Newbie Question: Need help diagnosing a problem, tubes on, music fades out quickly

[AndrewTosh]

Recently I built an S5 Electronics 8 watt push/pull stereo tube amp kit (http://s5electronics.com/gpage1.html). It was working perfectly until I put it into a small enclosure instead of just testing it on the breadboard.

Now it doesn't seem to be working right. I'm not sure if the problem is related to something coming loose while mounting it (I doubt it, nothing seems like it is loose at all) or a problem related to something I did wrong earler. When I first built it, I had the biggest capacitor in backwards by accident, which caused it to smoke a little. I discharged, desoldered and resoldered it in properly and it worked fine after that.

The symptoms I'm experiencing are: When turned on with a source playing, the music will begin to fade in, then fade out again rapidly. The tubes stay lit and get warm the whole time. Unplugging it and plugging it back in usually doesn't have the music come back, it only comes back if you let it sit turned off for a while.

I don't mind doing the legwork myself, but if any of you guys have some pointers on what those symptoms would indicate, then I'd be most appreciative.

Thanks,

Andrew

 

[fdegrove]

Hi,

It seems to be a PS problem...I suspect you're losing the B+ due to a cap discharging.

If you have a DVM you could easily verify this.

Another possibility would be a coupling charging up and letting DC through...It would be odd to have this happening on both channels simultaneously unless, perhaps you put in caps with way underrated insulation voltages: they just get enough time to recover a bit untill they give up on you again...
This is unlikely as it worked fine on the breadboard....

Hope it helps,

 

[ray_moth]

Maybe something's getting hot and causing the cut-out through a bad connection due to expansion. This may not be problem in bread-board layout because heat can more easily escape. You can try measuring the B+ and other voltages to see what's happening at the point when it cuts out.

 

[Sch3mat1c]

I would guess your capacitor is heating up and shorting... NEVER, EVER!!!!!!!!!!!!!!!!!!!!!!!!!!! use a cap that's SMOKED! It's utterly no good even if it smokes "just a little bit", they just don't work that way. Also avoid anything leaking or oozing. If using an older (exposed metal can, possibly twist-lock) cap, reform slowly or it may go off like a shotgun.

Tim

 

[AndrewTosh]

I would guess your capacitor is heating up and shorting... NEVER, EVER!!!!!!!!!!!!!!!!!!!!!!!!!!! use a cap that's SMOKED! It's utterly no good even if it smokes "just a little bit", they just don't work that way. Also avoid anything leaking or oozing. If using an older (exposed metal can, possibly twist-lock) cap, reform slowly or it may go off like a shotgun.

Tim

Thanks for your help This place is a bit intimidating at first, but I'm glad I could get some knowledgeable advice.

Saftey is pretty much my biggest priority when working on this stuff... so I much appreciate you letting me know that was a bad move. I'll try and pick up a brand new cap sometime this week.

I'm guessing you guys are right and that it is something power supply related, since like you said it would be odd for it to have the same problem on both channels at the exact same time.