Pignose G40V spontaneous meltdown

[tothemoonn]

Hi everyone,

I hope I've found the right area to ask this question. I'm hoping to get some opinions on why my amp practically caught fire today.

There's not much back story. Amp seemed fine at the store. I carted it home last night,and this morning I plug it in, flip it on, and head to the bathroom while it warms up. As soon as I walk out of the bathroom I smell that familiar smell of an expensive paperweight being born and notice that the LED has gone dark.

I've done a fair amount of electronics work, and know how to do things safely, but I'm pretty in the dark when it comes to amp design so I'm having a hard time troubleshooting.

In my handy photo gallery http://imgur.com/a/aklOp I've circled everything in red that's obviously melted or scorched, and I've done the same in the schematic. So far the most obvious components to have failed are 2-3 resistors (mostly 1/2 watt, and one larger one that looks to be about a 5W early in the circuit after the input transformer and near the bridge rectifier) and a 220 uf 50V capacitor. I'm also certain that the two yellow wires that are de-soldered in the photos only let loose because of the extreme current being passed, and was not the cause of the initial failure. Finally, the fast blow fuse in the back also went by the end of it.

Finally, I was using a two prong adapter so the amp wasn't connected to a true ground in a grounded three prong outlet...unfortunately I live in an old house and grounded outlets are not an option, the best I can do is GFCI and I'd have to be in the bathroom for that.

I've already ordered the parts I know need replacing but I'd love to avoid flipping this thing back on and watching the same thing happen again, especially considering this repair might require me to make a new PCB because of all the scorching and damage to the old. I also figure I'll replace both transformers with nicer ones while I'm at it, whether they check out OK or not.

Thanks in advance for any input!

[Stevem]

I have that same amp and I can make no logic as the that failure that you had take place but here`s my stab at it!!

If tube V1 failed by shorting I can see it taking out the cathode resistor R11, but that tube failing and the current it was able to pull when failing should not have taken out R3 and some of the diodes as that 1.5 amp should have gone south before that may lay!

I would rebuild it, get the output tubes tested and or prove them out in another amp and I would look on line for the info on how to test out a output transformer by pumping a low AC voltage into it

I would also replace C8, add a inline 200ma fast blow fuse in the OT center tap and confirm that the needed -43 volt or better bias is seen on pin 5 of both output tubes before you fire the amp back up with the stated tubes back in it!

Also while your at it resistor r24 and 25 should be upped to 1 watters just due to the voltage level they have on them at the power supply side.
You can PM me if you want and I can pass along my phone number if you still have issues.

[Stevem]

Also in looking at those pictures again, that amp has been messed with as all of those wires on that power supply board have been unsoldered from there factory thru connection and re soldered back on to the top of the board!

Doing it that way is fine as that is what I have done to work on mine, but just letting you know that it's been apart once before and the question is for what?

Can you still read the resistance value on the body of that green resistor next to those two yellow heater filament wires, is it a stock value?

Also confirm that r7 and r8 have not burned up.

[Stevem]

The best and biggest change you can make to get that amp to sing is to power it thru a 5AR4 recto tube by means of a new PT with a 5 volt winding for the tube, or a small transformer just for doing the tubes heater!

And do not let any one tell you you will not have tight bass with this mod as if you have 460 volts or so on the output tubes with the needed current for the tubes to pump out even 40 watts of clean power you will have the low end responce you are looking for!

[tothemoonn]

That's all very helpful, thank you!

You also confirmed my suspicion that the amp has been worked on before. When I started looking up info on them I noticed that I was missing the rear grill for the cabinet. Unfortunately I don't know that I'll ever be able to find out why. My brother gave this to me for Christmas this year and he picked it up from a vintage shop about 30 hours away from where I live. I know he played it there for a little while but I hadn't had the chance to touch it until yesterday.

Ill definitely check out R7 & 8 and up the wattage on R24 and R25. Visually R7 and R8 look good, no signs of heat as with other, but once I start pulling the thing apart Ill get a read on them out of circuit. The value on the green resistor (R3) is still giving me a stock 1K. As for the diodes I'm not sure whether they took much directly, or whether the indications of damage are purely from the big green resistor flaming out. I'm also worried about the integrity of C3-2 because of its proximity to R3 and some light burn marks on the casing right next to R3. Will these big caps still bulge on failure like the smaller ones tend to?

As for the 5AR4 upgrade, since I'm planning to replace the transformer anyways does the mod really only require swapping the 6L6s with 5AR4s and putting in a transformer with a 5 volt winding (sockets and pins are all still the same between these two tubes?)? Also, since all this amp stuff is foreign to me, you would be referring to the larger transformer (I think I've also heard it called the input transformer) when you say PT right? If so that sounds like something I'll definitely go for.

[David Root]

5AR4 is a rectifier tube not a power tube; Steve is suggesting you add a 5AR4 in the power supply, not replace the 6L6 with 5AR4s.

As you noted yourself, you do need to gain a basic understanding of amplifier topology and design. if you search in this forum you'll readily find sources of this information.

[Stevem]

Disregard that last post about the recto tube as that was ment for a different string and I do not know how it landed in yours,but yes that could be done to your amp if it had the real estate for it!

[tothemoonn]

Gotcha. Thanks guys. I think I'll keep it simple for now and try not to do much more than get the thing running again and beef a few things up like Steve suggested.

[ampdoc1]

Good luck with your rebuild. A friend turned me on to these and mine works fine,... very light and portable with a great Marshall 50 sound.

My buddy later had a power tranny meltdown. While rebuilding his, I noticed the rear fuse on mine showed a 1.5 A FB, but his amp showed a 2.5 A FB. I replaced it with the 1.5 A , and it's still working fine.

Reading some other info on the amp, a number of people recommended a better speaker. I bought a nice heavy 8" speaker, but have never installed it. The stock speaker sounds fine to me.

[tothemoonn]

Good luck with your rebuild. A friend turned me on to these and mine works fine,... very light and portable with a great Marshall 50 sound.

My buddy later had a power tranny meltdown. While rebuilding his, I noticed the rear fuse on mine showed a 1.5 A FB, but his amp showed a 2.5 A FB. I replaced it with the 1.5 A , and it's still working fine.

Reading some other info on the amp, a number of people recommended a better speaker. I bought a nice heavy 8" speaker, but have never installed it. The stock speaker sounds fine to me.

I'm really excited to hear it. Now that I've been searching info for it I see they've got a pretty good following. I don't think I'll modify it much, but I would like to build an isolation cabinet with another speaker since this would be an amp used pretty much exclusively for recording.

I think I'll take the tip on the smaller value FB, might have saved me a little soldering if that thing had gone sooner.

Cheers!

[Gaz]

Hey, sorry to be that guy, but someone's gotta say it: Playing or working on this amp without a grounded chassis is foolish. I just want you to live to see 2016!

[Phil_S]

Depending on how old your house is, you might actually have a ground available. My house was built in 1956 with a 2 wire system, but they ran armored cable to all the metal junction boxes and wall outlet boxes. This shield is grounded at the breaker box. I can install 3-wire outlets and connect the ground to the outlet wall box. I am not sure if this meets code, but my meter says it is a working ground and I've done it where I need 3 prong plugs. I suppose now someone will yell at me ;-(

I'll note that I've had a very competent licensed electrician working in my house who did not object to what he saw. Maybe he was just keeping his mouth shut, and I didn't want to ask. I took this as tacit approval. I may have assumed too much. I know that he took a feed for recessed lighting from one of the boxes that I converted to a 3 prong, so I know he saw it and checked it. He also installed a GFCI 3 prong on the exterior with the same config.

Unless you live in a really old place with no actual ground, you might consider running a new circuit from the breaker box. It is not difficult to do that. Find a place where you can do it with a short run of 14/3 and run a 14/3 extension cord [1] from it for your amps. (14/3 is for 15A. You need 12/3 for 20A)

[1] These are a little pricey but well worth it. You can buy a 14/3 40' cord on eBay for about $25 shipped. It's a bargain.

[martin manning]

C8 looks bulged at the top, so replace it along with R11 and R13. Be aware that the charred circuit board material can be conductive, so you should try to scrape away all of that blackened stuff.

Since R3 is toasted you probably had a power tube short (screen), so replace the output tubes. Why the V1a cathode resistor went up in smoke is a mystery.

[Stevem]

Yea, R3 still has me pulling my beard!
Many of these amps did burn up the PT, but I have had mine since 2002 with no issues, but than again I did not rebias mine so the outputs where idling at 45 ma like some folks have!

[Leo_Gnardo]

Disregard that last post about the recto tube as that was ment for a different string and I do not know how it landed in yours,but yes that could be done to your amp if it had the real estate for it!

Whew, thank goodness. The Piggie is already a very small chassis with a lot of heat generated. Seen a coupls semi-meltdowns but nothing like this. Sounds terrific when working right, though, and very portable.

Phil, I'll yell at ya for using the BX armor - HEY I do the same thing. I'm sure it's not code. If there was a major fault or lightning strike, can't really count on BX armor as ground. The steel armor is tough all right - keeps critters from biting the conductors - but it is steel and may be rusted and who knows how badly in places you can't see. Yes, deafinitely, better to run some new Romex if you don't have to worry about critter bites, or 3-conductor BX if you do. For the moment, we'll limp along with our dicey BX grounds.