Deluxe Reverb – one tube socket glowing blue

zebpedersen

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My Deluxe Reverb reissue (~3 years old) has started intermittently sounding a bit dull. As with lots of sonic things it's difficult to describe – it's not an abrupt change, it will sound okay for a few minutes and then gradually start sounding a bit lifeless, and then fade back to sounding a bit better, but never at '100% tone'.

I haven't ever changed the tubes so that was my first thought; on inspection in the dark I noticed one of them was glowing blue in unison with the chord I was playing to test:

ezgif-3-83d8b28687.gif



This morning I swapped the two 6V6s around, and the same thing was happening in the same socket with the other tube (i.e. the tube in the socket furthest from the rectifier glows blue independent of which tube it is), so it wouldn't appear to be an issue specific to that tube.

I use an attenuator with the amp (UA OX) all the time, but only ever run it around 4.5 on the volume (clean but just getting a hint of breakup if you really hit it). The GIF is recorded with the amp on 10 (and the speaker off!).

I was about to buy a new set of power tubes but this seems like it might be an amp issue? Or maybe normal behaviour?
 

Jon Snell

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If you are electronics trained, check the coupling capacitor on the affected socket.
With the 6V6s removes the voltage on pin 5, the control grid of the 6V6 valve.
You are looking for around -35volts on both valve bases with no valve inserted.
I suspect one will be the wrong value.
 

PCollen

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I assume you mean the tube is glowing blue, not the socket. Blue glow is normal.

I have a JTM45 clone with two KT66 tubes that put on a beautiful blue-light show when driven hard.....
 

King Fan

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Are you guys saying it's fine for just one tube to flicker blue when he hits a chord — while the other doesn’t? And the reason he says 'socket' is the behavior stays with the socket when he swaps tubes — smart test. And the amp sounds worse as he plays?

@zebpedersen , tell us about your comfort/experience level working on a live amp, measuring voltages, etc.
 

Phrygian77

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What happens when you turn it up more? Seriously. I suspect the reason it follows the 'socket' is because the output of the phase inverter isn't even (which is fine and typical), so that tube is getting hit with more grid voltage than the other.
 

zebpedersen

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I decided to drop the amp to a tech this morning. Happy working with low-current / low-voltage DC…not so much the big / deadly stuff.

To clarify, the symptom is not a loss in volume but 'dullness' to the tone, somehow more 'closed'. It would be much easier if it was a volume drop or distortion :)

Will report back on the repair…
 

King Fan

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Good for you. Discretion, valor. We see some ‘I’m going to fix this if it kills me' threads that run like a guy wrestling an alligator, coming to the surface over and over but for briefer periods each time, while we well-meaning observers shout encouragement….
 

Nicko_Lps

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Or maybe normal behaviour?
Yep. This Blue Glow is called Fluorescence and its a normal thing to see under low lighting, its not that visible otherwise and.. Ive seen a "scientific" explanation somewhere, if i remember correctly the blue glow is the electrons moving in the vacuum.

A simple explanation:
download.jpg

The image above is a HEALTHY tube operation where you can see blue glow(naturally) and Filament (red) only through the holes.
Now the only operation problem you can see visually on power tubes is this below:

red_plating_320px-1.jpg
 

peteb

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I take it back. The tubes glowing to the signal does not make sense to me.

I was aware that tubes look different in the dark, but most of my experience has been looking at idling power tubes, in the dark, for signs of ‘white plating’. Supposedly, before a tube red plates, it white plates.

now I start watching tubes in the dark, under signal conditions.

one amp tested. I only really noticed the orange glow of the heaters.
 

zebpedersen

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Maximum points to Phrygian77 – the phase inverter was 'producing limited gain on one phase'. Phase inverter, both output tubes, and coupling caps replaced, the amp is back and sounding healthy again.

I have given my Mark I Boogie to the tech now… 45 years on original caps and pots, and a few mods to reverse, fingers crossed ☺️
 




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