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XXX .... All new tubes..... Only 3 power tubes have purple glow under load...


Iron Lotus
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Okay so I just put a 4x12 together with a couple of v12's & a couple of swamp thangs. My xxx has matched tubestore kt77's & some select preamp tubes.

 

I noticed tonight when I was playing in dimmer lighting that the left power tube stayed lit like it was in standby while the other 3 to the right had that nice glow during normal operation. I switched tubes around, & the tubes are good, it only happens in that position.... What is going on? I'm pretty good with the technical repairs I've done it for years as a hobby, but I dunno what this might be. Anyone have any sugessteeeowns??sm-mic

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The blue glow is not an indication or proper or improper functionality. It occurs in some tubes and not in others. The Fluorescent effect is caused by the mercury vapor in the tube.

 

I would ask if you biased the tubes. If you didn't, it might impact the amount of grow. If you did bias them forget it. Blue glow is nothing to worry about. Only orange/cherry glow of the tube elements is something to be watchful of.

 

The Blue glow may or may not be there depending on where the getter was dispersed. That shiny mirror like coating on the inside of a tube is caused when the Getter is activated with a flash to vaporize it and depending on where that silver stain occurs it may be blocking the B+ corona from appearing.

 

So long as the tubes bias is correct the tubes are functioning the way they should be. The blue is simply an indication the B+ was applied but its not an indication a tube is malfunctioning if its missing.

 

You can read more about it here. This is written from the opposite view of the glass half empty vs the glass half full, the Glow being questionable, not the lack of it. Its again irrelative so long as the bias is good and you aren't overloading the speaker impedance rating of the output transformer.

 

http://www.thetubestore.com/Resources/Matching-and-other-tube-info/Blue-Glow

Edited by WRGKMC
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The blue glow is not an indication or proper or improper functionality. It occurs in some tubes and not in others. The Fluorescent effect is caused by the mercury vapor in the tube.

 

I would ask if you biased the tubes. If you didn't, it might impact the amount of grow. If you did bias them forget it. Blue glow is nothing to worry about. Only orange/cherry glow of the tube elements is something to be watchful of.

 

The Blue glow may or may not be there depending on where the getter was dispersed. That shiny mirror like coating on the inside of a tube is caused when the Getter is activated with a flash to vaporize it and depending on where that silver stain occurs it may be blocking the B+ corona from appearing.

 

So long as the tubes bias is correct the tubes are functioning the way they should be. The blue is simply an indication the B+ was applied but its not an indication a tube is malfunctioning if its missing.

 

You can read more about it here. This is written from the opposite view of the glass half empty vs the glass half full, the Glow being questionable, not the lack of it. Its again irrelative so long as the bias is good and you aren't overloading the speaker impedance rating of the output transformer.

 

http://www.thetubestore.com/Resources/Matching-and-other-tube-info/Blue-Glow

 

Well, every tube amp I've had... every power amp tube had a slight purple haze under load.

I checked the tubes with a tube tester & all were fine.

 

I am currently searching for my bias probe, its been lost for a while now though.

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Four power tubes is a push pull parallel configuration. If one of the tubes is not participating, I would think there would be a noticeable degradation in tone.

 

Well if it means anything, the 3 that were functioning uniformly were hotter to the touch than the left one that doesnt.

It was the same as if it were never flipped off of standby....

 

And the amp sounds good, but then again maybe it can sound much better if this is in fact an issue...

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You say you are "pretty good with the technical repairs I've done it for years as a hobby".

 

​Does that mean you are experienced and comfortable measuring high voltages inside a guitar amplifier?

 

Yes, I've been inside all my amps. I'm a welder as a dayjob so I work with some high voltage stuff. But as a hobby I'm a huge DIY'er & started soldering on boards at a young age because my grandfather was a tube tv repairman. Anyway I have measured 500 volts inside of the last 5150 I owned when I bias modded it. I seem to have to tear down everything I have & mod it somehow. So I know my way around with a meter & a soldering iron :thu:

 

I'll say I'm comfortable & capable of doing what needs to be done, once I know what actually NEEDS to be done.

So that is basically why I am here, because theres usually a good bit of specific knowledge to be shared around here on a good day.:lol:

I've never ran into this problem before, the closest was when an old 5150 didnt even light up on standby, so I replaced the screen grids & fixed it. Never had this particular problem, so I don't know exactly what to look for...

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Pin 3 is the plate and the voltage should be in the +400volt range

Pin 5 is the control grid and should be in the -50volt range

Pin 8 is the cathode and is normally tied to ground. It should read 0volts

 

The voltages from the good tube sockets will be actual expected readings

 

The output tubes are in parallel so the problem must be on or near the tube socket. ​One of the pins on the socket may have become physically disconnected from its power supply.

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