The AM Forum

This is a question about an audio amplifier. It has four 800 uf caps in it at 450 VDC. They are running at about 140 degrees F. Is that normal?  They are in parallel and when I listen to it on very good earphones I can hear no hum. One seems to be hotter than the rest to the touch.

if i'm right thats usually a sign that the cap is about ready to blow.

Heating caps indicate leakage current. The hotter they get the more current, the hotter they get the more current, BOOM !!!! Cap guts all over the place ! Yecch !!
Throw a variac on the AC, a ma meter after the rectifiers, unload the supply output and see what you have. Or check each cap with a variable metered DC source. I have a fully metered and adjustable current limited supply that will do 2.5 kv at 300ma.
 Makes a great cap checker. Has resolution down to 100ua and limiting at 5ma.
Bill

I agree with Bill, seems that the caps are leaking.  The hotter one being more leaky than the other three.

Fred

That sez it all! ! ! !

I remember that strange smell from that yellowy liquid coming out of a burst capacitor :P

What value equalizer resistors are you using and how much voltage is across each cap?

Did you test them individually for leakage under voltage before installing?

Are these caps rated for high ripple and low ESR or are they cheap Chinese crap?

Carl

maybe you have a bad rectifier leaking AC on to the caps

The four caps are in parallel so no equalizing resistors.

These are installed on an OB Audio Research amp that uses 8 each 6550 tubes in it in push pull parallel and I believe in AB1. It uses another 6550 as a low level and screen regulator.  




No, they were already installed. What's the best way to test for leakage? They are Mallory Caps, the computer kind.


Never heard of this condition. I just now tested it for ripple voltage and on my meters lowest setting in AC micro volts, I measure zero. I have pulled out the screen fuse and the amp is been on for about 1 hour and the caps are just barely warm. I think it's more the tubes now than bad caps making the heat but I will still recommend to the owner to replace them if they can be found. An amp like this will bring nearly a grand even if broken.

Here a couple pictures of this beast. It weights about 70 pounds

A leaky diode would place AC on the caps.

I have seen instances when new caps would get hot when first installed with full voltage on them.  Should they continue, one or more can be leaky and should be replaced.  However If they are starting to cool then they may form just fine.    Just watch them closely to be sure they cool.

I usually form caps before install, but I have a Sencore cap checker that checks leakage to 600 volts.  If you don't you can put a light bulb in the primary circuit of the amp to lower the voltage on the transformer which will protect it.  Remove the bulb when the brightness goes as low as possible.

that has got to be one of the nastiest smells there are. i remember that smell after some of the caps in my one ft-901 let loose, took a while to clear it out of the room, then weeks later could still smell it if i got close enough to the rig, probably still could smell it if i tried.

If I were sandwiched between eight 6550's, I'd be running hot too. Are we sure the caps are heating from within, or possibly getting warm from close proximity too a bunch of hot tubes?  :)

Jim
WD5JKO

That is not a very smart layout for those caps, what are their date codes? You mean the 6550's are running at less than 450V with 3200uF of filter? I bet the B+ doesnt budge.

If it was my amp Id fabricate a heat shield for each side.

Check the DC current before the caps with load, and load after the caps. 

Use two meters at the same time and see if there is a meaningful difference. 

If there is, they are making heat.  If not, they are being heated externally.

73DG

I would consider installing a cooling fan on the unit. One of those quiet computer types.

That amp reminds me of my Bogen MO-200 modded for stereo. It had 8417's. Now it has 6550's but it is going to get EL34's. The caps are nowhere near the tubes like that but it does not have as many caps either.

That's what it is. I'll be testing the amp with sine waves and music this week. The amp is rated at 100 watts per channel. I can only get up to that level with the dummy load which are those wire wound resistors shown in the last picture. I plan to use an amp probe and test the output at 60 cps and do the math from there. Maybe I'll stick one of my RF amp meters in there to see how they handle audio. I'll probably run it at 1000 cps to see what those meters read, and then maybe some 15 KC stuff. The use of RF amp meters is the way the US Air Force recommends as a method for setting up the 833A modulator in my Westinghouse MW-2 with large resistors and a scope.

I talked with the owner and he said he has what he referred to as a very rare fan unit that I guess was an aftermarket item intended just for this amplifier.

That would be a hard way to do it with this amp with the way that the circuits are on those circuits boards. I agree that would be the best way to test for leakage.

I love sandwiches  ;D ;D ;D

   I prefer a lower carb alternative. Still my question remains unanswered, are the caps getting warm from internal leakage currents, or from the proximity to a large quantity of 6550's where each might be dissipating 30+ watts. Tubes like that are radiation cooled and the caps are right next to them...

A temporary tin foil shield and a point & shoot IR thermometer should help figure this out.

Jim
WD5JKO

Similar to what I suggested 5 days ago

Thank you for your interest! By removing the screen grid fuse, the HV plate voltage stays on with about 430 volts on the parallel caps. I have it on for an hour and the caps barely get warm so I think the majority of the heat is from the tubes. With the screen grid fuse installed, the tubes are running around 265 degrees each. The single 6550 regulator tube runs at 295 degrees. The main power transformer runs at around 140 degrees and each output xformer runs around 100 degrees. 

Get a fan (large low-speed centrifugal blower) from a VAX6000. It is silent on 12VDC and moves a ton of air. On 24V, a little noise but it is quiet white noise. Seeing nearly all of those computers are 'as parts' today for those who still have to use them, the fans ought to be available cheap. Although designed to be mounted vertically, I have one horizontally on the top of each rack and they suck the heat right out.

Yes ,the owner has one. Thanks!

Thanks for all the suggestions guys ;D ;D ;D ;D