MESA/Boogie Studio .22

Intro

The Studio .22 is one of the Caliber series of amps introduced in the mid 1980s. It is a footswitchable dual mode (rhythm/lead) single-channel amp with reverb and optional graphic EQ, using five 12AX7s and a pair of EL84s. It was accompanied by the .50 Caliber (originally with four EL84s and later with a pair of 6L6/5881), and the advertised but not produced .38 Caliber. In combo form it came with a 50-Watt Vintage Black Shadow speaker. It was convertible to rackmount. After a few years it was revised and called the Studio .22+. This version was a bit less noisy, had a Lead Master control, moved the reverb out of the chassis to an external tank, and had black rocker switches rather than chrome toggle switches.

The Studio .22 is noted for a very saturated singing lead mode that has quite a "spongy" feel and somewhat dark midrange-heavy tone, as opposed to say a Marshall with its bright tone and aggressive immediate feel.

This page describes a few things I've run into with my Studio .22 combo. I bought it new in 1987. It's the version without graphic EQ. Here's a shot of the circuit board for reference.

small Studio .22 circuit board

Cooling

The Studio .22 runs pretty hot with its pair of "Class A" EL84s. The bigger Boogie combos have a fan built in. Why not do likewise for the Studio .22? I used an old 120V computer "muffin fan". With two bolts attached to the rear chassis grille, installed from the inside out so that they stick out the back, I can hang the fan from the bolts when using the amp. Before adopting this habit, the amp went through 12AX7s in the first two spots fairly quickly. After I started using the fans, the tubes lasted many many years.

Footswitch circuit

A common failure in the Studio .22 involves the footswitching circuit. In the early amps, a 680 Ohm 2 Watt resistor could burn out, sometimes taking a nearby cap and diode with it, and depending on what exactly failed, the amp could be stuck in the clean mode or could make weird noises. My amp had this problem once quite early on, and again after another ten or so years. I converted it to Studio .22+ spec (just for this part of the circuit) in 2016 while fixing other things.

There were a couple of successive factory fixes for this. Increase that resistor to 1K Ohms, 2 Watt or 5 Watt to handle the heat. Replace the 16V 470 uF capacitor at C9 proactively if it hasn't outright failed. Convert to the later Studio .22+ specs by using 3300 uF for C9 and add a 330 Ohm resistor in parallel with C9. For ease of service, you may wish to locate the replacement parts on the back of the circuit board.

By the way, if you forget your MESA footswitch, you can put the amp into lead mode by plugging a cable into the footswitch jack and grounding the tip of the plug to a convenient ground, such as the chrome handle caps.

Graphic EQ

If you don't have the built-in graphic EQ, you can still come pretty close with an outboard one plugged into the effects loop. The effects loop is not at precisely the same location in the circuit as the built-in EQ, but it is close.

Then the only trick is to mimic the EQ-Auto feature whereby the EQ is bypassed when in rhythm mode, and enaged when in lead mode. To do that you can replace the single-pole switch in the original MESA footswitch with a double-pole switch, and replace the jack with a stereo jack. Make your own special footswitch cable using mic cable or any other stereo (2-conductor plus ground) cable. Put a stereo plug on the footswitch end, and two mono plugs on the amp end. One of these mono plugs will go to the footswitch jack on the front panel as usual, and the other will go to the bypass jack on your outboard EQ. Presto: EQ-Auto.

Intermittent signal drop

This problem is not unique to the Studio .22. Any amp that uses switching jacks in its effects loop can develop the problem. If the amp starts cutting in and out, or suffering a loss of signal level, try plugging a patch cord directly from the effects send to the effects return. If that fixes it, clean the jacks, especially the switched one. Proactively, you might consider inserting a short patch cord permanently across the effects loop if you rarely use the loop. (Some sound technicians install shorting plugs in the insert jacks of mixing boards for the same reason.)

Ghost notes

My amp developed an unpleasant-sounding ghost note that would follow the notes I played, but at a lower pitch, in both the clean and lead modes. I replaced the capacitor in the bias supply stage of the power supply, and that cured the problem. I guess after almost 30 years, these electrolytic caps are getting a bit long in the tooth. This is an inexpensive capacitor and a replacement fits easily.

Hum

While addressing the ghost note problem described above I thought I would also try to reduce the background hum that the amp had developed. This hum sounded like it was 120 Hz, which suggests power supply filter capacitors. It wasn't offensively loud but why not try to fix it. There are three 450V 30uF electrolytic power supply filter caps. They are the large light-blue ones in the photo. The originals are MESA-branded and the exact part number 525450 is no longer available. MESA does have a substitute part 525535 that you can order directly from them or through your MESA dealer at roughly $6 USD each. Instead I ordered F&T brand from Antique Radio Supply (www.tubesandmore.com). They're more expensive but have a good reputation.

I thought the original MESA caps might be rebranded Sprague Atoms, but the Atoms are physically a fair bit larger. It turns out that the F&T ones are also larger than stock. They will just barely fit, but they'll contact both the circuit board and the chassis. I decided to mount them on the back of the board instead where they have some clearance between the board and the cabinet.

The hum was greatly reduced with the new caps.

(By the way, some people claim that Sprague Atoms these days are not like the originals. They're ordinary caps inside a large dummy shell that makes them look like the old Atoms for strictly cosmetic purposes. And they're expensive.)

Circuit board removal

It is awkward to remove the circuit board for service. Here's how I do it. (NOTE: Be careful! You can seriously hurt yourself with high voltage. I'm not responsible if you hurt yourself.)

  1. Unplug speaker wire and remove the four bolts holding the chassis to the combo cabinet.
  2. Slide the chassis out of the back of the cabinet.
  3. Unclip the tube retainer pan and carefully remove the tubes.
  4. Pull all the knobs off their stems. The stems are plastic. Pull straight back carefully so as not to break them.
  5. Using a socket, remove the nuts and washers from the input jack, the footswitch jack, and the control pots. No need to do the standby and power switches.
  6. Remove the Scotch tape that is probably holding the very thin green and black wires on the circuit board that go to the reverb unit.
  7. Using needlenose pliers, pinch the tabs of each white stand-off post on the bottom of the chassis so the tip of each post can be pushed back through the hole in the chassis.
  8. When the circuit board is free of the chassis, i.e. all the stand-off posts have been released from the board, lift the front edge of the board up a bit and push the input jack, footswitch jack, and all pots into the chassis. They have toothed washers on the inside so catch those if they fall off.
  9. With some fiddling you'll be able to get the controls and jacks out from behind the front panel and clear of the chassis. Now you can carefully flip the circuit board most of the way over, being careful not to pull those tiny reverb wires loose. There is very little slack in the wiring at the transformer end of the chassis.
  10. You'll want to rig up some supports to hold the chassis in various orientations so you can position the board as needed to work on it.
  11. Assembly is of course the reverse. Needlenose pliers may help for pulling the controls and jacks back into place, slipping the washers on, etc. Don't forget about the opening in the end of the chassis as a spot to reach through to position the jacks.

Other mods

Some people retro-fit the Studio .22+ features to the Studio .22, specifically the Lead Master feature. A Lead Drive is another common modification for both amps. I have not done either of those. I did move my amp to a Mark series cabinet in order to get the SUS-4 suspension mounting feature because why not. I investigated retrofitting a SUS-4 kit to the original cabinet but there are some subtle differences in the cabinet that make this impractical.