McIntosh MC3500 Legendary Vintage Tube Mono Amplifiers - Unmolested - Call For Restoration Options

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Please call us to discuss available restoration options and pricing.

General:

As equipment restorers our initial impulse is to cosmetically restore every piece of equipment that comes in. With this particular set due to the Woodstock notoriety, collectibility, and amazing patina, we decided to resist the temptation.

They are gorgeous as is, warts and all.

The legendary MC3500 amplifier has become even more popular since McIntosh released a reissue maintaining the same looks, aesthetics, and design characteristics. If you haven't seen the new model, be sure to google it as they are striking.

Most readers already know that this model was made famous because it powered Woodstock. This specific pair was definitely not at that historic event as those were modified to have some of the connections relocated to the front panel for quick access. However this pair likely still has some stories to tell as they appear to have been well traveled.

Right now both amplifiers are working, providing great sound and passing a signal with little hum or distortion. We have tested all the tubes and biased the unit to McIntosh factory spec.

However, if they are going to be used routinely, they should be restored for both reliability and performance.

Some collectors may choose to keep them as-is while others might want them restored, so we will leave it up the buyer.

Please call us to discuss available restoration options and pricing.

Manual:

McIntosh MC3500 Original Sales Brochure

McIntosh MC3500 Original Owner's Manual

The SkyFi Testing Process for Tube Amplifiers:

We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use an Amplitrex AT-1000 Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy. We document the results of each tube and replace any weak or suspect tubes before proceeding. When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable. If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged. Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1KHz test signal, slowly increase its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. For tube amps we like to run this test at least twice. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.

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