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Boom! Capacitor down!

So, I started my fire up procedures on my Blackeye 50 Deluxe build - and unfortunately I suffered a catastrophic capacitor failure.

First start up was with the standby off, no tubes in the build. I checked voltages for the switching and DC heaters - running steady at 12v DC. checked voltages out of the secondary to the standby switch...I was getting approximately 350v a side. Which was perfect. Shut the amp down. took a breath.

Then fired back up again. checked voltages again. This time I flipped a switch. Started to check voltages on the rectified voltage. Saw approximately 496. Which was definitely higher than expected, but the amp had no tubes - so without the tube load, I fully expected the PT to put off more stout voltages.

Then I shut off the amp again. Watched the voltages drain off the caps.

Fired up the amp again. Started checking voltages again, but I heard a sound and decided to turn off the amp again. a few seconds later....BANG!!!

C32 blew up ...and when I mean blow up, I mean there was paper and oil everywhere...and the can that used to be the capacitor was sitting next to the PI socket....blown completely out of the board. The axial leads still soldering in, but no cap connected to them.

Obviously, a setback and I wont be deterred.....But Im still scratching my head at this. I started tracing out every connection with my multimeter (which I did before this) and hopefully, I find something I missed. I'm hoping to find something wrong with my wiring, because that will put my mind at ease that a new cap will function OK.

One thing I did notice, was the ground point near the input. Not sure this is related....but do I need to be jumpering the CH lug with the ground point?

Any help or suggestions greatly appreciated!

Frank

5 answers0 replies

5 Comments

frankbenenati

Jul 20

Hey folks. A follow up. I've replaced all of the filter caps with brand new ones...and I still feel like I have the same issue.

One thing Im noticing is that I am dealing with very high voltage. Like 630-640v off that first filter cap (C34/C35) and 550v at C32. Now granted, this is without a tube load, but that still seems extremely high. Part of me thought that maybe tubes in the amp would bring down the voltages some....So I decided to put some spare tubes into the amp to see if the voltages get pulled down any....that was a hasty idea, because as soon as I flipped the standby, poof - I blew the mains fuse. OK, time to back off of that idea.

So I'm wondering outloud if I have a short on my B+ rail....could that result in the higher voltages, forcing a stronger draw? I traced continuity to ground with my DMM from practically every component on the B+ rail...and it all seems good. Also, the 630-640v on that first filter cap has me completely scratching my head. The most I should be getting after the SS diodes, should be 495 or so. - since I'm getting 350 a side on my HV secondary. One thing thats even nuttier, is that with the srtandby off, the HV secondaries are 350v. But with the standby on - the secondaries drop down to 300v or slightly lower....How am I getting more than 600v rectified DC on that first C34/C35 capacitor, with 300V per side on the secondaries. It doesn't make any sense.

Would love to hear some recommendations, if someone has any. I'm stumped!!!

Thanks,

Frank

Andrea Antonello

Jul 20

Replying to

Hello Frank, there’s definitely something weird going on!

I would suggest as a first step to check all ground points indicated on the layout. Also, check from the schematic that the components that are connected to ground are indeed beeping to ground with a multimeter. Then check all caps values and polarity and that your dropping resistors are correct (I had a couple of case in which I mistakenly used an R value instead of a K value). Also, check the orientation of the diodes and the caps in the bias sections

Report back your findings :)

Best

Andrea

AA Electronics

Edited

frankbenenati

Jul 21

Replying to

Hey Andrea! I figured it out!

Turns out the problem was staring me right in the face! It was my power transformer My PT has the following taps.

Ive build a number of 2xEL34 amps all based on the marshall 2204 power/rectifier platform. So when I sourced this PT, I thought nothing of grabbing a PT with 350v secondaries - especially one that would give me the 14v I need for the switching....because I always use PTs with 350v secondaries for these amps.

So I just wired it up without realizing your schematic and layout was designed for a PT with 180V secondaries and a voltage doubling circuit in the rectifier (which is similar to what's going on in the 2203 power section - an amp Ive never built). The HV secondary center tap going in between the two 100uf capacitors was doubling the voltage on the B+ rail. C34/C35 held up, because I think the fact that they were tied in series increased their power handling...but the very next filter cap in line was C32....and thats why it exploded....and thats why fuses were popping when I had tubes in the amp.

So I pulled the center tap from in-between C34/C35 and temporarily tied it to ground....and as soon as I did that, voltages on C34/C35 went right down to 485v!!!!

So I did a little rewiring of the rectifier circuit....I went to a 4 diode center tap rectifier like in the 2204, I soldered the HV center tap to ground and I bridged the positive sides of C34 & C35 to use single 100uf 500v filter cap in C34 (my actual Friedman BE50 has a single 100uf in this location).

The amp immediately came to life. After my usual start up testing, I put in the good tubes I had planned for this amp - and Im happy to report that it sounds just like my actual BE50.

I'll post some pics in a new thread in the other section. But I wanted to thank you for your help and guidance throughout this entire process. Even though I had this issue with the power transformer, the circuit board made this build SO MUCH easier.

I am VERY tempted to put together a Deluxe Reverb next.

Andrea Antonello

Aug 17

Replying to

Lovely!!

frankbenenati

Jul 15

Well an update....I've been through the amp/layout multiple times with magnifiers and continuity testers. Traced out pretty much every connection. The amp looks good.

Mustve been a leaky or faulty cap. Going to replace and hope for the best.

Anyone know what I should be doing regarding that ground near the input jack?