1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

The mighty Allen Organ 75

Discussion in 'Tube Audio' started by Brice, Oct 12, 2018.

Tags:
  1. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Hi guys,

    I wanted to share with you some work I did on this amps.

    20181012_083440.jpg

    As far as I know, this amp was produced from the beginning of the 60's to the 90's.
    It was mounted inside an organ and was design by Allen Organ and Acrosound.
    The schematic looks very similar to the published TO350 amp from the Acrosound brochure, except the PS and the entry stage with additional tubes.

    Some of you know my interest to the Acrosound family so I couldn't resist to try that one.

    Because Acrosound's involvement in this project, the early model have the desired Acrosound monster OT in square can. Then after Acro's debacle, Allen was forced to produce their own OT, in house and from other company, often with rounded can. I have both models and I can tell you that I did not measure or heard differences between the original Acrosound and the latter Allen OT.

    20170626_130803.jpg

    The older model have a PS with under chassis WW power resistors, poorly mounted in contact to the chassis for heat dissipation. Later model have modern resistor bolted to the chassis. There are slight difference in resistance value but that's beyond the point. I added an heat sink to provide more chassis contact and additional heat dissipation.


    20180424_112110.jpg 20180424_113800.jpg

    20180424_113738.jpg

    Everything in this amp has been design with reliability in mind.

    The PS with a big choke input is feed by an oversize power transformer, which barely gets warm after 5 hours of listening. The PS design is interesting, with a constant current "wasted" in heat, to get the choke going constantly.

    The amp design is excellent. Fixed bias, only 2 couplings caps to separate the 2 stages, some direct coupling driver tube/splitter, tube balancing via 6SN7, bias point. All is good here.

    For HiFi usage the first tube 12AT7 can be removed as there is no need for the super high gain factor the 2 entry driver tubes provide. I only use the EF86 as first stage with 470k to ground, 10K to grid, and relocated it to the 12AT7 which was originally mounted on a rubber platform. Very nice.

    20171010_185214_HDR.jpg
    20180410_140551.jpg
    20171009_185840_HDR.jpg

    The wiring is impeccable, with thick 1 mm solid copper plastic/cloth wire with good mechanical contact under the solder. On a funny note, the chassis were signed by Betty, Ana, Anne. These ladies did an amazing work and deserve our respect.

    This amp has 2 fuses, one on primary, and one on the HV CT to ground.

    So I rebuild the PS and bias supply with good caps, good SS diodes, added bypasses, relocated the balancing pot to the front panel as some version had it inside the amp with no throughout access, added RCA connector, removed the 12AT7 tubes etc etc..

    20171005_140108.jpg


    20180327_175952.jpg


    I used KT120 on this baby, and get a solid 68 W < 1% THD. The square wave is excellent, thanks to these monster OT.


    Now on the listening side, this amp is amazingly detailed and dynamic. Top to bottom, it is well integrated and the imaging is superb. It is one of my best amp.

    So I decided to make them pretty, I asked my friend Chris to make me some wooden enclosure. It really makes these amps pop.

    So in conclusion, this amp deserves much more credit than the little you can find on the internet.
    You can find the Acrosound's touch in this amp, with its distinctive dynamic.
    Once rebuild, it competes with the top amps of that era, without a doubt.

    Alright, I recorded most of the important info for this fantastic amp so you can get a pair and get going.

    Thank you for reading.
    Brice.


    My Notes as is:

    Modification Allen Organ 75
    C13 and C15 bumped from 140uF 350V to 680uF 400V Mouser 80-ALC40A681DL400
    C13 and C15 bypassed with 2.2uF 630V Panasonic film caps Mouser 667-ECW-FA2J225JB
    Old rectifier bridge removes, replaced with bridge IXYS 21 Amps 1200V added Mouser 747-VBO21-12NO7
    Rectifier bias supply replaced with UL4007.
    Bias supply caps 2x30uF 200V replaced with German FT-Cap 2x100uF 350V
    Added bypass Panasonic film cap 4.7uF 250VDC bias supply Mouser 667-ECQ-E2475KF
    Relocated internal bias adjustment pot to from panel.
    Input stage 12AT7 removed
    Relocated EF86 to the original location of the 12AT7 with vibration dumping grommet.
    R43 removed (supply 160VDC not needed anymore).
    Adjusted R42 from 12k1 to 39K as a result of removing R43: 205 VDC point 4 in specs.
    C15 40x10x40uf 475V replaced with an Authenticap KTL41 200x50uF 550VDC same location and an Authenticap KON5650 50uF 350V at the EF86 original position. Fits to cover the hole.
    Added 470k to ground to RCA input jack and 10K to grid to EF86.
    Added film capacitor 0.47uF 1200V before choke, to adjust the B+ and tame the choke buzzing.
    Added LED 6.3VAC with 3.9K resistor 1/2W serial.
    Added LED on 205VDC supply with 3.9M 1/2W resistor serial.
    Removed the 0.01uF 600V ceramic from primary to chassis: dangerous.
    Use the 125VAC red/black primary tap instead of the 115VAC white/black tap.
    Use of Tunsgol KT120 instead of 6550. Bias the same or higher.
    Change power cord and add aviation connector 3 prongs.
     

     

    Please register to disable this ad.

  2. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    schematics
    AllenType75amp.gif
    amplifier.jpg
    input_stage.jpg
    PowerSupply.jpg
     
    Horsey likes this.
  3. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    Oh man I'm drooling. It looks like a keeper that one. Amazing work there Brice. Looks like 6SN7's are used for the phase splitter and cathode followers?
     
  4. KentTeffeteller

    KentTeffeteller Gimpus Stereophilus!

    Messages:
    25,966
    Location:
    Athens, TN
    I heard one of these amplifiers in good stock condition pulled out of the organ. Modified enough for HiFi use. I love it's heavenly sonics. It's a nice amplifier, well built and engineered. I want one.
     
  5. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,045
    Location:
    Eastern Bamastan
    I want two, folks want ice water in hell and they will probably get that first. They are enticing, but too bulky in reality for my set-up. I can still admire them as best in class for organ amps.
     
  6. mjw21a

    mjw21a Super Member

    Messages:
    3,168
    Location:
    Brisbane, Australia
    Gorgeous. Really lovely. Wish I could hear them
     
    Pio1980 likes this.

     

    Please register to disable this ad.

  7. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
    1,181
    Location:
    Philly area
    That is some serious iron. Nice work, Brice!
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,781
    Location:
    Ball Ground, GA
    Great work Brice! Well thought out, and well executed as usual! If you get a chance, it would be great if you could really put one through its paces to know what its undistorted power response curve is, and the distortion produced over that frequency range. These amplifiers are much like the Bogen MO-200A I did a thread on some years ago -- so seductive in appearance, with an even better design than the MO-200A, and apparently built with all the right stuff -- but any real performance data beyond the general comments you've provided are still illusive for this amplifier. Having an Acrosound equipped model makes the data even more interesting since it sets a benchmark against which the other transformer offerings for the amp can be measured.

    With your comment about Acrosound's involvement with the design of the amplifier and their later debacle, it would be great if you could expound on those in greater detail.

    Super job!

    Dave
     
  9. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Thanks all for your replies.
    Kward, Keepers? Of course. Actually I restored another pair I gave to a friend of mine. I still have another half done I don't know yet.
    PIO, nothing is too bulky for good sound :)
    MJW, Sure swing by, from Australia ;)
    Dave, Yes I will run one to the torture chamber and report.
    From what I've read, end of the 50's the Allen company wanted an high power, high quality amp for their top of the line organ.
    At that time, Acrosound was the candidate they choose and they partnered on the creation of that 75 model.
    As I said, the design is close to the Acrosound TO350 amp published in their ads.
    However, the Allen organ does not use the TO350, the biggest from the Acrosound's line, it is something different Acrosound (Herb Keroes) produced especially for Allen Organ. It is much bigger and has difference in impedance.
    Allen produced quite a bit of AO75 with the Acrosound's OT. Then when Acrosound sadly closed the door, they had to create their own, in house and outside.
    The AO75 was produced until early 90. It was still their flagship product, then SS took over. Allen organ lost most of their records, but one person over there could match the serial number of one of mine to 1959 I believe.
    If anyone has better info, please jump in.

    Brice.
     
    Horsey and Pio1980 like this.
  10. Horsey

    Horsey New Member

    Messages:
    2
    I'm the fortunate friend bestowed these Mighty Allen Organ 75s completely rebuilt and retubbed by Brice.

    First, by way of background, I've owned some fine equipment and amplifiers. On the solid state side, I owned the McCormack DNA 300 Rev. A with upgraded wiring courtesy of Steve McCormack. There was a standard Rotel 200 watt amp prior matched with B&W 803 Nautilus and many solid state systems of commercial vintage along the way.

    Then I got more into tubes as Brice introduced me to a number of his rebuilds. Everything from Mcintosh 225, Eico HF-60, Acrosound 20/20 and the fine ULIIs among others. Each performed very well but then he said he was eyeing these Allen Organs.

    Since I had previously questioned him on obtaining the Acrosound ULIIs thinking it was a wasted effort before he bought them and been massively proven wrong; I held my tongue. When I saw the Allen Organ 75s, I thought little of their charmless industrial appearance. He pointed out the transformers were of Acrosound origin and on some others produced, the same design.

    Then Brice began working on them and sooner than thought, brought them out for a listen on his rig. It was an astounding improvement over every single amp he ever had in his system and not by a small amount. The only amp that competed with these Allen Organ 75s was Brice's complete rebuild of a Mcintosh 275. That's a special amp as many of you no doubt know and the only amp that could even get into the game.

    So about the AO 75 monoblocks. Imagine all the power you need with finesse that is unmatched and that's what I'm listening to right now with Dizzy Gillespie "A Night in Tunisia." Brice would tailor the power tubes for KT120s and they certainly do deliver. More recently, he brought out some NOS 6SN7 Sylvania tubes from his inventory and we agree it has heightened the performance to another impressive and unanticipated level. (Listen to Dizzy's horn at the end of the song above and if you don't get a rise from it; well you don't own these monoblocks sadly.) On the EF86 driver tube, NOS tubes offer some magnificence in matching and we've dueled with Amperex of French military vintage and Telefunken. They are deadly silent. There's no noise from your sitting position or at the speaker.

    About the sound, the articulation is simply some of the best I've ever heard and easily the best in my home. We're not talking about the overpumped audiophile sound big expensive rigs produce to grab hold of a listener. This is smooth and punchy, neutral, extremely detailed but finely integrated sound, relaxed and authoritative. The combination of separation and the soundstage where you can not only place but walk around the instruments is simply a marvel. That's not hype or hyperbole; that's just the way it is.

    The dynamics are what is above and beyond all other similarly powered amps I've had a chance to listen to for any extended period. The music is not pushed out, it leaps out but retains quality and instrument position throughout: from highs down to the lows. As many know, an amp either stands out here or it doesn't. Many a fine amp performs well but doesn't distinguish itself in this category from competitors. In this regard, the rebuilt Allen Organ 75 monoblocks stand alone.

    My friends who have had the good fortune to get a listen are amazed at the performance. Since they are not audiophiles, they are generally taken aback at what their hearing. But sadly, they haven't even heard them at their best. The Sylvania 6SN7 NOS tubes went in last month.

    May have more to report on this as there's certainly more to say but if someone has the ability to implement Brice's rebuild on a pair of these monoblocks they're set for life. I certainly am.

    Related equipment: McIntosh LS340 speakers (never sounded better)
    Schiit Passive Pre & Orei 34x DAC.
    Allen Organ 75s "float" on 27" Schrader tubes, board and steel balls.
     
    Last edited: Nov 10, 2018
    Crestwood23 likes this.
  11. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Dave, Please have a look at my results.
    THD at 5W output. Frequency response at 1.25W.

    I wonder if I should give a try with KT88 instead also.

    Thank you,
    Brice.

    AO75_THD+Hz.jpg
     
    Horsey likes this.

     

    Please register to disable this ad.

  12. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    While the amp was wired for test, I put 2 KT88 Gold Lions on it and bias them at 75% dissipation.
    This is interesting as I am getting a lot less distortion in the same condition in the HF spectrum.

    KT88-AO75_THD.jpg
     
    Horsey likes this.
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,781
    Location:
    Ball Ground, GA
    Thanks for the measurement work Brice. Based on their original intended application, the amplifiers are working very well -- that being a principle frequency range of from (about) 25 Hz to (about) 4200 Hz, based on the fundamental frequency range of a piano. Harmonics extend higher of course, but the range given is where any significant amount of power was originally required from the amplifier.

    If you try to improve the response characteristics on the high end (to flatten things out to 20 kHz), that will likely impact distortion at that frequency in a negative way. That's a pretty big transformer to push any significant amount of high frequency power through, so a compromise between distortion and response is necessary. The question is, are the compromises made in the original design for the original application appropriate for the the re-purposed application the amplifier is now used in? One way to start addressing that question is to establish what the current power response curve is. You know that it will be excellent on the low end, with plenty of power and low distortion available likely right up to your original (assumed) 1 kHz power output level and THD information provided. But knowing what the as-designed un-distorted power response of the amplifier is right up to 20 kHz will then provide the proper basis for determining what performance expectations/goals can realistically be set and achieved when the amplifiers are optimized for their re-purposed application.

    Did you do any stability testing to know how the amplifiers react to no and capacitive only load conditions? Full range speakers for reproduction work will likely present a much more complex load to the amplifier than the comparatively simple load that the original sound production speakers for an organ represented.

    Dave
     
    Horsey likes this.
  14. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Thank you Dave.

    No I haven't done that test. What would be the preferred approach? Small signal in, and small caps load up to 0.1uf to trigger oscillation?

    I haven't done square signal response yet.

    I let the amp on the bench, so I can do all sort of test, and that's a good excuse to get back online my Acro 20-20 :p

    I'll put back the KT120 as the amp produces less distortion at audio frequency than the KT88, surprisingly enough as the KT120 is not nearly close class A.

    Brice.
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,781
    Location:
    Ball Ground, GA
    The power response test is performed by measuring maximum power and THD at various points across the audio band -- including the 20 Hz and 20 kHz measurement points -- when properly loaded. It is typically done at the onset of clipping, or if poorly defined, at the point where deviation from a sine waveform begins.

    HF stability tests are performed using a 1 watt 10 kHz square wave (as measured when normally loaded) with scope hung on the output, and then the output shorted, opened, or operated in a capacitance only load ranging from .005 uF to 1.0 uF. At some value, the cap only load will certainly produce a maximized ring on the tops and bottoms of the square waveform, but should never cause it to break into full blown oscillation.

    LF stability tests are most easily performed by again hanging a scope on the output of the amp, and lightly pulsing the input of the amplifier when operating first into a normal resistive load, and then into no load. In both cases, the baseline of the scope should settle rapidly, with little after bounce. A baseline that is slow to settle or actually is stimulated into full blown oscillation obviously indicates trouble. You can pulse the amplifier with any convenient DC source, including the use of a simple battery. Just be sure to use a pulse voltage appropriate for the sensitivity of the amplifier, and an output level that would otherwise produce an output of about 1 watt.

    Dave
     
    Horsey likes this.
  16. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Thanks Dave for clarifying these tests.
    I'll do them and report.
    Brice.
     
    Horsey likes this.

     

    Please register to disable this ad.

  17. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    Hi Dave.

    Here is the Power response chart I measured for this amp.

    AO75_Power_Response.jpg
     
    Horsey likes this.
  18. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,507
    Location:
    Hoboken NJ
    For the HF stability tests @10k square 1W, I could not get this amp into oscillation.
    I've tried very small caps to 1 uf. The change in the curve gradually gets bad > 0.2uf.

    Here is where I started:
    10K 1W 0.0uf.jpg

    Then gradually increasing the loading cap
    0.1uf
    10K 1W 0.1uf.jpg

    0.2uf
    10K 1W 0.2uf.jpg

    And so on
    0.4uf
    10K 1W 0.4uf.jpg

    to 1 uf
    10K 1W 1.0uf.jpg
     
    Horsey likes this.
  19. multichamp12

    multichamp12 AK Subscriber Subscriber

    Messages:
    521
    Awesome job! Those are monster amps!
     
    Horsey likes this.
  20. 6DZ7

    6DZ7 Super Member

    Messages:
    2,216
    Brice,

    I'm curious about the shorted and open output tests. How long did your connection remain shorted and what did the scope show? And what showed on the open test as far as a change in the square wave amplitude or shape? How long did you run the amp with the open connection? Did you just disconnect the load resistor at the common terminal while the amp was running and then short the connections with jumpers?
     

Share This Page