The PrimaLuna ProLogue Two is a tube integrated amplifier highly appreciated for its musicality, robust construction, and technical solutions such as Adaptive AutoBias. Like all tube devices, however, it requires periodic maintenance: tubes have a limited lifespan, contacts can wear out, and some electronic components are not eternal.
It is worth noting that the Adaptive AutoBias system, often highlighted as a strength, is essentially a servo-bias circuit: an operational-comparator system that automatically controls the bias of the output tubes. A simple yet highly effective solution that ensures stable operation without the need for constant manual adjustments.
In my lab experience, I have found that these amplifiers, while carefully designed, tend to show some recurring issues:
- The servo-bias board sometimes develops drift and instability in adjustment, making it difficult to maintain proper bias for the KT88s.
- The octal sockets of the output tubes can loosen over time: the tubes become slightly “wobbly,” leading to intermittent contacts. This can cause dangerous situations such as red-plating of a KT88, resulting in overheating, burned resistors, or more serious failures.
For this reason, I offer a service of complete overhaul and maintenance: replacement of critical components, restoration of sockets, thorough checks, and calibrations to return the amplifier to its original reliability and sound quality.
PrimaLuna ProLogue Two – Complete Repair and Maintenance
A customer brought me a PrimaLuna ProLogue Two after one of the output tubes went into red-plate, overheating to the point of failure. To try a quick fix, the owner replaced the burned tube with another of a different brand, leaving the three original ones still in use. This kind of “mix” among different tubes, while understandable in an emergency, is never advisable: in fact, the amplifier continued to show faults and instability.
Initial diagnosis
The first step was a thorough cleaning with compressed air, as the interior of the unit was very dusty. I then checked the condition of critical components and found a common issue with these models: the octal sockets of the output tubes were too loose and slack, enough to make the KT88s “wobble.” This not only compromises contact reliability but can cause intermittent contacts leading to serious faults, such as red-plating tubes and burned resistors as a consequence.
Work performed
- Socket replacement: I removed the four faulty octal sockets and replaced them with new quality sockets featuring gold-plated contacts to ensure greater long-term reliability.
- Signal tube check: I tested all the preamp tubes. One of the original ECC82 was worn out and out of spec, so I replaced it with a new matched pair of ECC82 Tung-Sol. The two original PrimaLuna ECC83, however, were still in excellent condition and were retained.
- New output quartet: to restore balance and uniformity, I installed a quartet of new Tung-Sol 6550, chosen for reliability and tonal consistency.
Testing and burn-in
After the work, I proceeded with a controlled power-up under a Variac, monitoring current draw and bias stability. The amplifier responded correctly and began operating without any faults. As usual, I let the ProLogue Two play for several hours in the lab to verify its full reliability over time.
Thanks to this maintenance, the amplifier has once again become reliable, stable, and ready for many more years of music, with secure contacts, new tubes, and risk-free operation.
An amplifier poorly stored—very poorly!
Not all devices arrive at the lab in the same condition. Some have years of regular use behind them, while others have had a less fortunate life. This amplifier, for example, had been kept in a harsh environment: high humidity and likely chemical agents or aggressive fumes had compromised the interior. When it arrived, it showed heavy signs of oxidation and widespread corrosion.
The first step was disassembly. Many screws were so rusted that I had to re-tap the threads with new taps to reassemble everything safely. I cleaned and repainted the clamps of the smoothing capacitors and restored the mechanical supports, bringing order and solidity back to the inside of the amplifier.
The main fault had occurred right on the servo-bias board. Corroded solder joints had attacked the thin conductors connecting the board to the output tubes: two grids were left “floating,” causing arcing and irreversible damage to the circuit.
I tried to recover the original board by cleaning it with specific sprays and reflowing the solder joints, but the situation was compromised: broken traces, corroded vias, and solder that released abnormal substances when heated. Despite hours of attempts, the circuit remained unstable and impossible to calibrate properly.
At that point there was only one choice: rebuild the board from scratch. I reverse-engineered the schematic directly from the damaged PCB, salvaging only a few still-good components, and built a new circuit on perfboard. For practicality, I used two TL082 instead of the original TL084 (which essentially contains two TL082 in one package). Here is the result during the work phase:
The new servo-bias worked perfectly at the first test, restoring a stable and safe bias adjustment to the amplifier. After hours of testing, the unit played without issues, ready to resume its musical role despite the difficult starting conditions.
Here are the instrumental measurements on the sample unit with all new tubes:
Power: 30 Watts RMS
Bandwidth @ 1 watt: 15 Hz ~ 34 kHz ?3 dB
Bandwidth @ 25 watts: less than 10 Hz ~ 25 kHz ?3 dB with ?1 dB at 10 kHz
Damping factor: 1.5
THD at 1 watt: 0.3%
THD at 25 watts: below 1%
Spectrum analysis at 1 watt on a resistive load:
At 25 watts, still on a resistive load:
On the reactive load, odd harmonics become more pronounced
This is the bandwidth graph at 1 watt on a resistive load
1 watt on a reactive load…
The negative feedback is present, but in rather small amounts: this is clearly evident from the damping factor (DF) of about 1.5. This value is quite low and makes the amplifier less suitable for driving reflex speakers with large woofers or large internal volumes, because it easily exhibits boomy, poorly controlled, and annoying bass. This behavior is often superficially attributed to KT88s, whereas it is not the tube itself that determines this result, but rather the low damping due to the limited use of negative feedback.
The input, in fact, is very sensitive: only a fairly small signal is needed to drive the amplifier into clipping (although I did not precisely measure the sensitivity value). During the tests, I did not observe waveform distortion in the area between 8 and 15 kHz, distortion that would have been present if a high amount of NFB had been applied together with transformers having such a modest bandwidth. However, it is noteworthy that the unit shows instability with no load connected, a phenomenon that can indeed occur with transformers exhibiting these bandwidth characteristics.
Contact the lab
If you own a PrimaLuna ProLogue Two (or another model from the same series) that shows stability issues, worn tubes, or operating faults, don’t wait for the situation to worsen. Timely maintenance can prevent more serious and costly failures, and restore your amplifier’s full reliability and sound quality. You can contact me directly via the SB-LAB Contacts page to request information, evaluations, or a repair quote for your device.