Tuesday, March 4, 2014

1982 Mesa Boogie Mk1-2

This road warrior came to me the other day in sad shape. Really bad tone, volume drop, splatty distortion and noisy. Upon first observation one of the JJ 6L6 tubes had overheated, the red paint had turned completely brown. I figured maybe it had blown that tube but the fuse hadn't blown and the amp still worked. Each tube tested good.

The culprit? A number of issues. First, whoever installed these tubes did a really poor job of it. In the Mesa bias circuit there are 2 resistors running parallel to ground, a 10k and a 6k. There is a myth I've heard many times that Mesa amps are 'self biasing'. This isn't true at all. I've also heard that maybe Mesa perpetuated that myth so they can sell Mesa branded tubes that are selected to run safely in the amp due to what the bias is set at. Regardless there is no truth to this. You replace the tubes, you need to re-bias the amp. These do not have a bias trim pot so you need to replace those resistors or install a bias pot to get it right.

And, coupled with the fact that they replaced those 2 resistors with a 15k resistor taking the bias voltage from -52 to -60+ volts you have an amp that is biased way too cold. Then you have JJ tubes. You need to bias JJ tubes differently in general. They tend to run cold under normal circumstances. In this case they were drawing a whopping 2-4milliamps each. Barely enough for the tube to open.

So first we experiment with resistors. I didn't wind up installing a trim pot today, I did wind up installing 2x 4.7k resistors in parallel to get a bias of 29-30ma per tube.

So what about that tube that got too hot? At some point in this amps life a couple of very small value caps were installed from pin 5 (control grid) to ground. I assume these are to kill some very high end oscillation. One of them was shorting creating a bias voltage drop on that tube thus drawing far too much current. I pulled them out and tossed them. Next, replaced the 470 ohm 2 watt screen grid resistors, 2 of them were charred. Fortunately this is a good set of JJ tubes. If you get a good set they last, they are tough and can handle the abusive environment inside a high gain amp like this. Other choices are the good ol' Sovtek 5881. A Chinese tube would have been toast most likely. Choose wisely.

If you look carefully I did install a couple of 1 ohm 1 watt resistors from pin 8 to ground. This is the easy way to measure bias.

Next the preamp. Noisy and cutting volume. There were a number of things that needed to be addressed. The rice crispies and milk sound was simply a bad preamp tube, the dropouts were from cap failure.

I replaced the filter cap feeding the first preamp tubes and the cathode caps on the first stage tube. This cleaned that problem right up. The rest was routine maintenance. Tighten the pots and jacks, clean the sockets and pots.

Mesa amps are complicated beast. I enjoyed this one, up to the Mk2 I usually do. Once you get into the later amps you are dealing with double sided circuit boards, heat issues, a repair mans nightmare. I usually turn them away. It's very easy to spend as much time on one trying to track out one problem and it takes to overhaul an old Fender or Marshall. They are not a beginner amp! I like Mesa for this:


Publishing a repair manual for all to see. That's classy. I've dealt with them on the phone and they have been super supportive and cool. They stand by their products. That's rare in today's world. These older ones are BEAST. I remember the first one I ever played. It knocked a half stack away with that single 12" speaker. These early ones are built like a tank so if you happen upon one, they may be one of the most versatile amps you can find.

Thanks for reading and happy soldering! JB

Friday, January 17, 2014

1971 Vibrochamp

Okay, sorry for the crap photo:

This came in for some TLC. It was cutting out and sounding weak when it was working. Simple solutions on this one. Power switch was dying and replaced the 2 12AX7 preamp tubes as they were weak. Also replaced the cathode caps which helped the background noise. On the 6V6 470 ohm cathode resistor someone had replaced it with a Sprague Atom 25@25V. I've had those pop in the past in that position so I replaced that with a 25@50V cap.

The back panels were missing:

Had a new set made by Mojo Musical Supply. They do great work:

Added some "tube teeth" to prevent the 5Y3 and 6V6 tube from falling out:

It's not uncommon for these tubes to fall out during a performance no matter how much you re-tension the sockets. This was no exception. Amp has a heavy glass JJ 6V6. Gravity dig?

Here's some shots of the inside of the amp:

The infamous "brick drop" 70's CBS caps. These tend to be leaky but not in this case. I prefer to leave signal caps alone in an amp unless they are leaking or dead. Better to maintain the integrity of the amp.
And in this case, this is easily the best sounding VibroChamp I've had the pleasure of working on. Big and warm, full bodied tone. These can sound really scooped in the mids but this one is just majic.


Friday, January 10, 2014

RCA BA-2C rebuild.

This walked in the other day from a fellow who met me in Los Angeles. I sold him a preamp I built so I could make that months rent. Small world, found me here in Brooklyn through Linkedin.

In it's original form you can really see the folks at RCA were genius. Such beautiful wiring.

What amazes me is there is an absence of shielded cable in this, the inputs and outputs are right by the 120V ac in. How did they build these to be hum free?

First thing, the filter caps are DEAD:

You can see the now dried up interiors making their way outside. I'll also change the old power supply resistors, the main signal cap and the load resistor. One of the 1620 (super quiet 6J7 tube) tubes was dead too, I'm replacing both with NOS ones. The rectifier has years to go so I'll leave that.

My customer opted for the "spare no cost" correct filter cap way to go:

This job could be done much cheaper by wiring individual caps inside but ya know, this is really the way to go all out on such a beautiful piece of vintage studio gear. I did have to wire a couple caps inside only because I couldn't get the correct cans.

But the best part of course is the sound.......

And the last difficult to find part, these lovely tube grid shields:

Thank goodness for Ebay!

Voice sample:

And one of my trusty old Epiphone Spartan:

And some fingerpicking. Just a good old Yamaha FG-150:

Yeah, I want to build one of these now. Easy to fall for a great sounding piece of studio gear. Amazingly enough this baby is quiet, virtually hum free. Warm as can be but not fuzzy. That's what I like. Damn, I want one....