Bottlehead Forum
General Category => General Discussion => Topic started by: Alonzo on October 29, 2014, 06:21:28 PM
-
Just a quick question, had the amp playing in the den while the baseball game was on. 2A3 SET, basically 2 paramours in the same chassis. Came in to shut it down while we go party and saw this tube glow. Lots of blue and RED. Never saw red before. Took a pic and shut it down. Long night of celebrating ahead, what should I look for when I get the chance to check on the amp tomorrow? It was playing music fine. Chassis and power transformer very hot.
Thanks for the help,
Alonzo
-
Hard to tell from the picture, but if the red is from the plate then you are drawing too much current, most likely a grid bias failure but possibly a tube failure.
The blue appears to come from the glass, not the vacuum - again this is really hard to tell from the picture. The glass will glow when it is hit by lots of electrons, which is consistent with too much current but would not be a problem of itself.
How about them Giants?!
-
Yeah, how bout them Giants!!! We stayed away from the dumba**** in the streets and had a good celebration of the long season. Won't happen again for awhile unless they strike magic with some signings.
Looked at the amp this morning before work. Swapped tubes to test for a failing tube. The blue glow is from the glass, the red is the plate, definitely the plate glowing. Tube filaments come on, then the blue starts and flickers, then the plate starts to glow.
Shut the amp down and flipped it over for a quick visual. I f'd something up royally. There's heat damage visible on the power supply chokes (can't even read the C-7x on the paper anymore, it's all brown) and on the plate chokes (can't see Mike's writing, looks like varnish has run and looking down at the bobbin (?), it's not light yellow anymore, I can see brown thru it). I'm surprised I didn't smell anything before.
No other visible damage, fuse never blew, resistors, outputs and grid chokes all look fine.
Looks like it was overcurrent on both channels and one tube finally gave me indication. I'll email Mike to see how much of a beating the BH-6's can take and put in a order for new if needed.
Tubes are really forgiving, this amp was playing music for at least an hour before I noticed the problem.
Here's my plan, please let me know if I'm on the right track:
1) remove damaged parts, replace (resistor in place of the power supply choke, and a Hammond choke for the BH-6 just for testing)
2) inspect, reheat grid choke connections, grid resistor
3) check 2A3 socket pins (noticed the tubes weren't in flush to the socket when I removed them)
4) check power supply wiring, used a 302ax power supply transformer for the first time, to make sure I wired it in correctly
5) do a round of voltage checks
6) Come back for help!!
-
Voltage checks will always reveal where this problem is coming from.
It can be simple as one of the legs of your hum pot on that channel touching the chassis plate.
-PB
-
Bad coupling cap can cause current runaway too.
-
and a third possibility is the grid choke/grid resistor/grid stopper - if the grid does not have a DC connection to ground through something.
-
How not to treat ~$1k in iron... :'(
-
You may want to get a larger chassis....
-
I would suggest rebuilding the layout into something a little more organized. I think that might help sort out any issues.
-
I know it's a mess, didn't start out that way but it is what it is now. I took out one pair of bypass oil caps, that made it a little easier to get around to things.
Measured the damaged parts and others noted as possible failure points.
Plate choke left 125 ohms, right 510 ohms
grid cap left 98nf, right 90nf
both grid chokes read OL
Found a loose connection at the left power supply choke that got touched up. Gathered up the replacement parts (parafeed cap, resistors, etc)
Packed it away till the weekend, it's not the main system amp so there's no stress in getting it going.
Thanks for the help. I'll post some voltages on Sunday.
Happy Halloween!!!
-
If the grid chokes are open like they measure that could be the issue. IIRC the DCR should be maybe around 4K ohms. And you will need to determine if that choke that measures 510 ohms is cooked or if there is just a bad solder connection that is running the resistance up.
-
I can't figure a scenario that would blow out the grid chokes, but I agree that if they are truly open-circuited that would lead to excessive current.
510 ohms is about right (spec is 550) for a BH-6. It's the other one that appears to have an internal short.
Mike usually uses a special wax impregnant, which is probably what's leaking out.
-
Leaking coupling caps from the preceding stage could blow the grid chokes. I'll bet there is (was?) some hair fine wire inside those chokes.
-
"... hair fine wire..." IIRC, Mike once told me his winder is nearly the only person on the planet who can wind such fine wire any more - something like 44 gauge!
-
Sorry that was my mistake, my Fluke has auto range and OL signals over limit I think. On manual the grid chokes are 3.9K. Put in an order from Mike, but I'm deep in his cue. I have a pair of BCP-15's 50 H to try at plate choke. Started the strip down this morning. Hope to get to voltage checks by the late game today. I'm concerned that I didn't find an obvious smoking gun, no shorted parts or anything. Don't want to repeat the first failure.
Since this is for tweeters, I'm leaving out the cathode bypass cap and decreasing the coupling cap to some 0.68 clarity caps I found.
-
Rebuild complete, but no voltage checks yet. Getting a short on both A1's and A4's and the left pot reads X10 greater than the right. Don't know what's going on there so I'm going to replace the pots as soon as they arrive via Amazon. Here's the simplified build.
-
Smoked the 2 270 ohm resistors I put in, didn't check, they were only 5W and got pretty hot. Replaced them with C7 chokes from a Crack experiment I had and now I'm back up and running. Still don't know why I had such high current previously. The power supply transformer and the chokes are warm, but nothing like before, gonna get a IR thermometer and check the temps.
Ran some Valve Art 2A3 in the amp for a hour before switching to Sophia mesh plates. Rodrigo Y Gabriela 11:11 playing to start the break in.
It'll stay this way until I learn what output cap to sub inplace of the 0.66 clarity's in there now and I get some iron delivered from Magnequest.
Thanks for the help.
Alonzo
-
Burn in time, next is Russian K4 input caps and new output caps.