Blue Glow Question

[HJW]

I've been tinkering around witn my Epi VJ head, and I swapped out the stock tubes for a 60's Motorola 12AX7 and a '59 RCA 6BQ5/EL84.

Tone-wise, much better. I did notice an increase in the 60 cycle hum and a bit more hiss, but not much. I've got 5 other RCA 6BQ5's to try, and 3 other brands of 12AX7's.

Anyhow, the blue glow inside the plates of the RCA 6BQ5 dims a bit when I hit it hard (crank the amp). Is this normal ? These amps have solid state rectifiers. Is the dimming I see sag ? Failing tube ?

[pdf64]

Blue glow is normal
KCA NOS Tubes. Audio tubes for guitar amplifiers and audiophile equipment. 12AX7, 6L6GC, 12ay7, el34, el84, Fender amplifier,5AR4, 5U4GB, cv4004, 6SN7, 8417, 7027, gz34, Mullard, EF86, 6ca7, 6bq5, 7025, 12at7 - Content
Sag
Sag
Hum&hiss - maybe one of the tubes you've put in is bad, maybe they've got more gain than the previous ones so are amplifying the noise more.
Best to only change 1 thing at a time.

[HJW]

Blue glow is normal
KCA NOS Tubes. Audio tubes for guitar amplifiers and audiophile equipment. 12AX7, 6L6GC, 12ay7, el34, el84, Fender amplifier,5AR4, 5U4GB, cv4004, 6SN7, 8417, 7027, gz34, Mullard, EF86, 6ca7, 6bq5, 7025, 12at7 - Content
Sag
Sag
Hum&hiss - maybe one of the tubes you've put in is bad, maybe they've got more gain than the previous ones so are amplifying the noise more.
Best to only change 1 thing at a time.

Ok, thanks for the links. I haven't read through the info there yet, but yes, I know the blue isn't bad. I just wasn't sure if the dimming I see when I crank the amp was a bad thing. I posted the same question elsewhere right after I posted here, and someone replied that I was seeing corona discharge, and in his opinion, that's a good thing.

Edit: Read the articles, very informative. I live, I learn. Thanks again.

[melvin]

I believe the intensity of the glow is proportional to current. But I don't think it is wise to use it as a troubleshooting tool. Besides, it's cathode biased class A right? It may very well draw less (dc) current when cranked.

[HJW]

I believe the intensity of the glow is proportional to current. But I don't think it is wise to use it as a troubleshooting tool. Besides, it's cathode biased class A right? It may very well draw less (dc) current when cranked.

Correct, Class A cathode biased. I'm just new to tube amps and never noticed the dimming inside the plates. Then again, I've never had a class A amp either before this one.

I've been advised it's a sign of a class A amp working properly on another forum, and in an A/B amp I wouldn't see this dimming of the glow. Previously, all I had to refer too was my Sano amp, which isn't class A.

[Steve Conner]

I believe the intensity of the glow is proportional to current. But I don't think it is wise to use it as a troubleshooting tool. Besides, it's cathode biased class A right? It may very well draw less (dc) current when cranked.

+1. If you really crank one of these SE Class-A amps, they can draw less current than they do at idle.