Wind like noise coming from amp.

[Enthronedoath]

Hi. New member here. I have a Hughes & kettner triamp MK2 which has developed a fault. The amp is with a technician but it's looking like he might not be able to find the problem so I thought I would ask here to see if anyone has any experience with this fault or these amps. So I was playing then the 630ma anode fuse blew. I replaced the fuse. The amp works on all channels but on channel 3a and 3b there is a very loud wind tunnel like noise. I have a video if that helps I can try an post it somewhere. The noise is present with or without guitar connected. All the valves/tubes are ok. Any help would be appreciated as h&k are not answering any emails and it's looking like I might have to have a new main p.c.b or switching p.c.b to solve it. Thanks. I'm in the UK.

[nickb]

H&K will not hand out the schematic which makes things hard. I would start by noting what effect the controls have on the problem. So, for example, if the Ch3 gain affects the noise then you've narrowed it to before that control. This hopefully will reduce the scope sufficiently to make the problem easier to solve.

[Enthronedoath]

I know. I've tried to get hold of one but to no avail. The only one I can find is the triamp mk1 but I don't think it's the same. The controls don't have any effect on the sound only the volume. Which makes it louder. Even the technician said it's a pain.

[nickb]

Well that's narrowed it to between the CH3 gain and volume. There's probably one or two 12AX7/ECC83 tubes associated with that section so start pulling them to find out which one(s). Once you know that your tech can use various techniques to narrow down further e.g. he could put a scope on the grid of each one, he could try putting a 47nF capacitors from grid to ground of each one or he could use another amplifier to trace the noise on the grids of those tubes. That should narrow it to just one stage and he can then concentrate on just that bit.

BTW, you have tried the easy thing and swapped tubes, right?

[Enthronedoath]

As far as I know I'm sure he's checked all the valves/tubes. When I had it I swapped the four preamp tubes associated with channels 2 & 3 and it still did it. And I also tried the el34s. I'm not very technical but he's replacing some resistors which are on the preamp sockets or next to them. He said sometimes they arc across or something. By all accounts he's great working on amps and he's fixed my other amp. I think he's kind of saying to me that he could carry on replacing parts or just get a new p.c.b. now i know the p.c.b will probably cost a fortune but I really like the amp. I've got a video I could send if it might help. I know I'm clutching at straws but hopefully he'll ring up and say its fixed and not cost loads. 😂

[Gtr0]

I just had the same problem. I started pulling preamp tubes until the noise stopped. From there I was able to narrow down to the two stages that were culprits. Of course first I replaced that tube and that was the issue in this case... but from there I would start looking for, or simply retouching, possible solder joint problems. If that wouldn't work, I would try scoping it... but sometimes these types of issues disappear momentarily when you jam some signal through it... in which case I'd really concentrate on those solder joints.

Pulling preamp tubes 1 by 1 *should* offer indication of area of the circuit... as nickb said.

[R.G.]

Ultrasonic oscillation. Use an oscilloscope. Common cause: decaying power filter caps.