beautifully matched pair of classic Roberts model 770X monoblock all tube combination microphone preamps / power amplifiers in superb overall condition. As a retired old school recording studio service/calibration tech, with decades of experience in designing custom tube preamplifiers for demanding studio applications, I fortunately still have provision for the (fairly extensive) setup and calibration procedures required for proper reconfiguration and bench testing of these models. For those not familiar with these early mini rack units, first used in the excellent sounding reel to reel recorders marketed both as Akai and Roberts, and using the same basic circuitry in Models such as the 990, 770 X, M-8, etc., they have long been very popular for use as stand alone Mic preamps in both live and recording venues. The original circuit topography, and superior build quality, allowing both microphone, as well as normal line inputs, making them ideal for tweaking and modification. This pair has been set up in the following manner, utilizing a proven design I've used in many such units I've built over the years. These are the last remaining pair, I will have for auction, and will be sold mounted in the custom "Mini Rack" built for my own use.
First the original 6267 triode/pentode tube and related circuitry (used for the massive equalization and current requirements in the NAB tape recording and playback head circuits of the recorders) has been removed. This section, as well as the related 6AL5 tube used in the erase and bias section (also removed) effectively have nothing to do with the use of these units as designated, only adding noise and sonic degradation if left installed. What remains is the essence of classic Single Ended tube elegance, a huge power supply transformer, supplying well filtered B+ voltage and current reserves, tube rectified (using the proven 6X4 rectifier) to the remaining SE operated dual triode high gain and driver stages for the single 6BQ5/ EL-84 SE output pentodes, operated in pure class A tetrode mode. All superfluous wiring, cabling, etc originally installed for dedicated open reel interconnection, again has been removed, with only the front panel input and output jacks (standard 1/4 inch phone type) and separate, polarity reversable standard line cords (allowing true monoblock configuration) remaining, for easy, user friendly operation. All original power supply and active circuit components have been separately tested for value and tolerance with any required replacements done using original Akai Japanese exact equivalents. All critical coupling and bypass capacitors have been replaced using bench tested and VERY closely matched NOS high voltage American Sprague Black Beauty types, proven in this circuitry for their low noise, and low DC leakage capability. The single remaining variable potentiometer, (the critical volume/gain pot for both units) has been replaced with quality early generation Alps, again, hand measured for value, low noise, and tracking linearity. These are exceptionally quiet circuits in operation, with no prevalent PSU 60HZ, 120Hz, or higher order components, and only extremely low level, wide band (pink noise) measurable, but totally non intrusive in normal operation. This residual noise floor (common resistor and tube micro hiss) is much lower than is normally found in even many high end studio and home pre amps. The large blackface upgraded lighted analog metering, has been calibrated, with superb accuracy, channel tracking (better than 1/4 Db full scale) and currently showing 0 Db at the very onset of soft natural tube compression, and +2 Db at threshold of soft (and uniform) clipping. No Tube amplifier is any better than the components used in it's design. The phenomenal output transformers supplied in these early models is superior in performance in every measurable and audible parameter. Power output waveforms including demanding square wave analysis, reveal oscilloscope traces virtually unseen in lesser SE designs, with superb full bandwidth linearity, no ringing or overshoot even when driven into soft clipping, and transient response rivaling the best of P/P circuits. These are very good sounding amps, capable of all of the sonic virtues associated with Single Ended mythos, and seemingly none of the (often not spoken of) shortcomings. The properly biased Class A outputs, (Unbypassed Cathode bias) allow the supplied (low hour, matched used USA GE 6BQ5/ EL- 84s to couple through the 8 ohm rated transformers in such a way that appears uncanny in my experience. Square wave output wave forms are virtually uniform in linearity, bandwidth and harmonic detail from below 4 ohm dummy loads to open circuit conditions, with electronic and thermal stability unchanged regardless of load impedance or reactance. The 12AD7/12AX7 class of dual triodes specified for use in these circuits allows for a vast range of tube substitution tweaking. In my experience the finest design for use as HiFi amps, (with equally low noise, and bandwidth capability in high gain mic use, is the venerable British Mullard ECC83/ 12AX7, and as such these are supplied with low hour used and perfectly matched examples with intact silkscreening, ready to use. The supplied rectifiers are a matched pair of the proven Sylvania USA black plate 6X4. I will try to sum up this admittedly over wrought description: The multi position slide switch used originally to switch between record and play modes for the recorder is securely bolted into the correct fixed position. The cut type tone control is of course, no longer active and the original knobs are configured in this way. The first (left knob) controls volume/gain and the right set up operate the two position control, orig. used for speed equalization switching. As currently modified, this switch now allows use in one position for absolute flat response, with a second option giving a low order rise in high frequency response (begining at just below 11 Khz to a maximum peak of +4 Db at 17 Khz). Experience has shown over the years, that this relatively subtle curve is ideal for touch up of mic response from a large number of vintage designs, allowing just the right increase for miking higher detail for Piano, cymbal, etc. harmonics, from less than ideal microphone sources. This slight (but distinctly) noticable response change, also serves very well to compensate for the often time, too soft high end response of vintage full range and tweeter loudspeaker designs, the use of which is mandated by the efficiency requirements of the 4 watt RMS + output of these amps.
both home HiFi amplifiers and dedicated Mic to line pre amps, I have purposely left the input and output circuitry (and operating level) options for mic pre use. The Mic inputs are currently set up for use only with high Z unbalanced mics (with standard 1/4" jacks). The conversion to balanced input is of course, easily facilitated by use of any common Low Z (600 ohm) balanced to grid input transformer (either externally or internally mounted) and the required XLR or TRS jack for connection. The quality and price of such (ranging from quite inexpensive to vintage units often selling for several times the price of these units themselves) demands that the buyer set up their own choice. Traditionally the line outputs for these are optimized by use of the power output section (again 1/4" phone jack) for interface. The superb wide bandwidth linearity and stability into practically any load impedance, makes this configuration possible.This allows for the ideal current source, low impedance output capability to be used as is (for unbalanced input to vintage line level boards and mixers, etc.) and attenuated using the receiving units stepped or variable pad or input operating level attenuator. If lower source voltage is required only the addition of a simple fixed or variable voltage divider network is required to preset output and (purely resistive source impedance) to any standard operating level from extremes such as -20 db to classic +10 db levels. For balanced output use, again, a matching transformer and jacks are required, but the low output impedance provided, allows for use of the much less costly (compared to quality plate to balanced line) low Z to balanced low Z designs (600 to bal. 600 works nicely). These narrow faceplates allow for use of the remaining EIA rack width (when used with rack extensions) as excellent patch bay panels for installation of the desired components and jacks for custom setups.
For the recording engineer, home HiFi purest, or anyone desiring the versatility, and audio capability of these lovely little monoblocks, without the extravagant price demanded for the modified Designer capped Beautique versions (which are probably quite nice), but perhaps not worth the many hundreds each, often charged, here may well be an excellent opportunity.
It looks to me like it must not have been around long enough for the web archive to take notice, but it vanished long enough ago that all of the search engine's caches have cleared it out. :sigh:
Sent a message to the guy.
