

[Sonofastu]

Pics

[Sonofastu]

For whatever it’s worth...the smell I caught a whiff of when the smoke came out smells a lot like the inside of the doghouse. There is also some discoloration inside.

[sluckey]

On the positive side of the 70uF caps I measure 222V. On the three 20uF caps I measure 468, 445, and 378V.

One of those 70µF must have about 470V on it. Otherwise you would not have 468, 445, and 378V on the 20µF caps.

I think you need to put the tubes in, connect a speaker and guitar and gather some symptoms.
 

[Sonofastu]

Hahaha...okay, that’s what I’ll do then!  Probably won’t get to that until tomorrow.

Thanks!

[HotBluePlates]

Good to know about the heater wire myth, that was info I got from the link sent to me from this site. Page 4, 2nd paragraph

https://el34world.com/charts/fenderservice4.htm

I can’t say there was any hum issue to speak of anyway. Maybe some hiss.

[Mangled link corrected]  I don't know why Doug wrote that way back when.  "Heater phasing" was a thing brought up by Gerald Weber in his first book in 1994.  I tried it in the late-90s (by way of "fixing" an old Fender with "wrong phasing") and found it made no difference.  In the years since, I can't think of a reason it should matter.

But it's often repeated, and people learning about amps get quickly wrapped up with "lore" so I figured I'd brush that one aside.

[Sonofastu]

Happy to know. Glad it’s one less thing I need worry about.   

[Sonofastu]

Good day Folks!

After Sluckey's recommendation, I put'er back together, plugged in my Tele, and flogged her pretty good.  Plays and sounds great!  I'm still running the amp with two power tubes pulled and one speaker disconnected.

Now what?

Thanks! 

[pdf64]

..replace the 470 ohm resistors on the back of the power tube sockets with 3W metal film (what are these called?)...

Just to note that these are the screen grid current limiting resistors, or just screen grid resistors.
I see no benefit in uprating them, and, if there's a screen grid to cathode short inside the tube, a major downside of doing so.
They see hard service though, so occasional replacement is in order, eg when the ecaps are replaced. As with other power supply resistors, Metal Oxide with a flame retardant coating are the best choice.

[Sonofastu]

Put back together and patiently waiting for my new filter caps to arrive. She’s pretty clean...a lot cleaner than my studio right now