MkIII was HOT...maybe too hot

[kdorsey]

We had a private party gig last Saturday. A nice family party out in the country...swimming pool, pretty girls in bikinis, tons of beer and good food...good times. The stage was the entrance to their barn, so we had the building as shelter if, err, when it rained. But before the showers started, the temp was 95+ degrees and about 80 percent humidity and we were in direct sun some of the day.

Regardless of the blazing heat, the rig really sounded good - possibly the best ever. Running in class A mode, the power stage is just beyond the clipping stage. But when we got to set #3, the grit seemed to increase solidly. No, I didn't hit any gain knobs on pedals or touch amp settings, but I suspect the tubes or amp in general experienced a change due to excessive heat. Has anyone experienced this? The fan in my combo works really well but maybe it wasn't enough? Do I have a failing tube or two? Or worse?

 

[daveg62]

Well in that kind of heat, you're really just blowing hot air across the tubes. Better than no fan at all, but if the amp made it through, I would just wait and see how it works when it's in a cooler environment before assuming something is wrong with it. Is the amp working ok now?

 

[kdorsey]

...Is the amp working ok now?

Yeah, it does run fine now, but I'm afraid it's a tell-tale sign of needing a re-cap. Heck, it's 23 years old...it's probably time. Anyone know where I can buy 220uF 300V axial electrolytics to Mesa spec?

 

[Boogiebabies]

...Is the amp working ok now?

Yeah, it does run fine now, but I'm afraid it's a tell-tale sign of needing a re-cap. Heck, it's 23 years old...it's probably time. Anyone know where I can buy 220uF 300V axial electrolytics to Mesa spec?

Call Mesa for the caps. Cheaper than Sprague and more reliable than Ruby.
Plus, made in the USA.

In my exp. tubes sound the best right before they die.

 

[SonicProvocateur]

In my exp. tubes sound the best right before they die.

Ditto. Magic level gain and swirly overtones - then poof.

 

[kdorsey]

In my exp. tubes sound the best right before they die.

Ditto. Magic level gain and swirly overtones - then poof.

*sigh* great... :roll: Sounds like I should buy another pair of EL34s and keep 'em on hand.

Thanks for the feedback guys. It's good to bounce this stuff off of the group here even when the solution is painfully obvious. Meh, not too bad this time. 8)

 

[Boogiebabies]

In my exp. tubes sound the best right before they die.

Ditto. Magic level gain and swirly overtones - then poof.

*sigh* great... :roll: Sounds like I should buy another pair of EL34s and keep 'em on hand.

Thanks for the feedback guys. It's good to bounce this stuff off of the group here even when the solution is painfully obvious. Meh, not too bad this time. 8)

If you plan on gigging with your MK III, bight the bullet and buy a matched quad of 6L6 and EL34. If either duet goes down, you have a matching backup ready to go. For current production tubes, I have had remarkable luck with SED / Winged C 6L6 and EL34.

As for caps, without going through any arduous tests I look at the plate voltage under a typical 20-30mA draw. If it's over 485V on a MK III the caps are flaky. I once did a 60W C+ where I know the plate voltage should be 448V under a 30mA draw and it read 496V !!!
Cap job, same tubes 446V. In boogie's, the caps tend to over store voltage and burst. Look for any yellow goo leaking from the end / leads and check your bias draw and plate voltage.
It should tell you if it's begging for a cap job or running fine at the time. Still, 10 Years or older is a gamble.

 

[kdorsey]

If you plan on gigging with your MK III, bight the bullet and buy a matched quad of 6L6 and EL34. If either duet goes down, you have a matching backup ready to go. For current production tubes, I have had remarkable luck with SED / Winged C 6L6 and EL34.

As for caps, without going through any arduous tests I look at the plate voltage under a typical 20-30mA draw. If it's over 485V on a MK III the caps are flaky. I once did a 60W C+ where I know the plate voltage should be 448V under a 30mA draw and it read 496V !!!
Cap job, same tubes 446V. In boogie's, the caps tend to over store voltage and burst. Look for any yellow goo leaking from the end / leads and check your bias draw and plate voltage.
It should tell you if it's begging for a cap job or running fine at the time. Still, 10 Years or older is a gamble.

The SED Winged C 6L6s are in my Twin and are very VERY good power tubes. And since I've been steadily (read: weekly) gigging this over the past 5 years...reliably...there's no sense in taking any chances. It's a worthy investment.

Though I'm reasonably proficient at this stuff, I'll take the 'ol MkIII to my buddy's bench and follow your advice. In a previous business life I was a manufacturer's rep for electronic components, and specifically for Illinois Capacitor and know what their failures look like. Fortunately there's no bulging or rupture yet, but that won't show drying. And like you said, at 20+ years, it's a gamble. They will be replaced.

You were very generous with your time and talent...thank you for that Boogiebabies. If I ever get the chance, I owe you a beer or two. 8)

 

[mandoismetal]

FWIW, I've noticed that my amp's tone changes a bit (just enough so to be noticed) at different temperatures.

One time, I played on it after having left the amp in my car's trunk for a couple of hours on a sunny day, and the tone just didn't sound right.

However, given that your amp is about as old as I am, I would definitely think a bout recapping it.