Filter caps in series. What have I done wrong?

[Stratz]

Hi all. I had to replace a 55 year old 16uf/500v cap can that had only a single 16uf/500v in it.
Since I only had 450V caps on hand I wired two 33uf/450v caps in series which should give me 16.5uf/900v.
The problem starts when I bring the voltage up, I get a bad 120hz hum. I removed one of the caps and ran only one 33uf/450v cap and it's dead quiet.

Before installing the two 33uf caps in series I tacked in a 16uf/450v cap and it was dead quiet. I only ran my Variac up to ~100v because I didnt want the cap to explode as the voltage in the amp reaches about 465vdc.

Attached is a pic of the two 33/450 caps wired in series with a 220k bleed resistor on each cap. I followed the instructions from Hoffman's site for the bigger AB763 Fenders which I've attached a picture of as well.

I temporarily clipped a ground to the cap with the negative end facing the rectifier which is not shown in the picture and the work looks sloppy because I've disassembled it a couple of times.

The rectifier tube is a 6AX5GT in case the pins are throwing anybody off.
Thanks for any help.
Rob.

[R.G.]

Most likely it's not really wired like you think it is. I looked a bit and I can't see the issue, but the symptom screams that.

I am quite suspicious of the clip to connect ground.

It also occurs to me to check the OTHER 33uF cap that you didn't test the first time.

When you say "bad 120Hz hum", does the amp function otherwise and just have an objectionable hum, or is the hum overwhelming, all you can hear?

[Stratz]

It's VERY loud RG. The other 33us is brand new too but I'll check the capacitance and ESR just to make sure.
Thank you for the reply.

[The Dude]

It could be there and just not visible in the picture, but I don't see a ground connection on the low side cap.

[Stratz]

The second cap was bad! I put another 33uf/450v and it works fine.
I checked the ESR and it was out of spec.
Thanks so much guys!

[Chuck H]

I don't build much so my electrolytic cap inventory gets old. I use to keep those things around for years sometimes hoping I'd have a use for them. Then when it does come up that I needed a particular value I have on hand I look at those six year, ten year, more? year old caps I have and think "nope". It's too much work building an amp for me to install a cap of questionable integrity. I probably threw away about a hundy worth of caps a few years ago because I knew I would never use them. I still keep some lower voltage small ones for prototyping and testing, but I wouldn't install them in a finished build and all the big power supply caps went. I buy new electrolytics for each project now.

[Stratz]

Sounds like a inexpensive ESR meter would have been the thing to have then.
I have a couple of ESR meters Chuck and while some cost over $100 (My Peak Atlas was about $130.00) my favorite is this little $25.00 MK328 ESR/LCR meter from China. I try not to buy from China but it's such a great meter for under $25.00 delivered.
It measures ESR, LCR, transistors, diodes and other things. It also measures coupling caps for leakage which none of the big name ESR meters can do.

The only thing you have to remember is to discharge electrolytic's before you measure them. I blew out my first MK328 with too much voltage. I liked it so much that I bought another.
Well worth it to have around the shop.
Rob.

[Chuck H]

Maybe yes, but maybe no. It's not uncommon for someone here to ESR test a fifty year old, drooling cap and find it reads good. High voltage can do bad things to a cap that reads fine even though much of the electrolyte inside is failing. My cranky ideology on old electrolytic caps may actually keep me from ever getting an ESR meter. For the limited electronics work I do I just don't see that it would help me much. I don't do repairs and new amps get new caps. I did install some six or seven year old caps in a salvage build once. No smoke or fireworks but the amp was ghosting. I thought the hum for such a slip shod build was pretty normal though. When I installed new caps the ghosting went away and the hum was reduced. That's when I learned that electrolytic caps last a lot longer in service than they do sitting in a drawer.

[Mick Bailey]

+1.

Here's a dual section cap from a '64 AC30 during the re-stuffing process. The cap had a good ESR and value reading, but was a real mess inside and the end cap was bulging.



However, my ESR meter is invaluable and I use it all the time. It gets most use on solid-state boards - especially SMD boards with dozens of electrolytics. I wouldn't be without mine and wish I'd have got one 40 years ago. It's been responsible for countless easy fixes on some pretty complex gear.