Another couple for the tube gurus

Downunderwonder · #1 04-01-2011, 10:02 PM

When I power up my 50W'er the first of two 12AX7 lights up for an instant like a white hot lightbulb. This is with the amp set to standby, just turning the on/off to on. What's up with that?

JimmyM · #2 04-01-2011, 10:50 PM

i've heard that a lot of tubes made in europe will do that. don't know why, but it's nothing to worry about.

T-Bird · #3 04-01-2011, 11:23 PM

Hi.

There's been quite a bit of talk about this lately, perhaps the CP mfg. methods have changed.

Up until now, IME the only tubes that have kept on going while showing the start up flash have been the vintage Mullards. Every other tube with said flash have either bitten the dust immediately or after a few hours of playing.

The flash seems to be "normal" nowadays with some tubes, perhaps the manufacturers are trying to capture some of that Mullard mojo.

I wouldn't trust a flashing tube other than Mullard, but then again, I have enough NOS and vintage tubes to satisfy my current needs. And the flashing Mullard in my Fame still makes me a bit uneasy at start-up even after these few years it's sat in there.

Regards
Sam

Downunderwonder · #4 04-02-2011, 02:29 AM

The tubes in question are possibly as old as the amp. All the writing is gone from both of them but only one gets really bright. The rest glow a little bit of a flash but just the one goes all lightbulb on it.

T-Bird · #5 04-02-2011, 03:14 AM

Hi.

What's the secret amp, and how old is it?

Most importantly, have the tubes behaved like they do now for a long time, or just started to flash?

In most cases, there's also audible clues that the pre-amp tune is in it's last legs.

If the said tube has been flashing for a long time and the amp is used just for the rehearsals, no recording or gigging, and the amp behaves as it always has, I wouldn't swap the tube.
If the amp sees gig or recording duty, I'd replace, or at least test the flashing tube.

Regards
Sam

BassmanPaul · #6 04-02-2011, 11:38 AM

Pre-amp tubes have been doing this for as long as I can remember. It's caused by the inrush current into a cold filament. As the filament heats up its resistance grows until it stabilizes. I've seen it most in the 12AX7/ECC83 types. I have some that have been doing the same for thirty odd years and still test out into the green.

Mr. Foxen · #7 04-02-2011, 11:46 AM

I've had it with happy Mullards, and some other happy NOS european tubes. Wouldn't worry if they are still working fine after a few uses. Get those shielding cans if it worries you. Out of sight, out of mind.

Downunderwonder · #8 04-02-2011, 11:50 AM

It's a 30 year old Jansen. I'm its latest custodian and I'd like to gig it. I got it for the price of a service. I almost gigged it last night but chickened out and took sansamp and power amp instead.

It was recently serviced by a goto guy, friend of the last owner. I imagine tubes were tested. The 12AU7 pair were replaced along with the power tubes and sockets. All new JJ's. The 12AX7 are vintage.

It's a "Bassman" but it's a different amp to a Fender. They copied the general layout but not the circuitry behind the panels. They are favoured by guitarists for their distortion.

The other thing is the "BASS" channel has very litle going on with the bass EQ knob. The "NORMAL" channel is much more tweakable.

Downunderwonder · #9 04-02-2011, 11:52 AM

And some new posts while I was typing. Thanks for the reassurances. I guess I'll leave the power amp in the boot just in case.

Downunderwonder · #10 04-02-2011, 11:54 AM

Still don't understand why the cold filament gets white hot before the current stabilises at red hot.

BassmanPaul · #11 04-02-2011, 02:20 PM

If you consider a light bulb. Mostly when they blow it's when you turn them on. The filament is cold and has a low resistance and the maximum current flows. As the filament warms up its resistance rises and the current flowing reduces. It settles down at a point where the filament is giving out light. This characteristic is used in tweeter protection circuits in some of our cabinets. As they heat up they reduce the signal passing to the tweeter and so protects them. It's why sometimes our cabinets light up.

Downunderwonder · #12 04-02-2011, 08:24 PM

The high school physicist in me expects the steady state ought to be reached without a peak in temperature. I would think the current would always be dropping from the "cold" initial state so the temp would rise up from the excess current but would damp down the current straight to the steady state, applied voltage being constant.

You're right about the lightbulbs blowing when you turn them on. Seems my faulty logic is still faulty.

JimmyM · #13 04-02-2011, 11:58 PM

Quote:

Originally Posted by Downunderwonder

The high school physicist in me expects the steady state ought to be reached without a peak in temperature. I would think the current would always be dropping from the "cold" initial state so the temp would rise up from the excess current but would damp down the current straight to the steady state, applied voltage being constant.

You're right about the lightbulbs blowing when you turn them on. Seems my faulty logic is still faulty.

i guess that's the difference between high school physics and college physics

but at least you took it and got through it. i totally chickened out on it.

rickdog · #14 04-03-2011, 09:02 AM

Here's my guess - and it's just a guess, but an educated one, and a plausible explanation that won't violate anyone's high school or college physics.

The 12AX7 is a dual triode, with two separate cathodes, and each cathode has its own filament. The two filaments are wired in series, and thus both see the same current through them.

However, due to minor variations in manufacturing (in a new tube) and uneven wear (in an old tube), the two filaments may not have exactly the same resistance. The one with the higher resistance will have larger voltage across it (V=IR) and dissipate more power (P=I²R), and thus get hotter initially. As the lower-resistance filament heats up slowly, its resistance increases, cutting down the total current and letting the first filament cool down, decreasing its resistance. Eventually they both come into rough balance.

I've seen this happen too, but perhaps to less of an extreme (bright yellow, not light bulb white). If I were you, I'd carry a spare 12AX7....

Downunderwonder · #15 04-03-2011, 09:41 AM

Intersting. Of triodes I know nought. It's on standby when it's doing this, which I thought was putting power to the heaters before the rest got going.....

Downunderwonder · #16 04-03-2011, 09:43 AM

... but two heaters in series would be the same dealio....

BassmanPaul · #17 04-03-2011, 10:17 AM

The 12AX7 and its ilk are indeed twin triodes and do have two heaters. Most times these are wired in parallel and run from 6.3V AC not series connected for 12.6V.

Downunderwonder · #18 04-03-2011, 10:21 AM

And still doesn't explain why a regular lightbulb blows when you turn it on most often. Might have to google that....

Downunderwonder · #19 04-03-2011, 10:28 AM

http://io9.com/#!5628229/why-do-ligh...eyre-turned-on

figures, ok case closed, they aren't related in their glowings

rickdog · #20 04-03-2011, 01:53 PM

Quote:

Originally Posted by BassmanPaul

The 12AX7 and its ilk are indeed twin triodes and do have two heaters. Most times these are wired in parallel and run from 6.3V AC not series connected for 12.6V.

I should know better than to post before my second cup of coffee

I just pulled out a few random Fender and Marshall schematics, and sure enough, they all run everything off the 6.3V. Same for the classic Ampeg SVT. My SVT-3PRO runs them in series, though, so it's not unheard of, but you're right, Paul, that's not the common case.