Mesa Boogie TransAtlantic 15 TA-15 plate voltage

[pecorporation]

I have a TA-15 that keeps coming in because it keeps "eating" Mesa EL84s at all color ratings, 1 to 6 months on average. The unit has the R69 18K bias resitor installed putting the bias voltage at -13.2 VDC, which is where Boogie wants it to "run the tubes cooler". This puts the plate voltage at pin 7 on the power tubes at +407VDC in 25 watt mode (spec'd @ +385VDC on schematic). The output wave form looks clean on the scope and sounds great when played, it's just in a couple of months, it's sure to short one of the tubes again.

Has anyone else encountered a TA-15 that consumes power tubes like this?

[Chuck H]

That's a late model amp so I'm surprised the Vp doesn't spec. I haven't worked on one of those but I can tell you that Mesa used to bias at -15V for el84's at 400+Vp (something they love to do). I can also tell you that at over 400Vp you're guy is lucky if he get's 6 months. Especially if he actually uses it, especially if t's a combo amp and especially if you've been loading it with typical el84's. I've had much better luck using the Rusky 6p14p (sort of written 6n14n and often translated poorly). Research the different suffixes because there are models made specifically for longer life at higher volts as well as durability against microphonics, which is critical for el84 types in combo amps or they'll rattle apart in no time. They're starting to get scarce, but they're still worth finding IMO.

If this is a head amp I'd try to install some kind of anti "no load" protection like a shorting jack or a resistor. Though it may be impossible due to how Mesa often wires their secondaries on multiple jacks. I mention it because IME el84's pushed hard at high plate volts usually go rattly and microphonic such that they need replacement anyway before they actually short. So maybe someone forgets all connections at odd times? It does happen if you gig a lot. But it's been a long time since I've pushed el84's that hard and I haven't used any late model standard el84's in a while. So much could have changed about new tubes that I don't know about.

[Enzo]

Or also try EL84M types as more sturdy than regular.

In a working amp, as in regular gigs, like Blues Junior or Classic 30, I expect about 6 months on a set of EL84. Does this bounce to and from the gigs in the trunk of a car or back of a van?

EL84s were made for table radios, not hard working amps.

[pdf64]

Is there a schematic lurking somewhere online?
For a degree of protection from the open secondary condition with that output arrangement, consider the YBA1 / AC50 method of a resistor across one of the sockets, eg 470 ohm 5 watt on the 16 ohm output.
http://bmamps.com/Schematics/vox/Vox...0Schematic.pdf
http://www.0rigami.com/vb/traynor_ba...mkii_yba1b.pdf

[Chuck H]

Something else you could do is reduce screen voltage. There are many amps that run el84's at or around 400Vp, but the ones I've known as tube eaters usually used a choke and have screens very near the plate voltage AND the power tubes were able to be overdriven hard. An exception would be the Peavey Classic 50 which seems to get pretty good life from a set of plain Jane el84's. But I don't think that amp pushes the grids too far beyond clipping. Anyway... If that amp will really drive the grids then getting screen voltage down may help. It may also change the tone, if it would be noticed. We don't have a schem so I'll just say that Mesa often uses a choke in el84 amps and there's no harm in adding a resistor in series with the choke. Check your preamp voltages first so you can reduce the resistance after the choke/resistor and restore it to stock. Otherwise there will be a more noticeable change in tone. Reducing rail resistance is pretty easy in some Mesa amps because they often use a string of small resistors in series. All you need to do is jumper a couple of them.

[The Dude]

FWIW: Mesa is the last "brand' of tubes I would use in anything. My experience is that over half of them are bad out of the box.

[Chuck H]

FWIW: Mesa is the last "brand' of tubes I would use in anything. My experience is that over half of them are bad out of the box.

Maybe they should put the website for The Lube Store on the boxes then

[pecorporation]

Thanks for all the replies, everybody. The TA-15 in question is still under warranty, and as such, I need to adhere to Mesa Boogie's stringent 'no alterations' and 'Boogie ONLY tubes' guidelines; otherwise, we void the warranty. My advice to the customer will be to take it up with Boogie. The last Boogie tubes that were installed only lasted a month under regular practice and performance.

[PurpleStrat]

pecorporation do you have a schematic for the TA15? If so could you please share it with me?