HF-81 resto

Discussion in 'Eico Place' started by interalian, Dec 5, 2016.

  1. interalian

    interalian Super Member

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    I have an HF-81 that my dad hand-built in 1959 that's gathering dust in my basement. I 'inherited' it when he replaced the 'old tube stuff' (*) with some Kenwood separates in 1979 and ran it for a number of years after that as my bedroom stereo when I was a teenager.

    Now, the thing is, in my youthful and amateur teenage zeal, and with access to Radio Shack, I pulled and replaced all the original Eico/Mullard tubes and tossed them, replacing with "The Holy Grail RS Lifetime Tubes" which were Japanese tubes of questionable provenance. Ugh.

    In order to atone for my youthful indiscretions, I want to do a full and proper resortation on the little beauty. Any tips for best sources for parts (Canadian source preferred), and any suggestions for the best replacements for the old carbon comp resistors and signal capacitors? I intend to keep all the original wafer switches.

    I'm tempted to ditch the EZ-81 tubes and replace with diodes to ease the load on the power transformer. Any concerns about doing that?

    Dad is almost 80 now, and it would be nice if I could finish it for him to hear again.




    * = junk...
     
  2. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    Stick with vacuum tube rectifiers (IMO). Why do you want to change them? You'll have to add some sort of soft-start (or standby switch) to avoid cathode stripping and you'll have to pad the power supply to allow for the higher efficiency of ss rectifiers.

    Cannot help you with sources in Canada. www.partsconnexion.com is a good source IME (and they're in Canada) but maybe not what you need or want in terms of parts quality/price.

    Personally, I don't replace old carbon comp resistors a priori. I do (i.e., I did) replace the ceramic coupling capacitors.
    The HF81 is a very nice pp EL84 amplifier by any standards (except aesthetic!).
     
    drtool likes this.
  3. interalian

    interalian Super Member

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    Fair enough about the SS vs VT rectification, but I've not seen much out there in EZ-81 land. The rect tubes are original as RS didn't ever have the LT tubes in that type.

    Since I have the original assembly manual, I'm really tempted to strip it to the chassis and build it up with all new R&C. You know, in for a penny...
     
  4. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    I reckon the rectifiers in mine are original, too -- they'll last a good long time (pushing 60 years in the case of my HF-81, which is roughly the same age as me, I believe).
     
  5. Patrice B

    Patrice B AK Subscriber Subscriber

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  6. interalian

    interalian Super Member

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    I've seen that site. They don't appear to publish any manufacturer names or specs for the parts they're selling. The 40uF/450 axials for about 4 bux? Atoms sell for 10x that, no?
     
  7. Patrice B

    Patrice B AK Subscriber Subscriber

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    They sell yellow caps, don't know what brand, but they're good enough for me.

    To be honnest, I didn't replaced the ceramic caps in mine.
     
  8. interalian

    interalian Super Member

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    I'm talking about the electrolytics. I'll give them a shot for some though.
     
  9. Patrice B

    Patrice B AK Subscriber Subscriber

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    interalian likes this.
  10. interalian

    interalian Super Member

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    As I think about this, I recall that Eico made the HF-86 which was essentially just the stereo amp section of the HF-81 although it uses different driver tubes (12DW7 combo vs 12AX7/12AU7). I always wanted to run the HF-81 as a monoblock and have an HF-86 as monoblock to drive the other channel. Good plan? Does anybody make decent 12DW7s anymore?
     
  11. interalian

    interalian Super Member

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    Just noodling here, but I'm missing a couple of the knobs for the 81. A friend has a machine shop, so I'm thinking a non knurled/ridged set in the same basic shape anodized black, with an indicator notch. My old dual concentric bass and treble pots are toast and I have some nice Ohmite replacements to use but are common shaft.
     
  12. fdrennen

    fdrennen Organist in Residence

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  13. gogofast

    gogofast Addicted Member

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    I used justradios.com extensively when I first started rebuilding tube receivers and amps. They're fantastic overall. I believe they source the capacitors from a manufacturer that Cornell Dubilier source as well.

    Just don't buy capacitors from radio shack. Most of their film and 'lytic caps are junk! Legs come off the film caps, super light weight 'lytic caps pop and leak at slightest stress.

    HF-81's power tranny is a known weakness, and there were beefier replacements available for around $100 (not sure how it is now). Fantastic amp for sure. There's definitely a close to cult-like following. If you can, replace the rectifier with NOS since it's not unreasonably expensive still. It may help if your current one is tired.
     
  14. interalian

    interalian Super Member

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    Thanks for the recommendation. I'd never buy parts from Radio Shack (not the least due to their absence in the Canadian market now), rather parts from Mouser and the like.

    I've had the 81 forever and have heard about the 'weak' PT. I'm going to keep the EZ81 rectifiers and hope the PT stays intact. I do recall it always ran very hot. That said, the original manual states the PT and OPTs run very hot, and 195F is not an unusual temperature.
     
  15. tube-a-lou

    tube-a-lou Super Member

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    Heyboer makes the upgraded power tranformer which I'm really thinking of getting because the amp does
    have that magical sound!
     
  16. Shadowdog

    Shadowdog Super Member

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    Flin Flon, Manitoba
    I have HF-12's which are said to have the same OPT's except set up to lay horiz. The OPT's shouldn't even run warm (mine stay cool) unless caps are bad. PS trans. runs very hot so I run a fan on the amps to help things last.
     
  17. Mossback

    Mossback New Member

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    Location:
    The Great Pacific NW
    Highly recommend what Tube-a-lou said, new Heyboer power supply transformers are a great improvement safer as well.
    ~$100.00
     

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