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#1 2012-05-25 06:18:08

sewage666
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From: Oakland, CA
Registered: 2010-11-19
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Internal fuse blow before external?

So I was on stage tonight, and my V4B powered on and turned right off. I checked the 6A external and it wasn't blown. I borrowed a head and figured out that I had only one speaker working on my 2x15 (pretty sure it blew when I lent the cab to the openers). Knowing that the speaker blew, I also borrowed a cab and finished the show.

I prayed the internal fuse was blown, and I've taken it apart. The internal fuse did blow, so tomorrow I'm going to replace it and the speaker and hopefully all will be right with the world again.

My question is... What happened? I thought the external would blow first, and then the internal. Instead, just the 10A internal fuse blew. I figure it has something to do with the Ohm mismatch, but I thought it was relatively safe to mismatch upwards with a tube head, or at least that's what I've read.

Not that I would've intentionally mismatched, but I just never thought it would blow a fuse because a speaker went out. Just looking to see some theories on the why... Thanks!

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#2 2012-05-29 17:18:56

sewage666
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From: Oakland, CA
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Re: Internal fuse blow before external?

Well after having a dog of a time resolderin the internal fuse in, my V4B made a quiet buzzing noise then stopped. The light never went on.

Guess I'm borrowing for the rest of the tour and my amp tech gets some money to figure out what I can't. Boo.

I'm also bumming on the lackluster state of the big box stores like Guitar Center and Sam Ash on their lack of stock on any replacement speakers in store. Not just for me, apparently they carry nothing at all in store. Luckily, a local shop in Richmond, VA (Backstage) carries a 15 eminence delta that'll have to do.

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#3 2012-05-30 00:02:48

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Internal fuse blow before external?

does your v4 have the flyback diodes still?

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#4 2012-05-30 02:15:39

sewage666
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From: Oakland, CA
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Posts: 79
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Re: Internal fuse blow before external?

hangman wrote:

does your v4 have the flyback diodes still?

Dammit! Didnt check that in the kitchen of the bar I had to work in... And you mention it here a lot. I'll look at that tomorrow when we have some time and check back in.

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#5 2012-05-30 02:42:05

hangman
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From: Seattle Washington
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Re: Internal fuse blow before external?

They would be on top of the circuit board connected to a small two contact terminal strip.

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#6 2012-06-04 16:08:22

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

sewage666 wrote:

What happened? I thought the external would blow first, and then the internal. Instead, just the 10A internal fuse blew. I figure it has something to do with the Ohm mismatch, but I thought it was relatively safe to mismatch upwards with a tube head, or at least that's what I've read.

It's not inconceivable that either your slow blow is REALLY slow to respond...or you got just the right current draw spike in there that the 10AMP to blew instantly response to it before the slow blow had time to do it's calculations over time.  Or maybe the slow blow is elderly...

That is all theory though, I don't know the ins and outs of fuses...I was always told that the 10AMP fast blow fuse was a 'dummy fuse' so that if anyone put a nail or a fuse wrapped in tin foil into the external fuse slow 'just for this gig' that the amp would still have some protection.  Not sure which fuse is more common to blow first; I've never blown one myself, interestedly enough.   

This is a random though, but why don't tube plates and or screens etc ever get fused? Not in production amps, no, but no one has ever entioned it that I recall.  I mean, if the fuse was sized right, could it make a red plating tube or pair kick out of business instantly and potentially make the tube still usable in cases that it wasn't directly because of the tube, such as bias error, wall voltage, etc?

Last edited by Liquids (2012-06-04 16:10:15)


Matthew

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#7 2012-06-04 16:50:41

hangman
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From: Seattle Washington
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Posts: 1848

Re: Internal fuse blow before external?

Marshall, has fused the cathodes in pairs,  problem is that players would just keep using their amp with fuses blown and them sounding like crap.

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#8 2012-06-11 23:59:22

Liquids
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Posts: 491

Re: Internal fuse blow before external?

I'm so curious what that would sound like, now.


Matthew

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#9 2012-06-12 02:55:55

hangman
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From: Seattle Washington
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Posts: 1848

Re: Internal fuse blow before external?

What it sounds like when the cathode fuse blows?

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#10 2012-06-12 11:06:40

Liquids
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Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

yeah


Matthew

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#11 2012-06-12 16:09:15

hangman
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Posts: 1848

Re: Internal fuse blow before external?

It sounds awful.
Ratty distortion,   Sound weak too.   Asymmetrical output.   

Ever heard an amp that as a dead triode in the phase inverter? Pretty much the same deal,

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#12 2012-06-12 18:07:27

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

Never heard an amp with a dead triode in the phase inverter either...

I imagine it sounds awful...but if marshall players would keep using them, maybe there should be a switch for it to disconnect the cathodes smile


Matthew

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#13 2012-06-12 18:38:29

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Internal fuse blow before external?

ha ha... except the problem is the players never notice,  so who would flip the switch?   and what if the switch got flipped on accident and nobody noticed?   also... would have to have one for each cathode... 4 switches that had almost no useful functionality.   (not to mention the fact that the impedance selector would also have to get switched to get full power)

let the mains fuse or HT fuse blow... thats what I say.

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#14 2012-06-12 18:51:59

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

but then you don't get access to that secret mojo distortion tone!  You're limiting features!


Matthew

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#15 2012-06-12 19:39:51

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Internal fuse blow before external?

ha ha!  lets get you a job at mesa-boogie making amps that have every possible feature nobody always wanted!

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#16 2012-06-12 19:44:18

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

Apparently they have the position filled!


Matthew

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#17 2012-06-15 06:51:32

sewage666
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From: Oakland, CA
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Re: Internal fuse blow before external?

I'm finally home and had time to open up my amp... I disconnected the flyback diodes, as I saw hangman suggest on another thread. Sure as shootin', my amp fired up again fine. Success!

I'd still like to replace the flyback diodes. I found this thread:
http://www.ampegv4.com/forum/viewtopic.php?id=686

I just want to check with the resident experts again on what I should be getting to replace these and how many to string. I found the above thread a bit confusing in the descriptions.

I'm hoping to source the parts locally, either from the ma and pa electronics shop with the not friendly hours, or a Radio Shack if possible. Thanks for all the help!

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#18 2012-06-15 07:29:51

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Internal fuse blow before external?

You can use three 1n4007 diodes in series to replace place the two diodes currently in there.   
I maintain that they are not needed,  and your amp wouldnt have broke down on the road had those diodes not been there.   But I have voiced that opinion many times before.

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#19 2012-06-15 12:48:02

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

There is a point to what Steve says, however, they are also there and in some cases, protect the transformer.   It's whichever risk/pain is less evil to you and your situation.


Matthew

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#20 2012-06-15 12:50:05

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

And should you decide to want say, 6x 1N4007, but for some reason are getting crazy prices or have difficulty sourcing them locally, one of us with them on hand may be able to ship them to you so you don't pay crazy amounts.


Matthew

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#21 2012-06-15 14:15:16

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Internal fuse blow before external?

Liquids wrote:

There is a point to what Steve says, however, they are also there and in some cases, protect the transformer.   It's whichever risk/pain is less evil to you and your situation.

Good point... Never seen an ampeg with a blown output transformer...oh wait... Yes I have, a number of times actually, and they all had flyback diodes in them...  Must've been a fluke though, since those transformers were all protected.

How many other amps used flyback diodes?   Hiwatt?, Marshall?... Fender?... I can't think of anybody that did... (ok... Musicman did)  Yet I don't see fewer ampegs with blown output transformers.
On the other hand ,  I see a lot of v4swith blown flyback diodes.

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#22 2012-06-15 14:46:26

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

I thought I said "some cases" and not "all cases."  Oh wait, I did.

Pardon my ignorance...and inferiority.

Maybe you can explain all the reasons why flyback diodes fail, and explain that their utilization protects nothing at all, ever.


Matthew

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#23 2012-06-15 15:16:33

sewage666
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From: Oakland, CA
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Posts: 79
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Re: Internal fuse blow before external?

http://img.timeinc.net/time/2007/la_riots/king_rodney.jpg

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#24 2012-06-15 15:42:11

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

Just to offer a perspective outside of my own (which is purely from what I've been told from other people I respect as well - I'm far from anything but a novice to think for my own on this subject), I just now asked Merlin, this guy, for his take:

http://www.freewebs.com/valvewizard/links.html

Me: "Merlin,

whats your perspective on using flyback diodes on the output transformer of a push pull high powered amp?"

Merlin: "They're a good idea as far as I'm concerned!"

Me: "Would you say that they sometimes protect an output transformer from blowing?"

Merlin: 'Well, theoretically they should work. Of course, it's hard to be sure if they're really doing anything useful, just like the rock that keeps tigers away, but they don't hurt, so its cheap insurance."

Just a third voice in the mix. 
Resultant conclusion: I personally don't think always or never are good policies here.


Matthew

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#25 2012-06-15 15:47:58

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Internal fuse blow before external?

Maybe we should upgrade them to fuses!   =^)


Matthew

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