Mesa V Twin problem.

[davefromtheattic]

No, that's cool. The power supply is only rated at 300mV. Doh! I'll pick up a new one tomorrow and see how it fares.

Thanks a million guy's.

[dbtk44]

Power Supply AC-1 for the V-Twin is 12vac, 1000ma

[dbtk44]

If you can't get a replacement wall wart from Mesa ( and they're kind of expensive IIRC ), you could take a 12vac 1a ( or larger) transformer from Radio Shack, wire a power cord to it and the appropriate plug on the secondary, and you'd be in business as far as power goes.

[Enzo]

If you apply DC, the only failure mode I forsee is if the DC through the transformer primary burns it out. I bet it survived. The three low voltage supplies all have rectifiers, so reverse DC is blocked and forward DC passes by and powers the supply. You can generally apply DC where AC belongs, the result will be one polarity of supplys will be absent inside the unit.

Rather than wire something up, I'd just get a 12vAC 1A wart and plug it in.

It is a cinch that with a DC wart, the tube will see no B+. Snce your LEDs come on, then I suspect the heaters do too, but you are missing the -12VDC. ANd of course the B+.

Get a proper wall wart, see what happens, and then come back.

[davefromtheattic]

Thanks for the comments guys. I have ordered a new psu, I was in fact using a DC supply. I foolishly thought that if you changed the polarity it turned it from DC to AC. Spot the moron (yes that's me).

Once again, many thanks for the help.

[davefromtheattic]

I got the correct power supply and the v-twin is working just fine. Once again thanks for the help in resolving my problem, simple as the resolution was!

[bulldogguitars]

Im glad to hear that it was that simple.

[ragnarisaprirate]

Mesa V twin killing power supplies

Hello,

I know that this is an old thread, but, it came up in my google search, so I thought I'd give it a whirl.

I have a v-twin pedal that has killed not 1, not 2, not 3, but 4 power supplies in the past 6 months. They work for a bit (getting hot the whole time), and then give out. They are all AC units, that put out 1 amp or more. The pedal works fine until the power supply gives out.

I took the pedal apart to replace the switches (all to Carlings) as some of them stopped working, and have also replaced some LEDS, but other than that, have done nothing to the pedal.

What could cause this?

Thanks in advance.

[J M Fahey]

Probably that "1 Amp" rating is marginal, in the best case.
Buy a 12V 2A (or 3A) transformer, straight from a reputable supplier, and mount it on an approppriate case yourself.
The PSU market is a cutthroat jungle and undercutting a competitor by even a cent is their way to survive, so try to bypass that by specifying yourself what (quality) transformer to use.

[soundguruman]

The Mesa is actually a solid state preamp with tubes stuck on the outside.
Inside, there are many IC chips, containing hundreds of transistors.
If the power supply is reversed, it will blow the transistors. OR sometimes these FET transistors fail, and draw too much current, killing the power supply.
If you do have a working power supply, power the unit up, wait 15 minutes, check the temperature of the IC chips one at a time.
The IC chip that contains one or more blown FET transistors will be very HOT. Install sockets and replace the HOT IC chip(s).

Then the unit will draw less current, and the power supply will last longer.

Being that this is a double sided circuit board with plated through holes, only an expert should attempt to remove IC chips from the board. Some of these units have sockets for some of the IC chips. The chips installed in the sockets are the ones that are expected to fail. On some of these units, the sockets make it easy to change the failed IC chips. On others, there are no sockets, you gotta install them.

[J M Fahey]

The Mesa is actually a solid state preamp with tubes stuck on the outside.

Please check the schematic.
It not only has one *tube* preamp inside, it has *two*.
Actually independent, you can pull both triodes belonging to one of them, you kill it but do nothing to the other, and viceversa.
Talk about independence.
The first preamp has a "Fendery" preamp, which distorts on its own, no SS distortion involved; which is impedance-buffered , both before and after (good design practice) by two unity-gain op-amps , U1A and U2, which of course do not alter the sound in the least.
The triodes involved are V1A and V1B,
The second preamp also uses 2 triodes in a typical "tone stack driver" (V2A and V2B), they are impedance buffered by U3B and U2 , and they are also driven by SS preamps U4A and U4B, to achieve very high "Metal" gain.
Most of the distortion is still provided by a tube: V2A, although some is also added, above certain levels, by some clipping diodes.
Nothing very different to what's seen in many high gain tube amps.
In any case, the last link in the distorting chain, and the one that imparts its own flavor, is still V2, so I guess that their function is a little more important than impressing kids.

Back to the original problem: any of those Op Amps plus a couple logic ICs used for switching or any of the FETs might have died , but not because of "bad polarity" because the VTwin uses a 12V AC wall wart.
Try it with the correct type one, it might even work at once, no further trouble.

[soundguruman]

"The first preamp has a "Fendery" preamp, which distorts on its own, no SS distortion involved; which is impedance-buffered , both before and after (good design practice) by two unity-gain op-amps"

In other words: It's a solid state preamp with a tube stuck in between two op amps.

"no SS distortion involved"

Without the transistor buffers, there is no distortion, therefore, transistors are integral. Stop pretending it's tube distortion.

[J M Fahey]

J M Fahey said:

"The first preamp has a "Fendery" preamp, which distorts on its own, no SS distortion involved; which is impedance-buffered , both before and after (good design practice) by two unity-gain op-amps"

The self appointed Sound Guru said:

In other words: It's a solid state preamp with a tube stuck in between two op amps.

"no SS distortion involved"

Without the transistor buffers, there is no distortion, therefore, transistors are integral. Stop pretending it's tube distortion.

Is that so?
OK, please explain in technical terms:

1) how do you get a unity gain TL072, powered by +/-15V rails, passing a straight guitar signal (100/200mV average, 1V peaks) to distort/clip/limit , you name it.
To make things less confusing to you: please explain how a clean guitar signal as defined above, applied to U1A, pin3, appears distorted on U1A, pin1. ("integral SS distortion" as you call it).

2) explain how said cascaded tubes, when driven by a 1V peak signal (as defined above), straight into V1A, pin2 (grid) will not distort; yet when that same signal is passed through U1A, obviously retaining its 1V peak, now will magically distort.
Of course, the clipping/distorting stage would still be a Tube.(V1B)
That would define it as "tube distortion", wouldn't it?

I'm all ears.