Bassman 100 eventually blows fuse

[Kenzo1979]

Hey everyone, I have a silverface fender bassman 100 that works perfectly for hours, then out of no where gives an audible ‘feedback-ish low/mid octave’ hum that increases quickly in volume and then everything goes dead (fuse blown).  Throw a new fuse in and good for many more hours until it happens again.

About the amp:  I got it from a kid about six years ago who was probably unloading it because of this problem.  First time it happened I noticed there was a fast acting fuse in the holder, so I replaced with the correct slo-blo hoping that was the only issue.  Nope.  So of course I got a brand new set of tubes and the problem still persisted.  Fast forward to a few years later I took everything apart and did a blackface mod to it. All new electrolytics, flame proof grid resistors, reflowed solder on EVERY spot, checked all of the wiring to the schematic, everything checked out right.  Both sides bias are set around 25 miliamp.  (Which I believe is on the cold side, so I don’t think anything is running too hot.) At the same time, I blackfaced my 1973 silver face twin reverb which had no issues before or after the mod.  The bassman however, still has the original problem and I don’t honestly know where to begin looking for what could be causing it. I know that in the past six years I have tried at least three completely brand new sets of power tubes to no avail.  Visual inspection of components does not show anything burnt, bubbled, loose, cracked or otherwise.  Nothing obvious is standing out.  One thing is for sure and that is that the problem was there before the mods and after the mods, before new tubes and after new tubes, so it must lie somewhere outside of any of the sections I touched.  If I had to make a guess I would imagine something is opening up when it reaches a certain temperature from being on for a certain period of time. But I’d like to hear what other people have to say about it.

Hoping anyone has some insight to this specific symptom and can point me in the right direction. 

Thanks!

[Raybob]

Filter caps.

[Latole]

Remove all power tubes, test amp ON for many hours.
Report

[Kenzo1979]

@ Raybob -  brand new sprague atoms.

Latole, will do, stay tuned.

[Latole]

No I check whats wrong before replace anything.

I do not replace parts that do not need it or by chance.

[Kenzo1979]

Remove all power tubes, test amp ON for many hours.
Report

Running for 24 hours straight, still has power. 

[J Fletcher]

 Wonder if there is an issue with the bias supply failing on one or both output tubes . Can you monitor the bias voltage at the grid of the 6L6's ?

[pdf64]

Repeatedly undergoing fault current incidents will cause damage eventually. So it’s best to use the amp via a light bulb limiter, in order to limit current to acceptable levels, until you get this sorted.

[Kenzo1979]

Repeatedly undergoing fault current incidents will cause damage eventually. So it’s best to use the amp via a light bulb limiter, in order to limit current to acceptable levels, until you get this sorted.

I can do that, What wattage bulb do you recommend?

[Kenzo1979]

Wonder if there is an issue with the bias supply failing on one or both output tubes . Can you monitor the bias voltage at the grid of the 6L6's ?

I’m sure I could.  Refresh my memory, will there be four separate readings simultaneously (one for each Pin 5) or two readings  (1 reading for each pair of 6L6’s?). Also, do I have to measure voltage or can I just monitor bias current from centertap to Pin 3, Which is how I adjust bias.  I usually set up 2 multimeters for this and my readings are for each pair.

[mresistor]

What did you do in your blackface mod?  Is it now a different amp configuration?   

[pdf64]

Repeatedly undergoing fault current incidents will cause damage eventually. So it’s best to use the amp via a light bulb limiter, in order to limit current to acceptable levels, until you get this sorted.

I can do that, What wattage bulb do you recommend?

Try 100W in the first instance.
In the non fault mode though, with it being such a beefy amp that may drop voltage too much for the fault to manifest.
So if it runs fine for much longer than normal, maybe move up to 150W, or whatever you can get hold of / rig up.

[2deaf]

What did you do in your blackface mod?  Is it now a different amp configuration?

I was wondering about that, also.

[Kenzo1979]

What did you do in your blackface mod?  Is it now a different amp configuration?

Removed Hum Balance, removed suppressor caps, added adjustable bias, also some resistor changes in the dog house as well as new filter caps.  Basically everything in the pic although I kept my MV control.  I did the Gerald Weber list of things to do.  I will pull the chases and take some pics.

***Again, this issue DID exist before the modding.  And I did an identical BF mod to my SF twin reverb which presented NO issues before or after modding.

[2deaf]

I had some big silverface that would red plate a pair of 6L6's after operating for awhile.  I took the screw out at the end of the fiber boards and pried the boards apart a little and it stopped red plating.  There was no readily apparent path to ground upon disassembly and inspection, so I entertained the possibility that the bottom fiber board itself was conducting enough to lower the bias voltage on one side.   

[Kenzo1979]

Chasis and cap housing...

[sluckey]

That's a terrible soldering job on those caps!

[Kenzo1979]

That's a terrible soldering job on those caps!

I think they look worse in the picture than they do in real life. All the connections have nice shiny solder joints. You may be seeing the excess flux that kind of bubbled up around the joints and almost looks like corrosion in a low resolution picture. Happy to email full HD close-ups.

[Kenzo1979]

That's a terrible soldering job on those caps!

Just brushed off the joints with a nylon brush to get the excess flux away. Busted out a spot lamp and tried to take the best picture I could.  That being said, could that be a possible cause of this problem?... Bad connection somewhere in the tank there? if I have to take everything in there apart, Remove all the old solder, polish up the eyelets and reconnect everything to solve this problem, then great!  The only thing I keep coming back to is this problem existed before I touched that.

[66Strat]

I think that you may have the first cap shorted to ground.

[Kenzo1979]

I think that you may have the first cap shorted to ground.

I don’t believe that positive and negative connection goes to ground. Just to be sure, I googled “silverface and blackface filter cap board layout” and every picture or a drawing that I see seems to be wired that way.

[66Strat]

I think that you may have the first cap shorted to ground.

I don’t believe that positive and negative connection goes to ground. Just to be sure, I googled “silverface and blackface filter cap board layout” and every picture or a drawing that I see seems to be wired that way.

It's been awhile since I was in a big fender, but I seem to remember the filter caps and balancing resistors where oriented and connected as notated in the attached photo. This agrees with the schematic. Or my failing memory is worser than I thought. It may be worthwhile to double check your connections with a DVM.

[DummyLoad]

That's a terrible soldering job on those caps!

Just brushed off the joints with a nylon brush to get the excess flux away. Busted out a spot lamp and tried to take the best picture I could.  That being said, could that be a possible cause of this problem?... Bad connection somewhere in the tank there? if I have to take everything in there apart, Remove all the old solder, polish up the eyelets and reconnect everything to solve this problem, then great!  The only thing I keep coming back to is this problem existed before I touched that.

post a photo of the other side of those two big caps and a shot from looking down on the whole array.


--pete

[2deaf]

seems to be wired that way.

It's been awhile since I was in a big fender, but I seem to remember the filter caps and balancing resistors where oriented and connected as notated in the attached photo. This agrees with the schematic. Or my failing memory is worser than I thought. It may be worthwhile to double check your connections with a DVM.

He's got it right.

[Kenzo1979]

That's a terrible soldering job on those caps!

Just brushed off the joints with a nylon brush to get the excess flux away. Busted out a spot lamp and tried to take the best picture I could.  That being said, could that be a possible cause of this problem?... Bad connection somewhere in the tank there? if I have to take everything in there apart, Remove all the old solder, polish up the eyelets and reconnect everything to solve this problem, then great!  The only thing I keep coming back to is this problem existed before I touched that.

post a photo of the other side of those two big caps and a shot from looking down on the whole array.


--pete

[Kenzo1979]

As I read these posts and search google images of the capacitor tank, I now see that I have: 2 -22uf's, 1-40uf, and 2-80uf's.  Can't find any reason why I have a 40uf instead of 3-22uf's.  Most of the pics I'm finding of Fender Amps BF and SF, as well as 2deaf's photo, seem to have 3-22's.  Could this higher value capacitor (the one in dead center, only one of it's size) be causing this problem? 

[66Strat]

seems to be wired that way.

It's been awhile since I was in a big fender, but I seem to remember the filter caps and balancing resistors where oriented and connected as notated in the attached photo. This agrees with the schematic. Or my failing memory is worser than I thought. It may be worthwhile to double check your connections with a DVM.

He's got it right.

My apologies. The layout that you posted is very helpful.

[acheld]

When I think of intermittent short circuits, heat may be playing a role, and possibly vibration. 

Consider 2deaf's thought about conducting through fiberboard -- we have seen that occur. If you can get a peak under the board, maybe you could see evidence of arcing.

[Kenzo1979]

Thanks everyone for your input.  I have tested  my filter caps and also my rectifier diodes, all checked out fine. I then went through and again verified all of the connections.  I found no errors.  I do apologize for the Layout pic I posted earlier, the original was taken from another site which I edited to show the mods - there are 2 errors on that drawing.  1: The jumper wire linking #6 pins on the middle power tubes is not depicted.  Also, the Bass and Treble pots are mislabeled (flip-flopped) on the Bass Instrument Channel.  I am glad that somebody out there drew that layout though, because I searched for the life of me and could not find a genuine fender layout for the basement 100. I found the schematic, just not the layout.

Anyway, been researching some other forums for similar topics and came across a lot of people talking about this conductive fiberboard.  If anyone has some tips on the fiberboard situation I would love to hear it. I’m about to take everything apart clean everything up and put everything back together. After that, I plan on firing it up and testing all of my voltages while I monitor.

About the boards... I noticed that my main board is pretty well waxed, but my rectifier board does not seem like it has wax at all. Should it?  Does the eyelet board become conductive or is it usually The spacer board underneath conducting to ground?  Is there any benefit in trying to add some other form of insulating material to keep the fiber boards from conducting against something?  Also, Which board is usually a culprit?... filter cap, main, or rectifier, or can any of them suffer from moisture etc.?

[sluckey]

The main board is usually the problem. When the board becomes conductive it will pass voltage between adjacent eyelets. Really messes things up. No easy "one solution fits all". I would just replace the boards with good G10 fiberglass boards.

[SoundmasterG]

Another thing I would suggest is to look at the power tube sockets and make sure there is no evidence of arcing. I had a Bassman 100 a few years back that would bias up fine and idle for hours with no issues, but get it in a gig situation and after about 45 minutes of use it would start arcing on a tube socket and the tube would red plate and the amp would blow the fuse. Took me going to the guy's gig and watching the amp from the back to see what was going on after the 2nd set of power tubes went south. Replaced the socket and the tube and no more problem.


Greg

[Kenzo1979]

Another thing I would suggest is to look at the power tube sockets and make sure there is no evidence of arcing. I had a Bassman 100 a few years back that would bias up fine and idle for hours with no issues, but get it in a gig situation and after about 45 minutes of use it would start arcing...

Thank you, I have read about that symptom.  Perhaps it was even your story.  I’ve been bookmarking a dozen different entries it’s hard to keep track.  I will keep that in mind.  Definitely going to be a process monitoring this thing.  Just took some readings with the power tubes out AND in - will post next.

[Kenzo1979]

Here are my readings with NO power tubes.  Bias pot is set at minimum, about -46vdc (but it has range up to -72.5vdc).  All measurements were taken with controls at zero and no input signal.

[Kenzo1979]

Here are my readings with the power tubes installed.  Bias pot is untouched, set at minimum.  All measurements were taken with controls at zero and no input signal.  Next I'll drive some signal, not sure if that is a factor or not.. 

[PRR]

Why does it say "Remove 2000pFd cap"?

Some of these builds were marginally stable. Without that 2000p it may randomly sing radio waves and burn a gut.

[Kenzo1979]

PRR, thank you, I am taking any and all advice into consideration.  I am using my SF to BF layout guide as a template for my current amp layout.  The 2000pf capacitors were originally there between pins 1 and 8 on V5 and V6;  from what I know they were there to prevent parasitic oscillation due to CBS lead dress issues. BF mod says to remove them because they suck out some of the "sparkle."  This is said to pose no issues, but perhaps that's not ALWAYS true. The problem I am experiencing happened pre-mod (before 2000pf cap removal), but is still present. I did keep all of the components I removed;  Maybe one of the 2000pf caps was bad before the mod and contributed to this problem and removing them only continued it. Going to test the old caps.  I will look at this from every angle. Here is the layout prior to modding and a copy of the original schematic.

[Kenzo1979]

So after some monitoring, here's what I've found happening...

Amp ON, 8ohm speaker connected, volume up some so I can listen for that ominous hum right before it pops the fuse.  Btw, there is no given time this will happen, but never on startup and you can't set a clock to it.  1st time I hear it, I didn't have the meter connected, just visuals - The pair on the right are glowing red.  Shut it down, connect my meter to pin 3 of the pair that was red plating (blue wire) right side, no more than 5 seconds to attach the meter, flip it back on, tubes are fine, back to normal again.  Waited about 45 minutes, didn't happen again.  At this point I swap the tube pair from the right (blue wire) with the left (brown wire) side.  Since this doesn't happen very immediately, I had to come back to this amp a few times for 1-3 hour periods before it happened again.  So now, with the meter on the blue wire side, and the tubes that red plated are now on the brown wire side, I hear it happening, now the brown wire side is red plating, ahhhhhaa, it's following the tubes!  Right side readings fell from a steady 445vdc to about 400vdc.  So now I put the meter on the brown side and leave the tubes the way they are.  But the next time it happens it red plates on the blue wire side with the tubes that did NOT red plate previously.  So it seems to be losing bias randomly from side to side    And killing power with the standby switch even for a second and back on seems to almost "reset" the issue. Any more direction into what could cause that would be great.  Meanwhile, I'm keeping that conductive fiber board theory in mind.  Just want to make sure I'm not missing anything here. 

[PRR]

The way it happens suddenly and randomly makes me think about hardly-touching bad joints. Bright light and magnifier.

[tubeswell]

Have the e-caps in the bias supply been replaced? (It is just as important to replace these, as it is to replace the main PS filter caps)

[Kenzo1979]

Have the e-caps in the bias supply been replaced? (It is just as important to replace these, as it is to replace the main PS filter caps)

Yes, the e-cap in the bias supply was replaced as part of the BF mod.  But I am investigating the Bias supply further. 
After more observation, I monitored the bias voltage on the bias pot wiper.  Stayed  around -45V.  Got it to red plate again and as it did, the bias voltage dropped to about -40V before I shut it off. So it does seem to be losing bias.  Not many components in the bias supply.  Possibly the diode leaking some AC through as it breaks down?  Other than that there is the 470R tied to the diode and the bias pot itself.  More testing continues.

[Kenzo1979]

The way it happens suddenly and randomly makes me think about hardly-touching bad joints. Bright light and magnifier.

Thanks.  Been through the board checking every joint, chopstick tested with no discrepancies, flowed some new solder once again.  Still persisting.

[Kenzo1979]

Well, after much consideration, testing and monitoring, it would seem (knock on wood) that at this point the culprit was the single diode in the bias supply on the small board before it hits the bias pot.  Originally I had changed the 470 ohm resistor, that the diode is tied to, when I noticed that it was only a 1/2W resistor and my notes called for a 1W.  Since that resistor showed no signs of damage and was giving me good readings, I didn’t think that would actually change anything, which it did not.  My E-cap was already new and was giving me good readings, so I decided to replace the old tin can diode with a new 1N4007.  Had it running for about four hours yesterday evening and it never blew the fuse.  I think it’s safe to assume that this was the problem. I think I’ll also go ahead and replace the six can diodes in the Power supply rectifier circuit before I put everything back together. Thanks to everybody who contributed suggestions. Hopefully it holds up.

[Kenzo1979]

Aaaarrrrrggggghhh!!!

Well, disregard the last post.  On for the last 15 minutes and it’s red-plating again !!!!  Back to the drawing board.

[drew]

I would just replace the boards with good G10 fiberglass boards.

[dunner84]

Have you examined or replaced the wire that feeds the screen resistors?

[66Strat]

You could check the filter cap board for conductivity using your DVM.

[Kenzo1979]

Have you examined or replaced the wire that feeds the screen resistors?

It’s never been replaced, but it’s intact, free of nicks, melts and burns and is connected on both ends.

[Kenzo1979]

You could check the filter cap board for conductivity using your DVM.

I was originally told this problem would exist on the main board, but I will check for voltage on all the fiberboards at this point.  Thanks.

[Kenzo1979]

Bought some new boards and sockets from the site and I'm in the process of rebuilding/rewiring. Couple of questions for those in the know:

I've noticed that in a blackface Bassman, the cathode bypass caps in the preamp section are all 25uf paired with 1.5k Resistors, but in the Silverface Bassman 100, there are 5uf paired with 820 ohm, 5uf paired with 1.5k ohm and also a 25uf/1.5kR.  Anyone know why this would be necessary, or can I use the 25Uf/1.5kR across the board with no ill effect? 

Other question:  The .1uf cap (AC Shunt) going to the PI pin7 has a 200V rating.  I have new .1uf 600V caps, like the ones used for the Coupling Capacitors.  Could I use a 600V rated cap in it's place or is the lower voltage rating necessary for protection reasons, etc.

Thanks!

 

[sluckey]

I've noticed that in a blackface Bassman, the cathode bypass caps in the preamp section are all 25uf paired with 1.5k Resistors, but in the Silverface Bassman 100, there are 5uf paired with 820 ohm, 5uf paired with 1.5k ohm and also a 25uf/1.5kR.  Anyone know why this would be necessary, or can I use the 25Uf/1.5kR across the board with no ill effect? 

Different amp. Different builder. Different recipe.

Other question:  The .1uf cap (AC Shunt) going to the PI pin7 has a 200V rating.  I have new .1uf 600V caps, like the ones used for the Coupling Capacitors.  Could I use a 600V rated cap in it's place or is the lower voltage rating necessary for protection reasons, etc.

600v cap is fine. Fender used a 200v cap because the voltage rating was adequate and the cost was less..