OT blow speakers?

[Lennart]

Hi!
A friend of mine has a Music Man 410 HD that makes him very unhappy. It has blown a bunch of speakers.
After replacing the blown original speakers the same shit happened. He describes it as when he had played for a while the amp suddenly got silent for three seconds or something and then sounded again, but with distortion at low frequences. And the speakers were hot! Two of the new ones blown. I´ve heard it could be short circuit from primary to secondary. When I measure there is no DC at the output jack. Could it be an intermittent problem caused by vibrations? And is there any other possible cause to the problem?

[Lennart]

I believe that there should be no DC on the output jacks. 

Normally, no.

What kind of instrument was being played?

  Guitar

[Jack1962]

OT is probably shunting DC voltage to the speakers , turn amp on check output jack for DC voltage.

[Lennart]

OT is probably shunting DC voltage to the speakers , turn amp on check output jack for DC voltage.

As I said before there is no DC on the output jack...
1. I suspect something intermittant as a result from vibration. Could I be right here?
2. If not 1, is there any other possible cause?

[Willabe]

That amp as with other MM amps has pretty high output tube plate voltages and has SS (current?) drivers on the cathodes of the output tubes. when they first came out Clapton and J. Winter were playing them on the road along with others. Well until they started catching on fire on stage anyway.

Has any work been done on the amp before, like new filter caps? It's old enough to need new ones at this point.  

I wonder if something is wrong with the SS cathode drivers?

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

If it has a 12AX7 for the PI it's an older model that was favored by many. 



                              Brad      

[Lennart]

Has any work been done on the amp before, like new filter caps? It's old enough to need new ones at this point.  

I don´t think so. But would new filter caps protect from the speakers to burn?

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

Yes,the amp is an HD 130. 410 means 4X10" speakers...

If it has a 12AX7 for the PI it's an older model that was favored by many. 

This one has IC in the preamp.
But nobody here seems to have any answers to my questions. So far.


                            

[Willabe]

The output stage is more complex on those amps than standard tube only output PA's that are the norm that most guys here have experience with.

I used to own a RD50 with 1x12" speaker but never worked on it. It had a pretty good sound to it.

Setting the bias on those amps is also a little different than what most guys are used to as far as -voltage value readings.

Why is the amp burning up speakers?       PRR, HBP or Sluckey (maybe some others) will be able to help you.

But as in any amp if the filter caps are old and are starting to leak they will not be able to support the dc voltages that the amp needs and as they leak more and more they will get hotter which will speed up the aging process. Then at some point 1 or more of them will blow up/short out to ground. That's when things can go from simple fix to big money fix many times.

Anyway if a filter cap went out that could have messed with output stage including the SS driver. That would then mess with the PA's bias which would mess with the OT? All of this is speculation on my part, but filter caps are the kind of thing that is the 1st domino to fall that leads to catastrophic PA failure. But why this would kill speakers?     


                   Brad     

[Lennart]

But as in any amp if the filter caps are old and are starting to leak they will not be able to support the dc voltages that the amp needs and as they leak more and more they will get hotter which will speed up the aging process.

Yeah, there are a lot of parts in an amp...

[Willabe]

This one has IC in the preamp.
But nobody here seems to have any answers to my questions. So far.


It would be of great help to know exactly which amp it is. It's not just a simple run of the mill Fender Champ or even a 5E3 deluxe.

The MM amps in Doug's schemo library are not listed as HD 130.


              Brad     

[Lennart]

This one has IC in the preamp.
But nobody here seems to have any answers to my questions. So far.


It would be of great help to know exactly which amp it is. It's not just a simple run of the mill Fender Champ or even a 5E3 deluxe.

The MM amps in Doug's schemo library are not listed as HD 130.


              Brad     

http://www.el34world.com/charts/Schematics/files/musicman/musicman_2475-130_&_2275-130.pdf

[Willabe]

Ok, that amp does have the SS cathode drivers.

Schemo says 700vdc for OT plate PS but the cathodes are lifted by about ~60dcv. So you end up with ~640dcv on the output tube plates. Maybe a little more because the wall voltages now area little higher then back in the 70's when the amp was made?

That amp has a high dcv PS with enough current to drive 4x6CL4 (EL34) to ~130w. Who ever works on it should have a good amount of experience because of this. That amp can kill you.


                    Brad     

[PRR]

> short circuit from primary to secondary

That should not be fatal to speakers.

Short from B+ to one side of speaker winding.

One end of speaker winding is grounded, and speaker winding has very low DC resistance (<1 ohm). So about 7/8 of any DC current from B+ goes through winding, not speaker.

What is the most current that B+ can supply? 130 Watt output, say 240 Watts of DC from B+ normally, so 0.34 Amps steady-state. A transformer can maybe supply 10X rated current into a short. 3 amps.

7/8 of that 3A in OT speaker winding, 1/8 in speaker. 3/8 or 0.375 Amps in speaker. 0.375A^2*8r is 1.13 Watts of power in the speaker. A "130 Watt" speaker array can stand 1.2W forever.

Assume a *second* defect. OT speaker winding has gone open. All the 3 Amps possible from B+ flow in speaker. 3A^2*8 is 72 Watts of heat in the speaker. A "130 Watt" stage-speaker array "should" survive 72W for many seconds, though if DC the huge excursion will stretch the surround and just-maybe jump the voice coil out of the gap. It is also a race between speaker damage and melt-down of the Power Transformer supplying 10X its rated output.

So it could happen; but I'm real doubtful on the chance of two different faults (short P-S and open S) and in an amp which often works fine.

> amp suddenly got silent for three seconds or something and then sounded again, but with distortion at low frequences. And the speakers were hot!

Something is going on, but I don't have a glimmer.

The 3-second silence, then part-life, ought to be a clue, but I'm blank.

I assume the speakers are good hi-power jobs. (I've seen 100W amps with $30 30W speakers installed thoughtlessly then blown.)

[Lennart]

Something is going on, but I don't have a glimmer.

A picture of my friend:  
What about a parallell universe influence?  

I assume the speakers are good hi-power jobs. (I've seen 100W amps with $30 30W speakers installed thoughtlessly then blown.)

Hmmmm.... When the original speakers burned he installed Jensen JCH 10/35, $45... made in Italy. Mistake? They are really light weight = tiny magnets.
But we are not talking heavy metal.

[Willabe]

Hmmmm.... When the original speakers burned he installed Jensen JCH 10/35, $45... made in Italy. Mistake? They are really light weight = tiny magnets.

Doesn't matter, you still have an amp that's putting out 130w.


               Brad       

[Lennart]

Hmmmm.... When the original speakers burned he installed Jensen JCH 10/35, $45... made in Italy. Mistake? They are really light weight = tiny magnets.

Doesn't matter, you still have an amp that's putting out 130w.


               Brad        

Not with the volume knobs halfway. And there are four of them. But what speakers would you suggest?

[6G6]

Better speakers are a good idea, but if youhaven't found the problems upsteam of them
they may not last any longer.

[Lennart]

.........seems like there is only one solution here...a new amp...
But is it really 100% sure the OT can´t be the cause? The speakers are connected only to the OT. The current that burns the speakers has to come from somewhere, and certainly not from somewhere over the rainbow. Or can it?

[mat janssen]

The speakers were too hot!
May be the amplifiers is oscillating at a high frequency at full power. Speakers do not like this.
Take an oscilloscope an look!
Regards,
M.

[Lennart]

The speakers were too hot!
May be the amplifiers is oscillating at a high frequency at full power. Speakers do not like this.
Take an oscilloscope an look!
Regards,
M.

Someone told me that an amplifier of this kind could oscillate with the bias set too hot. Now it´s not too hot but was at first. With the bias at a correct level, what would make the amps oscillate?

[Willabe]

Not with the volume knobs halfway.

When they would list an amps output it was supposed to be full output measured clean. Now they could cheat that number but an map with every thing turned up full could put out 2x that output because the output sine wave is now square. So a 130w amp could put out 260w.


               Brad     



 

[Lennart]

Not with the volume knobs halfway.

When they would list an amps output it was supposed to be full output measured clean. Now they could cheat that number but an map with every thing turned up full could put out 2x that output because the output sine wave is now square. So a 130w amp could put out 260w.


               Brad     



 

But now we have a situation where the original speakers suddenly burned, and the new replacement speakers burned too. Would it not be very stupid just to buy a set of new ones and hope for the best?
I want to hear more about the oscillation issue.

[Willabe]

I'm just saying don't think that an amp with the knobs set at half only puts out half the wattage rating. It's probably closer to around it's full wattage rating.


                  Brad     

[stratele52]

I did not read all the answer .

The only thing I see is the negative feedback send DC voltage to speaker jack . But Lennart said there is no DC voltage; intermittent , please check in the amp , any mods ?

loose wire ...touching output speaker jacks ????

Amps with Output transformer can't sed DC by the transformer itself

Pictures inside the amp please

Or you friend always put to low powered speakers ?

[Lennart]

Testing testing, one two,
http://www.flickr.com/photos/99357732@N02/9361660898/http://www.flickr.com/photos/99357732@N02/9358928095/
http://www.flickr.com/photos/99357732@N02/9358953703/

[stratele52]

Look original , not mods , everything is clean .

 Can you tell us what is the speaker your firiend put in and blow .

This not explain why original speaker blow , maybe too old ?

[Lennart]

I succeeded in adding three of five.  

[Lennart]

Look original , not mods , everything is clean .

 Can you tell us what is the speaker your firiend put in and blow .

This not explain why original speaker blow , maybe too old ?

They are already mentioned in the thread. Jensen JCH 10/35

[stratele52]

I should read all the post , Thank's

[stratele52]

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

                               Brad      

Yes Music Man 410 exist .

I have Music man amp book, it is a 75 watts RMS amp, I'll read it but no shematic . I'll comme back later

[Lennart]

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

                               Brad      

Yes Music Man 410 exist .

I have Music man amp book, it is a 75 watts RMS amp, I'll read it but no shematic . I'll comme back later

The schematics for the amp is linked earlier in the thread...

[stratele52]

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

                               Brad      

Yes Music Man 410 exist .

I have Music man amp book, it is a 75 watts RMS amp, I'll read it but no shematic . I'll comme back later

The schematics for the amp is linked earlier in the thread...

The schematic link I see is for the 130 series ;  the HD 130 , 212 HD130 and the 210 HD130

Not same amp as 410 sixty five or 410 seventy five listed in  my book

[Lennart]

I did a search for Musicman 410 HD and it came up with that it's probable a HD 130. Which amp is it exactly, there is no MM 410 HD listed in Doug's schemo library.

                               Brad      

Yes Music Man 410 exist .

I have Music man amp book, it is a 75 watts RMS amp, I'll read it but no shematic . I'll comme back later

The schematics for the amp is linked earlier in the thread...

The schematic link I see is for the 130 series ;  the HD 130 , 212 HD130 and the 210 HD130

Not same amp as 410 sixty five or 410 seventy five listed in  my book

This is a 130 W amp with 4X10" speakers.

[stratele52]

In my Music ma book I found 410 HD at 150 watts. Yes the 130 shematic could be the good one.

[stratele52]

What the output nominal ohm rating for the 130 HD?

Schematic show 8 ohms !!!

If you use 4 X 8 ohms speaker ; two in parallele IN series with 2 in parallele = 8 ohms

[stratele52]

Music Man amp link


www.pacair.com/mmamps/Misc__Info/Model_Chart/model_chart.html

Service bulletin ;

pacair.com/mmamps/Documents/MM%20Service%20Bulletins%20All.pdf

[Lennart]

It´s wired series/parallell 8 ohm.
Is this discussion really necessary?
Can you tell me the reasons an amp would oscillate?

[Lennart]

The speakers are 8 Ohm and wired in series/parallell.
And all knobs go from 1 to 10.

[stratele52]

I would put a new good quality phase inverter tube ; 12AX7

As I read here ;

www.mk-guitar.com/2009/02/15/music-man-guitar-amps/


Originally there was one pre-amp tube – a 12AX7 – used for the phase  inverter stage of the amp. This is the stage between pre-amp and power amp. This way a certain amount of tube distortion was added. In about 1977 this tube was replaced with a solid-state version. The reason was that a certain malfunction of this tube could cause severe damage to the complete output section, including a damage to the power tubes and the expensive output transformer. The amps with the new solid-state phase inverter still sound very similar to the earlier models, but are nevertheless by some considered as sounding not as warm as before.

[stratele52]

The speakers are 8 Ohm and wired in series/parallell.
 

You are sure of that ? Most  people don't what it is serie parallel.

[Lennart]

I would put a new good quality phase inverter tube ; 12AX7

Read post#6

[Lennart]

The speakers are 8 Ohm and wired in series/parallell.
 

You are sure of that ? Most  people don't what it is serie parallel.

I do.
 

[stratele52]

I would put a new good quality phase inverter tube ; 12AX7

Read post#6

Sorry I did not understand this mean the need to replace the 12AX7

"...If it has a 12AX7 for the PI it's an older model that was favored by many.  ......"

[PRR]

> Not with the volume knobs halfway.

Volume control just means you have to play harder; you can always max-out an amp. (I've had systems clip with knobs at "2".)

> And there are four of them.

Four 35W speakers is 140 Watts. Amp is 130W? If they are good speakers, marketed for serious guitar-amp use, that should be OK. I'm not over-impressed with that series of Jensen. OTOH, I know full-well that a re-speakering of a quad box is BIG money.

I'm still disturbed by the 3-second silence and recovery.

I'd try a 200 Watt dummy load resistor, so the amp can be worked HARD in the shop without attracting cops from the next county. Across that some good speaker with a 100 ohm 10W resistor in series so it plays at part-Watt level.... you can hear OK, but won't hurt the speaker or disturb the peace.

Also clip-lead a DC voltmeter to the amp's B+. Watch it when idle, when working hard, and especially if you manage to get to to crap-out. Both in the silence and the subsequent fuzz. If you have more meters, monitor the amp's bias (if you can figure it out on a MusicMan; on those plans, the 3.9 or 10 ohm resistors are useful points), and the 350V to the screen grids.

[Lennart]

I'm still disturbed by the 3-second silence and recovery.

I´d like to follow the oscillation track for a while. Let´s say the amp starts oscillating at a certain moment, the speakers get hot, short circuit in two of the speakers, all current run through them and no sound is heard until the coil wire is melted in the two, and the current can run through the other two. 
So my question still is: what creates the oscillation?
To check for oscillation a oscilloscop is needed.

[stratele52]

So my question still is: what creates the oscillation?
To check for oscillation a oscilloscop is needed.

You can read/ see oscillation with Voltmeter on AC mode

[Lennart]

You can read/ see oscillation with Voltmeter on AC mode

OK. And what would cause this oscillation?

[stratele52]

You can read/ see oscillation with Voltmeter on AC mode

OK. And what would cause this oscillation?

I did not have a chance to read you before about that oscillation. Now I must go for the day .

[mat janssen]

Oscillation can come from a problem in the feedback circuit. Semiconductor(s) problem. Bad capacitor(s). Problem in powersupply (stabilisation component if used). etc. etc.

[Lennart]

On the board, what seems to be the tremolo section, there are two transistors. One of them are shorted base-collector, both directions. I can´t find it on the diagram. 2nd page to left (Page is turned 90 degress clock wise) is 2N 3391. That one is OK. Could the tremolo unit oscillate uncontrolled some way, even if not used?

[mat janssen]

Yes that is possible, and its also possible that then the oscillation is brought to the powersupply.