Mesa Boogie Express - pre amp tube probelm?

[donnyb]

Hi, I have a MB 5/50Watt and in the last year have replaced all the tubes, main power tubes (2) and the pre-amp(5).

I noticed a kind of crumbling of my solo notes when in distortion mode at a gig over the weekend. A bad distortion, like a condensor microphone as its battery starts to go weak.

Does anyone know if all 5 x pre-amp tubes are meant to be lit up when this amp is switched on, with stand-by turned off - ie the amp under no load, but will put out sound as soon as a guitar is plugged into the amp's input. ?

I noticed that 2 of the 5 pre amp tubes are not lit when in this situation , namely, the 2 tubes on the left hand side of the bank of 5, when viewed from the rear of the amp.

Thanks,

Don

 

[AR-305]

Yes, the filaments to all five tubes should light up when you power on the amp, whether in standby or not.

 

[phsyconoodler]

Some russian preamp tubes are difficult to see if the filaments are on,but if all functions are working on the amp they are working.

Your problem may not be tube-related and you should seek a tech or take it in for warranty.

 

[donnyb]

Thanks for that. It may be that the heat glow is hard to see on those 2 tubes. I will first try substituting each with a spare 12AX7 I have and report back to the forum. COme to think of it, my last thread eventuated in me repalcing all tubes with a set form Dougs tubes. The amp sounded like it had never sounded, even new (or at least, as it sounded in the shop). But I havent been able to find that tone and response over the last few months. Put it down to having a bad day or a quirky pedal or guitar lead. I agree, life's too short to put up with bad tone.

 

[donnyb]

I said I'd report back on this problem. It appears it was caused by me adding another new pedal to my daisy chain, without adding up the current draws of the 4 devices.

I haven't tried to replicate the situation I had when the problem occurred, but when trying the amp out just by itself today,( after checking the pre-amp tubes wear all warm, and yes, you cant always see the glow with some tube tops), the amp by itself sounded great.

So, I assumed it may be the pedals or leads, I checked all leads and then the total current draw of all the pedals, which I found exceeded the Boss power supply's 450mA.

I added another power supply for just one of the pedals, tried that, and the amp sounded fine. Also noticed the compression pedal sound much better.

Incidentally, I noticed at the gig, that I needed to turn the compressor's volume up more that usual, so I am assuming now that it was not getting enough juice due to the current sharing.

Also, in my original post, I said that the problem distorted sound I had at the gig was reminiscent of a condenser microphone battery running flat, which is (in hindsight) a pointer to the power supply as the problem (?).

Again guessing....I am thinking that as the gig got going, the single power supply got a bit hot having its guts sucked out of it, which would have made matters worse , with the heat -resistance relationship (?).

So I think I have sorted it. If anyone' s running multiple effects from a daisy chain, it pays to check you're not at or over your power supply's current draw limit, as like I said, the compressor pedal is noticeably better ie brighter than at the gig.

As I am not electronically gifted, if anyone who is reads this and thinks I've added 1+1+1 and got 4, please add a post.

Will post again if the problem re-occurs at the next gig.

Thanks, Don.

 

[Koop]

Sounds like you're on to something, but I have to ask: Were you running all of the pedals at the same time? If the pedal is "off" it shouldn't be drawing much if any current.

 

[phsyconoodler]

Sounds reasonable.When you want help in the future,details like pedal boards should be removed to get at the amp's functionality first.
More information is always best when describing issues.
We would have your amp in pieces long before you mentioned pedl board issues.

 

[donnyb]

Koop, I was running 3 of the 4 pedal at once at times, but dont recall which combination at the time of the problem sound. This was the first gig I had used 1 new pedal ( a delay stomp unit) but it has a stated small current draw of 65mA. (its a rockton boutique series). Im still not 100% confident the issue is sorted. It may be that something else is 'getting hot' (etc) as our set list proceeds. But I will post again after our next big gig in a weeks time. 2 x 1 hour concerts. That should flush it out ! And yes Phsycondoodler, I did ask a bit too soon. Got me there.

 

[Koop]

Please let us know what you find. Some pedals, such as my StarTouch A/B/Y will draw a small amount of current any time it's plugged in (to light the LEDs) but generally your board will only draw current through the pedals that are active.