Tonewheel General Hospital
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Tonewheel General Hospital

There are lots of aging Hammond Tone Cabinet and Leslie amplifiers out there, as well as Hammond organ preamplifers. Some of these go back to 1935! In most cases, as long as the power and output transformers are in good condition rebuilding or repairing these units is feasible.

What is involved in an amp or preamp rebuild?

To rebuild (replace everything), or repair? That, is the question. Very few amplifiers, and even fewer preamplifiers, need to be completely torn apart. The goal with either should be the restoration of the unit so it reproduces sound the way it did when new. This is all about the sound, right? If the goal is to come out of the repair with a completely different sounding unit, then maybe the funds should be directed at a completely different sounding unit instead. There are a few items that are good candidates for replacement in both cases, and it seems most cost effective to begin with them, then evaluate the preamplifier/amplifier for further restoration.

The philosophy at Tonewheel General is to perform a complete evaluation of the unit before any work is done, making note of all voltages and tube conditions. Yes, this takes a little time, but, again, it is all about the sound, right? How can we know that any changes performed have made an impact if we don't know how the unit was performing before we started? Add to that any specific complaints made by the owner, and a picture of what needs to be done begins to emerge.

At the very least all the electrolytic capacitors should be replaced in these units. Leslie used a plug in module and expected it to be replaced at frequent intervals. Hammond used the same style modules, but made them less easy to replace, requiring they be soldered in place rather than plugged into sockets. Tonewheel General installs new CE Distribution tubular can capacitors to maintain the stock visual appearance of the unit.

Most electrolytic capacitors are used to reduce the AC component (called ripple) from the DC voltage that results after the rectification process (where AC voltage is turned into DC voltage). When they fail a distinct hum or buzz is heard through the speakers. You can verify these are the source of the noise by playing a low B flat tone while listening to the hum; a full wave rectifier will turn the 60 Hz. ripple into a 120 Hz. tone which is close to the frequency for B flat. If the two sound nearly in tune (beating slightly) the capacitors probably should be replaced.

As these capacitors reach the end of their life (fifteen years was their expected life), the electrolyte inside dries out, causing the two sides of the capacitor foil to short circuit. The units keep going (usually) but the resulting sound is distorted or buzzy from the lack of filtering.

Tubes should be checked for emission, mutual conductance and shorts; tube sockets should be checked and cleaned. Hammond organs are generally pretty easy on the tubes, so blindly replacing all of them is rarely the best approach unless there is no way to evaluate them individually; same with Leslies. It is not uncommon to find 50 year old tubes that test (and sound) good. Hammond tone cabinets, however, seem to wear out tubes at a greater pace, particularly the rectifier and power tubes. In the case of power and balanced input driver tubes, they should be replaced if they no longer provide a symmetrical amplification of both phases of the input signal.

This is good place for a word about tube testers. There are a number of tube testers out there, some very good and some not so good. Regardless of their state of goodness, they are old and subject to the same ravages of time that our musical instruments are. Every "serious" tube tester has a calibration procedure; if yours (or your tech's) has not been calibrated recently use the results with caution.

Plate and screen resistors are next in line. These items are subject to hundreds of volts and over time they increase in value, causing the tube to operate at a less than optimal level, resulting in lowered output, distortion and poor sound quality.

At this point in the process the unit is reassembled and run through the same evaluation as when it arrived. It should be well within the operating parameters specified by the manufacturer and should sound good. Any pops, clicks, thunderstorms or wave actions that remain can be localized to specific areas and targeted for removal; good troubleshooting procedures (and experience) require a relatively small amount of time to isolate and eliminate these. When this is finished the unit is back to factory specifications and ready for return to the customer, usually with just a handfull of parts replaced.

How much can you expect to pay for a rebuild or repair?

Performing the above procedures on a Hammond AO-28 (B-3/C-3/A-100 style preamplifier) typically runs around $240. This does not include tubes, tube sockets, transformers, and other parts (resistors, capacitors, etc.) not involved in the initial analysis and correction phase described above. Complete rebuilds use the Trek CPR-28 kit and are $700.

Performing the above procedures on a Leslie 122 or 147 amplifier typically runs around $150. Complete rebuilds are $225 plus parts.

Turn around time is generally five business days or less. All units received for repair will be evaluated prior to being worked on and an exact price quoted.