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Akai M-8 R2R SE pentode amplifiers

Discussion in 'Tube Audio' started by Squidward, Aug 21, 2006.

  1. Squidward

    Squidward Scrappy Mod Moderator

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    I just scavenged an Akai M-8 that had a pair of SE EL84 amps in it. I remember seeing a page about a guy who homebrewed some amps out of something similar, but I can no longer find it. I know that's vague, but if anyone knows that page I'd be glad to check it out again. Also, any information about these amps in stock form would be appreciated, too!

    Thanks!
     
  2. goldear

    goldear Certifiable Audio Junkie

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    I know exactly the site which you are referring to. And I'm searching for my link now. Stay tuned...
     
  3. goldear

    goldear Certifiable Audio Junkie

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    Ok, I have some bad news, and some good news:

    The link to Conrad's Tube Stereo page is dead: http://rummageads.com/tstereo/ There is something about a server crash on the home page. It looks like they lost everything.

    But here is the good news: I found a version to be still around on the wayback machine. :banana:
    http://web.archive.org/web/20050331085218/http://rummageads.com/tstereo/

    This was a really cool site. This guy would have been right at home on AK, but if I recall correctly, he died from cancer several years ago.
     
    Last edited: Aug 21, 2006
  4. Squidward

    Squidward Scrappy Mod Moderator

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    Goldear, you are THE MAN!!!! :thmbsp: :banana: :music: :smoke:

    That's EXACTLY what I was looking for!

    Thank you so much. Sad to hear that the original author has passed on, he seemed like a cool guy!
     
  5. PakProtector

    PakProtector AK member

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    Ann Arbor, MI
  6. goldear

    goldear Certifiable Audio Junkie

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    Sorry Douglas, no luck with that one. I can't even find any cached pages for you in any of the search engines. :no: 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:
     
  7. tracker-x

    tracker-x Active Member

    Messages:
    140

    I was doing some research on old threads and saw this. I have one of these as my main amp. The only mods needed are, remove the (2) 6267 tubes, and the 6AR5 tube. Put the deck in "record" mode. Turn the capstan motor "off" (center position on speed switch, top of front panel)- and disengage the capstan (lever on front panel, just to right of ff/rew knob). Input your source through the "line in" jacks on left side. You will need a 1/4" to RCA adapter, for each one, available from Radio Shack. Output your speakers from the right side, using same adapters. (or use the accessory kit that came with the AKai deck, which supplied these cords- I found one on Ebay for $10). You now have a great sounding single-ended pentode amp, for under $100. In this mode it will play tape, CD, or any other line-level type input.

    To play vinyl from a turntable input, you have to leave the 6267 tubes in. These amps sound GREAT with vinyl !

    high efficiency speakers are a must.

    make sure all the tubes test good first, of course.

    added benefit- you can use the reel to reel, which will play/record up to 7.5 IPS open reels.
     
  8. tracker-x

    tracker-x Active Member

    Messages:
    140
    interesting mod

    this guy made his own cabinet and feet for an M-8, removed the reel section, and added high end pots, VU's, etc, and locked the deck into record mode internally. But this level of modification is not really necessary- just remove the 3 tubes, lock the deck in record, and it will sound the same as this one.

    [​IMG]
     

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  9. tracker-x

    tracker-x Active Member

    Messages:
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    more info on M-8 mods for dedicated SEP amp

    I'm afraid that I no longer have an open shop to do repair or modifications
    on audio equipment, but I am enclosing a fairly extensive (and proven) list
    of procedures you might wish to consider implementing to improve the
    performance of your units. You’re on the right track in use of these M-8
    amps, but you might find that you've only begun to tap the potential of
    these great little SE amps. The following is a brief rundown of what is
    required for maximum results:
    If you are using the amps solely as line level preamp/amplifiers, or using
    also as high gain microphone pre/power amps this might help:
    1. The configuration your using is suitable for non-critical HiFi use.
    However for use as stand alone monophonic amps (dedicated for use only as
    mic preamplifiers, and stereo line level input power amps) with true flat
    bandwidth audiophile response, noise floor levels, and even more dynamic
    headroom the following modification should be performed.
    2. First of all, these amps are self powered, with each having its own
    power supply transformer, capable of independent use, and with only minor
    changes, can be used outside of the heavy recorder case. Each amp can be
    directly connected to the wall socket by installation of standard two wire
    (unpolarized preferably) line cords. If you have basic soldering experience,
    one of these bare end power cord leads should be soldered directly to the
    transformer tap labeled 117 volts, on the power supply transformer of each
    unit. These taps are accessed by removing the two screwdriver head bolts,
    holding the top (gray painted) cover to the transformer. The current wire
    soldered to this tap should of course, be removed. The second lead should be
    soldered to the input terminal of the power switch (again removing any
    existing wiring), and making sure the second (switched lead) wire from the
    switch returns directly to the transformer tap labeled 0 or common. When the
    redundant unused wiring leads (Octal plug on one of the amps and three
    conductor lead on the second) are removed (unsoldered or simply clipped
    close to their respective terminations), the amps can then be used simply by
    plugging each into a wall socket and switching on. Note; this, as well as
    the following procedures will make these units unsuitable for use any longer
    as recording amps in the original recorder, without first reversing these
    procedures.
    3. Next, the operational mode should be switched on both amps to what was
    originally the record mode. On the top of each chassis is a metal slide
    switch assembly held into the right position (looking at the face of the
    amps) by a large spring. This spring should be removed completely, and the
    multi position slide switch moved to the far left position of its travel. If
    you wish, the metal slide assembly can be locked into this position by
    adding a suitable washer onto each of the two bolts, which operate as guides
    for the switching assembly, and retightening the bolts.
    4. The amps are now set to operate solely as pre and power amps, and the
    top two jacks (left side on the vented area, on standard M-8 preamps) are
    now no longer utilized for signal input or output. For standard line input
    (approx 1 volt RMS) the upper of the two remaining jacks (lower left side)
    is used, and for high gain mic input, the lower.
    5. The original tone control circuitry (orig. used in tape playback mode)
    is now effectively bypassed and is non-operational, and not needed for flat
    response.
    6. The units now are essentially flat (With much superior bass and mid
    linearity and hugely improved headroom) in the lower half of the overall
    wide range response, requiring only one last critical change to effect what
    is ultimately almost perfect response linearity (+1 db 20 hz to 20 kHz).
    This change is effected at the two-position switch (inner right knob
    actuated) used formerly to alter the NAB response for 7 1/2 and 3 3/4 speed
    operation.
    7. With the amps turned upside down, and the small partial metal panel
    (hum shield) removed, you should be able to see all of the active circuitry.
    Now, with the faceplate and knobs facing away from you, the back side of the
    two potentiometers are now visible and accessible. The volume control pot,
    (still on the left as viewed) will have the outer of the three solder
    terminals with a .02 MFD gray tubular electrolytic capacitor soldered to it.
    To this terminal (again, the one closest to the adjoining tone control pot)
    you will need to solder one wire leg of a 375 Pico farad (PF) high voltage
    capacitor. Any common type such as a silver mica or even a higher voltage
    tantalium will work nicely, but DO NOT use a ceramic wafer type (higher DC
    resistance can negatively affect performance). The second lead of this cap
    should be soldered to the ground leg of the formerly used tone control pot
    (handily the closest tap on the second pot). No wires or components soldered
    to either pot should be removed, only this one cap added.
    8. Prior to this simple modification, the response curve would have a 6 db
    per octave rise in the high end beginning well below 10 khz and peaking at 8
    to 10 db at 20 khz. Now with the still active tape speed selector (inner
    coaxial to the tone control) switched to the 7 1/2 speed position, the
    treble and overall response (depending on original condition and tolerance
    of the other active and passive circuit components) should be virtually
    flat, and when switched to the 3 3/4 position will have a slight rise,
    peaking @ 17 khz. The versatility of this switching was covered in the text
    of my auction ad for reference.
    9. You now have a pair of true HiFi single ended tube amps. When any
    suitable line level signal (normal .775 volt RMS) the amps should have
    excellent (and controllable) gain characteristics, with clipping into normal
    speaker loads occurring somewhere around the 12 o'clock to 3 o'clock
    position of the volume controls. Using both this input and the mic input you
    should have very low overall noise floor characteristics, and huge dynamic
    headroom that is now uniform across the entire audio spectrum.
    10. At this point the amps should be ready to use, or at least now suitable
    for any degree of traditional tube audio blueprinting or tweaking desired. I
    would highly recommend at least the following:
    The 6267 combination triode/pentode and the 6AR5 tube ARE NO LONGER
    UTILIZED IN ANY WAY. These can now be simply removed and left out of the
    amps. Doing so will lower the inherent noise of the units, and give the
    power supply slightly greater current reserves for more dynamics from the
    remaining compliment.
    All of the .02 Mfd coupling and bypass capacitors should be replaced.
    Even if the amps you are building are NOS the almost inevitable drying out
    (with related loss of tolerance and DC leakage) to the orig. electrolytic
    (gray axial lead cans) has occurred during storage. These should be replaced
    with 400 to 600 volt types of your choice. Sprague orange drop poly caps are
    ideal in these circuits, they are commonly available, have excellent
    tolerance, sound great, and will last for decades. When replaced, you are
    now assured of no DC leakage to offset the grid bias of the next stage,
    lower noise floor, and correct coupling capacitance for circuit operation
    and linearity. Transient recovery and slew rate improvement should be
    noticeable, with better dynamics, and phase coherence (point source and
    depth).
    The last arena of tweaking (which is mistakenly often approached first)
    is tube replacement. If your units still have the original NEC, or possibly
    Matsushita tube compliment, and they are undamaged, they should serve
    nicely. These were Japanese clones (very well made) of Amperex designs,
    rugged, reliable and usually well matched at the factory. Remember the
    common 12AX7 will work great in these units in place of the harder to find
    12AD7 and if chosen, nothing sounds better than Mullards in this stage.
    Although factory rated at 6 watts per channel (8 Ohms), by low distortion,
    full bandwidth, HiFi standards, I call all of these series a more realistic
    4 watt output.
    Last (finally) I would ask that if at all possible, please bench test
    these units if at all humanly possible, using suitable (8) ohm dummy loads
    and checking waveforms with a decent oscilloscope. Even a few minutes
    observing 1 KHZ square wave response and gain traces, will give more actual
    information about the above changes, as well as the overall circuit and tube
    condition and matching, than hundreds of hours of qualitative listening.
    When doing so, it would be a great time to calibrate the meters. This can be
    done internally in the meter cases, but is very time intensive and delicate
    work. A simple series resistor (start in the low kilo ohm range) on the
    supply side of the meter wiring of the higher reading unit or both, can
    serve very well to match the meters for accurate reading of peak levels.
    They should be set to give identical 0 Vu reading at the onset of 400hz to
    1khz sine wave compression. The original volume control pots have probably
    aged as well, with the deposited film resistance changing both in value and
    linearity. I they are quiet in operation, they probably won't need
    replacement, but don't expect tracking accuracy between the two units, even
    if the amps are perfectly matched in gain.

    I honestly believe that the preceding information is accurate. The
    procedures sound more complex than they really are, and with only the
    investment of about 10 capacitors, and less real world time than it takes to
    write these tips down, you may well be pleasantly surprised at having some
    of the finest small SE amps around.

    Long live DIY Glass audio, and good listening,
     
  10. tracker-x

    tracker-x Active Member

    Messages:
    140
    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.
     
  11. goldear

    goldear Certifiable Audio Junkie

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    Location:
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    I"ve got an old Roberts 997 (same as an M7) that I used once or tiwice as a backup amp when I was a kid. I do remember thinking that is sounded better than the stereo 70 which was my main amp back then, but I never really listend to it that critically (I found the lack of power to be a problem back then). I was still a dumb kid who was under the mistaken impresion that tubes were lousy back then, and so I guess that I didn't really believe my ears. It was probaly another couple years after that that I discovered that it was actually SS that sounded inferior.
     
  12. tracker-x

    tracker-x Active Member

    Messages:
    140
    (chuckle) same here...I listened to high-powered solid state receivers for 25 years. I stumbled upon an M8 for $10 around 2002 or so- it was sitting on a neighbor's front enclosed porch for years, taken apart. Brought it home, soldered a few loose wires on the jack terminals, when it started working, and I inputted my linear turntable, my exact words were "ho-ley sh-t". The Stones record I was playing, it made it sound like Jagger was standing in my living room, spitting the lyrics at me. And Nicky Hopkins' piano was sitting to the right banging away right there- not from the speakers, but in the room.

    That's when I finally realized what "soundstage" is. Music coming from thin air- not from the speakers. Also realized, all my solid state stuff had no soundstage, or a flat, crappy soundstage.

    I had no idea what I had been missing all those years. Just goes to show, how media and marketing can confuse consumers, and keep us in the dark. What happened was, they pulled all the tube units off the market across the board, and replaced them with solid state, and pushed the "no warmup required, no maintenance, no changing tubes, better specs" sales line. I talk to audiophiles who remember it, and they say at first, there was a lot of resistance to transistors, they sounded too harsh.

    They did the same thing to analog tape and vinyl, with the CD.

    For that matter, they did the same thing to the 78rpm shellac record, with the vinyl record and analog tape.

    A clean 78 that's been electrically recorded, will amaze.
     
  13. spaceman

    spaceman Registered Lunatic

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    Location:
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    Thanx VERY much for the great info, tracker-x! I picked up a Roberts 770X last night, & had an idea of it's potential, but your info has me pretty excited now. :banana: I think these amps will be perfect with the LEAK Sandwich speakers I have. :yes: This thank you also serves to bump the thread, so others can see it.
     
  14. Andyman

    Andyman Scroungus Stereophilus Subscriber

    Sounds great!!
    I just picked up a Roberts 990 fo rhtis very reason. Are these mods appicable to the amps in it also??
    Pics will follow this evening.
     
  15. vinyldavid

    vinyldavid Proud Jaguar Owner! Subscriber

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    Location:
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    oh damn....I better got on the Roberts 770 that I have a lead on......


    EDIT: Just Jumped. :D Sent a message to the guy.
     
  16. spaceman

    spaceman Registered Lunatic

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    Location:
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    I hope so, Andyman. I imagine if yours is tubed monoblocks, it's gonna at least be very similar. I haven't touched mine yet, had a speaker re-foaming project underway, but I'll definitely be peeking inside later today. :yes: I need to hit ratshack for those 1/4" to RCA adaptors too. BTW, can anyone confirm for me, if I can run the pre-amp line out from a Sansui integrated into it's inputs? I don't want to do anything stupid, lol. If not, a tuner with variable output would work for testing, I'm pretty sure.
     
  17. vinyldavid

    vinyldavid Proud Jaguar Owner! Subscriber

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    Location:
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    I read somewhere that they were supposed to be used off of pre-outs....I'll prolly be getting an Advent receiver sometime and 'll run the ones I might get off that.
     
  18. Endspec

    Endspec Super Member

    Messages:
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    Location:
    Drumright Ok.
    need to make this info a sticky.
     
  19. Endspec

    Endspec Super Member

    Messages:
    1,477
    Location:
    Drumright Ok.
    Locked in record mode the volume on the units work but the tone controls don't.
     
  20. Andyman

    Andyman Scroungus Stereophilus Subscriber

    The promised pics...

    [​IMG]

    I have the tube grilles, but need nuts for the bolts to hold them down; the recorder used T-nut in the case. The Vu meters have a nice look to them.
     
    Last edited: Oct 24, 2010

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