The Graaf GM50B is a tube integrated amplifier born in the early 2000s — a product of refined Italian engineering. It stands out for its remarkably natural and dynamic sound, thanks to its fully balanced architecture and carefully selected components. However, many misconceptions circulate about this amplifier, especially regarding the correct output tubes to use.

Many enthusiasts install KT88 or 6550 tubes without much consideration, assuming they are equivalent. In reality, the Graaf GM50B drives these tubes well beyond their safe dissipation limits: the plate (anode) reaches up to 50 watts, while KT88 and 6550 types are rated for about 35 watts maximum. This leads to the so-called “redplate” phenomenon, where the tube plate visibly glows red from excessive heat. Note: this should not be confused with the normal internal glow of the cathode. If the plate itself turns red, the tube is operating beyond its limits and may suffer irreversible damage. For this reason, the only tube truly suitable for the GM50B is the KT90 — designed to handle higher dissipation safely under such demanding conditions.

I was recently entrusted with a GM50B that had been operating reliably for about twenty years, for a general check of its tubes and electrolytic capacitors. The amplifier was equipped with 6550s, all completely worn out. After a thorough inspection, I found all the signal tubes in good condition except for one ECC88, which was weak, while the 6550s were totally exhausted. I replaced all 150 µF electrolytic capacitors, which showed very high ESR and compromised the proper supply of the power stages.

In the input stage, I installed a matched pair of Siemens E88CC gold-pin tubes, while for the output stage I fitted a new quartet of KT90s, thus restoring the amplifier to full performance.

Unfortunately, I wasn’t able to perform the usual full set of instrumental measurements due to time constraints, but the unit is now sounding perfect again and ready to deliver many more years of top-level listening pleasure.