Blown fuse... Why ?

[dude]

I blew a fuse in my 18 watt Marshall and can't find any reason for it.     All voltage are fine, nothing I can see that could have caused it.

I've been playing the amp hard for about three years every Saturday night for hours at a time. I've replaced the power tubes a few times when I hear them starting to make a scratchy sound that gets worst when I rotate them in the socket or sometimes it leaves and comes back, new tubes always stop that noise. I do run them a little hot, 130 ohm bias R.

I was playing the amp extra hard on 10 with the gain up which I've done several times before and no problem but the fuse just pooped out. It's a 2 amp slow blow.

I guess my question is do fuses wear out, I always thought if a fuse goes there's a reason....?

New fuse and new el84's and she plays great on 10 for half an hour so far. Should I figure it's fixed?

Thanks in advance,

al

[sluckey]

New fuse and new el84's and she plays great on 10 for half an hour so far. Should I figure it's fixed?

Yes. Play on Dude. And carry a box of fuses in your gig bag. 

[stratele52]

Power tubes biased too hot , or / and contact pins in socket loose

[dude]

New fuse and new el84's and she plays great on 10 for half an hour so far. Should I figure it's fixed?

Yes. Play on Dude. And carry a box of fuses in your gig bag. 

Well, I didn't get so lucky. After a few more short plays the fuse went again.

stratele52 The 130 ohm R has been in the amp since I built it six years ago and never a problem, no red plating and el84's didn't seem to be real hot but on the other hand I had been noticing a lot of hum that wasn't there and some crackling sounds that disappeared when I rotated the tubes while in the socket, lazy man's retentioning , 

New power tubes didn't stop the blowing fuse after a few minutes of playing at med volume, maybe the pin's are arcing...? And I should retention them, they seen loose in the socket..?  I just don't want to keep trying something and keep blowing fuses till I find what's causing the short after a few minutes of playing.

I know it could be the rectifier tube but don't have another and it's only a few years old, it fits tight and looks ok but that means nothing. The filter caps are only six years old and look ok no cap tester either.

My guess is since for a while I've been noticing some hum and slight crackling noise is :

1. Arc on tube sockets as stratele52 suggested, I checked the wiring but plenty of space, I'll retention the sockets.

2. Loose connection shorting to the chassis but that would blow the fuse right away...?

3. Could it be a preamp tube?

Any suggestions what to do first? I only bought a four pack of 2 amp fuses

al

[stratele52]

First :

Remove power tubes ; do a test ( same test as if you use the amp ) with new fuse .

 Fuse don't blow ; check bias first . Bias OK ; check power tubes , tubes OK check speaker , and its connection. Output transformer short little bit .
________________________________
Second :

You said from many year same 130 ohms and no problem ;

This do not mean nothing ; TAKE SOME BIAS READING.   Do you know how ? It is so easy why don't you do that before ?

You think bias is not too hot ;WHAT BIAS DO YOU READ ?
________________________________
Third :

Other sugestions : DON'T THINK IT IS ALRIGHT , GIVE SOME PROOF ; TAKE REEDING

Check for some bad solder on ALL B+ rail

It is easy to fix but you must trouble shoot the right way .

[dude]

OK, the first part is easy.

Second part: I'll bias the tubes to 70% dissipation for an el84, I'll take voltages and do the math change out the bias R if needed to get 70%.  But I'll have to have the el84's in to do that...?

I'll check all solder joints along the rail but I'll do that first.

Can an OT be fine and then short out after three minutes of playing...?

Something probably came loose after at least 500 hours of hard playing in a recording studio over three or four years. I never moved the amp around. I'll dig in, your right. No "magic wand" to wave over the amp.  

Thanks, its been a while since I had an iron in my hand, I've just been playing or recording. You guys are pro's here, I've always fixed anything I tried with your help of course.

I'll let you know what it was.

al

[stratele52]

First part ; blow or not ?

Second part ;how can I know you do bias right ? What is your reading ; Plate voltage and cathode current on each EL84 please

Check solder ; how ? Just looking ? Yo must reheat them .

[alerich]

I do run them a little hot, 130 ohm bias R.

I'll bias the tubes to 70% dissipation for an el84, I'll take voltages and do the math change out the bias R if needed to get 70%.

EL84 bottles sure do sound good running hot, don't they? My Goodsell runs them hotter than a jalapeño douche. There are about a jillion 18 watt Marshall clones out there and the 130R shared cathode resistor is pretty standard to most of them. You have been running this particular amp balls to the wall for three years with no problems until now. I don't think your bias (if it's working as designed - i.e. the cathode resistor and/or bypass cap aren't pooched) is the inherent problem. I calculated the dissipation on my amp once and about had a stroke. It's a good thing sturdy new production EL84's are available and economical. I'll be interested to see what you think of the tone once you dial it back to 70% and get the other problem solved.

[dude]

First part ; blow or not ?

Second part ;how can I know you do bias right ? What is your reading ; Plate voltage and cathode current on each EL84 please

Check solder ; how ? Just looking ? Yo must reheat them .

I'll post the voltages as soon as I get some free time, in a day or so.

Fuse didn't blow with only the rectifier tube in.

Thanks, al

[stratele52]

If Bias is 70% of tubes power dissipation , this is with no signal . right ?

Check bias when playing guitar with your volume at 10 like you use it . Bias could be 110 % ( and more  ) of power dissipation if not real Class A . This is too much current.

[dude]

I haven't checked the bias yet as I'm still trying to find the short but yes the 70% dissipation would be set at idle, although I usually go to 80%. I'll check out the bias while playing too but first I need to find the short.

All power R are good, sockets are good, re-tensioned them too.

I checked all the tubes, 12ax's and el84's and rectifier on a tube tester. One of the 12ax7's has an inter-element leakage on one cathode, it's max is no less than 1 meg according to the manual, all other preamp tubes are over 15 megs except the one is about 4 megs, in the range but the manual says a stricter standard for high reliability application would be no less than 10 megs on any test so this tubes passes but not for high standards.

Could this be the reason I'm blowing fuses after a few minutes on playing...?

After I know I have all good tubes, I'll check all voltages and do the math for around 75/80% dissipation at idle.

So a slight leak in the cathode on first preamp, tube pin 3... cause blowing fuse?

al

Well that isn't it, use all good preamp tubes.  Turned on amp to stand-by with all tubes in after "five minutes" the fuse went...? No never turn on just standby and it blew, any suggests where to look?

Well, the fuse didn't go but looks like the PT did. NO smell, nothing. Before it went I checked all power R and the power rail without power, checked all tubes then plugged in on stand-by for about five minutes and was getting ready to turn on and check voltages when the pilot light went out. I thought I blew a again fuse. I have no voltage except 120 at the fuse, no voltage from the PT anywhere. I smelled nothing.

Could all this have been my power transformer? A short that finally fried the PT..? No burning, no smell. Now I'm thinking what caused my PT to go. It a Weber PT that he sells with his kits, I have heard of people running four el84's with that same PT and no problems.

Now I have played the amp very hard for years, and did notice that it was making more hum then usual last few months. also, when the first fuse went I though I heard a slight ticking noise or something then the the fuses started blowing after a few minutes. I haven't changed any wiring recently or even open it up in a few years. It just kept cranking out the tone ,I have at least 500/800 hours on the PT, I have a VVR and away used that to cut some voltage, probably ran it at around 300 volts and never dimmed, gain and volume half way or a little more.

I hate to put another PT in and have that blow. ???    Basically the PT went just sitting on Stand-by but I was having the above problems of fuses blowing after a few minutes. Before that noticeable hum for a few months. I have the normal 18 watt Marshall 1974 amp, two el84s, three 12ax and rectifier tube but I did tap off the heater supply for a small PC fan on the VVR, changed the ac to about 6vdc for just the small fan but the amp had been running with that small fan for a few years.

al

  

[sluckey]

Turned on amp to stand-by with all tubes in after "five minutes" the fuse went...? No never turn on just standby and it blew, any suggests where to look?

Pull the rectifier tube out. Does the fuse still blow?

[dude]

Sluckey, I just added to my last post as you were writing.

Thanks, al

[stratele52]

Dude , please follow trouble shoothing I show you . Do only the test I ask you.  You waste your time checking power resistor .

I need Bias reading as I ask you before ; Cathode current , and plate voltage.

Tubes tester don't test amps's  tubes . Forget that . Amps tubes work at high voltage and "high" current , tube tester use low voltage an low current . Stop to say your tubes are good beacause tube tester .....

Bias reading will tell more or put new tubes if you follow what I write before.

[dude]

stratele52,

I know you're trying to help me and I appreciated that but if I can't get any power to the tubes I can't give you any readings. Before the PT went (I'm assuming as I have no voltage from the PT) the fuse would blow before I could get any readings.

I pulled all tubes except rectifier and the fuse didn't blow but I only checked it for a minute. Then I put know good tubes back, checked for any short that I could see on the B+ rail, sockets, etc. Checked power resistors all checked out OK.  I turned on the amp and left it on stand/by for five minutes and the amp went out, I never put voltage pass the stand/by, never turn the amp to ON. I thought I blew a fuse and was going to check the fil. taps but when I looked at the fuse it was good. I pulled all tubes except the rectifier and took voltage readings, nothing, no 270-0-270 ac, no 350vdc no 6.3dv fil, nothing dead PT. NO smell, no hissing the PT just went sitting on stand/by.

Either all my problems were from my PT going south or maybe I had a short off the filament supply. The reason I was thinking filament supply is that I tapped 5vdc to a small PC fan keeping my VVR cool (it's in a hot place). I have some diodes off the filament taps to get the 5vdc for the fan. I was thinking maybe one of the diodes went bad causing the short, very doubtful though.

I just looked at the amp again, the amp has a 5v tap along with the 6.3 filament, I used the 5v tap rectified to dc for the fan.

At this point the PT is blown, maybe I blew it by running the amp without finding the short or it was it's time... But it blew on stand/by  and the rectifier tube is know good, the switch isn't shorted either.

Could all the problems I'm having have been a PT going bad and it finally went...? How can a PT blow on stand/by unless it had a short inside...?

al

I'm losing it, now I have voltages. Must be something wrong with my power/stand/by switch. I have a DPDT switch as I'm using only one switch hole the other had the VVR. Maybe the switch has an intermittent short, I'll get back once I straighten things out.

[dude]

Plug on end of power cord had loose and broken prong inside casing, causing here and there power, ha. Fixed that.

Plugged in amp with rectifier tube only, 290vac on each sec. no fuse blowing but only 260vdc going in to VVR, coming out it's 35v. VVR isn't working.

At first, after I fix the power cord, I got 350vdc to the VVR in and it dropped 25v out to 325vdc..? It was working then stopped.

Now, the rectifier tube only has 260vdc out. I'm stuck, maybe I should disconnect the VVR and check voltages then?

I have another mosfet, and all the zeners and resistors needed to replace all the parts in the VVR but how can I find out what's wrong with the VVR without replacing everything, could it be the zener? I have one of Dana Hall's cathode bias bds.

Besides the broken power cord prong, I've got a feeling the VVR caused the fluse blowing incidences.  Once Dana Hall said the the zener sometimes goes with the Mosfet.  Any suggestions?  Sorry for the confusion with this amp but that broken prong was driving me crazy, never had anything so simple cause me so much misery. One minute power, the next nothing....

Before I replace all the VVR parts just asking for any guidance.

I'm using the search options here, I have the VVR after the recto no filter cap, then filtered after, should the filter be, before the VVR?

I searched this:  But I'm doing the whole amp as I never turn down the voltage that much to cause any scratchy pot issues.
 
 Dana's VVR is designed for DC operation, not pulsated DC after the recto, it should be inserted after the first filtering node following the recto and the nodes you want to scale. A typical amp use the first node for plate supply, so you need to add a cap after the VVR and take the plate supply from there or add a cap between the recto and the VVR.

You need to add a diode after the VVR and before plate/screen filtering nodes when the whole amp isn't scaled. It'll block cap discharge thru the VVR at power down. You may add another diode between your first filtering node and preamp filtering node(s). Dunno about the third supplied diode - could be used in reverse as a VVR bypass or is simply a spare.

Consider also adding bleeder resistors - 220 or 270K 3W flameproof - across the first node(s) delivering voltages to tubes, the one after the VVR will also help stabilize it.

Schematics of your amp PS would help if you want more specific info.


Thanks, al    

[Willabe]

Plug on end of power cord had loose and broken prong inside casing, causing here and there power, ha. Fixed that.

Nice catch!

At first, after I fix the power cord, I got 350 vdc to the VVR in and it dropped 25v out to 325vdc..? It was working then stopped.

Now, the rectifier tube only has 260 vdc out. I'm stuck, maybe I should disconnect the VVR and check voltages then?

I would. Get it running with out the VVR and then trouble shoot the VVR.

Dana uses or at least used to use a smaller die mosfet than Kevin uses for his Power Scaling. The larger die mosfet can take a little more heat dissipation. I don't know if Dana has changed to the larger mosfet package.


                 Brad        

[dude]

Thanks Brad,

I'm a musician not really a repair guy but like "paint by numbers" anyone can copy a layout, ha. I've got some experience repairing amps but sometimes it can be a chore. I know this amp design inside and out but I'm finding out it pays to know what your doing rather then "paint by numbers".

I'll disconnect the the VVR get the amp going then like you say trouble shoot the VVR.

I always seem to have problems with my VVR's blowing, maybe it's the filtering after the VVR and not before..?  I just search some info.

I'll get back without the VVR.

Thanks,

al

[Willabe]

I'm a musician not really a repair guy but like "paint by numbers" anyone can copy a layout, ha. I've got some experience repairing amps but sometimes it can be a chore. I know this amp design inside and out but I'm finding out it pays to know what your doing rather then "paint by numbers".

Hi Al,

Good to know both, they both help. Depends on what you want/need to do. Everybody learns different things in different orders and then add to what they feel their missing. That's how I learned to play guitar. Pretty much paint by numbers then started to see/hear what's missing and went after that.

I always seem to have problems with my VVRs blowing, maybe it's the filtering after the VVR and not before..?  

That could be a clue that you keep having them blow up. Are you using steel or aluminum for your chassis with VVR? IIRC Kevin said that putting a ss diode right before the PS (VVR) input to feed it pulsating dc is easier on the mosfet. At least that was for the old style PS which I think is what Dana still uses/sells.

Take a look at London Power.com for Power Scaling info. Kevin O'Connor IMO is the guy who came up with it, not Dana.


                      Brad      

[dude]

Brad, thanks for taking the time to help me along with everyone else.

I got the energy to disconnect the VVR and...  WoW!  the amp's alive, again...  But,

I love that VVR for recording a live band in one room, cuts the volume from bleeding without cutting the tone...

I got to get it back but correct.

thanks for all the help,
al
 

[sluckey]

What kind of heat sink are you using? Can you post a pic of your VVR?

[dude]

I'll sent pic's but don't laugh at the wiring, it's not a rat's nest but it "ain't like what I see here". The fan is an outdated p/c 12vdc fan that cools the memory or processor chip on that  p/c a with heat sink. I took all that , heat sink included and mounted it under the Mosfet with the fan on top. Worked fine for years till now. I never had a filter before the VVR only right after. Why I'm having problems, I don't know...?

Give me a day. I've done this on a few amps of mine to keep it running cool, No problems. I'm using a NTE2973 Mosfet, 14A Max. This Marshall is an aluminum chassis. 

al

[dude]

What kind of heat sink are you using? Can you post a pic of your VVR?

I used the 5v filament for the pilot light and have a bridge rectifier off that to the 12vdc fan. I'm getting about 5vdc, just enough to get the little fan going with low noise. Fan comes on with the stand/by switch. I know the wiring needs improvement, one of the first builds I did. I have the Paul Ruby mod and some R's hanging in case I want to go back to them.   

The VVR feeds right off the rectifier tube, no filter or diode before, output is filtered by 32uf filter then down the rail. I've had several mosfets blow so I use the fan on all my VVR's. I'm doing the whole amp and did the scratchy pot thing too. I found that tightening the screw on the Mosfet too tight can give you noise, squeal.

Maybe someone can direct me me to the best installation for a VVR bd. This one was done about five years ago maybe there's a better design..?

Thanks, al 

[sluckey]

Your VVR and heat sink installation look good, although I don't see any heat sink compound (grease) between the MOSFET, mica insulator, and chassis. The grease makes a BIGGG difference with transferring heat from the MOSFET to the chassis and heat sink. I highly recommend using some. Radio Shack has it.

I would put a 32µF filter cap on the input to the VVR board.

[Willabe]

Your VVR and heat sink installation look good, although I don't see any heat sink compound (grease) between the MOSFET, mica insulator, and chassis.

I think he might be using a silicone pad? Can't use therm. grease with them. Can't quite tell from the picture, it may be mica?

And yeah that heat sink is a good sized one plus with the fan they should keep the mosfet plenty cool, even on a steel chassis. But if that is a mica insulator with no grease even with the heat sink and fan the mosfet might not be able to transfer the heat to the chassis and heat sink. Looks like he's got grease between the chassis and the heat sink.


               Brad    

[dude]

I used the grease on the fan's heat-sink as you can see, as far as a mica pad or silicone pad, I don't know the difference.

The one I used was clear and I used no grease but I have a gray pad too in stock that seems like some kind of rubber base material. Which one should I use and should I use grease with that one? 

I'll just use a new mosfet and zener, check the pot and three resistors, too.  Do I twist the leads from the mosfet to the vvr board?

So, I should add another 32uf filter after the rectifier tube and before the VVR input? Can I use a 22uf cap, if 32 is better I think I have some of both.

Looks like I'm back in business, these amps ready run nice with a 5751 tube up front, and a few mods to clean things up. I like a clean amp, you can always dirty things up with the gain and the VVR and even a pedal but you can't get a good clean rhythm tone if it's not there from the beginning. That Weber Blue is nice too, 30 watts and doped.

Once I get the VVR going, I'll check the dissipation wattage but I like things a little on the hot side. Also, notice the big cathode bias cap, it's 1000uf. A guy over at 18 watt dot com, zapoid phil, says that that big cap gives a much cleaner bass with less mud all round and he's right. Kind of tightens things up, like fixed bias.

Thanks for all the help,

al

[Willabe]

I used the grease on the fan's heat-sink as you can see, as far as a mica pad or silicone pad, I don't know the difference.

The one I used was clear and I used no grease but I have a gray pad too in stock that seems like some kind of rubber base material. Which one should I use and should I use grease with that one?

Looks like Sluckey nailed it. You have to use heat sink compound between the mosfet and mica insulator and between the mica and chassis too.

The clear 1 is mica and the gray 1 is silicone. With the silicone pad you don't use any grease. Plus they won't/can't crack like the mica 1's can.

Have all the VVR mosfets you've blown up have the mica insulators with no thermal grease? If so you probably now know why.

Do I twist the leads from the mosfet to the vvr board?

I'd say yes, Kevin's very strong on doing that. Even with short leads.

                
                   Brad          

[sluckey]

The one I used was clear and I used no grease but I have a gray pad too in stock that seems like some kind of rubber base material. Which one should I use and should I use grease with that one?

I only have experience with the mica insulators (clear) and you need grease with those. Put some on the metal tab on the back side of the MOSFET. Smear it around a bit then lay the mica insulator on the MOSFET and slide it around a bit until it lays flat. Then put another dollop of grease on the mica insulator and smear it around a bit. Then just bolt to the chassis. You'll end up with grease on both sides of the mica. There are many different ways to apply the grease but this method puts less grease on me and everything else I touch.
 

Do I twist the leads from the mosfet to the vvr board?

No need.

[DummyLoad]

I have a gray pad too in stock that seems like some kind of rubber base material.

cleaner solution to mica washer and thermal grease. kapton pads do not require use of thermal grease. they have a 4KV insulation breakdown rating so should work well in this application.

http://www.wakefield-vette.com/resource-center/downloads/brochures/thermal-management-accessories-wakefield.pdf

[dude]

I would put a 32µF filter cap on the input to the VVR board.

Could I go from the rectifier tube to the first 32uf cap can filter with the OT and highest B+, then to the VVR and out to the board or just add another 32uf at the input of the VVR?

This has probably been asked before but why does the voltage drop about 20v at the output of the VVR when it's on the highest voltage setting?

I have some time today to get the amp back together.

al

[sluckey]

Could I go from the rectifier tube to the first 32uf cap can filter with the OT and highest B+, then to the VVR and out to the board

No

or just add another 32uf at the input of the VVR?

Yes. You could just connect the positive lead of the additional cap directly to the rectifier socket and connect the negative lead to the same point that the original first B+ cap is grounded to.

[dude]

Thanks, sluckey

One more question, if I have a 2w 220K bleeder R over at the cap can, that would bleed all the voltage from all the filter caps, right? So, no need to have more than one bleeder in the power supply?

Although, if the VVR gets it's own filter at the rectifier and since that feed goes to stand/by, the bleeder on the cap can would bleed the VVR filter only if the switch was on? Off would disconnect the can filter bleeder from getting the whole power rail...?  So, in this case I might need another 220K bleeder at the VVR filter...?

al

[Willabe]

When the amps tubes heaters are fully heated/warmed up, if you leave the stand by on and just turn off the power on/off switch the tubes will drain the filter caps. The tubes will try to pull their B+ dcv from the filter caps since the wall voltage is off and since it is shut off the caps will not be recharged. You'll hear it happen if the volume is up. After you hear no sound from the amp the caps are then discharged and you can turn off the stand by until the next time you use the amp.

It's a nice little trick to drain the filter caps when working on a BF/SF Fender amp or any other amp who's PS B+ is wired up that way.   

The new 1st filter cap, the VVR and the 2nd filter, ie, 1st 1 after the VVR along with the PT B+ CT should all be grounded together. Then run a wire from there to the next ground star point, which is probable the next filter cap and it's tube/tubes circuit.


               Brad        

[dude]

I follow you completely but I should have mentioned that I have only one on/off/standby switch, it's a DPDT as I'm using the hole from the on/off for the VVR. I have so separate standby switch so I might need the bleeder on the new VVR filter.

I'm only mentioning this as I got a 350vdc shock from a 67 Bassman head A165, no bleeder in that amp. I drained the caps but they have memory. I should have left a clip on bleeder hanging on the cap. I got 365v in one hand and out the other. I felt screwed up the rest of the day. After the shock my heart was in some kind A-fib for about 20 seconds till it came back. Since then I'm extra careful and a strong believer in bleeders.

Thank, al

[Willabe]

I got 365v in one hand and out the other. I felt screwed up the rest of the day. After the shock my heart was in some kind A-fib for about 20 seconds till it came back. Since then I'm extra careful and a strong believer in bleeders.

     That's why the old saying of "keep 1 hand in your back pocket when working/testing inside of a live chassis."

Depending what R's are going to ground on the input of the VVR they will bleed off the 1st filter caps dcv but it may take a lot longer to do so? It won't hurt to put a 220K across the 1st filter cap. Or you could put a 330K across the 1st and the 2nd filters caps. Takes a little longer to drain off the dcv but it's slightly less drain on the PS than using 2 @ 220K. Not a big deal either way.


                  Brad             

[dude]

I had the amp off for an hour, wasn't live or at least I thought that since I used a blade of a screw driver to drain the caps. Of course this was in my inexperienced days. I kept the cap shorted till all the voltage was gone, a few minutes. The meter read 0 volts. I lifted the screw driver and began working on the amp after fifteen minutes my palm touched the B+ where the cap was and the other hand, God knows where it was inside the amp. Zap! I flew about five feet and my heart was pounding irregular. I thought I was having the big one then everything when back to normal and I felt washed out for the rest of the day. Thought about the ER but... I took the meter across the cap I was near 350vdc.

The cap had a memory and went right back to high voltage and I had no idea thinking I drained the cap. So I now either have bleeder installed or I use a clip on bleeder, no chances. I don't have 9 lives but looks like I have at least two, 

al