HF-87 restoration

Discussion in 'Eico Place' started by RS Steve, Jun 6, 2017.

  1. RS Steve

    RS Steve Tube Junkie Subscriber

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    I have two of these to restore, both have one OPT cooked. So I decided to swap the good OPT from one and send both bad ones out to be rewound. I hear great things about these amps so saving them both seems the thing to do. One unit has the short leads from the PT hoked up, and an adjustment pot installed, the other doesn't. I'm hoping I can get advice and direction on these since I don't have much experience with these amps. The pictures below show what's been done, most small capacitors have been replaced.

    20170606_105124.jpg 20170606_105805.jpg 20170606_105838.jpg 20170606_115323.jpg 20170606_115350.jpg 20170606_115447.jpg
     
  2. primosounds

    primosounds Single ended KT66/KT88 with 6SL7 or 6SN7 driver Subscriber

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    Hi RS, i have rebuilt 2 of these and also an HF89. I like the 87 better than the 89 soundwise. But this maybe my choice of caps in the power supply. i used some very big uf caps and perhaps they don't sound as good as smaller units. So, i would suggest maybe no larger than 470uf for the voltage doubler. Also for that 89 i installed a choke and made the final cap a 100uf x 500vdc film cap. I think this is a good move except maybe the film cap is overkill. I also used all metal film resistors and this may have affected the sound some. If i did the rebuild again i would sprinkle some carbon comps in the grid stopper and if you put 100 ohm stoppers on the G2. For coupling caps i used vitamin Q from Sprague which sounds nice with these units. If you are going to rebuild followiing the stock circuit, you need to be careful of the 12ax7 input tube as it has so much gain that it can be noisy. But the circuit is excellent in stock form and will wipe all the Dynaco st70 head to head.
    About that pot in your amp, the HF87 only had the volume pots, so i am not sure what that is for. In the fixed bias 89 there are 4 pots. Also if you can keep the surgistor slow warm up device, that is superior to a CL90 or similar. You need to check if it is working properly and sometimes it may need to be adjusted so that the contact points meet precisely.
    In some amps the bias voltage from a pair of el34 is used to heat the 12ax7 filament. This is a good thing but i am not sure if doing it that way is better because the later circuit had regular AC heating of that tube. What i did do was install a small filament trans and powered the ax7 that way. I also tried using DC on all the signal tubes but my knowledge of tube circuits back about 15 years ago was not good at all and i had trouble implementing that. But, if you find that the stock circuit with AC heaters is a bit too noisy it is probably worth considering doing that.
    Well, good luck with your efforts.
     
    Last edited: Jun 8, 2017
  3. RS Steve

    RS Steve Tube Junkie Subscriber

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    Thanks for your information, turns out both the HF-87's have a bad OPT and they are from different companies. I could still put one together with the different OPT's, but I'd rather wait to see if I can find the correct ones for each unit. So if you have any leads on HF-87 OPT's please let me know, these two amps will be put aside again until I can find what I need. Not sure if they made any mono amps with the same iron or not.
     
  4. primosounds

    primosounds Single ended KT66/KT88 with 6SL7 or 6SN7 driver Subscriber

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    OK, well, the OPT were made to the same specs , although later ones have secondary taps to 16 ohms while the earlier versions are tapped out to 32 ohms. In the picture the opt are EICO HF87 from a basket case that i bought awhile ago. The amp i made with them is a class A PP with a transformer phase splitter. The circuit is simply an "RH" type universal amp with the signal first undergoing a phase split before seperating into 4 rh circuits. My intentions were to test the effect of phase splitting on sound quality. I would say that even this type of phase splitting, as accurate as it may be, left a sonic fingerprint on the sound quality. There is a slight "haze" around the notes that are not present when the exact same circuit is used single ended. This is something that i now notice on every pp amp, instead of the sound being clean without artifacts. .So, if you want to buy these 2 transformers i would be willing to sell them but as a pair.
    As far as EICO EL34 mono blocks they did have 2 models , afaik, one was the HF59 with a potted chicago trans, and the other the hf60, with the Acro OPT. So, for some reason they did not use the HF87 opt for a mono amp, afaik.
    DSC05437.JPG
     
  5. RS Steve

    RS Steve Tube Junkie Subscriber

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    From what Dave Gillespie has told me, the last version of the HF-87 had two B+ leads and all earlier versions had just the one B+ lead. You are welcome to PM me with details on your OPT's, could you post what numbers are on them?
     
  6. primosounds

    primosounds Single ended KT66/KT88 with 6SL7 or 6SN7 driver Subscriber

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    M32019
    352025
    IS the number on both transformers. These are the early version with 4,8,16 and 32 ohm speaker taps.
     
  7. RS Steve

    RS Steve Tube Junkie Subscriber

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    That's definitely the type I need, PM sent.
     
  8. RS Steve

    RS Steve Tube Junkie Subscriber

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    Still searching for OPT's , turns out each HF-87 I own have different make transformers. So either number will work, but I'd like to not mismatch if possible. M32019 and S32019 are what I am searching for. These amps will get set aside I guess until I find the needle in a haystack.
     

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