Firstly, my apologies if this belongs in customising, since essentially it does involve some amp dismantling.

OK, I've got my old practice amp, a Hughes and Kettner Thirty, sitting in front of me (the hairy one), and it sounds wonderful. For about 20-30 minutes. After that, the sound seems to cut out for a few seconds - anywhere between 2 and 20. I can sometimes force it back by turning the volume up/smacking the strings infinitely harder than normal, but sometimes it needs a quick flick of the power switch.

All cables and guitars I've used so far with it have been tested in other amps, no problem. So, it's deffo something in the amp.

The amp's all transistor-esque, so no blown tubes.

Now, I've got the amp head attached to a different speaker, and am currently cycling Still Unbroken by Skynyrd through it to test out if the problem still happens. Still, I've got the feeling that I might still have the problem with a guitar, so will be testing that out.

I've also noticed the speaker is a Celestion G12L-35. So, 35 watts. However, my reasoning (and feel free to correct me on this) is that the peak voltage of the amp, as a 30W amp RMS, is somewhere around the 50W mark. Could this affect the speaker, or is the speaker's wattage also RMS?

If it's the amp, does anyone have any suggestions as to what the problem might be, and how I might go about fixing it?

If it's the speaker, does anyone have any suggestions for a nice speaker to switch in. It must be:

12" size
Able to handle a 30W amp
Not too expensive, since I'm low on fundage at the mo.
From a UK supplier. I don't want to have to pay import tax on it.

Thanks for all the help I'm sure to receive in advance.

"I use strings hand-made by old blues players who are dead" - WHERE DO YOU FIND THESE PEOPLE???
A quick update for the 40 people who took a look at this thread, and may be interested in what happened next:

The amp was dismantled, and the head was hooked up to the speaker in the back of my Marshall (100W) - worked flawlessly.
The speaker was hooked up to a 15W head from a beaten-up Fender Frontman 15W - again, no problems.

I've since put head and speaker back together, and had no problems at all from it. I'll keep testing, but for now, I'm thinking it might be a solder joint that's causing some problems. Could a solder joint build up static and stop noise from being made like that? Or am I just being stupid, and there's something else I haven't quite thought of?
"I use strings hand-made by old blues players who are dead" - WHERE DO YOU FIND THESE PEOPLE???
I know this thread is really old but I have exactly the same problem! I have a Hughes & kettner ATS120 with matching cab, loaded with celestion G12L-35's wired in series. They cut out after 20-30 minutes with either my H&K or my Marshall. I have replaced the wiring and re-soldered everyting and still the problem persists.

I have tried with 5 different guitars, and 6 different guitar leads, and two speaker cables (one of these was brand new)

All the speakers check out on the multimeter, I have even tried them individually with the H&K amp wired straight to the individual speak terminals and they work. Once they are connected together again the problem occurs...

Any ideas?

I've got a Hughes & Kettner Thirty (the hairy one), and it does the same thing too. I've popped it apart, touched up solder joints & put it back together 8 or 10 times. It will work for awhile, but it always cuts out again--usually at the most inopportune moments! If anyone EVER hears of a permanent fix, PLEASE let me know!

Sounds like something H & K has an ongoing problem with! Wish someone from the factory coulD let us know how to fix this. For what it is, this is an amazing little amp. Small, lightweight, but it KICKS--when it works! When it doesn't work, it's WORTHLESS! Hughes & Kettner, ARE YOU LISTENING???
This is a speaker fault not a fault with the H&K amp... I think what we need to find out is the cause of the fault in the speaker... is the voice coil going?

My issue is slightly more complicated that you guys with the Thirty's, i have the hairy 4x12 and the G12L-35's are wiring in series so it's hard to find the faulty speaker
I have fixed it!

What you need to do is reflow the solder on the wires coming from the speaker cone going to the terminals, use some long nose pliers to push the wire from the cone through the terminal a bit further and make sure it has a good connection.

Good luck with getting your combos going again