Now THIS is going to be interesting (Unknown-origin Dual-Mono Mullard 5-20 home-build?)...

Discussion in 'Tube Audio' started by GordonW, Jun 9, 2018.

  1. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,765
    Location:
    Marietta/Moultrie GA USA
    Someone meant serious business when they built THIS amp!

    IMG_1929.JPG
    I mean- Acrosound TO-300 outputs- and dual mono construction THROUGHOUT!

    Got this from a local guy here, to check out for him- I haven't gotten to trace out the circuit yet- but I'm guessing that, from the tube complement (EF86/12AX7/EL34s) that it's probably a Mullard 5-20 design.

    IMG_1931.JPG
    I think those are probably paper-in-oil coupling caps?

    This looked to almost be a "cost no object" build- the power transformers, the oil-filled power supply caps, the huge chokes (6H 200ma each), and the output transformers- this amp probably cost about as much as a good used car to build, back in the day!

    From the looks of it, nothing was left to chance in this amp. An adjustable-tap resistor for cathode bias adjustment, a DC balance pot, an AC balance pot, and an input gain pot, for each channel. They even used an aluminum chassis for the audio circuit parts- a great thing for fidelity (no magnetic flux leakage in the chassis).

    I'm looking forward to getting into this one. The owner was pondering whether to part it out (heck, the transformers themselves are pretty super-valuable), or to get it going- I'm thinking that it would be a shame to NOT see what it can do...

    Regards,
    Gordon.
     
    bobabode, maxhifi, jeb98 and 8 others like this.

     

    Please register to disable this ad.

  2. drtool

    drtool It might get loud In Houston Subscriber

    Messages:
    2,147
    Location:
    Houston Texas
    Wonder if the oil caps still check good.:lurk:
     
  3. soundmotor

    soundmotor super modified Subscriber

    Why didn't he just build 2 mono amps?

    :crazy:
     
  4. kward

    kward AK Subscriber Subscriber

    Messages:
    3,183
    Location:
    Utah
    I'd sure agree with that.

    Maybe he thought separating out the power supply into its own chassis would reduce noise. Not an entirely bad idea.
     
    Brice likes this.
  5. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,491
    Location:
    Southern NJ
    That is lovely construction. I love the symmetry of it all. Certainly would be a waste to part them, they're just too nicely built.
     
    Pio1980, Dandy and TENER like this.
  6. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,765
    Location:
    Marietta/Moultrie GA USA
    I think that the prevailing logic probably was, that getting the power supply into its own chassis, is more important than separating the two channels.

    Also, once you get past two chassis- things become really unwieldy in practice. Who usually has the room for four boxes, just for a stereo amp?

    Regards,
    Gordon.
     

     

    Please register to disable this ad.

  7. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,765
    Location:
    Marietta/Moultrie GA USA
    I'm definitely going to find out. This is one that's worth taking the time to gradually bring up on the variac, over however long it takes...

    Regards,
    Gordon.
     
    drtool likes this.
  8. TENER

    TENER Active Member

    Messages:
    190
    Man those are very nice! I bet the guy that owns them can't wait to put some sound through them.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,416
    Location:
    Ball Ground, GA
    Very nice project Gordon! Looks like the only electrolytic caps are in the can under the amplifier chassis. With only oil caps on the power supply chassis, it should come to speed rather immediately. It will be interesting to see what you find out circuit wise, but the great odds are that it will be very close to the circuit as shown in the Mullard book "Tube Circuits For Audio Amplifiers", first published in 1959. That circuit employed an EF86, 12AX7, and EL34 tubes, and an output transformer with the specifications of the Acro TO-300 (6600Ω primary impedance, with screen taps at 43%). That circuit did not include any AC or DC balance controls or gain control, and used individual output tube cathode resistors and bypass caps, and also a GZ34 rectifier, but otherwise, the circuits are likely very similar. If you can't find the original schematic source for this unit, the Mullard circuit from this book will be a good one to reference in its place.

    Good luck with it!

    Dave
     
  10. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

    Messages:
    2,237
    Location:
    Terra, 21st century CE
    Building a separate power supply chassis has many benefits. The AC nasties can be physically isolated from the sensitive amplification circuits. Transformer vibration is also isolated. It is easy to manipulate your PS by tweaking component values and just dealing with only the components you need. About the hardest thing to deal with is the necessary arrangement and connections necessary for the umbilical cord. The picture doesn't clearly show what the designer chose to do. Many use male octal plugs. There are also cinch type or beau plugs. Some use military style connectors.
     
  11. jdsalinger

    jdsalinger Well-Known Member

    Messages:
    806
    Location:
    KCMO
    That would be a travesty. The sum of the parts can be more than the whole, on the market. But only if it was un-repairable or a horrible DIY circuit. Not the case here.

    Love the tag board amp design. To me, that also hints at Mullard.

    :lurk:
     
    Last edited: Jun 10, 2018

     

    Please register to disable this ad.

  12. knockbill

    knockbill Addicted Member

    Messages:
    7,255
    Location:
    SE PA
    Nice,,, looks to be a lot of military parts...
     
  13. pdm4606

    pdm4606 Super Member

    Messages:
    2,867
    Location:
    Las Cruces, NM
    Watts?????????
     
  14. soundmotor

    soundmotor super modified Subscriber

    Makes sense but cannot recall ever coming across one similar.

    Power supply & vibration has to have been his goal and remain on 2 chassis. Most would have opted for 1 channel & 1 ps on 1 chassis for that era. Glad they weren't stripped. I tore down several back in the day for their iron and wished I'd just let them be later.

    I get that but it is quite unusual construction practice for the time. Not sure I've ever seen that done, not saying it isn't cool. Back when I was actively hunting stuff down I stripped many a homebrew for its outputs. Builders haunted the surplus stores and military connectors such as Cannon type weren't uncommon to see for the umbilical.
     
  15. Dave451

    Dave451 Super Member

    Messages:
    1,005
    Location:
    Philly area
    Interesting indeed, Gordon. Oil caps? Wow! Intrigued to see how it sounds when you get it going. I have the book Dave G mentions, so I'll study up on the circuit as homework. Good luck with it!
     
  16. Dandy

    Dandy Super Member

    Messages:
    4,356
    Location:
    North West, UK
    Interesting use of dual power supplies. (I have a similar Mullard 5-10 stereo build from the same era with a separate power supply, but mine has only one very large PT and two chokes.)

    What tubes are used for the rectifier? What umbilical power cable and connectors are used? The power supply is not a straight copy of Mullard, as they specified a 10H 180mA choke. For ease of reference, the Mullard 5-20 circuit and construction manual is here: http://www.r-type.org/articles/art-003d.htm

    Will be following this thread with interest as a pair of these were my first home builds. It would be great to have the schematic when you get to it, as the original is a classic design, with impressive specs, yet this builder has added refinements. I remember when I got my first one finished and powered it up, I thought I'd made a mistake as it was completely silent.
     

     

    Please register to disable this ad.

  17. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,765
    Location:
    Marietta/Moultrie GA USA
    5V4 rectifiers.

    The umbilical cords use Jones plugs.

    Regards,
    Gordon.
     
    Dandy likes this.
  18. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,291
    Location:
    Middletown,New York
    Wow!, nice build, I really like that one, beautifully made.
     
  19. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,765
    Location:
    Marietta/Moultrie GA USA
    I just traced out the circuit, and quickly hand-drew a schematic- it looks like it is indeed a pretty exact Mullard 5-20. The only changes, were the addition of the input pots, the AC and DC balance pots, and the change from separate to combined cathode resistor and cap on the output tubes. Every other value matches the 5-20 schematic exactly.

    I still need to trace out the power supply- but it looks like the original topology of the 5-20 too. The cap and choke values are a little different, and it's using a 5V4 rectifier instead of a 5AR4- otherwise, it's very close.

    Regards,
    Gordon.
     
    Dandy and Pio1980 like this.
  20. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    2,457
    Location:
    Edmonton Alberta Canada
    Some early Williamson amps had outboard power supplies, heathkit for example.. I did an amp this way too, it works out great because it completely removes ground loops from the amplifier chassis, greatly simplifying the challenge of getting the amplifier totally hum free.
     

Share This Page