68 Bandmaster AB763 Popped a Cap

[Playingw/fire]

I've been working on reconditioning a 68 Bandmaster AB763. I replaced all the filter caps and purchased a new set of tubes. I fired it up with the old tube set and it seemed to function just fine for about 20 minutes. I checked the voltages and everything looked fine except for pin 3 on V4 (the vibrato coupling cap?), it's low about 0.1v, should have been 2.1v. So I played it, on the vibrato channel, for a bit and it sounded good but I never really pushed it. So I put the new tube set in and checked the voltage at pin 5 on V1 and V2 and they were 46.9 which was about a 1 volt drop from the old tubes. I just let it alone because I figured it was close enough.

I played at low volumes for a few miutes and everthing seemed fine but when I turned it up and cut the guitar vol it blew one of the 100/350 TAD filter caps. A big poof sound and a cloud of white smoke to follow.

Anyone have any idea what rookie mistake I made? I'm going to go ahead and replace the five 25/25 caps on the board while I have the soldering iron fired up. Could one of my new 6L6WXT (the power tubes were Peavy 6L6GC) tubes be bad or one of the preamp tubes? They are Solvtecs and JNJs.

Any insight would be appreciated.

Sean

[52 Bill]

Welcome to the board.

I checked the voltages and everything looked fine except for pin 3 on V4 (the vibrato coupling cap?), it's low about 0.1v, should have been 2.1v.

Pin 3 is the cathode, so that is the cathode bypass cap. If the cap goes bad, it will reduce the gain of that stage. Changing all of the 25uF at 25v caps is a good idea.

So I put the new tube set in and checked the voltage at pin 5 on V1 and V2 and they were 46.9 which was about a 1 volt drop from the old tubes. I just let it alone because I figured it was close enough.

Pin 5 of the output tubes should carry the bias voltage, so the 46.9 volts were negative? The bias voltage should be set based upon the output tube wattage, not only the voltage level.

I played at low volumes for a few miutes and everthing seemed fine but when I turned it up and cut the guitar vol it blew one of the 100/350 TAD filter caps. A big poof sound and a cloud of white smoke to follow.

Do you mean the cap exploded? Was the cap installed with the correct polarity? Those two caps are wired in series pos to neg.

[Playingw/fire]

Hi Bill,

Thanks for the welcome. I posted this message here and on AX84 and I've gotten great responses everywhere.

I took a real hard look at the schematics and my little drawings today and I find that my drawings and memory have to be wrong and I solderd that cap in backwards. The way I have it looks nice and balanced but the inboard cap had to have been laying the oppisite direction. You are correct it's two sets of caps and resistors in parallel in series and I have them wired some weird half parallel half series screw up. I'm just glade it didn't take anything else with it. I'll make it a point to check those resistors before I go live again.

I just ordered all the caps form Mojo including the 25/25s. Where do you usually pick up parts. Mojo seems to have decent stuff for reasonable prices. Tube Depot sent me three matched sets of 6L6 tubes when I only ordered one. Both had great turn around times for shipping. Parts is Parts has served me well also.

It's nice to know there are people out there that want to help others!
Thank you,
Sean

[52 Bill]

I just ordered all the caps form Mojo including the 25/25s. Where do you usually pick up parts. Mojo seems to have decent stuff for reasonable prices. Tube Depot sent me three matched sets of 6L6 tubes when I only ordered one. Both had great turn around times for shipping. Parts is Parts has served me well also.

Sean:
Blowing up a filter cap is something that you hopefully only experience once!

I work in a guitar shop where we order parts from Mojo, Part Is Parts, New Sensor, Mouser, Allied, DigiKey, etc., as well as from the manufacturers like Fender, Peavey and Marshall. We tend to order in quantities and shop for the best quality parts at the best pricing.

Bill

[Playingw/fire]

Hello Bill,

I was just looking over your comments again. Yes, the bias voltage was negative. I need to learn a little more about my new tubes as I believe they have a slightly higher output and that voltage needs to change a bit I think. I'll check the difference between the GC and the WXT, but won't it just be biased a bit cold if I just leave it alone? Just from a point of how they looked I think the were fine. They weren't making much light. Just a deep orange and I didn't see any heat going out into the plates or anything. They did get a blue glow when I flipped the stand by switch on but I understand that is normal for new tubes and it should go away for the most part after they have been in use for awhile.

You were also correct about the two resistors. I belive they are both fried. I get inconsistant readings from both of them right now. I haven't gotten them off the board yet but I think I'll have time tomorrow and check them out of the circut. It sucks pretty bad because I already ordered the caps and now I need these resistors and get another shipping fee. I can't find anyone in my area that has parts even for horrible prices which might be cheaper than paying shipping again. The good news is will give me an excuse to go ahead and get new grill cloth. The speaker cabinet has some holes and I'd like to spruce it up a bit. I might even wrap it up in some new Tolex eventually. I belive this unit saw some heavy use at some point in it's life. It's pretty banged around.

It's my first tube amp and it sounds differnt then all my solidstates and way different from my VS8100 head and 4 x 10 cab. Sooooo much more bottom end. I can't wait to get it back up and play with it somemore.

Thanks again for all the help! I'll let you know how the next round of repairs goes and hopfully I won't be picking your brain any more.

Sean

The cap didn't really explode. It just kind of vented. It appears that it was designed to fail that way. I had to clean up the mess inside the "doghouse". There was a nice puddle of oil in there.