B-52 AT-100 repair!

 This B-52 all tube amplifier head came in for inspection and repair. It had a few issues and we wanted to open it up and see what's going on in there. It's a high-gain 24kg monster with a brutal overdrive sound and quite a few features. It's got three transformers (for the output, AC input and reverb effect), a choke coil and a tri-mode rectifier circuit!

 Here are the official specs of the B-52 AT-100 head:
• All-Tube Signal Path
• Tri-Mode Rectifier: Tube - Class A or AB,
• Solid-State mode - Class AB
• Overdrive Channel: Gain 1 and Gain 2
• Each with Independent Master Volumes,
• 3-Band EQ, Assignable Contour Control
• Clean Channel: Master Volume, 3-Band
• EQ and Bright Switch
• Global Controls: Volume, Low-Res
• (Resonance) and Reverb
• Effects Loop with Level Controls
• (Footswitchable)
• Line Out with Level Control
• Selectable 4 / 8 / 16 Ohms External
• Speaker Jacks
• 4-Position Footswitch: Channel, Gain,
• Reverb and Effects Loop
• Ground Lift Switch
• Dimensions: 27" x 11" x 11"
• Weight: 53lbs

Source: http://www.b-52stealthseries.com/at100.html

The problems:
The amp made a few delay-like noises, the tone had changed on both OD channels and the reverb effect did not work at all. The At-100 uses twelve tubes and the preamp tubes were my first suspects.
Here's the list of tubes used:
4 x 5881 power tubes
7 x 12AX7b China preamp tubes
1 x 5AR4 rectifier tube

 The Chinese made 12AX7B type tubes are responsible for the high gain sound of the At-100 but, like all tubes, they tend to fail at some point. This was the case with three out of seven preamp tubes: V1, V2 and V3 which became microphonic or useless over time. We decided to change these tubes with JJ and Sovtek branded 12ax7s. The delay noise and tone change was fixed but the reverb effect was still off.
 I inspected the electronics for signs of damage or wear (especially the electrolytic caps) and cleaned up the pots, jack and switches with some contact cleaner. Everything seemed to be working fine apart from the reverb tank. One side of the on-board reverb tank was out. This meant that the coil used to drive the reverb springs did not work anymore and no sound signal could pass through. A new reverb tank is a bit expensive so we decided to disconnect the faulty tank and leave it off.

Here you can read a lot about how reverb tanks work:
http://www.amplifiedparts.com/tech_corner/spring_reverb_tanks_explained_and_compared

Here are a few pictures of the AT-100 head:

-Thanos