NAD: It's here! AFD100 in the house!

[ToneMonkey]

Only if you have a green guitar plugged in it

[Twinfan]

Well I've just dropped the AFD100 back at Sounds Great, it'll be heading back to Marshall today.  It seems to be the Power Scaling that's on the blink, with random volume fluctuations when it's dialled in.  Wound out on full power, just using the Master Volume it's REALLY noisy - proper hissing.

I used it at rehearsal last night and at times it sounded really good, but it's definitely a one trick pony.  A great, great rock amp.  Looking forward to getting it back again, and hopefully fixed this time!

[purpleplexi]

Mine is doing exactly the same thing  :?

I plug it in, it sounds great then the volume starts going up and down. At some point it suddenly sounds loud and very shrill.

Took it back to Marshall ( I live reasonably near) they fiddled around with it, replaced some tubes and set it up for me. Plugged in and it sounded great. Took it home. plugged in and it still sounded great. Suddenly the volume starts going up and down again then the tone turns to sh*t.

Tried a different guitar, different leads, different mains lead. No better. Anyway my other amp with the same kit doesn't do it.

Marshall gave me a shipping number and it was sent back by local courier. That was Tuesday, today is Sunday. I found a guy on youtube who had the power control replaced because of the same thing. So that's 3 of us...

I'm not bashing Marshall, they are IT as far as I'm concerned but this is getting annoying. It's not as though it's a cheap amp. My Dad always used to say 'never buy the mark one version of anything'......

[keith]

Wonder if Slash has had any problems!

[Alex]

Maybe Slash doesn't use the power scaling...

[purpleplexi]

I know he has 2 amps running on stage plus a spare.....

[Twinfan]

So that's three of us!  :o

Mine came back from Marshall twice with the same fault they said they couldn't replicate.  I've used it at home a few times since, and at rehearsal, and it's behaved itself perfectly.  There's clearly some sort of intermittent fault, and I think it's related to the biasing procedure.

I'm watching mine for a bit to see how it gets on before sending it back for trip number 3...

[keith]

I know he has 2 amps running on stage plus a spare.....

and probably half a dozen techs after they read the BKP Forum about the problems Dave has been having

[purpleplexi]

My money (not that I have any now....) is on the power scaling pot or circuit. I tried different tubes in mine before I sent it back and I re-biased it several times. Worked perfectly every time.

If I knew the first thing about electronics (which I don't) I'd say the pot or circuit were leaking current somehow. The change in tone in mine is really apparent when it starts playing up. If you've ever had a loose connection on a guitar pot and suddenly the thing sounds really tinny that's the effect I'm getting.

If I get it back with the 'sorry we couldn't replicate the problem' message I'm going to be cross.

[Twinfan]

You've got the same issue as mine dude  :(

I don't think it's the pot.  I spoke with a Marshall tech the first time it went back and the power control is more of a "trigger" for the increments in power, not an infinitely variable pot.  If you look in the manual you'll see the power levels with fixed trigger points.

My guess is that as the amp gets warm the resistance track of the pot changes a little and it ends up on the cusp of a trigger point, dropping to the next point down.  It seems to be when the power is around 8:30 to 9:00 o'clock on mine - same with yours?

[purpleplexi]

Yep. Well to be honest I haven't tried it any louder. I expected to be able to get a hot tone in the house as well as at gigs/rehearsals - that's the reason I chose the amp. I'm not a Slash worshipper - I just want a good reliable hotrod Marshall and this amp ticked all the boxes.

Does the tone change on yours when it plays up - mine goes really ice-picky  :(

[Dmoney]

I'm not bashing Marshall, they are IT as far as I'm concerned but this is getting annoying. It's not as though it's a cheap amp. My Dad always used to say 'never buy the mark one version of anything'......

+1

I remember we all had a chat about the bias technology with relation to the hughes & kettner maybe? I've read and seen information about Orange using it too, and also making a standalone auto-bias unit to retro fit older amps. the DIVO I think.
If this unit is used for the 'power scaling' (as far as I understand the term power scaling is a trademark of London Power and relates to a particular method of power reduction which I think you should pay a fee to use) then that might make sense. I guess the pot is triggering some digital device registering the resistance changes in steps. maybe its something else and not that bias engine.
I'm not sure about the pot getting so hot the resistance changes. Most pots have a pretty big tolerance. +/- 20% for example. I'd imagine the steps in power are arranged in a way that would make it really hard for you to get bang on half way between.

anyway. its all really interesting. sorry about your amps though.

 

[Twinfan]

I'd imagine the steps in power are arranged in a way that would make it really hard for you to get bang on half way between.

I'm not sure it is, as I reckon that's the issue....

[purpleplexi]

When the amp is on you can feel a 'step' as the power control goes from one level to the next so I imagine it's more of a rotary switch than a pot as such.

I still don't think it's to do with the bias thingy because the first time it went wrong I immediately re-biased it to make sure. No difference. Also I'm sure we all know that bias can change quite a bit on a regular amp without making a sudden noticeable difference. Besides if there's a bias problem it's supposed to flash to let you know. Mine hasn't flashed.

I'm sure the voltage in my house varies from in the day when no-one is using much juice to the evening when everyone has the telly and the PCs and lights on. On my other amp (which is a 50watt handwired plexi clone with external bias points) I can hear a subtle difference but it's nothing like this. Anyway reducing the input voltage is a way to get a 'browner' sound - people have been using variacs to achieve this for years. In this case the sound is going thin so I doubt it's a fluctuating voltage issue.

Interesting point though - I don't have a way to measure the voltage coming out of the wall. Anyway the problem is random - day, evening - any old time. I'm not happy to use the amp at a gig though. Which makes the whole thing start to look a bit pointless unless someone can say categorically that the problem is <this> and we've done <this> to fix it.

I go to a local jam where the backline kit is all Fender. If I take this down there and it plays up I can just imagine the jeers from the Fender benders.....

[Twinfan]

It'll be a bit odd if the amp is that sensitive to voltage fluctuations won't it?

I rehearsed with mine last week (in an old mill with what must be cr@p wiring) with no issue, and I'll be taking it again tomorrow night.

Mine hasn't played up for a little while, which is strange...