A different twist to the Eico HF-85

Discussion in 'Eico Place' started by BrokenHill, Oct 29, 2011.

  1. BrokenHill

    BrokenHill Well-Known Member

    Newhall Pass CA.
    As most have mentioned The Eico HF-85 in stock form has some issues that others
    including myself find to be a nuisance, as far as I know nobody makes any upgrade
    kits or better replacement potentiometers that fit nicely in place of the original ones.
    Most of the ones that do get highly praised are the ones that have been rebuilt and
    modified by D.I.Y. type owners and there are a several here on the AK forum that
    have achieved that.:thmbsp:
    The HF-85 can be a very nice sounding preamp but it will require a lot searching
    and reading with lots of work to be preformed.
    The ones you see on ebay are usually stock and in need of new caps and most likely
    other upgraded components to be real good performers yet they do seem to fetch a
    lot of money in that condition.

    I got my Eico as a basket case in very poor condition and acquired it a lot cheaper
    then they normally sell for so it made a great candidate for what I did to it, basically
    it was just a good parts donor, even the tube sockets were junk and had to be
    I am only using three 12AX7 tubes and the one 6X4 rectifier tube the other tubes
    have been removed from the circuit and so has the selector switch, the unit has been
    simplified and is just a bare bones preamp with tone controls, and yes I did add some
    double diodes in parallel each other in opposite direction to bring down the heater
    voltage on the remaining tubes and that works extremely well and I thank one of the
    members here at AK for that trick.
    Along with the preamp out jacks I did keep the tape out jacks because they bypass
    the tone controls and the level out is controlled by the volume pot and that was the
    way this preamp was designed stock and I liked the way that works and can be used
    with or without tone control.
    All pots have been replaced with much better ones, I used a ALPs pot for the volume
    and that was a major improvement over the cheap crappy stock one, I had to make
    short shaft extensions for all the new pots to fit with the Eico's metal enclosure.

    I did make a new face plate because I moved the tone pots to where was more room
    on the other side with the new circuit boards for the tone controls mounted over the
    opening where the selector switch used to occupy, that worked out very nice.

    The encased printed circuit wafers that sit just behind the tone pots can be upgraded
    to individual film caps and resistors mounded on a small PCB or breadboards from
    your local Radio Shack, the schematic for them can be taken from the Eico HF-81
    schematic it uses the same printed circuit wafers that also sit behind it's tone pots, for
    some reason the HF-85 did not show those in detail on it's schematic.
    I used sheilded wires that are grounded from the new tone circuits to the tone pots,
    I believe this is part of the problem with this preamps stock noisy characteristics.

    While I was rebuilding the HF-85 over the past year I kept thinking if this is was going
    to be worth all the effort and work that had gone into it, but being that it's just a D.I.Y.
    hobby and after hearing what it sounds like now I think it was worth every bit of it.

    Attached Files:


    Please register to disable this ad.

  2. Ohighway

    Ohighway Wannabe Minimalist Subscriber

  3. gogofast

    gogofast AK Subscriber Subscriber

    wow, that's really nice. can't hear what they sound like, but sure looks awesome. any chance you can show us the inside? and under?

    so you're just using for one source? i can understand getting rid of the phono, because they are indeed terribly noisy, but why not leave another input for two line level source? anyways, love the minimalistic look of the hf-85.
  4. BuzzK

    BuzzK AK Member Subscriber

    Very nice work!
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    Broken -- very nice looking unit in deed! It retains its original identity, but with a fresh new look you gave it. Very cool.

    Is your circuit basically stock, or did you deal with any of its design deficiencies as well?

  6. mhardy6647

    mhardy6647 AK Subscriber Subscriber

    I like the looks of it!

    I also have a basket case HF-85 (as well as one fairly nice one)... maybe this will give me... ideas; thanks!


    Please register to disable this ad.

  7. Dmannnnn

    Dmannnnn Up Late

    Denver, Colorado
    It looks great. Did you keep the stock PEC cards on the tone controls?
  8. Kegger

    Kegger R.I.P. 1/12/1966 - 6/1/2017 Super Mod

    I've still got 2 of these to finish.

    My main mod is deal with it's gain, changing the main gain stage tube from
    12ax7 to 12ay7 and redoing the feedback circuitry so that a 12ay7 has the
    same amount of feedback as the 12ax7 did which will drop gain.

    How the gain/feedback work originally is that even if you swap to a different
    an lower gain tube then a 12ax7 the output of the device stays the same is
    because when you use the lower gain tube the feedback lowers along with it.

    So I am going to modify that feedback to have the same amount of feedback
    you got with a 12ax7 with a 12ay7 in it's place so the gain will then be lower.
    (the rest of the circuitry in the 12ay7 stage will need be tweaked very minor)

    We'll see how it works, but I have high hopes. ;)
  9. zekk

    zekk Active Member

    Gorgeous! :thmbsp:

    Mine was done almost like your's.

    Bass pots used 2x Alps wired in mono.

    Treble pot used back original but was totally dismantled and serviced, both sides calibrated for balance, locked-in, before soldering back.

    Finding replacement for PC-1&2 is mission impossible!

    Resort to fabricating it based on HF-81's drawing and using good quality componants butchered from my Radfort SC2 preamp. :D

    Results, sounds louder, cleaner, but lack dynamics!


    From the above working layout, I can check-out the condition for the two 100K and the 4.7nF between #2 & #3.

    Guys ..... thanks for bearing with me!
  10. BrokenHill

    BrokenHill Well-Known Member

    Newhall Pass CA.
    The top inside was a mess when I got the HF-85 and I did not focus to much on
    making it pretty because you really don't see it when the top cover vent is on.

    I use a A/V Switcher box to select different audio sources, a switch inside the Eico is
    one less thing to take up space.

    the circuit is basically stock, I am not as advanced in modifying like some of you AK
    members are and the Eico sounds very nice now so right now I am enjoying listening
    to it, although in the future I might open it back up and try some of those circuit mods.
    I did use a 250K linear ALPs pot for the volume control because I had an extra one
    and it made the control on the volume more tame easy to adjust and not so touchy,
    it even tracks evenly at low volumes, I know it's not the right value but somehow it's
    working very nicely in this circuit.

    good luck with Eico HF-85 project.

    those darn stock PEC cards is where I was getting a lot of noise from, after I built
    the replacement boards I still had some noise then I used shielded wires to connect
    them to the pots and grounded the outer shield conductor as you can see in the pics.
    It really did bring that issue under control.:)

    you always amaze me with the stuff you do and it will be interesting how your mods
    work out.

    the dynamics are very good with the PC1 & PC2 replacement boards, I did use a
    lot of Wima Film caps in this rebuild because I have had great luck with them in
    other projects and it's easy to find the values I need from Mouser, it's most likely
    the combination of everything that I did to this HF-85 to still have nice dynamics
    and good soundstage.

    I still have AC to the tube heaters but you have to put your ear right next to the
    woofer when the volume is all the way down to hear any hum at all, it's very hard to

    Attached Files:

  11. gogofast

    gogofast AK Subscriber Subscriber

    wow, very impressive under the chassis as it is outside... very clean and nice! i've built some headphone amps with those wima's and they sound fantastic.


    Please register to disable this ad.

  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    Keg -- I took the approach to the HF-85's gain problem you describe several years ago, and it works wonderfully. Basically, (for my purposes anyway) the line stage only needs a gain of about 6 (~ one half of the original) for use with the sensitivity of most power amps. In my version, I replaced the 12AX7 with a 12AU7, and also addressed the the ability of the second stage to properly drive the lower FB resistor required. Using this tube and a 20 db loop, distortion is unmeasurable, output impedance is less than 500 ohms, and it will drive a tank.

    Even with stock units, there are three basic issues everyone should address with their HF-85, that amount to design errors in the original design:

    1. The original B+ filtering is inadequate (120 Hz hum can be heard with sensitive power amps). The best answer is to split the 10K B+ dropping resistor into two 4.7K resistors, and add an additional C of filtering at the new junction.

    2. The tap for the tape out jack is taken from within the FB loop of the line amp. This causes an EQ error with the tape out signal (bass boost). Correct this by moving the source for the .02 tape out coupling cap to the output side of the main coupling cap (.1 mfd) for this stage.

    3. DC from the line stage flows through to the tone amp stage, making the tone controls noisy. Correct this by adding a .25 mfd cap between the output of the line stage and the input of the tone stage. An added benefit of this approach is that the main output jack coupling cap (.25 mfd) can now be removed, as it is no longer needed.

  13. ferninando

    ferninando Lunatic Member

    San Jose Ca.
    Brokenhill, did the removal of the low level preamp tubes have any effect on the PS voltages, either heaters or B+?
    What Did you do about it/?
  14. BrokenHill

    BrokenHill Well-Known Member

    Newhall Pass CA.
    Yes it did, the tube heater volts went up to 6.9 volts I used two diodes to drop
    it back down to 6.1v.
    The voltage on the 12ax7 & 6x4 went up also, and no I have not done anything
    about that yet.

    Tube V3 (12ax7)
    pin (1) 219 volts > should be at 180 volts
    pin (6) 174 volts > should be at 140 volts

    Tube V4 (12ax7)
    pin (1) 215 volts > should be at 180 volts
    pin (6) 163 volts > should be at 140 volts

    Tube V5 (12ax7)
    pin (1) 164 volts > should be at 140 volts
    pin (6) 169 volts > should be at 140 volts

    Tube V6 (6x4)
    pin (1) 299 volts > should be at 275 volts
    pin (6) 299 volts > should be at 275 volts
    pin (7) 355 volts > should be at 340 volts

    It's a hot potato, do you think those juicy voltages are causing the vocals
    and voices from the radio DJs to sound like they are coming out in the air
    instead of the speakers?

    The rainy and cold days are coming soon so I will most likely find the time
    then to try to figure out the additional mods that dcgillespie is suggesting.:thmbsp:

    B.T.W. this HF-85 is a match made in heaven to the Magnavox 9303 amp.
    I have had two Fisher 400 tube receivers (still have one of them) and these
    two units together sounds better then those :whip: Fishers.
  15. ferninando

    ferninando Lunatic Member

    San Jose Ca.
    thanks for that. and so you dont feel any harm comes from slightly increased PS voltage. I like the double diode fix. gonna try it when I get a chance on my friends 85 Ive worked on.

    PS: Id say you're experiencing soundstage openess/expansion and the speakers dissapearing. thats a GOOD thing. Just IMO tho.
    Last edited: Oct 31, 2011
  16. BrokenHill

    BrokenHill Well-Known Member

    Newhall Pass CA.
    Well if others chime in and think it's going to cause some issues then I could take out the
    diodes on the tube heaters and try a thermistor on the AC line in, they cause a voltage
    drop and might bring all the voltages down to closer to what the Eico HF-85 instruction
    manual shows, I did that on another tube amp and it worked out nicely.


    Please register to disable this ad.

  17. BuzzK

    BuzzK AK Member Subscriber

    Dave -- Thanks for sharing some great advice.

    Do you recall the changed cathode and plate resistor values for the 12AU7s?

    Also . . .

    1) Did you have the volume / tracking problem with the level control, and if so, what did you do?

    2) Did you install fewer RCA jacks in the back (e.g. remove tape head input) so that you could have more room for modern cables?

    3) Make any loudness switch changes?

  18. Augjer

    Augjer Music Appreciator

    Los Angeles, CA
    WIMA all over that thing...
  19. BuzzK

    BuzzK AK Member Subscriber

  20. ampex6973

    ampex6973 New Member

    Kent Ohio
    I love my hf 85 stock sound

Share This Page