It would help to know which amplifier you are talking about. I assume when you took your break, you switched everything off? I always thought the fuse is there to protect the amp from power surges, etc. Do you use an electric filter or surge protector before the amp?
3. After some time I see a spark near the IEC inlet and check the fuse - it is blown.
Did you see a spark or was it a clear glass fuse that blew? As far as blowing the line fuse I would think something is wrong with the amp. Something is causing an over current condition. Since you mentioned 1 of the transformers being hotter than the other would make me believe that maybe a tube is starting to short out whether a rectifier or output. Its very rare for transformers to go bad but they do.
Do you have a captive power cord? I might try changing the power cord before anything else?
Sounds like a bad capacitor or a tube with an internal short is causing excessive plate current to flow through the left channel output tube(s) and transformer. See if the plates of any of those tubes are glowing red, which would confirm this.
If the coupling capacitor between the driver stage and output stage of the left channel were breaking down, it could result in a large positive bias on the grid of the corresponding output tube(s), causing this excessive current flow. As could a marginal short in a tube, that might be temperature sensitive and therefore only appear after operating the system for some amount of time.
A short in a speaker cable or connection, or in a speaker driver itself, also come to mind as possibilities, although that would not seem consistent with the intermittent nature of the problem.
I agree with Almarg. My first thought with a tube amp which blows fuses is to check out the tubes. I keep spare tubes on hand (at a minimum a replacement set) and when a channel starts to have sound issues, glowing tube plates, or blows fuses I can just sub in a new set in the bad channel and see if that cures the problem. 90% of the time it does.
Hope that is all that's going on.
Newbee: The amp does not show tube plates glowing red, and there is no strange sound. I will try to insert the stock output tubes (KT-66) to see what will happen next.
Cyclonicman: I use aftermarket power cords (VH Audio Flavor Four and AirSine).
YOu might want to check to make sure you have a secure connection at the IEC, esp. w heavy or inflexible power cords. I have been told that if these sag and/orget loose, and not have a nice, tight connection, it can cause problems. I often have to wrap the male end that goes into the IEC w eletrical tape and/or support from below.
...Inserted the stock KT-66 tubes... So far, so good. The X'formers are barely warm - but the left one is still a bit warmer than the right one.
Have you re-checked the bias settings? Could be that its gotten out of whack a tad (or more).
My observation: the connection at the IEC is often poor because the position of the IEC socket at the back of the amplifier is often very awkward and inconvenient - e.g., at the side and not in the middle of the amp's back, which causes excessive tension of the power cord.
Whatever may be wrong always blame it on the tubes.
Ther amp has an autobias feature.
I presume the amp is New Audio Frontiers Legend II, a Danish amp.
1st blown fuse last week when this thread was posted :
This is not a Danish, but an Italian amp.
If the fuse is blowing then you have a power supply problem - a component has failed somewhere or is failing once it gets real heat (things can get very hot on standby on Class A designs - they actually run cooler when operating). When the device fails the power supply draws too much current and blows your fuse. The fact it works again (once you replace the fuse) suggests that the component is not quite dead...yet or that your amp manages to work in a crippled mode until it overheats itself.
Remember the amp cools quickly during the time it takes to replace a fuse and then once you play music again it may run cooler on whichever side of the amp (+ve or -ve signal) that is tending to breakdown.
Yes, exactly. One side still runs cooler.
Transl, FWIW it is possible that one or more of the tubes in the amp when it was blowing fuses had a 'bias' point which exceded the values of the bias circuit. A personal experience (on two different amps - same tube set). One amp had 'auto bias', one tube ran hot (plate turned red). Sub'ed in another set and this same location (V3) ran hot again. Sent the amp (under warranty) back for servicing. They reset the bias - the amp has worked fine ever since, but with different tubes - I thought the initial tube that tuned red might have been the problem.
Next I tried to use that same set in an amp with fixed bias. I reduced the bias pots to minimum as normal. Three out of the four tubes had a normal (minimum reading), but the tube that had turned red in the other amp registered 50ma on my meter with the pot in minimum position. I subsequently figured out that the correct bias point on this tube should have been 75 and it was somehow mixed into a batch with a 50 bias point.
How, or if that applies to your amp, I don't know. Just don't assume that auto bias circuits and tube bias points assigned by tube matchers are foolproof. I'm very carful now when I buy matched tubes to insist of bias points of about 40 and have had 0 problems since.
Or, it might have been an short caused by an undersized pin, or loose contacts in the socket, causing a short too (although with the latter it was always noise, popping etc, and was corrected by tighting the contacts in the socket.
Lost any fuses since you went back to the original tubes. BTW, what were the tube types and brand of the tubes you were trying to roll in?
The stock output tubes are Golden Dragon Retro KT66 (Shuguang?). Good tubes, slightly warm.
I also tried vintage grey glass GEC KT-66 tubes. As they are very old, I admit that though they were sold to me as NOS/NIB, some of them may have lost some vacuum or might have undergone other deterioration due to old age. They sound marvellous, more airy, deep and intimate than the stock tubes.
I also have clear glass GEC KT-66 tubes. Unlike the grey glass variety, they sound very dynamic, fast and articulate, if a bit harsh and clinical. Quite unexpected for GEC tubes (unless they are faked).
The stock tubes don't have such air and depth as the grey glass GEC KT-66 tubes, but sound very deep and consistent,
The amp also uses two 5687 driver tubes (in came with NOS Raytheion "orange paint" tubes which I replaced with NOS TungSol tubes) and one 6922 tube (I use Amperex JAN 7308 "green paint").
I did not have an opportunity to listen much with the stock tubes, but so far it does not blow the fuse.