The Graaf GM50B is a tube integrated amplifier born in the early 2000s, a high-level Italian design. It stands out for its fully balanced architecture and, over the years, it has earned an excellent reputation among enthusiasts. However, a lot of incorrect information still circulates around this unit, especially regarding the output tubes to use.

Many enthusiasts install KT88 or 6550 as if they were equivalent. In reality, the Graaf GM50B drives them beyond their intended limits. The plate, meaning the anode, can dissipate up to 50 watts, while KT88 and 6550 are designed for a maximum of around 35 watts. This leads to the phenomenon known as red plating, meaning when the tube plate becomes visibly red due to excessive heat. Be careful not to confuse this with the normal internal glow of the cathode. If the plate turns red, it means the tube is operating beyond its limits and risks irreversible damage. For this reason, the correct tube to use in the GM50B is the KT90, which can handle this type of operation without issues.

Recently I was entrusted with a GM50B that had been in operation for about twenty years, for a general check of the tubes and electrolytic capacitors. In this amplifier, 6550 tubes were installed and already at the end of their life. After careful inspection, I found all the signal tubes in good condition except for one ECC88 that was worn out, while the 6550 were completely exhausted. I therefore replaced all the 150 µF electrolytic capacitors, which showed very high ESR and were compromising the proper power supply of the output stages.

In the input section I installed a matched pair of selected Siemens E88CC gold pin, while for the output stage I installed a new quad of KT90, thus restoring the correct operation of the amplifier.

Due to lack of time I could not perform the usual complete instrumental measurements, but the unit returned to perfect working condition.

Disastrous shipping, incorrect packaging and full restoration of the Graaf GM50B

A client contacted me after purchasing a used Graaf GM50B online. Unfortunately, the unit arrived with heavy transport damage, but in cases like this it is important to be honest and say things as they are: automatically blaming the courier is too easy. Very often the real problem starts earlier, with poor packaging.

In this case, the seller had packed the amplifier in a completely inadequate way, with too little protective material between the unit and the box, and most importantly without securing it on a pallet. The result was predictable. During transport the GM50B suffered violent impacts that deformed the chassis in the rear corner, bent the top cover and even twisted the speaker terminals. Internally, due to inertia and repeated shocks, the transformers literally tore and deformed their mounting points, bending both the chassis and the transformer covers themselves.

This needs to be clearly stated, because there is still far too much carelessness around this topic. An amplifier like this is not shipped like a toaster. The box must be significantly larger than the unit and filled with a proper amount of protective material, not with a couple of random pieces of cardboard. And when heavy transformers are involved, the package must be secured on a pallet with straps. Without a pallet, packages are handled manually in sorting centers, thrown around, flipped, and dropped from significant heights. With a pallet, they are moved with forklifts and handled much more properly. If you ship a unit full of heavy iron bolted to sheet metal without a pallet, you cannot be surprised if it arrives with a bent chassis and transformers that have torn everything apart.

After taking charge of the amplifier, I was fortunately able to source a replacement bottom plate and top cover from Mariani’s old workshop. This was very lucky, because original Graaf spare parts are now running out. The few remaining pieces will not last forever, and once they are gone, there will be no more available. This is something buyers of such equipment should keep in mind, because today finding original mechanical parts is far from guaranteed.

I started by completely disassembling the amplifier, but another issue immediately appeared, one of those things that makes you want to hang certain “geniuses” of hi-fi on the wall for display. The previous owner, apparently convinced to be an expert in electronic maintenance, decided to “clean” the tube socket contacts not with a proper contact cleaner, which was probably too difficult, but by spraying lubricant over much of the board. The usual masterpiece of the weekend technician. Oil and lubricant on electronic boards is about the same as washing dishes with honey and expecting them to come out clean.

That lubricant, over the years, collected dust and dirt, then dried out, turning the inside of the amplifier into the usual greasy, sticky mess that takes hours just to bring back to a decent condition. I had to thoroughly wash the board with degreaser to remove all residues, especially around the socket areas, where over time burnt and conductive deposits could have formed.

Unfortunately, this is not even the first time I have seen such a mess: I already found the same disaster in a Graaf GM5050 and also in a Conrad Johnson. Of course, even after degreasing everything, in areas not reachable with a brush there were still hardened deposits of dirt and dust, which I had to remove patiently, working little by little wherever possible.

So let me be very clear: do not use oil, lubricating sprays or similar junk on electronic boards, for any reason. If you need to treat contacts, there are proper products for that. Everything else is just dirt that ages badly and can create further problems.

After the complete cleaning, I straightened the transformer mounting tabs, cleaned the main chassis from all the accumulated grease, and had to restore all the threaded rivets in the mounting holes. Those that had not already been torn out by transport impacts were still on the verge of failing. I then replaced all the electrolytic capacitors, as the existing ones had low capacitance and were no longer in acceptable condition, and finally reassembled everything using the recovered chassis parts.

At that point I also checked the full set of tubes supplied with the amplifier. Fortunately, apart from one pair of ECC88, the rest was still usable. And here is another small masterpiece of carelessness: the two ECC88, which should have been a matched pair, were actually two different tubes. This says a lot about the level of certain “maintenance” jobs done just for show. After replacing the incorrect ones, I powered on the amplifier and let it run for a full afternoon burn test.

In the end, the GM50B was fully restored. However, one important thing must be said: even if the transport had not caused all that mechanical damage, the customer would still have received a unit with several issues to fix. There were capacitors to replace, a board covered in greasy residue, a mismatched pair of ECC88 installed without any criteria, and in general all the signs of the usual sloppy work. This kind of carelessness is exactly what I cannot stand, just like I cannot stand the habit of certain people spraying oils and lubricants on boards as if they were rusty hinges. On equipment of this level, improvisation has no place. And indeed, when people improvise, the results always end up on the bench, dirty, bent, and needing to be redone from scratch, with hundreds of euros to be spent.