Marantz 8B Vintage Tube Amplifier - SkyFi Serviced and Gorgeous

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General:
This is the Marantz 8B Amplifier from the mid 1960's. It was a fierce competitor to the McIntosh MC240 amplifier and vintage Harman Kardon Citation amplifiers. Weighing in at an impressive 56 pounds it is built very well.

Not many of these are in good condition today, due to their age and chassis rust. Plus because they are heavy and hard to handle, many were dropped, dented, or dinged.

This 8B luckily has none of those negative attributes and is in great condition. 

Plus for peace of mind, it was serviced in-house at SkyFi where the "known to fail" components were replaced.

Be sure to watch this video from our YouTube from a previous 8B listing for more information on this piece of HiFi history:


SkyFi Cosmetic Notes:
This amplifier is in really great condition for its age. No significant marks or scuffs to make mention of, which is really something considering this is around 60 years old.

SkyFi Technical Notes:
This 8B underwent preservative servicing here at SkyFi.

This included the replacement of known to fail components in the bias supply, and the main power supply. We replaced the output barrier strips which were damaged. A new set of feet was also installed.

Important to note, the signal path components are all original with the exception of a couple of electrolytic caps which were replaced with modern equivalents. 

The desirable tube set  included with this amplifier is as follows:

A new old stock (NOS) pair of GE 6BH6, a strong vintage pair of 6CG7, and a set of current production Svetlana EL34. 

The bias and AC balance were set on our bench, and the amplifier was tested for proper output power and distortion performance.

It then moved to our listing area where our staff and clients enjoyed it for well over a week, paired up with modern Klipsch La Scala speakers.

I'm listing to the 8B as I type this right now, with period correct Frank Sinatra raising the hair on the back of my neck.

Ownership:
Single Owner

Connections:
RCA, Speaker screw down terminals

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
8/10 = Very Good. Excellent front faceplate, one minor flaw on chassis side or top. See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Just the unit and power cord.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.Specs:
Power output: 35 watts per channel into 8 Ω (stereo)
Input sensitivity: 1.3V
Speaker load impedance: 4 Ω to 16 Ω

Dimensions:
3.5 inches wide, 7.25 inches high, 10.5 inches deep

Weight:
56 lbs.

Approximate Age:
1965

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Testing Process:
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 a modern Amplitrex AT-1000 Professional 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 1 KHz test signal, slowly increasing 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 1 KHz square wave check and a 20 Hz to 20 KHz 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 minimum.

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.