Did I really get away with this?

[hmbscott]

I am working on a build of a Moreplay and just got to the point where I needed to perform the Raw B+ Voltage test.

Some background: when I performed the Heater Wiring test (glow test), I discovered on of my tubes was cracked. I used the cracked tube anyway, hoping I was seeing things, but unsurprisingly, if didn't glow, because it's really cracked and thus down-to-air. The good tube glowed in both sockets so the amp passed the Heater Wiring test.

So back to the Raw B+ test, fortunately I had bought some upgrade tubes (6V6GT  J.A.N. Sylvania brown base matched pair) that I could populate the sockets with for the Raw B+, but I wanted to confirm glow with these tubes before flipping the amp for the Raw B+ test. I did confirm glow, but then I noticed smoke coming out of the vents over the PS filter caps. "That's not good" was my thought, and promptly unplugged it. The amp was powered for a minute at most for this glow test.

Pressing onward, I flipped the amp and examined it closely for signs of a toasted component, but found nothing suspicious. I then performed the B+ test and observed less than 50VDC and dropping, after staring at it drop for maybe 10-15 seconds, I unplugged the amp again. It was down to 35VDC by then. Just to be clear this was across terminals 21 and 23. Also, no smoke was observed during this test.

It was then that I noticed I had reversed the polarity of one of the 220uF 350V filter caps, which presumably explained the weird voltage and maybe the smoke.

I unsoldered the reversed cap and examined it closely and didn't observe any signs of damage. Just for kicks, I checked its capacitance with my Fluke 110 DMM and rather surprisingly it read 220uF, what the !?

So I put it back in the amp, correctly this time and reran the B+ test, and measured 321VDC after maybe a minute (no smoke observed). The B+ voltage didn't really settle, but was continuing to slowly drift higher and so I don't know how high it might have eventually risen.

Did I get away with exposing that electrolytic cap to reverse polarity for maybe 1-2 minutes? Is it safe to use in my Moreplay or do I need to toss it and get a new one?

[Paul Joppa]

The cap was damaged - the smoke probably came from venting as the insides boiled.  It didn't  barf its guts out, so there's probably a bit of electrolyte still left inside, and the surfaces are reforming which is why the voltage is drifting up. (There are some scored lines on the top surface which are there to allow it to blow without exploding if mistreated; fortunately you caught it before that happened.)

It may work for a while now, but its life has been severely shortened. It should be replaced as soon as practical, preferably before turning on the amp again.

[hmbscott]

Thanks Paul. Before receiving your reply I turned the amp on again and watched the B+ voltage for 11 minutes. I timed it. Once it charged up, in a few seconds, It fluctuated slowly over that time between 320 and 321 VDC with no obvious trend. Obviously that doesn't change anything, I need a new cap.

[Paul Birkeland]

If the smoke came from the power supply resistors, you may have gotten lucky.  If the smoke came from the capacitor, that's a much bigger problem. 

Capacitor smoke does not smell particularly good at all, and resistor smoke smells a lot more like really hot cement.  It's certainly reasonable to replace that cap, but I wouldn't necessarily let that stop you from having a listen while you wait for the new cap to arrive.

[hmbscott]

Thanks PB,

That was my thought, "... but I wouldn't necessarily let that stop you from having a listen while you wait for the new cap to arrive", which is exactly what I did. By the way, I don't recall noticing the smell of the smoke. There actually wasn't much, just a couple wisps.

Completed the build today and passed both resistance and voltage tests with no issues. For having a lightly toasted cap in the PS, it sounds pretty dang good, and the cap lasted through a 3 hour listening session.

It's my first totally stock build with one small exception that you may want to consider making standard for the Moreplay. In mine, pins A1 and B1 are tied to chassis ground via terminals 8L and 18L, which allows me to run metal 6V6's. I ran across a mention of doing this in one of the forum posts, probably from you. Have to say it sounds mighty sweet with those RCA Metal 6V6's, a matched pair from the good folks at VacuumTube dot net, $15 each, which included matching. (shown in the pic)

[Paul Birkeland]

You can get away with running the metal 6V6s with pins 1 and 8 tied together, though that's not optimal.  I suspect I had a different tube rolling option available that needed G3 tied to the cathode, and that may have motivated that connection.

[Larpy]

To use metal tubes on my Moreplay, I soldered a short piece of wire from pin 1 to the middle lug of the terminal strip used by the heater wires.  That connects pin 1 directly to the chassis ground.

[hmbscott]

Hi Larry,

I did the same but chose a different center pin.

Scott