Marantz Model 5 Monoblock Amps Restored/Serviced

For your consideration, we have a pair of the rare and amazing Marantz Model 5 Monoblock Tube Amplifiers. These units came to us from the son of the original owner who has passed away.  We chose not to power them on, and instead had them serviced as we had no idea of the history. While we know that some collectors prefer to leave these units completely original, you must keep in mind that this amp was manufactured in the 1960's, so we prefer, in part for safety reasons, to go through and selectively restore them using modern components. All parts that were carefully removed, have been saved and will be included in the sale. They could be reinstalled if one would choose to do so. While the outside of the unit has some scratches and blemishes, it still presents itself very well. Inside the unit is very clean.  All tubes tested well on our Amplitrex AT1000 tube tester. 

After we restored the unit, I took it home for about six months and ran them in a modest system in my living room. I really enjoyed having them. This unit is highly collectable, but it also needs to be used and enjoyed. It is set up for another 60 years of faithful service.

While we service most of the units that we sell, we were fortunate to have the extremely talented Ben Hase of Novalux Stereophonic do the work on these highly collectable amps. Ben is very experienced, talented, and extremely intelligent. Anyone who is able to get a piece of gear that he has restored is very fortunate, as his work is impeccable. 

The following service and restoration work was performed on these amps:

Make: Marantz
Model: Model 5 (Monoblock Pair)
S/N: 2342 / 2667
Restoration Date: July 2024

Restoration

S/N: 2342
● Leakage test fail on 2 section negative bias cap - got hot during leakage testing
● Replaced selenium rectifier
● Replaced .1uF filament supply bypass cap - trace leakage
● Tested PS Mylar caps - Perfect for Value & Leakage
● PS Oil Cap - 6.82uF - 530uA Leakage at 500V - OK to Stay
● All sections of multi-section can test OK for value and leakage

2 - 47uF - Leakage Pass
3 - 51uF - Leakage Pass
4 - 27uF - Leakage Pass

● Negative Feedback .022uF cap - .032uF 22uA of Leakage - Replaced
● C1 .1uF Input Cap - .26uF 312uA Leakage - FAIL - Replaced
● C2 022uF Input Cap - .026uF 2uA Leakage - PASS - Replaced
● Replaced Power Cord
● Replaced bias dual cap
● Rebuilt Multi-section cap under chassis
● Checked meter with bias probe - bias needle indicates 50mA plate current.
● Set Bias and AC Balance with test set of Mullard XF2 EL34

Restoration

S/N: 2667
● PS Oil Cap - 6.72uF 730uA Leakage at 500V
● Multi-section Cap - FAIL

2 - 51uF - Not Tested for Leakage
3 - OPEN
4 - 30uF with High ESR

● C1 .1uF Input Cap - .2uF 236uA Leakage - FAIL - Replaced
● C2 022uF Input Cap - .0355uF 36uA Leakage - Value Fail - Replaced
● Replaced power cord
● Replace bias dual cap
● Rebuilt multi-section cap under chassis
● Found extra resistor added between pin 8 and pin 1 on B output tube. This took the
cathode resistance for the output tubes to ground from 15K to 7K. Solder looked like it
had been manipulated, no other pictures of 5’s showed this change and neither does the
schematic. Removed extra resistor, now both amps measure 17 ohms from Pin 8 to
ground.
● Checked meter with bias probe - bias needle indicates 51mA plate current.
● Set Bias and AC Balance with test set of Mullard XF2 EL34

Bench Tests:

● 2342 shows a bit of high frequency ring on 1KHz square wave test. These amps are
fairly far apart in serial number and this is likely due to discrepancies in the output
transformer or negative feedback components. The effect is minimal and only worth
mentioning because there is a slight difference channel to channel
● Slight channel imbalance which was corrected by swapping the 6BH6 tubes from amp to
amp.
● Both amps test to around 35W before hard clipping
● Tested distortion (THD+N) at 1 and 30W (Rated Power) Left = 2667, Right = 2342 -
Panasonic VP-7725
○ 1W 1KHz = .0204% / .0269%
○ 1W 20KHz = .1912% / .1098%
○ 1W 20Hz = .0460% / .0338%
○ 30W 1KHz = .0823% / .0781%
○ 30W 20KHz = .7600% / .7600%
○ 30W 20Hz = .4504% / .2911%
● Specification: .1% Mid Band, .5% 20Hz - 10KHz, 1% at 20KHz - PASS

Listening Tests:
● (2) 10 hour stints of continuous running
○ Bias stable
○ No excessive heat
○ Listening Test - PASS

The amps will be carefully packed in heavy duty boxes, and surrounded on all sides with several inches of polyethylene foam. Water activated, fiber reinforced tape will be used on all box seams.