Triple Rectifier blowing fuses

[rp3703]

I just picked up a REV C Triple Rectifier that the previous owner was told by his local shop tech needed a new preamp board. I opened it up as soon as I got it home and the inside looked like new, untouched and no mods. Plenty of mouse turds though. Not believing the bad preamp board diagnosis, I plugged it in to try it out. I did pull two of the 6 power tubes to keep the wattage a bit more manageable. I got nothing from channel one, channel two sounded pretty good, switched to channel three and it sounded about the same as channel 2 but I was hearing a few pops and scratches so like a dumbass, I reached underneath(had it sitting upside down) to wiggle the cream tubes and pop. I saw a flash and the fuse blew. I pulled the main preamp board and there was some char on the input side of the board and all the solders were dull like a cold solder looks. I wiped the board clean with rubbing alcohol and put everything back together. I fired it up and started with channel 1. I was able to hear guitar for a few seconds and then it went to a low hum a flash from underneath and pop, the fuse blew again. I pulled the board and no char, so I’m assuming it’s not an issue with the preamp board or at least the section that was originally charred. At this point I decided to employ my lightbulb limiter(60 watt Bulb). I turned on the power but left standby off and the bulb was bright so I shut it off immediately. I then pulled the EL34 power tubes and turned it on again with standby off. The light was dim. I then put in a quad of brand new 5881’s that I had. The light was bright, so I turned it off. Sorry for the long winded thread but it’s been my experience that the more details the better.
So, I guess my question is, what does the bright light mean? If old power tubes and new power tubes make the light bright in standby mode, where should I be looking next?

[ViperDoc]

i was just reminded of this on my current build. Using the bulb limiter without power tubes in will help diagnose a short: dim means no short, bright means short. If using a light bulb rated below your collective power tube rating, when putting power tubes in, the current draw will fully light the bulb and make it bright. When limiter testing with a 4 x 5881 (i.e. 80-85 watts) and the limiter, choose a bulb that is rated at that power or slightly above. Martin suggested two 40W bulbs in series to make 80 watts for this type of rating. I'd been using a 40 watt bulb for the lower power amps I was building prior and simply overlooked that I was using a weak bulb for the 80 watt amp I just built.

[Stevem]

Let’s confirm that the high current drawing output section of the amp is ok first.

You might try loading your good 5881s in and unhooking the center tap of the OT and seeing if the fuse blows.

In making test of the amp and to ease the load on Pt so it does not fry due to blowing fuses you should derate the main fuse by 25%.

You should also test out the 4 screen resistors , these amps when the experience a shorted output tube will take out the resistor associated with it.

Also to make this trouble shooting easier I would do this.

Locate the 2.7k resistor after the choke and unhook it’s down stream end ( the end that feeds the next 2.7k resistor) this will unpower the whole preamp section of the amp other then the heaters to the preamp tubes.

Note that a could not find a triple recto schematic, so with what I just posted I am assuming that the power supply in your amp feeding the preamp section is the same a the 50 watt smaller duel recto model that I looked at the schematic of.

Please also post up some pictures of the top and bottom side of the preamp board where things where flaming up.

[rp3703]

I do not have access to a 100W incandescent light bulb at the moment but I did pull the OT center tap as well as the 2K7 resistor. I was already using a 3A fuse since I was out of 4’s. I turned on power to see if there was any difference in how bright the light was and there was not. It was still bright. Here are some inside pics of the top of the PCB and I did not take pics of the bottom when I had it removed. Since I’d have to unscrew about 50 potentiometers to get it back out, you’ll just have to take my word on the burn marks on the bottom for now. Besides, I cleaned it with alcohol and it looked pretty much like new except for the solder when I was done. I checked for continuity in a few spots and everything was fine so I left it alone. The theory I’m going with is that an insect or rodent got inside the amp and blew up on start up at some point. I could not see any arcing on the board, just the soot. There was also a soot spot on the chassis below the board. The area where the flame up happened was under the channel switching IC on the opposite end of the amp to the PT, if that helps at all.

[rp3703]

Here is the schematic I've been using. So far it's been correct.

[Stevem]

I think you may have some carbon tracked output tube sockets from the results of shorted output tubes or other carbon tracking going on.

Mouse turd’s found in there means they where pissing in there also and high voltage may have followed the remains of that and now a board could have internal carbon tracking going on also.

If this is the case then you really need to use a Veriac set for a low voltage to test for voltage points that should only have a certain voltage, or no voltages at all.

One test you can make with all the tubes out of the amp and with that 2.7k resistor out of the supply path is to see even a 1 amp fast blow fuse gets taken out, because without a bulb limiter it should not.

[rp3703]

So, I bought a 100 watt incandescent bulb and turned the amp on(still with the OT center tap and the 2K7 resistor disconnected) and it was dim.
The mouse piss theory makes sense. The chassis sits at the bottom of the head box so if the mouse pissed while inside the amp, it would leak through onto the board through one of the holes punched in the chassis for the tubes to poke through.
I will pull the power tube board to see if there's any carbon on the underside of it as well. I do not have a variac, Is there another way I can diagnose internal carbon tracking? All PCB traces looked like they were still gold from what I could see.
Does the new 100 watt lightbulb being dim add any clues?

[rp3703]

I tried a 2 amp fast blow and it turned on but I turned it off after a few seconds because I realized I was not plugged into speaker. When I turned it back on, the fuse blew immediately. My speaker cab was plugged into the 4 ohm jack if that is somehow important information. There were no tubes in the amp at all. I did not remove the power tube board because I could see through the power tube sockets that there was no carbon. All 1K screen resistors measure 1K and showed no signs of burn.

[rp3703]

With B+ not running to the preamp, I got suspicious that this may be a problem with the filament power. Just for shits I took resistance measurements from pin 5 of one tube socket to pin 5 of the next and likewise with pin 9. All were around 2 ohms except for V1 from V2. That measurement low and would climb to 101 ohms on pin 5 to 5 and 41 ohms on pin 9 to 9. I'm not sure if this is a clue or not but I will pull that PCB once again and take another look at the traces between the tubes. I will also check the filament resistance between the power tubes as well.

[sluckey]

All were around 2 ohms except for V1 from V2.

Look at the schematic.

[rp3703]

Yup, I pulled the board and realized V1 does not share the same filament as the other preamp tubes and the schematic confirms this.

[Matthews Guitars]

Charred spots are conductive. Sooty spots are conductive. Places where mice peed, are conductive.

It need a good bath and cleaning of ALL char, soot, and carbonization. Clean and scrape it off until there isn't a trace left,
no matter how deep you have to go to get there.

Tube sockets, too.

After a thorough cleaning, test again.

Triple Recs are highly reliable. Aside from occasional switching system issues, which can be rectified by swapping out a handful of J175 FETs, they don't usually experience failures other than those induced by bad power tubes and unwanted intruders on four or six legs.

[rp3703]

Again, I cleaned it with alcohol and there is no sign of carbon residue anymore but the solder still looks gray like a cold solder. Here are some pics I snapped of the underside of the main PCB. The bottom pic around where the 18 pin IC is located is where the charing occurred. All the circuit traces still look to be shiny gold under the green, so I doubt any charing occurred on the traces themselves. That 18 pin IC is the main switching circuit control, how would one check for a failure there or in one of the JFET's?

[thetragichero]

That 18 pin IC is the main switching circuit control, how would one check for a failure there or in one of the JFET's?

i think you may have misinterpreted the post. some (likely cmos?) logic switching ic and fets are small power problems, when you're having big power problems blowing fuses. don't get sidetracked tuning the car stereo system while the engine is on fire

[pompeiisneaks]

As far as the solder joints looking greyish, that's common on more modern electronics, they use non leaded solder which never gives you a bright shiny joint like a well soldered lead mix will.

~Phil