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    Anode fuse keep blowing TM18 head.

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    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: Anode fuse keep blowing TM18 head.

    Post by bordonbert on Thu Mar 31, 2016 9:01 am

    Yes, you MUST use a SLOW blow fuse, a fast blow will not stand the surges at switch on and in heavy use.

    Does the fuse blow when you are using the H&K 8ohm 112 cabinet?  H&K say that any impedance between 8 and 16ohm is acceptable but I wonder like you whether the 16ohm Marshall cab is perhaps a little beyond the upper range and the amp doesn't like it.  With a valve amp it is important to get the impedance matched as closely as possible to the correct value for the output stage.  This means less energy will be wasted by being released in the output stage as heat.  The further away from the correct match you get in either direction, the more energy is wasted inside the amp's output stage. H&K are unusual in allowing that wide range of impedances.  Their output stage must be very well engineered to cope with it.

    It may be as simple as the cabinet being too high impedance.  You are right that if you measure it weith a meter you will see a very different value to its stated impedance.  You can't measure the impedance of a speaker correctly without specialist equipment.  A meter will only tell you the DC resistance, and the impedance is made up of much more than that.  It has high inductance and capacitance involved too.  H&K models some of that in its Redbox setup!  If you look at any speaker's impedance as given by the manufacturer you will see it is a complicated curve.  Here is a typical curve I picked up if you are interested.  It explains what makes each region take the value it does:



    You can easily see that the impedance is nothing like a standard value across the frequency spectrum.  He also notes that the quoted impedance is above the DC resistance.

    If your problem is that cabinet, and I think that is a possibility, then a simple solution is to not use it.  If you felt confident you understood enough to get this right then you could consider putting a power resistor in parallel with the speaker to drop its impedance a little.  That is wasteful of a little power however but it would make it a closer match.  I have always said that, if the amp likes 8-16ohm, its design output impedance must be somewhere between the two values.  If you were to put a 33ohm resistor in parallel with the cabinet, that brings its impedance down to just under 11ohm.  You would need a big one but aluminium cased resistors are only a couple of pounds, cheaper if you have a RS account!  Here's an example of the sort of thing that would be best just so you can see what you are dealing with:

    Arcol Power Resistor

    My first line would be to only use the amp with the H&K cabinet for a while and see whether that triggers it.  If it stays ok for a couple of heavy outings then I would say we have the answer.  In their defence, H&K are unusual in giving us this ability to run with a range of impedance and there is a limit how far you can go with that.
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    Fabio65

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    Join date : 2016-03-30

    Re: Anode fuse keep blowing TM18 head.

    Post by Fabio65 on Thu Mar 31, 2016 1:53 pm

    Bordonbert,

    Your post makes sense. I do appreciate your help a lot. Yesterday I've blown the fuse when I was plugged into the Marshall 4x12 and the previous (and first time) I was on the H&K 1x12 but the day after a three hours rehearsal session on the 4x12 (the marshall cab it's not mine, it belongs to the studio where my band do practice, I use it to avoid carrying the 112 cab since I'm already loaded with two pedalboards and two guitars... I'm confident with basic electronics, I've built a few classical tube amps and still I'm building but I can't mod the cabinet of the studio...
    I will put the head on test with its 112 cab for a while and see what will happen... Thanks, thanks a lot indeed! Fabio
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    Fabio65

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    Re: Anode fuse keep blowing TM18 head.

    Post by Fabio65 on Thu Mar 31, 2016 5:09 pm

    Edited... my 1x12 Tubemeister cabinef is 16 ohm as well....
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    bordonbert

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    Re: Anode fuse keep blowing TM18 head.

    Post by bordonbert on Fri Apr 01, 2016 1:34 am

    Sounds like you're getting on great with what's going on so you won;t need too much more from me now. One thing I would point out is that you don't need to modify the cab in any way to fit that power resistor, it can easily be attached outside the cabinet. Just make up a lead with an amp plug, a length of cable to the resistor which just goes straight across it, then continue both of the cable wires on about a foot to the cabinet plug. That way your cab doesn't even know it's there, though we hope your amp will of course and will thank you for it. It isn't particularly high voltage and you would only need to take care it couldn't short out on anything metal nearby. Usually just hanging it down the back of the cab with the cable will be enough.

    I was surprised to find the 1X12 TM cab is 16ohm. Then when I looked I saw that the 212 is 16ohm as well! I had always assumed they were 8ohm. I don't really know why but I find that odd. Maybe unexpected is a better word.

    Anyway, get to grips with testing, you've got a logical outlook so you'll get to the bottom of it. And we're still here if there is anything more ewe can do. Keep us up to date with your progress, it helps other people who may have the same issue. Good luck.
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    Fabio65

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    Re: Anode fuse keep blowing TM18 head.

    Post by Fabio65 on Mon Apr 04, 2016 2:20 am

    Thanks Bordonbert, the trick of the resistor to decrease the cab impedance is interesting, I could
    build a dedicated power cable for this... Thanks a lot!

    Yesterday I've also tried to replace the anode fuse with a new (T type) and this has fried the very
    moment I've switched the amp out from stand-by... Thomann is going to render a new assistance
    service, I do hope they will locate the problem... Thanks again for your help, I'm really scared of
    those pcb based amps with small components and crowded chassis...

    Fabio
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    bordonbert

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    Join date : 2015-01-28
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    Location : Southern England

    Re: Anode fuse keep blowing TM18 head.

    Post by bordonbert on Mon Apr 04, 2016 8:14 am

    You're obviously in a good place with going forward with this now Fabio so I'll not go on about the problem.  However, I will make a comment on what you have said at the end of your post because I think you are not alone in being a bit scared of new technology in the music realm, there are a lot of people out there who feel the same as you.  I've opened a thread in the "Anything else to discuss" area of teh forum for us to discuss this aspect.  Have a look there and see what you think.

    Don't forget to keep us up to date with your progress.  What you find may still be of help to someone else.

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