Fuse blown - replacement?

HI all,

I just bought a Grandmeister Deluxe 40 and after a couple days of playing, the amp started making some popping and crackling noises until one last loud sound and then no sound at all.
All the lights on the TSC were lit. I researched about this issue and after opening the amp I discovered a blown 400mA fuse.

My question, where can I get a 400mA 250V glass fuse? Everywhere I look they only have either 200mA, 250mA or all the way to 500mA.
Can anybody confirm if this is a slow blow or a fast blow? Unfortunately, the manual doesn't say.

From the 400mA current I assume you are in the UK as that is what is needed for 230/240V.  The GMD40 should have slow blow fuses in it.  They are both shown as "T" types which is short for "time delay" or slow blow.  The mains fuse is a 250V T800mA as shown in the user manual spec page.  The manual does not rate the smaller HT fuse as it is supposed to be only a "service technician" replacement and there is a good reason for that.  The HT fuse should only blow in response to a fault deep in the circuitry and just replacing it without addressing what caused it to blow would not be "productive".  (That could be read as "safe" in terms of both to yourself and further damage to the amp.)

The GM40D schematics are not available in the wild (yet) so the best we can do is to go from the GM36 which are available.  That clearly shows a 250V T400mA fuse for the HT just as you have found so the value looks correct.  You MUST stick to the designation and not be tempted to put in a higher value.  At the very least it would void any warranty.  In the UK you can get these from suppliers like RS Components, a really comprehensive trade supplier with retail units in a number of places.  You need an account with them to buy through the post and that is difficult if you are just a member of the public.  However, you can just get fuses through ebay.  Have a look here: ebay fuses  That's not overly expensive and it will come through in a couple of days.

Now to the other issue which is also relevant, perhaps more so than the fuse.  Why did this happen?  You have the classic symptoms of a valve going down.  The popping/crackling sound is often a feature of a valve going out and to take out the HT fuse it is most likely an output valve, the EL84s.  If you have just bought this amp brand new I would seriously put the fuse back, carefully replace the bottom panel and take it back to the shop where you bought it.  You need to preserve the warranty and that means NOT opening up the unit yourself.  The valves themselves will most likely not be replaced free under warranty.  If it is out of warranty then you can continue and fix it yourself as it is already open but do take care as there are high voltages in there and they hang around a long long long time!

The original valves going out quickly is relatively common.  Most electronic component failure is in the very short term.  Once you get them past a settling in period they usually then last for a very long time.  H&K and many other amp manufacturers use a cheaply sourced Chinese valve type as stock.  Their reasoning is that most people now believe a lot of hype about valve makes and will swap out the valves for their own preferred magic brand almost immediately no matter what is put in there by the makers.  It makes no sense to put in a set of valves which cost more, put the price to the customer up a little, and which will likely be thrown out at the first click on a guitar forum button.  That makes engineering and marketing sense despite what every critic will say.

There is actually no real problem with the Chinese valves.  Many people prefer their sound, (in truth it's only very subtly different if you can detect it at all), but they are scared off from saying so by the flak that admitting that brings from other "golden eared" types who know better.  This has been so long standing now that everyone just "knows" that valve selection is a key issue in your amp sounding amazing or c**p!  I call good old British "bollocks" on that and I now have the engineering proof to show it.  This is from a technical type who actually set out to prove there ARE major differences between types.  His published results from a well designed and implemented test setup show how stupid that argument actually is, despite his attempt to frig the display of the results to try to prove a difference!  There is actually a good reliability argument for using a Chinese type over the others in one of the slots in the amp which is set up as a Direct Coupled Cathode Follower but that is a point for the tech types to discuss really.

So the advice is...  If it is under warranty, put it back together and take it back for the shop to fix the fuse problem and replace the faulty valves for you (which is technically a warranty requirement too though you would be fine if you follow the replacement process to the letter).  If it is not under warranty, replace the fuse and the valves yourself.  If it is at all possible I would first get your hands on a spare set of known good EL84s and a known good 12AX7 and replace the valves in turn to prove there is one which is faulty.  You can expect to possibly take out a fuse or two doing this. As I said, it is highly likely it is a power amp EL84.  If that is the case you must replace at least that pair with matched ones to keep the balance in your output stage.  It will not have any real impact to replace these with any make you like but leave the other Chinese pair in.  Of course you could bite the bullet and buy a full set of matched ones of any make you decide on.  For myself and 95% of the other members here JJs are great.  They are cheap, reliable, good sounding workhorses which can be found virtually anywhere.  Or you could take out a mortgage on that set of NOS pixie dust and leprechaun rainbow beam Muggards from the 50s?