Bios after Changing Tubes

Please can someone help me?

I have a GM36 that i've had for 6 years. I'd never had any problems with it until September last year when 2 of the Tubes died.

I replaced them but it still didn't work. After searching for weeks for anyone that could potentially fix it I found a guy about 2 hours drive away from me.

He fixed it and said that the Bios needed setting up.

Everything has worked fine since..... until Sunday when 2 tubes have gone again.

I don't really want to have to take it back to the same guy, as he only works Monday-Friday (same as me) so I would have to take a day off to take it down there and then another to pick it up.

I can buy the tubes and fit them myself, that is no problem, but I cannot find any information on Google or Youtube about the Bios settings.

Can anyone help me with an explanation of what I need to do please?

Any help would be very much appreciated.

Thank you

Hi Maiden_London, welcome to the place!  I have been on this site and a GM36 user for many years now.  I have no idea what he is talking about when he says BIOS, and I programmed to a high level in my day job for many years!  Are you sure that was the term he used?  Now, in the computer world BIOS stands for Basic Input Output System.  It is the absolute lowest level of programming a PC uses to start up.  It gives it the ability to access its memory and read data from its hard drive.  It can then load a normal operating system like Windows which gives the machine the ability to call up other programmes and do complex tasks.  I wonder if he is using the term as a way of saying that the software in your GM36 had become corrupted and he applied a factory reset to set it up again from scratch?  That is a possibility and you could start with a factory reset yourself.

DANGER WILL ROBINSON!!!  It will wipe out all of your presets and other settings - forever!  If you have presets that you use and cherish you should take copies of them before you do a reset.  That could be by saving them via the app, or it could just be by writing down all of the control settings by hand.  If you have no app and MIDI interface, manually copying each one will take time but it is the only way to prevent you from losing them.  Do you use the app in any way?

A factory reset, should you choose to use it, is performed by holding down the [Store] and [FX Access] buttons together while you switch on the GM36 with its Power/On switch.  The button LEDs will flash three times to indicate success in the reset.  Remember, don't do this if you have presets you wish to save and you have not copied them in some way.  They will be lost into the pit of unholy tone for all time!

Now tell us, which two valves are the ones which have recently gone faulty?  How do you know they have gone faulty?  Is there something you can see which is wrong?  You have preamp valves V1 under the aluminium can, V2 in the middle, and V3 on the other end to the can.  You then also have four output valves in a line behind them acting as two pairs.  The outer pair is on all of the time and the inner pair only when in 36W mode.  I would have thought it would be most likely the outer pair of output valves, but that's only me gambling with myself.  If you are constantly using the amp in 18W mode or lower then the inner pair will never be used and should stay good.  It may also be that he replaced say the outer pair but put them in as the inner pair and swapped the old inner pair to the outer slots.  This could mean that the pair which have now gone pop are the original partners to the ones which died last time.  Anyway, I'm just thinking out loud here, in reality I'm guessing.  Can you give us any info regarding which were changed and which have now died on you?

Are you able to call this guy and find out what he meant by the BIOS?  The amp does have microprocessors inside it controlling everything but strictly speaking it doesn't have a BIOS, it simply has firmware which is the very basic programme which controls all of the amp's functions.  That is a very ultra simple task for a microprocessor, (kind of the equivalent of Rachel Riley's Oxford 1st Maths degree being used in Countdown    which I'm sure you will get being London linked!)

Thank you for your responses.

When switching the amp on (but in standby) all 4 LEDs stay on.

When switching the standby off then the two middle LEDs stay on and there is no sound coming from the amp. So the inner two tubes have blown.

At the time of this happening I was connecting the amp (using the Red Box) to my mixing desk for recording but no sound is coming out.

When I remove the connection from the Red Box and plug the amp back into my cabinet there is still no sound.

I've just checked the Manual (again) and it's BIAS, not Bios (apologies for the misspelling).

Thank you

Aha!  That makes perfect sense.  And no need to apologise for being unfamiliar with technical terms.  When my wife talks to me about her sewing I haven't a clue what she is saying.  We aren't all tech types.

"Bias" makes perfect sense.  This is the small amount of current the valves are set up to pass when they are just idling with no signal.  It's kind of like getting them ready to go immediately a signal comes along.  Without it there would be a dead zone as the output stage swaps from one valve which amplifies the positive part of the signal to the other which amplifies the negative part.  It causes really severe and rough distortion.  Your H&K amp has automatic bias, a small microprocessor constantly measures and adjusts it for you.  Other amps need you to go inside and adjust it manually with a meter to set it up correctly.

Ok, now we know that we can go into things a little more.  As you know, one of the red TSC (Tube Safety Control) LEDs on means that valve is not currently working.  That does not necessarily mean it is damaged, (though it could be).  Is the power soak set on the 36W setting with all 4 of its blue LEDs off?  If it is set on any of the other power settings, (i.e. if any of its blue LEDs are lit up to show the maximum power available), then the inner pair of red TSC LEDs WILL be lit up, and that is correct.  For 1W 5W or 18W settings on the power soak the amp itself switches off the inner pair of valves in order to help reduce the amp's power.  Can you confirm that you have the power soak set on the full 36W with all of its blue LEDs off?  We must have it in that state before we know what is happening.  If any blue LED is lit up then simply press that switch again to put the amp back into full 36W mode.

In the meantime try pressing the TSC switch on the back with your pick.  The LEDs should then flash.  Count the flashes for each valve, that means four counts to take and report on.  That will tell us the state of the valves.



EDIT:  Oh, and here is a tip.  When you are using the Redbox for silent playing, don't remove the plug from your cabinet!!!  Just press Silent on the Power Soak on the back.  That's the switch with the crossed speaker symbol.  The amp is then still left running with its internal load.  When you want to play out again just press the power soak silent switch again and it will revert back to 36W mode.  It isn't a good thing to use any valve amp without a load on its output.  The GM36 should immediately detect that condition and automatically switch to silent mode but there will inevitably be a slight delay while it does this.  It is just possible you may have blown the HT fuse inside the amp and the valves have no power.  If that is the case you should not get output from the Redbox or any other area of the amp.

Ok.  I hope you have had a bit of time to take in the last post.  I have a few more simple tests you can perform at home.  When I say silent during this post I mean not a scooby do!  No background hum or hiss at all.  Nothing to hear even with your ear pressed against the speaker.  Check the amp and see if there is any sound coming from the speakers at all, no matter how slight, with the amp turned on, turned up and no lead plugged in.  This cannot be checked via the Redbox as that adds in other gear outside of the GM36 which will add its own hum and hiss.

You say you use the Redbox out and that is currently silent.  Can you set things up as you have done the Redbox but using the output from the Preamp Out?  That uses a standard jack/jack lead of course.  I would guess that is also silent.  Now check the Fx Loop Send socket in exactly the same way.  I would guess that is also silent.  Now the killer!  Put an ordinary guitar lead into the Fx Loop Return socket and either just touch the end of it with your finger to generate the buzz you get or play your guitar through it.  That is going direct into the power amp section and missing out all of the preamp so any sound will be very reduced in volume and would sound flat.  I'm guessing that will also be silent.

If I am right in all of that I reckon you have blown your HT fuse and the supply voltage to every single valve is now out.  Do not confuse this with the valves lighting up, they will still heat up and glow!  That is no sign that the valve is working correctly or even that it can work.  Also don't assume this would turn off all of the LEDs and indicators and the lighting, they are on different low voltage supplies to the HT with their own fuses.  It isn't a good thing for a valve to be run for long periods with the heaters up to temperature but no current running through it so don't leave it powered up for long periods at the moment.  Running a few tests for a half hour or so will not do it any harm though.

For anyone interested that is why you should NOT use your classic Standby switch on older style amps.  It is precisely what that design of Standby does, it simply removes the HT from the output valves.  This makes the amp go silent but leaves all the valves heated up and ready to go as soon as the HT is reapplied.  And in the meantime they are being killed by cathode poisoning!  When a valve is left heated it is kicking out electrons from the cathode as it does in normal working.  With HT applied this means the electrons (-ve charge) are attracted and swept off to the anode (the +ve terminal).  When there is no HT applied they cannot be swept away and they stay built up around the cathode in a cloud.  This makes a microscopic layer of resistive material (interface resistance) grow in the area where the cathode metal base and its oxide coating meet.  That means an unwanted resistance grows in the cathode circuit which cannot be removed and which slowly strangles the valve action.  Even Sweetwater (<- ) has come to agree that this is a real issue!  (Oh, and as for the Standby, why not just turn your Master Volume fully down?    )  For the record H&K use a completely different and safe way of applying their Standby.  That is why you can often still hear a very slight signal even with the Standby applied.  People often complain about that as if it were a massive problem!  

Can you check that out and if I am right I can talk to you about the HT fuse and changing it out.

Bordenbert, thank you so much for your information. There was a lot to take in, but much more information than I can find on the net.

Thank you for letting me know about keeping a slcabinet plugged in but set the Powersoak to silent. I've got a feeling that this is what happened last time, as it was as I was about to start recording, so I have possibly blown something (both times) by unplugging the cabinet and plugging in the Red Box.

Please see the following:
With the amp on standby all LEDs are on.
In 1w, 5w and 18w the two middle LEDs are on.
In 36w all LEDs are off.

There is no sound from Line Out, Red Box, Cabinet or FX loop.

When switching the amp on just now to check this, all Valves have a glowing light on them, except the middle two are not as bright. I've never noticed this before so don't know if it's because of this issue or if it always like this.

Well done, all tests performed and results clear and unambiguous.  Did you check the Fx Return with a cable and touching the tip?  That won't give you a shock at all, it's only signal line stuff, you just need to know if the power amp valves are powered, (I'm sure they are not but it's best to test if you can).  The fact that the middle two valves don't glow as brightly is not important.  It's very common to find that, and it's a valve by valve thing nothing to do with manufacturers.  As long as they all glow somehow then their heater voltages are fine.  I do reckon this is down to your HT fuse blowing and I would next recommend just checking that out.  So you have a decision to make.  The power supply fuses are on a PCB incide the amp and are not the ones you can access from outside the case, you have to open up the amp to get to them.  Do you want to go ahead and check and if necessary change the fuse if it is blown?  It's nothing more than that!  Assuming you decide to do that, (read this bit before finally making your decision as it will tell you how difficult the job is), let's go ahead.

Now its the HT!  That's almost 400V!!!  That can kill and I do mean kill.  Keep that in mind but don't worry, to change this yourself is a perfectly feasible thing to do and it could get you into understanding your amp a lot better than you do at the moment.  Can you change a fuse in a mains plug confidently?  This is exactly the same task except it is a slightly smaller fuse and fitted on one of the PCBs inside your amp.  Safety first always so you ABSOLUTELY MUST do the following before you go any further:

1. Switch off and unplug the amp from the mains completely.  Switching off alone is not enough.  Take the mains cable out of the back of the amp and there can be no "surprises" in store.

2. Wait at least 5 minutes before going any further, having a cup of tea and watching something on the TV after the amp has been disconnected is great if overkill.  I like overkill in safety matters!  This is to allow the voltages in the amp to drain down to 0V.  The capacitors (caps) store the charge fed to them by the mains transformer and allow the circuitry to tap in and use it in any way it wants to when it wants to.  Think of charge like the water in a bucket.  When water flows in a stream it is called a current.  Hence an electric current is just electric charge being moved through wires or circuitry.  When you switch off, the circuitry runs down and stops taking out charge.  It is possible that some of this charge and hence the voltage can hang around for a long time.  This can sometimes be at very high voltages.  The H&K amps have resistors placed across the power supply capacitors to continue to gradually leak away all of this charge gradually pulling the voltages right down to 0V.  That must be given time to complete.

Then and only then can you get to the fuse.  You simply take off the bottom cover of the amp, you do not need to touch the top cover at all for this work.  Turn it over and there will be a series of 10 small screws underneath the amp which you can see are holding the bottom grille panel on.  If you remove these you will then see that the bottom panel slides sideways a little to release at each end.  If it is very tight I think you may sometimes have to loosen the end cheeks a couple of turns for a touch of slack in there.

Once it is off you can see a bewildering array of PCBs and components in front of you.  Don't be too put off by that, you only need to identify a fuse and you should be able to recognise that.  I have added a link to a picture which shows the bottom of the amp with the cover off.  GM36 Interior Lower Cavity The location of the two fuses is ringed in red.  They are tucked under the top PCB, sitting in holders on the lower PCB so it isn't a simple pull to get them out.  As long as you have left time for the voltage to leak away there will be no risk involved.  Gently lever the fuse out using something sensible and insulated to your hand if possible, (best to still take precautions wherever you can).  Pop a new fuse in there and you're good to go.

The fuse you need is the less accessible one which is tucked further under the top PCB, not the one which is easier to access.  You may be able to read its designator, FU1, on the PCB silkscreen, the easier one is FU2.  Your FU1 is a 20mm T400mA fuse, (that's 20millimetre size and 400milliamp slow blow) and it MUST be replaced with only a T400mA fuse, near is NOT good enough.  You will need to source these and ebay is littered with them.  A box of 10 is about £3 here in the UK, (probably cheaper in the US).  I always keep a box of them in my gig toolkit as they are always lifesavers when things go sour.  If you have a box of them it makes sense to just change it for a new one and have done with it.

Look at the old fuse.  Is it obviously blown?  Blackened glass, broken wire, balls of melted metal, these are all clear signs.  Unfortunately fuses don't always blow like that.  They often break inside the end caps where you can't see it.  They can even break in the wire and make contact until they heat up in use then droop apart, coming back together when they cool down.  Swapping in a known good new fuse is solid!  And if we can see the fuse is definitely blown then that helps too as we know this was the problem.

Have a good think about this and let me know what you decide.  You have enough to see what is involved and you know how confident you feel to do this.  In truth it sounds a lot more complicated than it really is and its very safe as long as you follow the two rules I set up.

bordonbert, I've tried the FX Return and there IS a signal coming from that, but nothing else.

I've had a look on ebay and they have the T400ma Fuses as you have mentioned. However, they have "Slow Blow", "Quick Blow", "Time Delay" or "Antisurge". Would it be the "Slow Blow"??

I think (unless the fact that there is a live signal coming from the FX Return means something different) I will just buy some fuses and give that a try first.

I'm away next week, so by the time the fuses arrive and I get a chance to change it, then it will probably be the following weekend until I can let you know how I get on.

Thank you so much for going out of your way to help me, and especially with so much information.

Now, we have a problem with that first bit of info re the Fx Return test so we had better clear that up.  What you should have done with the Fx Return is to put a guitar cable into it to feed a signal in, not take a signal out from there.  When you touch the bare tip at the other end with your finger you should hear a loud buzz from the speakers as the signal is being fed into the Fx Return and on to the power amp.  That would show whether the power amp is working or not.  The Fx Return sits just before the power amp input.  Is that what you meant, that you could hear a noise from the speakers with a signal going into the Return?  I don't want you to open the amp up and have to do the fuse swap if the HT is actually there and there is no point to it.  If the power amp is working that would mean there is HT.

For the fuses, I would go for Time Delay or Slow Blow.  They should be labelled T400mA250V stamped into one of their end caps.  Higher voltages are fine but the current must be exactly 400mA and they should be designated "T".  The Time Delay and Slow Blow might have have slightly different characteristic curves but basically do the same job.  They stop the fuse from blowing when a short transient happens.  Definitely not Quick Blow which would blow as fast as possible once its rated current has been passed so it would not be resistant to blowing on a non-destructive transient.

I put a Guitar cable into FX Return, I switched the amp on and turned the volume up, tapped the other end of the cable and can hear the taps/buzzing from when my finger is touching the cable.

Would this suggest that the HT fuse is OK? And that it is the tubes? Or could this be something else.

I believe it is because of (like you mentioned) when I was plugging in the Red Box I had unplugged the cabinet and this has caused some kind of shortage.

If you can hear buzzing then the HT fuse is ok. The Fx Return socket is directly before the power amp input, The HT fuse must be ok for the power amp to be working to produce your noise. It also means that the output valves and V3 themselves are working as they make up the power amp circuit. Damn! That removes the idea of an easy fix with just a fuse swap. Ok, we have to move on and look for something else. There is no point in opening up the amp yet.

If you had a couple of valves changed last time that does suggest it may be down to a valve problem again, but what is worrying is that it has happened only a year after replacing them. Do you have any spare valves to swap out, or maybe another amp which you could borrow a valve or two from temporarily? We are only talking about V1 and V2 here, the one under the aluminium can and the one next to it in the middle. The easy way to do this is to check with a different valve by swapping it for one of the suspect ones in turn. You must put each valve back in its original place after checking. Sometimes V1 is a specially selected valve for low noise, and V3 can be specially selected for close balance between its two halves. Try not to mix them up, it won't cause any problem but you will lose that benefit.

One other way to check this if you don't have a spare is a little more involved but easily doable if you get the logic of it. We know that V3 is working because the power amp works. If we test by putting the other valves in the V3 slot and it stops working we know we have a bad valve in there.

Start by putting put your cable back into the Fx Return socket and plug your guitar into it. We want to hear if there is sound but also if it is heavily distorted this time. Then carefully pull V3 out of its slot. Wiggling it a little as you pull helps but be careful not to bend it too far to the sides in order to not bend the pins. If you now remove V1 and put it into the V3 slot you can turn the amp on and test it through the Fx Return. If you hear a quiet but plain sounding signal out of the speakers then V1 is ok. Take it out and refit it to its original V1 slot. Now take V2 out and put it in the V3 slot in the same way. If you can hear yourself play and the sound is without obvious distortion then you know that V2 is good. Put V2 back in its original slot and replace V3. Don't forget to refit the can onto V1.

That way you should be able to identify a bad preamp valve. I'm hoping you will be able to find or borrow a different valve or two to simply swap in and test. Remember, we have already shown that V3 and all of the output valves are ok by getting sound from a cable in the Fx Return socket so this is only V1 and V2 we are now looking at.

Thank you, I don't have any spare valves so I'm going to try as you suggested and swap them around.

If this doesn't work then I will buy 2 more valves when I get paid at the end of the month.

My concern though is, last time I changed them out I still didn't have any sound from the amp, which is why I had to take it to the guy I mentioned in my original post, where he had to adjust the Bias. If I replace the valves and still don't get any sound then would I have to take it back to him (which is what I'm trying to avoid, as it's an extra large expense) or can I adjust the Bias myself?

Thank you once again for your help.

I didn't want to say anything about it earlier as it was irrelevant to fixing the problem, but there is something going on with what he told you which is not right.  He didn't adjust the bias at all, there is no bias adjusting mechanism on the GM36, it has a microprocessor controlled automatic  bias setting system.  The amp itself reads the bias setting continuously and adjusts it to the optimum level for your valves.  And that is only for the output valves, the four big ones.  The other three smaller valves have their bias set by the other components around them and that is always fixed.  Adjusting the bias of any valves in the amp is absolutely not necessary in any way.

The output valves seem to be working fine.  You proved that by actually playing into the Fx Return.  That relies on only V3 and the four output valves.  If you got clean undistorted sound out under that test then the whole power amp is working.  It seems we are looking at some problem around the V1 and V2 preamp section.  Like I said, the bias of the preamp valves is fixed at a high level by fixed value resistors and it can't drift or be tweaked.  No amplifier ever has a problem with its preamp valve bias setting.

I'm hoping your tech guy is just simplifying what he is telling you for you to be able to understand.  It may be that the auto-bias circuitry had blown, it occasionally does, and he needed to replace the transistors that control each valve.  Let's not worry about this yet but at the moment I would suggest you don't go back to him until you know what he actually did sort out.  We could do with a little communication on that.  Can you email him and ask him what was wrong last time so you can get him to write it down and you can get his own words clearly?  I know it is very difficult remembering technical stuff people tell you when you are not a tech yourself and the words mean nothing to you.

Let's just check out those two other valves by swapping them and see what comes up.  From your username, are you London based?

I've just got home from work and thought I would check the Guitar through the FX Return first.

There is sound, but it's not distorted, it's crystal clear and at a normal volume.

I haven't taken it apart yet to try the valves.

If I read it correctly, and looking from the front: V1 is on the left, V2 is in the middle and V3 is under the metal cap?

When I remove V3 and put V1 / V2 in it's place am I leaving V3 out totally? or putting it in the other's place?

Yes, South London (well, Surrey but within London's Carpark - the M25)

First point, yes you have proven that the V3 and the output valves are all good. Well done. We know we can substitute a different valve for V3 and check it out in the same way.

Yes, I think I'm not sure but I may have got it wrong and told you the other way around and you have corrected that. Looking from the front the preamp valves are the line in front with the 4 bigger output valves behind them. V1 is under the metal cap on the right, V2 in the middle and V3 is on the left. (My apologies, It's hard to keep things right in your head when you are looking at a structural drawing of the PCB which is in isolation, upside down and who knows which way around. )

That's a really good question. It should be unnecessary to put V3 into the V1/V2 slot. For the test to give us results you must check only the power amp which we know is working. The input must be your guitar through a standard cable directly into the Fx Return socket. That way the only circuitry which you will be using is the power amp which we now know should work with a good valve. The Fx Return socket comes after all of the preamp circuitry so that will not be involved. If the power amp works then the substituted valve is good. If the power amp then stops working or it distorts badly then the substitute valve is bad.

The only reason I asked about your location is that I'm not a million miles away from you between Guildford and Portsmouth. (You were right to not be specific in open forum, best keep it a bit vague.) If we had been a little closer we could have met up and I could have given you an older but working valve or two to test with. Mind you, I do have a friend from the Croydon area who I will be seeing tomorrow morning. Perhaps I could give them to him and you could pick them up from his work place?)

Anyway, do the tests and they should tell you whether those two 12AX7 valves are good or not.

Good afternoon,

I've tested these valves and it seems to be the one on the left (when looking from the front) as there is no sound whatsoever when that one is put in the place of the one on the right (under the Metal can).

As everything else that you told me to check seems to work ok and this one doesn't then I will order a new one and replace it when I get paid at the end of the month and see if that solves it.

That's very kind of your offer to lend a tube and you have been been very helpful with all of the information you have given already, especially to someone that you don't know.

I work in Croydon so very familiar with the area. We're on holiday tomorrow for a week, but if you didn't mind letting me borrow a tube to see if that fixes things then I could pick it up from your friend when I got back, test it and take it back to him, if this wouldn't be a convenience? If not, then no problem as I can order one when I get paid.

Thank you

I've seen him this morning and I've set this up.  Don't worry about the valves, I wasn't going to lend you them, I have a lot of used 12AX7s in my kit from replacing them in amps I've bought over the years.  Putting old valves into an amp when it gets sold on is a common way for people to ditch their "on the slide" ones.  You should always take into account the fact that you may have to buy a few valves to get a newly bought amp up to scratch.  I have given him a couple and you can just keep them as spares.  They have got a few hours under their belt though they should still work.  They will be able to tell you if it is definitely and only the valves which are the problem.  It's always a good idea to keep one or two in your tool bag just for this very reason.  Valves can and do go down on you occasionally as you might have seen.  I'll PM you his address and phone number when he has returned home and set things up.  It should be later today.

If these valves have just been swapped out for new ones less that a year ago then there may be something else wrong causing them to go down.  Do you know if these were the same valves that the tech swapped out last time?

Ok, now that we have arranged the valve pickup, on to your testing. V3 is on the left and V1 is under the can on the right when viewed from the front.

From what you have described I think you may have got this wrong. Putting V3 into the V1 slot, that's the left hand one into the right hand slot, will only stop the amp from working. V3 is the power amp phase inverter valve and without a valve in the V3 slot the power amp will stop working no matter what is in the other two slots. That means that your test would have to fail as it is simply missing that valve. We need to use that slot to test the others.

Try the V1 valve into the V3 slot, that's the right hand "capped" valve into the left hand slot. If that works then pu it back into its slot V1 and try the V2 middle slot valve put into the left hand V3 slot. Remember, we know that V3 works because the power amp gives good sound with your guitar into the Fx Return. We want to take out the V3 valve and put one of the others in its place to see if they give good sound in the same way.

If you can get to Mark you should have spare valves at some point. Simply swap my working valves in for both V1 and V2, (the right hand with the cap and the middle one). We know V3 is working so that should show whether your own are duff. If I were you I would get a permanent marker and just write a simple little "1" or "2" or "3" somewhere on the glass of your valves so you don't get them mixed up, (we definitely won't do that, except we have all done it ). There is a long standing and completely nonsensical myth that that is harmful to your valves. It's total bunk!!! They aren't halogen bulbs with a sensitive coating, they are simply thick hardened glass and that isn't harmed in any way by fingerprints, ink or fish shop grease!

So, I've changed both V2 and V1 (under cap) with the valves that you very kindly supplied to me.

Both of these gave the same result:

• No sound when plugged into the input.
• Clean sound through 1w, 5w, 18w and 36w when plugged into the FX Out.
• All lights off when in 36w.
• Middle 2 lights on when in 1w, 5w and 18w.

I was really hoping that changing the valve(s) would sort the problem but it seems not to be the case.

Thanks Gavin.

That is bad news. I had hoped that this might have solved things but it was an unusual set of symptoms to come across for valves to be the answer. (I am assuming you changed both V1 and V2 out at the same time and left V3 in place.)

maiden_london wrote:Clean sound through 1w, 5w, 18w and 36w when plugged into the FX Out.

I hope you meant Fx Return there. Remember, the Fx Out is the socket set up to send signals to a pedal input. It won't work having a signal pushed back in and your guitar shouldn't really have a signal pushed into its circuitry either. However, you say that it is working when you do this so it must surely be a simple naming mistake. Everything that you report about the power amp working and the LED responses is just as it should be. This proves our power supplies, power amp and output power soak are all working as they should. The problem is before that in the circuitry.

Are you able to take the signal from the Fx Out socket and feed it into another amp? That could be into the other amp's input or its Fx Return socket. That would test the preamp section but I'm pretty sure it would only show that that is where we have the problem. You could even feed it into the Aux input of a home music amp just to test it.

It is beginning to look like the problem will need someone to open the amp and do a bit of diagnostic work to see where the problem lies. That isn't something I would recommend you to try. The voltages in there w2ith the amp running are lethal, (yes I do mean they kill), and you have to know what you are doing to get a result and for you and your amp to stay safe. I'll go back into the schematic and look again but I'm afraid there is very little information you can help with from outside the case and there are many possible faults which could cause it.

It keeps getting stranger.........

Throughout this whole time I haven't had the Footswitch plugged in. I thought I would give it a try this evening and the following happens:

When plugged into the main input I can here a very faint distorted sound coming out through the speaker. It doesn't matter which wattage I am on and there is a louder hissing.

When I switch ANY of the channels on the footswitch it immediatley switches to 36w and I can play (through the input) all 4 channels as if there is nothing wrong.

However, there is a loud clunk when I change channels and there is still the hissing.

If I then change the wattage back to 1w, 5w or 18w it goes back to the faint distorted sound. As soon as I hit the footswitch again it switches back to 36w.

I don't know if this has made this issue better or worse?!

It is definitely better in terms of a diagnosis simply because it gives us more information to work with, perhaps not for a fix.  If you can get normal levels of sound under any odd circumstances that means the basic amplifier is working.  Can you just confirm that, when it works as you describe do all of the other controls such as the bass, middle and treble work as they should too?

I wonder if there is something wrong with your Power Soak circuitry.  If you look under the amp into the ventilation grille I think you can see the power soak resistors.  They are very large ones soldered so they are "floating" in notches on the edge of their printed circuit boards.  Can you have a look at them and see if any of them are badly scorched or oddly marked? Have a look at the second picture on this page of the amp opened up. You can easily see the large Power Soak resistors there:

GM36 Power Soak Resistors

If you can't see them through the grille or you aren't sure of what you are seeing we can talk through taking the bottom cover off to just take a couple of pictures and post them.

I've checked through the grill and everything seems as it should do. I may just go ahead and take the bottom panel off anyway to check and post some pictures.

Everything works fine (Bass, Treble, Mid, Reverb etc.), however, the first bank on the footswitch doesn't omit any sound (no matter what I do to the amp), whereas all the other banks seem to work perfectly fine, which is very odd.

Did you perform a factory reset of the amp which I suggested earlier?

This is starting to sound like it may involve the software to be honest.  If your FSM432 is producing odd results too it's not too likely that both the amp and the footswitch would develop faults at the same time.

The power amp is fine, we know that because it works with a signal in the Fx RETURN.  You can get life out of the preamp section on all channels even though it is under weird conditions so the preamp is basically working.  The FSM432 powers up from the amp and the digital control sections basically work with the amp responding to them which shows the power supply is up and running too.  Still something weird is happening.  I would try a factory reset, even if you did it before just to rule it out.  It will purge any presets you have created.  Remember, these are only on the amp, there is nothing stored on the FSM432.

Have you created a lot of presets that you can't afford to lose?

If you have not then perform the reset.  Switch the amp on with its Power/On switch while you hold down the [Store] and [FX Access] buttons together.  The button LEDs will flash three times to show the reset has been completed.

If you have a lot of presets then I would try overwriting the first bank of four presets with the problem, one by one, with new settings.  It doesn't matter what they are, it is only to see if we can purge the problem presets out of the set.

Update:

I've done a reset as you suggested and so far so good.

All banks on the pedal are working. I've saved 4 channels on 2 banks.

All wattages are working.

The amp is working through the input.

The only thing is, when I switch the amp back on it goes straight to 36w. I've never noticed this before, is that the norm??

I'm going to try it again in the morning to make sure that the patches that I've saved are still there and that there are no other problems and I'll send an update.

Thank you for all your help.