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Strange amplifier problem

Dear mates,
I started to experience a strange problem with my amplifier. Here is the description.

1. I'm listening to the music, everything is OK. Then I make a break for about on hour or two. Want to listen again, but there is no sound, though all tubes are glowing.

2. I check the CD player (change tracks, switch off and on, etc.), it is OK.

3. After some time I see a spark near the IEC inlet and check the fuse - it is blown.

This problem occurs for the second time during the last week. It is obvious that something inside the amp makes the fuse blow.

Some time ago I noticed another problem - when the amp is running, the left output transformer runs warmer than the right one. I wrote the dealer and the manufacturer, but company's president answered that this was normal because the left transformer is closer to the tubes (!!!), though this is not the case - they are symmetrical. The company's president either does not know what his company makes or deliberately ignors my problem in order to rule out any attempts to bring the amp back to him.

My numerous questions about the transformers and fuse remain unanswered.

Before bringing the amp to the dealer, I will appreciate any opinions of those who encountered similar problem and to whom such symptoms are familiar.
transl
almarg's avatar
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almarg
9,670 posts
 
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.

Regards,
-- Al
newbee
4,775 posts
 
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.
transl OP
156 posts
 
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).
swampwalker's avatar
swampwalker
5,138 posts
 
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.
transl OP
156 posts
 
Good idea!

...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.
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