Lumley M100 Fuse Blows

[Jason]

Help! I'm a newbie and I know nothing about tube amplifiers or vintage electronics.

I recently picked up a pair of lumley M100 monobloc amplifiers. I was able to hear them play before I bought them and they sound incredible on a pair of big Bozak speakers. I decided to buy them without the tubes because I wanted to source my own and make sure I knew what I was buying and where I was buying it. So I bought a quad matched set of Mullard KT 88's and just install them.

No I really don't have a good set of speakers because my Polk lsi25s or modern and have a built-in amplifier for the low end.

I decided to bring my Cerwin Vega D9s down from storage, even though they don't work very well and probably need some service. I know they're not exactly the most sophisticated speakers, but they didn't crank in the 80s when I was in college so I figured it was a reasonable match for testing until I get something truly awesome.

After warming up the tubes and slowly bringing the bias up to about 1.5 on the meters, I brought up the volume on my Sunfire theater grand preamplifier and then my right shuttle failed.

The main power light on the lonely amp remained on, meter went dead on both tubes when selecting by us, but the tubes remain lit , The other channel continue to function perfectly.

I then found what I believe to be a blown fuse? But the fuse seem small and I don't understand why the amp would have power with a blown fuse? I've always known fuses to cut power, but maybe the fuses on the output side going to the speakeror maybe the fuse is in between the power source which is AC 120, but why would the light for main power remain on? The meter was dead on both sides.

Obviously, I could go out and just buy a new fuse, but I'm more concerned as to what I did wrong and how this functions and why a fuse would blow when I'm not even cranking the volume up very loud. Anybody can still carry on a conversation in the room, I know those speakers are capable of deafening a small house.

I certainly don't want to simply replace a fuse if there something wrong with what I'm doing, everything seems to work and I don't think I'm using inexpensive components or have it set up incorrectly.

Any help would be appreciated!




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[mep]

Jason-For someone who admittedly knows nothing about tube amplifiers, I'm surprised that you decided to buy a pair of tube amps that you heard working and sounding great with the tube set it had installed and you decided to have them remove the tubes and you jumped in and bought your own tubes and installed them. Now you are having a problem....

You probably have a bad output tube or you didn't bias them correctly. Something is drawing too much current which is why the fuse is blowing.

[Jason]

The guy wanted $400 more with the tubes, and I assume the tubes have a useful life. When I did a little research, it seemed that KT 88's should not have cost $400 extra, so I thought it made sense to buy them direct for a fraction of the price. Anyway, I don't know anything about tube amplifiers, all I can remember about tubes is smashing them as a child in the 70s which was probably a big mistake. Anyway, I increased the knobs very slowly that adjusted the bias, kept both amps at the same level, both sides which I assume one side matches each tube. But I still don't understand why the fuse blowing would not have killed the main power other than the fuse being upstream for some other purpose.

Could the speakers be the problem? Should I be using a slow blow or fast acting fuse, I picked up both this morning at the hardware store? I'm kind of scared and thinking that maybe this was a big mistake


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[Jason]

Also, what's an output tube? Each amp has two KT 88's which are the ones I just purchased, and too much smaller tubes and I have no idea what those are for. The small ones, two per amp, came with the amp?


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[mep]

The output tubes are the large KT-88 tubes you bought. The two small tubes should be the input tube and the phase inverter. As for fuses, you should be using exactly what the manufacturer recommends for each application. Again, it's possible you have a bad tube which is causing the fuse to blow. Or, you talked about your CV D9 speakers not working properly (whatever that means) and needing servicing, but yet you hauled them out of the attic and lashed them up to your tube amps. It's possible that one of the speakers is shorted which is shorting out the amp. Try moving the speaker from the blown fuse amp over to the other channel that is working and see if you can blow that fuse too.

[Jason]

I have a pair of klipsch thx, I think I'll try them. Unfortunately, I don't believe the amp slow blow or fast blow, it simply says 500 milliamperes. So I really don't know what it calls for. I hope the tube isn't bad, but I guess I should probably get another set as a spare anyway


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[Jason]

Thanks


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[blackdog]

That fuse is the high voltage or plate fuse for the KT88's (output tubes). The only thing between the power supply and the tubes is that fuse. If it is blowing you have at least one bad tube or you biased it too high.
The fuse should say F500ma or T500ma. F for fast, T for time delay (slow blow).

Not to be critical, but please read your posts or turn off the auto correct. It's tough reading your posts.

[Jason]

Thank you for the information. Sorry about any typos, but I'm handicapped and cannot type. I have to use voice recognition.


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[mep]

Thank you for the information. Sorry about any typos, but I'm handicapped and cannot type. I have to use voice recognition.


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Sorry about your handicap. I am curious as to how you are biasing tube amps if you can't type though.

[Jason]

Perhaps I'm not doing anything right? All I was told was that there were knobs on the front of the Lumley amps, I need to let them warm up slowly and adjust the knob so that both the left and right side tubes of the amp have the same reading. Then I can bring up the volume, being careful to periodically turn on the meter on the left and right side to make sure the meter is reading the same. The handicap is not prevent me from turning the knob or switch, but it does make it extremely difficult to type.

I have the klipsch connected and the fuses replaced with fast below, they sound pretty good except for the lack of base since the speakers only have a horn and two drivers no bigger than about 5 inches. They sound better than the Vegas, I don't have a low-end. Maybe I just had a bad fuse, I was told never to bring the meter past halfway which is five


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[blackdog]

Hopefully you won't have any more issues. Occasionally you will get a new tube that has some impurities in it. When the tube heats up it can arc, and you will blow a fuse. This should stop, and the tube will be fine. If it continues to happen you will need to find the bad tube and replace it.
Keep and eye on the bias. One tube may still be bad, and the meter will show it as it rises. Member "Albert" has a pair of these amps, hopefully he will chime in and can tell you exactly where the bias meter should be set.

[Jason]

Thanks for the information, very educational!


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