Help! My amp's dying.

mart

Anybody know a good amp tech in the Swansea area? My amp seems to be dying.

It's a Hughes & Kettner Edition tube 25th anniversary 40W 1x12 combo. I leant it to a friend for a few months, and just got it back on Saturday. As soon as I started playing it something wasn't right. At low volumes it was ok. But when I turned the volume up, even just half way, it started producing a nasty distortion as if there were a tear in the speaker cone or a loose bolt rattling badly. 

When I first heard that I took the back off and tightened everything in sight, and checked that the valves were seated properly. Putting it back together it was the same, except sometimes I got an octave-doubling sound - just like having an octave pedal running. This seems to have stopped now, but I've still got the distortion at anything above low volumes.

And it's especially bad at certain frequencies - a low G triad sets it off very badly. And on the lead channel if I play that triad, the distortion settles into something like feedback noise, and this keeps going even when I put the amp volume down to zero. So there's obviously some sort of instability going on. 

I guess it has to be a tech job, but I'd be interested if anyone can diagnose it on here.

(Edited title in a shameless bid to try and get some response )

dandl

Hi Mate,

I'll PM you the number of the guy all the local shops use. He's very good, but not cheap!

mart

Thanks mate.

ICBM

Sounds like it might be a knackered power valve.

mart

ICBM said:

Sounds like it might be a knackered power valve.

Thanks. Can I just drop in a new one, or do I need a tech to re-bias things? Sorry, a bit clueless about this stuff.

ICBM

Ideally you should fit a new pair and check the bias, and adjust if necessary. If you want to troubleshoot it first, it's safe to change just one power valve and see if the problem goes away - then change the other one (putting the first original valve back) to confirm. Leave the new valves in and keep the good old one as a spare (assuming both aren't knackered, which is not very likely). Adjusting the bias means going inside the chassis on this amp I think, so best left to a tech if you're not sure what you're doing. It's safe enough to fit new valves and not check the bias immediately, but for the best valve life it's generally better to when you can.

mart

Many thanks! I'll try out a replacement valve before giving in and handing it over to a pro.

As it happens, on this amp you can't even <i>get</I> at the valves without taking the back off and extracting the chassis from the cab, so probably the biassing point(s) would be accessible, but I wouldn't know what to do with them anyway.

ICBM

Oh... another Euro-nanny "no user serviceable parts inside" valve amp...!

I'm sure there will come a time when you're not even allowed to change the strings on your own guitar in case you cut yourself.

mart

Yep, fraid so.

Phil_aka_Pip

ICBM said:

Oh... another Euro-nanny "no user serviceable parts inside" valve amp...!

wasn't the AC30 one of the first of those?

ICBM

Phil_aka_Pip said:

ICBM said:

Oh... another Euro-nanny "no user serviceable parts inside" valve amp...!

wasn't the AC30 one of the first of those?

No, they're easy - four screws (or just a couple if you don't want to bother with all of them...) and the entire chassis slides out along the side rails.

If only the designers of the last few series had looked at how it was supposed to be done, rather than copying the look but not the function! They're a complete pain to get into.

mart

Finally got hold of a pair of new EL84s to put in and ... It's still not right. There's very little headroom on the clean channel - even at 10 o'clock it's breaking up. And there's a hint of that octave-doubling sound I was hearing before.

It mostly sounds not bad, if you like a slightly-breaking-up sound. But I want to be able to get lovely cleans at reasonable volume, like I used to be able to.

ICBM

Try changing the phase inverter as well. It's not a certainty but the sound you're describing does sound like what happens when you lose one side of the output waveform, which is why I thought output valves first. A phase inverter that's dead on one side will do the same thing. After that, it sound more like a component failure in the circuit - an earlier valve in the preamp would generally just cause silence or very poor sound, rather than early or odd-sounding breakup.

The silver lining is that if that doesn't fix it, you do at least have the spare set of valves you need (2x power valves, 1x preamp) for a 2-EL84 amp.

Travisthedog

Is it as loud as before or overall quieter?

mart

Thanks. So, once I've figured out which of the two 12AX7s is thePI, I can just drop a replacement in like I did with the EL84s? 

It's hard to be completely sure because I hadn't used it for some months before this trouble happened, but I think it is as loud as it ever was, just with much less clean headroom.

ICBM

mart said:

Thanks. So, once I've figured out which of the two 12AX7s is thePI, I can just drop a replacement in like I did with the EL84s?

Yes. It will usually be the one closest to the power valves or furthest from the input jack, or both.

If you really can't be sure, just change one and try it, then the other if that doesn't fix it - it can't do any harm.

mart said:

It's hard to be completely sure because I hadn't used it for some months before this trouble happened, but I think it is as loud as it ever was, just with much less clean headroom.

With one side of the output waveform missing it will seem to work OK and be as loud as before at low volume, at medium volume it will seem as loud as before but have less clean headroom and nasty distortion, and at high volume it will be much quieter and have less nasty (but possibly more squashed-sounding) distortion.

The problem is that even if it's definitely the problem, there are other things than valves which can cause it, but it's always worth trying them first.

mart

Thanks. It certainly sounds ok at low volumes, and only when you turn the volume up more is there any sign of a problem. I've not tried it at anything like full volume because it's already too loud in the house. 

So, yes, it does fit with your description of half the waveform missing. Cheers!

Travisthedog

Screen grid resistors? If it has any???

mart

Er, not sure! I'll try replacing the PI first, and then see if I can find a schematic and work out if it has screen grid resistors. Ta.

mart

Finally got a new 12AX7 to drop in, so I opened it up and ... struggled to figure out which place to put it, because the preamp valve nearest the power amp valves was also the one nearest the input. 

But when I got closer, I noticed that one of the new power amp valves I'd put in was showing signs of discolouration on the label, as if it had been getting overly hot. As you can see in the photo, the logo is looking slightly sizzled on one tube, while on the other it is pristine. Is this natural? 

http://i327.photobucket.com/albums/k455/mart206/Wiring diagrams/IMG_0785_zps4ba3cc8f.jpg

I couldn't see anything else obviously wrong, but here's a pic of the board, just in case I'm missing something.

http://i327.photobucket.com/albums/k455/mart206/Wiring diagrams/IMG_0783_zps69fc3d91.jpg

ICBM

No, it's not natural. That indicates that probably the bias is very wrong on that power valve. That would also cause a highly asymetrical output waveform. It could be something as simple as a cold joint not feeding the bias voltage to the valve, or a faulty component.

At this point I would take it to a tech - it doesn't look like a particularly easy amp to work on, or even troubleshoot - the layout is (as you've realised) somewhat tricky. I think the phase inverter is the lower valve in the pic, but I'm not actually 100% sure!

Chances are that if you haven't been running it like that for too long, the cooked valve will have survived, although its life expectancy has probably been a bit shortened.