EL84 Bias Values, crackling, etc....

[stoneattic]

So I have a Mesa Studio Caliber (DC-2) that I have been using for years as my main gigging amp and at practice the other day it started making a crackling noise. The crackling is intermittent, random and with about 10-20 seconds between. It seems to happen regardless of if I'm playing or holding the strings, etc.

I have a few new spare matched pairs of (EI) tubes that I had picked up a while ago so figured that the Mesa EL84s were old anyway, so I was going to put a pair of EIs in. But since Mesa wants you to be their "pre-selected" tubes I measured the bias on the Mesa tubes first using my Amp-Head bias probe and got the following results with the Mesa and EI tubes:

Mesa
V7 I=38mA V=390V
V8 I=29mA V=391V

EI (set 78)
V7 I=51mA V=365V
V8 I=46mA V=365V

EI (set 104)
V7 I=44mA V=374V
V8 I=40mA V=374V

All pairs are supposedly matched. It looks to me that the Mesas are not matched, at least not anymore. But the large differences in current and voltage readings between the Mesas and EIs surprised me. Any thoughts about the suitability of the EIs without a re-bias? The EI current number are higher than I expected based on various references regarding EL84s.

I have no idea if this is even related to the crackling issue but I want to get this sorted out first before tackling the crackling. (poetry unintended)

I have a gig tomorrow night and would prefer to use the Mesa, but I have spare amps so it's not the end of the world to use another.

Thanks!

[BiBi]

I haven't had any luck with "matched sets" of output tubes. From my experience and from what I've read it doesn't really matter so much anyhow, at least for a guitar amp.
As to the crackling, that is much more of a trouble than the output tubes. Usually, crackling comes down to a bad solder joint. The best way to tackle this is to drain the filter caps first then go over each solder joint with an appropriately hot iron. What you need to see is the solder melting again-- don't over do it especially on a PCB board.

Good luck!

[MWJB]

I doubt the crackling is directly related to the standard of matching of your output tubes, first thing is to try substituting preamp tubes...it's more likely that one of these is crackling, rather than 3 sets of output tubes.

As Bibi says a cracked solder joint could possibly be to blame...other than that it looks like you need a tech.

A reasonable match on power tubes is desirable, especially if you run the amp hard. If you just noodle at low volume, you may find mismatched tubes are acceptable. Plate current matching has more sonic effect in your guitar amp than the PI tube, or OT deficiencies.

[BiBi]

Hi MWJB, I'm curious about the matching of output tubes. Could you explain that a little bit i.e. why they would make more of a difference when run "hard"?
Is it because the crossover distortion becomes more noticeable at higher levels?
I would actually have assumed the opposite-- that to get as clean a sound as possible, for instance in hifi, matching is more essential.
thanks, and sorry to hijack the thread...

[MWJB]

If tubes are mismatched, but still at reasonable current, so that they sound warm, they take the edge off fidelity/highs. Some amplifiers use specific controls to unbalance the PI, specifically to introduce a more "lo fi" tone. Upshot is that you lose headroom, so if you need to play the amp at volume, whilst maintaining decent/typical headroom you are better off with matched tubes.

You often still have quite a margin for differing currents before you start to suffer with crossover notch distortion, but yes, at low levels with cool bias, you might just suffer a weak/thin tone, without audibly hearing the effects of crossover distortion.

[stoneattic]

I only had the crackling happen at practice on Wed. I didn't play it at all on Thurs and on Fri I just did the bias measurements without playing. Then I put the EIs with the 40/44 bias numbers in and played for a bit and it sounded wonderful. It shimmered with great harmonics and, at least for me, sounded better than with the Mesas. There was no sign of the crackling, but then again I only played it for half an hour, not four.

I worry that the higher bias current will be really rough on the tubes and perhaps the amp. Any thoughts on that?

I have plenty of 12AX7s so I may throw some new ones in, but I tapped each one last night with a chop stick and they were all silent, so I don't think they are the cause of the crackling.

Either way I will bring a spare amp tonight, as usual.

[MWJB]

EL84 are run over max rated dissipation in many amps that are reasonably reliable, 28/29mA per tube is perhaps as low as I would go, they can go sterile much under 30mA. So I'd aim for 28-35mA, making sure that tone at the lower end is acceptable.