Help! Mesa Stiletto is motorboating and then fuse blows

[rabies]

So I bought a brand new Stiletto Deuce stage 2 about three weeks ago from Mesa Hollywood.

They replaced two EL34's in the store after play test right off the bat.

I blew a fuse after I cranked the master volume to noon and output volume to noon (played fine for about 30-60 min's). There was a 250ms delay effect (w/o a delay pedal!) or motorboating before the fuse blew.

I reproduced this same thing after Mesa replaced a screen green resistor that was 5% off spec and replaced all four EL34's.

I reproduced the motorboating and blown fuse a few days later.

So three failed power tubes in less than three weeks on a brand new amp.

It's not the speaker cable or spkr cab as I tried my Mark IV at nearly the same volume (85 watt amp, noon on master and noon on output) with the same cable and cab.

Any ideas as to what's going on here? The amp seems to have a load issue. The motorboating did not occur when the output volume was around 10 o'clock...

thx.

[PRNDL]

Can you return the amp?
That's the easiest fix. Get one that works right.

I would check the return policy closely, and see if you can get another one, because something is wrong, and who wants to wait while it's shipped to Mesa for repair.

Most likely the output tubes are mis-matched.
Some believe that you can't rely on Mesa matched tubes - you have to measure them in the amp.

If you really want to keep the amp, find someone that specializes in tube guitar amps to bring it to.

PS
Mesa's aren't easy to fix, and a lot of repairmen hate them.
I've noticed that people like the heavy metal tone, but few builders like them.

[anatech]

Hi rabies,
Get an exchange if you can. If the next one does similar things then I wouldn't trust that design. You've been through enough already. If the store will not exchange it, call Mesa direct and tell them your story and request a swap. They do not want bad press and you don't want a lemon.

Personally, I like Fender amps the best. I find 6L6 types to be really tough. KT-88 models are real animals, but how much power do you really need?

-Chris

[rabies]

thx for the feedback guys. I dropped it off @ mesa hollywood and they gave me a full refund.

I'm still dying to know what caused that motorboating effect at higher load (output = noon).

I'm just wondering if somehow the amp was seeing a load less than 4 ohms due to the spkr cable and/or cab......

They weren't able to explain the root cause of that motor boating symtpom but I've read it can be a bad filter cap. who knows....

That amp was a hot-rodded JCM 2205. But I have a Mark IVB I like a lot.

I've never had any power tube shorts/failures in the past with my Mark III or RK1. Those were all Mesa power tubes. They use Robotube technology and the cool thing with Mesas is they're fixed bias so you don't need to bias your amp when re-tubing.

My TSL100 when I had it I used four E34L JJ's. No issues with those for about a year solid (cranked at 7 out of 10 routinely).

I was thinking about getting some KT77 or E34L's from Bob @ eurotubes, their tubes are cheaper than Mesa or GT.

KT88 I heard will knock you down. Be careful!

[rabies]

BTW, Mesa's policy with tubes is as follows: you can use JJ's or another brand, but if the power tube shorts and causes damage to another part of the amp, your warranty is void.

that's the problem if you own a Mesa under warranty, you're basically stuck with their tubes and that means no KT77, for example, b/c only JJ and GT sells them AFAIK.

if you use GT with a Mesa, they recommend medium (4-7).

[anatech]

Hi rabies,
I'm glad to see it worked out for you. You may never know what was going on with that amp because it may be a design wrinkle they haven't worked out yet.

Stay with what you like. Conservative designs will treat you the best over time (conservative guitar amps - Ya, right!).

-Chris