univox u-1011 woes

[freak scene]

off topic i know but this amp is driving me nuts.

it keeps blowing fuses/tubes.  i have had it in the shop twice and it is still doing the same thing.  i only kept taking it in because the guy was looking at it free since he hadnt fixed it, but he is an hour or so away from me and gas prices being what they are id like to fix it in my basement.

the best i can determine, the voltage seems to be doubling after the rectifier.  the high voltage winding is putting out about 300 volts, and after the rectifier its 720.  which is a bit much for 4 6l6's and i can only assume a symptom of my tubes blowing.  i replaced the diodes thinking that one was out somehow creating a voltage doubling circuit, but alas that wasnt the problem.  the cans are suspect, one is bad and someone did a somewhat ham fisted repair job with 3 20uf electrolytics (radial lead) all soldered to the common terminal on the bad cap.  i plan on replacing these down the road, but one is 30 bucks and the other two are about 10 bucks a pop.  im not a fan of having caps floating on the terminals like that.  they seem to test alright but functioning is not the same as functioning well.

here is the schematichttp://www.univox.org/pics/schematics/u1011-2.gif

one difference is that the 2 100 uf caps in the power filtering are actually 2 2x50 uf cans, each wired in parallel.

[Sir H C]

Yup, 700+ volts there.  Shouldn't kill good 6L6s as the screens are at 350 volts and so the tubes should be safe.  What is the bias current running at?  You might want to look at some of the diode methods to keep the tubes plates from going negative (The Trainwreck pages have that circuit).

[AL]

The first thing I do in any old amp I pick up is a cap job. Whether or not it looks like it needs one. That may or may not be your problem but the caps are old (most of 'em) and they will need changed eventually if they don't already.

You can replace the caps with single caps instead of the cans and save yourself some cash. There are different price levels on the caps obviously. Xicons are inexpensive and sprague Atoms have moved on to the ridiculous price range. There are several in between - F&T, Illinois etc.

AL

[freak scene]

i planned on recapping anywho, been saving for the cans just cause i guess.  but its drawing fairly appropriate milliamperage (less than 30 but i dont remember).  i pulled the tubes a while back to take voltages from the power section to see if anything was obviously awry.  nothing seemed so aside from what i thought was high voltage.  i put the tubes back in last night to bias them, and a brand new (less than 6 hours of use) set f winged C's that i ordered a couple of weeks ago and then poof, smoke, frantically reaching for off switch...  i think the socket is worn out, i havent tested the tube to see if it was damaged, but i fear the worst in that aspect.  so yeah, new sockets will probably help as well.

[AL]

I'm sure this may be obvious to you now but it's usually a good idea to keep a spare set of tubes around just for that occasion. I have a few sets of worn, but working, tubes that I use to power things up in case there is a problem that I'm not aware of. Sorry that happend to you - those are good tubes.

AL

[freak scene]

yeah, the thing is, i have a set.  i blew some sovteks in this amp a while back.  i let it sit for a while, and then finally got around to testing it out.  i popped my old pair in and it seemed to do fine for a few practices and all of the current draw looked fine.  so i popped in the winged c's and 3 practices later a fuse blew. 

also the winged c's came from the tubestore.com.  they are a matched set and they write the current draw of the tubes on the base, they were all 18 milliamps.  i wrote them before putting them in my amp saying that seemed like an odd number for 6l6's and that wouldnt i have to have 1000+ volts to achieve 18milliamps.  they told me they would be fine if i just biased them to around 30 so i did so.