The GRAAF Venticinque, also known as the Graffiti 25, is one of the most compact integrated amplifiers produced by the Modena-based company founded by Giovanni Mariani. It is a push-pull tube amplifier capable of approximately 25 watts RMS per channel, designed according to a philosophy quite different from that of GRAAF’s better-known OTL power amplifiers.
The output stage uses four 5881 tubes, two per channel, while the input and driver stages employ three 6922 / ECC88 tubes. The output stage operates in pentode configuration with cathode bias and a very limited amount of negative feedback. The power supply also features a rather unusual choice for a tube amplifier of this period: instead of the traditional laminated-core power transformer, there is a large toroidal transformer, a solution that allowed GRAAF to keep the amplifier relatively compact.
The amplifier is built in a fairly rational way, but things become considerably less enjoyable when work has to be carried out on the main circuit board: reaching some components requires dismantling a large part of the amplifier. Over the years I have had several of these units on my workbench, and on this page I want to document two repairs carried out on the same model, many years apart.
First repair: worn-out capacitors and a failed power transformer
The first unit arrived at my workshop completely dead. The amplifier had never been serviced and, after many years of operation, had simply stopped working.
After opening it, one of the first obvious problems was the condition of the electrolytic capacitors in the power supply. One of the four main capacitors was visibly swollen; the split in the plastic sleeve visible in the photograph was made by me to check more closely whether the capacitor had actually vented.
The problem is that replacing these components is not simply a matter of opening the cover and desoldering them. To remove the main circuit board, practically the entire amplifier has to be dismantled. Once I had access to the PCB, I therefore replaced all four main filter electrolytics.
At this point I expected the amplifier at least to power up again. Instead, nothing happened.
Continuing with the checks, I discovered the second and considerably more serious fault: the primary winding of the toroidal power transformer was open-circuit. Most likely, the faulty capacitor had not caused a hard short circuit sufficient to blow the fuse immediately, and the transformer eventually failed. A particularly unfortunate situation, because what had begun as a relatively straightforward power-supply overhaul had now turned into a much more substantial repair.
Fortunately, at the time I managed to obtain an original GRAAF replacement transformer, bearing the manufacturer’s markings, which was personally supplied to me by Giovanni Mariani.
With the new transformer installed and the power-supply overhaul completed, the Venticinque finally came back to life. After the final checks and tests, the amplifier was once again stable, quiet and fully operational.
Another GRAAF Venticinque on the workbench
Many years after the first repair, another example of the same amplifier arrived at my workshop. This time the GRAAF was operational, but suffered from an annoying high-pitched noise coming from one of the two channels. As soon as the cover was removed, it became obvious that someone had already worked on the amplifier. The circuit board was very dirty and showed clear signs of a previous repair. In particular, the large electrolytic capacitors in the power supply had already been replaced.
At first glance they appeared to be Kendeil capacitors, but a closer inspection revealed something quite different: they were actually Samsung capacitors rebranded with Kendeil sleeves. Counterfeits, in other words. This is something worth documenting in the photographs, because it is a good example of what can unfortunately be found inside equipment that has already been repaired in the past.
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Regardless of their rather questionable origin, these capacitors had reached the end of their useful life and needed to be replaced. And this is where the Venticinque immediately reminds anyone repairing it just how inconvenient it is to work on its main circuit board.
Complete disassembly and circuit board cleaning
To remove the capacitors and work properly on the PCB, I had to dismantle practically the entire amplifier. Once the PCB was finally accessible, I removed the old electrolytics and was also able to deal with the dirt that had accumulated on the board. In cases like this, cleaning is not merely a cosmetic matter. Residue, dust and contaminants accumulated over the years have no reason to remain inside equipment operating at high voltages.
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The circuit board was then thoroughly washed and cleaned, the new capacitors were installed, and I could finally begin the lengthy process of reassembling the amplifier.
New capacitors and a new set of tubes
Once the work on the power supply was complete, I moved on to the tubes. Here too, there was little point in putting a freshly serviced amplifier back into operation with a tired and mismatched set of tubes. I therefore retubed the GRAAF with a new set. The only tube I kept was one ECC88 which still measured in good condition and showed no concrete reason to be replaced. Replacing perfectly serviceable components simply for the sake of saying they have been replaced is not the way I work.
After reassembly, the usual electrical checks, operational tests and listening tests followed. The noise that had been present when the amplifier arrived had disappeared, and the unit was once again operating correctly on both channels.
Bench measurements
Once the overhaul was complete, the GRAAF Venticinque underwent the usual bench measurements to verify not only that the amplifier was operating correctly, but also its actual performance. Maximum measured output power was approximately 23 watts RMS per channel. Naturally, this figure may vary slightly depending on the output tubes fitted, their condition and their actual operating point. Measured total harmonic distortion (THD) was 0.2%, a very good result for a tube amplifier of this type.
The frequency response is particularly interesting: the -1 dB bandwidth extends from approximately 5 Hz to 70 kHz. This result is above all a testament to the quality of the output transformers, which are capable of maintaining a very wide response at both low and high frequencies.
The measured damping factor is approximately 1.5. This is rather low in absolute terms, but not unusual for a tube amplifier designed with a minimal amount of global negative feedback.
To summarise, this particular unit measured 23 watts RMS of actual output power, 0.2% THD, a -1 dB bandwidth of 5 Hz – 70 kHz and a damping factor of 1.5. These figures describe the philosophy behind the Venticinque quite well: a straightforward tube circuit with a very wide frequency response and deliberately limited negative feedback.
Conclusions
The GRAAF Venticinque is an amplifier well worth keeping in full working order. Over the years, capacitors, tubes and other components can deteriorate, while previous repairs that were not carried out correctly can introduce problems that may only become apparent much later.
In my workshop I can repair and overhaul GRAAF amplifiers, from tracing an individual fault to more extensive work on the power supply, tubes and internal electronics. Once the work has been completed, the amplifier is checked and measured on the test bench to verify correct operation before being returned to its owner. If you own a GRAAF Venticinque, or another GRAAF tube amplifier in need of repair or servicing, you can contact me through the website and describe the problem you are experiencing. I will assess the work required to restore it to the best possible operating condition.