Vintage Modern 2266 Power tubes issue

The Dose of Harmony

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HI Guys,

Need some help here , i need some direction its the second time that the VM , burns the 2A fuse under the power cable and one of the power tubes , but only does it when i max out the master ,

last year was the first time it had the original , I remember that i crank the master volume to the Max a few times no issues , one finally one of the tubes burn out burn the fuse so i thought might be a power tube failure

next time was last week same thing only this time it did not lasted that much time with the master volume up , burnt tube burnt fuse



so i decided the to check some things I have a Eurotubes probe and it reads around 425 V on the plate voltage on Both sides , Now

the current plate on one side was 45ma but the other side at first was reading 48ma but when i max out the master volume started to increase and at the very end when up to -175 ma so i turned off as soon as i saw that

today i opened put the Eurotubes probe but i tested with my multimeter at the testing pins too , but todays has not done that its been steady


any ideas what to check?

thanks
 

The Dose of Harmony

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Was the amp just hissing or putting out guitar signal?
I’m confused by -175mA. Do you mean that the valve current flipped from +48mA positive to -175mA negative?


you know i just tested right now with and without audio signal(guitar ) and it looks like the Eurotubes was giving a false reading ( it was showing only the negative sign only no number )because when i max out the master with the audio signal the multimeter read +172ma

without audio signal Everything its steady but i just notice that it sound very noise the whole amp
 

The Dose of Harmony

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Sounds like the amp needs to be seen by a qualified technician. I suspect a bias stability issue.
i had almost an hour on but the plate current was steady

looks like the issue its when i max out the master and have the guitar plug in can the problem be on the pre amp but just showing symptoms on the power side?

checked R90 and reads fine
 

Matthews Guitars

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Understand that bias current changes as the tubes are driven harder. The tubes run hotter as you drive the amp harder.

This is like that classic squawk to an aircraft mechanic: "Altimeter misbehaves when flying above 20,000 feet." Mechanic's statement: "Checked altimeter. Can't replicate altimeter problem on the ground."

Stable bias at idle is one thing. But what's happening when the amp is dimed? Totally different scenario.
 

The Dose of Harmony

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Sounds like the amp needs to be seen by a qualified technician. I suspect a bias stability issue.
Understand that bias current changes as the tubes are driven harder. The tubes run hotter as you drive the amp harder.

This is like that classic squawk to an aircraft mechanic: "Altimeter misbehaves when flying above 20,000 feet." Mechanic's statement: "Checked altimeter. Can't replicate altimeter problem on the ground."

Stable bias at idle is one thing. But what's happening when the amp is dimed? Totally different scenario.
thanks man

checked all the resistor on bias side all tested good too!
 

The Dose of Harmony

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I notice that the light Led its blinking more often and a lot of hiss when mid boost and HRD are engage!
 

ken361

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I just have the bias probe going into a meter I used it on my new 5150 and my deluxe reverb it seems to work Great. Haven't done my VM yet . The volume should be set very low if that matters when biasing. I never maxed the volume yet usually at half the most. Chinese power tubes are recommended By the designer.
 


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