by bordonbert Fri Mar 05, 2021 7:52 pm
That does sound microprocessor related. If there is a fault in the digital control section I'm afraid there is very little you can do other than take it in for a PCB replacement. If that is the case then the parts may be pricey by classic amp standards but the good thing is they should be plug and play, that is easy to fit, for a tech. Parts - a bit high, labour - fairly low. If this is a new amp with only 5 months on the clock is it not still covered under warranty? Can you not take it back to the place you bought it from for them to check?
There should be no connection between the two areas but you could still rule out other issues like valves, the original supplied ones can occasionally go west fairly quickly in your GM40D. Have you fitted new valves to the amp? Do you know how to open up the top section to change the valves around? There is a simple but ESSENTIAL procedure you must follow to open the amp up so as to not invalidate your warranty. On each end cap, remove the top 2 screws completely. Remove the middle rear screw completely and ignore the middle front screw. Loosen the lower 2 screws only a few turns, do NOT remove them completely, to allow the end caps to flop outwards just enough to remove the lid. On refitting make sure the LED PCB strip is correctly fitted and clipped along the top of the perspex panel and make sure the leads to it do not get pinched when replacing the lid and sides. Make sure you follow that. Valves just need clean hands to pull and fit them and possibly a wipe with a dry cloth if you are very sweaty, they do not need sterile wipes and chemical washes! Guys on the internet who advise never touching them with your bare hands and scrubbing them clean them with alcohol and other shit do not know what they are talking about. Valves were used for decades without anyone treating them in that way and there was never a problem. They are just glass bottles, they are not halogen lamps!
If you can then I would swap output valves T4<->T5 and T6<->T7. (That is simply the Left Outer<->Left Inner and Right Outer<->Right Inner). Simply taking them out and replacing them may help with a dirty base contact and if there is a problem in one of them then you should hear it change on your 20W power soak setting where one pair of valves is turned off. You can then try swapping T1<->T2, check if there is any change then swap back. Then T2<->T3 and again check and if there is no change swap back. The output valves are not an issue as long as they are kept in their matched inner pair and outer pair, but the preamp valves should be kept in their original positions other than for testing as they may be selected for noise. You can safely run the amp with the top off for short periods as long as you keep your fingers out of it when it is fired up. As you can see there are no accessible voltages in there other than the LED strip so keep that clipped down.
You are looking for any significant change in volume, tone or noise with the valves in different positions in the amp. The output pairs can be tested by listening in 20W mode as this turns off the inner pair of valves. When a faulty EL84 is in the inner pair 20W mode will be perfect as that pair is switched off but when it is in an outer slot you should hear the problem occur. If you have say a noisy second preamp valve for example, this will get much louder when it is swapped with the first input valve as it now has much more preamp gain behind it which amplifies the noise much more.
I wouldn't have thought that a valve issue would cause the symptoms you are seeing, it seems to be microprocessor related, but it wouldn't hurt to try the simple swaps and let us know what you find. It's 10 minutes well spent.
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