Fender ProReverb Blowing Mains Fuse

[nevetslab]

One of my steady clients sent over an older generation Fender ProReverb Combo amp that was blowing fuses. Until I got it up on the bench, I was fearful it was the newer generation that are a real PITA to work on inside. This is the older black tag-board hand-wired variety. 5AR4 Rectifier, pair of 6L6GC's. Initially, once I got the 2A fuse replaced with a 2A Slo Blo, it powered up ok. RCA-based foot pedal, and similar exposed-braid RCA tank cables. I wasn't getting any Reverb, and verified tank DCR was ok, looking down thru the tank cables. I had to spray all of the pots, as they were noisy as the dickens but after exercising them, that solved that. I just had the Reverb system to sort out, and the Tremolo was ticking loudly.

Before I could get a signal injected into the Reverb Return path, the mains fuse blew. It blew when I switched out of Standby. So, I removed the power tubes and tried again. Blew again, switching out of Standby.

I turned the chassis over and opened up the doghouse. They all look recent, as did the cathode axial leaded bypass caps on the tag board. Nice large Sprague Atom Filter caps....80/450 x 2 for the stacked pair that the 5AR4 charges up ahead of the standby switch. I had one more new 5AR4, so thinking that was it, as I saw a bright flash in the amp's 5AR4 installed (J/J 5AR4, J/J 6L6GC's). Powered up again, now using 2.5A Slo Blo, as my stash of 2A SB's was running low.

It blew again. I replaced the first filter stage following the Choke with an F & T 22uF/500V Axial Electrolytic. Tried again, and fuse blew again. I stopped at that point to ponder my options.

What I needed was a 2-3A Magnetic Circuit Breaker to end this destruction of fuses. I found all I had was a 5A Magnetic Breaker...Airpax. Then 10A, 15A and 20A rockers, all Airpax. I found a 2A Magnetic Breaker this morning on ebay and bought that, but won't see it until middle of next week.

I found an old failed 5AR4...Ruby...and un-soldered the tube pins from the octal base, so I could turn that into a Solid State Octal Rectifier using 1N5408 3A rectifiers.

I then looked for a Nema 5-15R AC cable Receptacle to wire up the on-hand 5A Magnetic breaker. I'm out of those now.

Looked around the shop, and found I didn't have any die-cast boxes large enough to mount the 5A breaker, and have room for the 2A breaker that's in-bound. The nice box I had bought at Home Depot for that Light dimmer box I made is too shallow to install the magnetic breaker. I guess I'll stop there on the way home, just to see if they have anything suitable.

Meanwhile, I found Hammond Manufacturing had a 4.7" x 4.7" x 3.7" die-cast box...1590V box. Pacific Radio down the street has stopped carrying all but a few small sizes of the Hammond boxes, so that's the last place that USED to have suitable hardware for such occasions. RATS. Everything now is order-on-line and pay shipping and wait. Grrrrrrr! I found Mouser had the box, and ordered that, but won't see it until after the 2A Magnetic Breaker arrives next week.

So, I'm still, at present, without a magnetic breaker wired up to use, as I change from using the last 5AR4 rectifier to the 3A 1N5408-based plug-in rectifier module....to try and find WHAT is causing this high current failure. I ruled out the stacked input caps (80/450 x 2 series-connected filters, fed from the 5AR4). I tested the pair of 6L6GC power tubes in my Yamaha T100 Power Tube Test Fixture, and find those are working just fine. I had removed them from the ProReverb, just in case I had one that was going bad.

That leaves the Choke type 125-C1A 4H/90mA that follows the Standby Switch, and feeds the 1st 20uF/500V Buss Cap, which I've now replaced. After that filter, there's decoupling resistors to the rest of the filters, so I'd rule those out. Also following the Standby Switch is the Primary of the Output Xfmr...it now without power tubes plugged in.

The new 5AR4 Rectifier tube I had plugged in, replacing the one that had been installed, which I saw Flash when the Mains Fuse blew, this new one ALSO Flashed inside when the Mains Fuse blew last.....did THAT rectifier tube now get added to the DEAD pile?.

I'm kind of at a loss just how to proceed in finding WHAT is shorting out and blowing the mains fuse. My thoughts were to have the 5A magnetic breaker as the resetable protection device, with a 20A fuse installed in the fuse holder. The 5A breaker has a trip current rating of 6.25A.

pro_reverb_aa165_schem.pdf

[Jon Snell]

I woulld have firstly replaced the fuse for the correct value and type, removed the rectifier valve and if still blowing fuses suspected the secondary wiring of the power transformer then working towards the mains, the transformer to confirm it is at fault.
If no blown fuse, a quick resistance check between ground and the standby switch will confirm any short circuits. Then remove the output valves and fit the rectifier. Test again.
From that point on, it is straightforward to fault find.
Either a faulty valve, a leaking capacitor, choke or the output transformers insulation, has broken down internally.

Try using a 100W incandescent household light bulb in place of your mountain of blown fuses.
Unfortunately due to the EU, incandescent lamps are no longer available in Great Britain, that is why I use a ressettable 3 or 5A fuse link.

[Mick Bailey]

I like to use a bulb limiter because it allows the fault to hold for long enough to be able to see what's happening when monitoring voltage or current using clip leads in various parts of the circuit. I use the limiter just as much as the Variac - both are equally useful. Mainly I use a 60w bulb.

Rough Service incandescent bulbs are still available in the UK. I stocked up on incandescents for amp work before they were finished, though the one I'm using right now has been in service for 20 years.

[Tom Phillips]

In cases like this I have sometimes resorted to temporary installation of fuses/circuit beakers or even low wattage resistors along the power distribution path. Then you can get a clue of where the high current is flowing when it happens again. Another handy method is to monitor the voltage across various resistors along the path, including the resistance of the choke, with a DVM set to Min/Max mode. If you have one or more DVMs with that feature of course.

[nevetslab]

In cases like this I have sometimes resorted to temporary installation of fuses/circuit beakers or even low wattage resistors along the power distribution path. Then you can get a clue of where the high current is flowing when it happens again. Another handy method is to monitor the voltage across various resistors along the path, including the resistance of the choke, with a DVM set to Min/Max mode. If you have one or more DVMs with that feature of course.

While I do have one DMM with that feature (Philips PM2535), it has a 300VDC Max limit! RATS!

I did just finish wiring up an AC Mains cable with a 5A Magnetic Circuit Breaker a couple feet from the attached Nema 5-15R Load connector.

I do need to build myself a Light Bulb Limiter. Hard to think all this time I've not done that already, but this would be very handy on this amp. I was going to stop off at Home Depot on the way in this morning, but to add to my life complications, my Left Ankle is now acting up again. Sigh..... I have kept one of my crutches in the back seat of the car, just in case either ankle problem crops up again. I did let that be an excuse to get in to the air conditioned building to spend the day dodging the 102 deg heat.

So, for now, I'll move forward with the 5A breaker in-line, and a 20A Mains Fuse in the fuse holder. I'll begin without the 3A Rectifier module and cycle the Mains Switch. I wasn't getting any indication before that would lead me to suspect the Secondary side of the Power Xfmr, Granted, the 5AR4 takes a good 10 seconds before you start seeing the filters on it's output start charging up. I ahd cycled the Mains switch once it had heated up a few times, and didn't have any issues there. Though there was NO Load Current flowing at that point, other than the charging current of the 1st filter stage. With that 3A Rectifier module installed in place of the 5AR4, I know that will increase the supply voltage.

I think once I verify all is well still on the unloaded Rectifier side in Standby, I'll power down, and then close the Standby Switch and bring up the AC mains with the variac and see if there's any sudden fault current. I still have the 'now-tested' power tubes removed. The Driver Tube 12AT7 does have plate resistors. It, the Reverb Driver Tube V3 (with it's small transformer in the plate circuit) is also on the output of the choke, as is the Tremolo tube circuit V5. I've replaced the 1st filter stage following the choke, but I was still getting fault current flowing some place.

I'll see how far this goes before I stop and go fetch the materials for building the Light Bulb Limiter.

[nevetslab]

I have the amp powered thru the 5A Magnetic Circuit Breaker wired into my Test Mains Cable I just built, and placed a 20A SloBlo Fuse into the Fuse Holder of the amp.

I've got the amp powered back up using this 3A Rectifier Module I cobbled together. Without any power tubes installed, I've been able to bring up the mains, after first heating up the preamp tubes, then shut it down, switch out of Standby and brought the mains back up, now having 483VDC (instead of 463VDC) on the Primary of the O/T and other side of the choke to run the rest of the preamp tubes. After being able to switch the S/B switch on and off repeatedly without any hint of fault current flowing, I powered down, installed my bias probes and the tested 6L6GC Power Tubes and powered back up. No issues found switching out of Standby, other than I had to turn the bias down to restore the 30mA Plate Current I had dialed in prior to the 5AR4 rectifier tubes started shorting out.

I haven't tried to plug the new 5AR4 rectifier tube back in, since I saw it once Flash with the 2.5A SloBlo Mains Fuse installed up and blow. I'm still at a loss on that, as I have yet to find any fault current flowing in this present set-up. It;s been running correctly now for an hour, switching in and out of S/B, as well as from Off to On-S/B and Out of S/B, letting it run with signal, then powering back down.

I did at least find the Reverb Return path is working, being able to inject signal thru that RCA Reverb Return jack. So, that leaves V3 Driver tube circuit suspect. And, I still have the Ticking Tremolo noise to deal with.

I either go out and buy another 5AR4 Rectifier tube, or produce a cover plate to glue onto the 3A Rectifier Module I built out of a bad 5AR4 tube. Can't leave that lethal voltage exposed for a client.