NAD PRS Archon 25w combo with some issues

[Dennyz]

This week ive got an used PRS Archon 25W combo, the handwired version.

The amp was in good condition in general, a bit of dust inside the cab, some tolex/grill marks but nothing to worry about.

The first day that i fired up the amp, i was playing normally in the clean and dirt channel fpr about half of an hour, then sudddenly the volume started to oscilante, lowering, then stabilizing. It did this between intervals of 30/45 min a few times.

The amp came with the speaker swapped from the owner. It has a 16 ohm Greenback. The speaker cable isnt soldered to the tabs of the speaker. It has those brass clips that hook into the tabs. One odd thing is that the amp before testing it was set to 4ohms and Half Power. Ive inmediatelly set it to 16 ohm to match the speaker's impedance.

So, what could cause the malfunction? loose speaker cable/tabs?

Burned/defective power tubes? It has the stock 5881 Tungsols which i assume they been for at least 8 or 10 years on the amp. One thing that got my attention is that the left power tube has some hazy blue light when i play. Dont have those gray/burned marks btw.

Should i swap em for brand new JJs?

Thanks!.

 

[DreamTheaterRules]

First thing to always check on a used amp is the tubes. If you don't have "known good" replacements for them, I'd start by buying a new set.

 

[Dennyz]

Small update.

Fired the amp for an hour and a half, no volume drops.

Ive measured the bias setting on the power tubes with a multimeter. It reads 28mV both tubes.

Also ive managed to take some pics of the power tubes with the room's light off. They look a bit exaggerated because of the night mode setting on my phone camera.

Are they cooked?

 

[fingermystrings]

If you continue to have issues with this amp, it may be worth contacting PRS customer service. I had a pretty longstanding issue with my archon (watergall noise in the lead channel. I have another post on this subforum) and, while it took quite a few moons, they perservered and finally resolved my issue, with Doug Sewell (I assume. They referred to him as the "designer") himself looking at and working on my amp. They're great people who stand behind their products (in my case, I bought mine second hand off of reverb with an aftermarket speaker (v30)).

 

[S2C22]

Try some preamp tubes as well. Both power and preamp tubes do funny things that are unpredictable.

Personally, I'd retube it completely, being that it's used. That's something I always do, regardless. I have spare tubes at all times. Multiple sets of power tubes, and 20-30 preamp tubes on hand. Anybody who owns a tube amp should have spares.

 

[DreamTheaterRules]

Yep, new tubes are usually a requirement when you purchase a used amp.