EICO HF-81 REBUILD TIPS: Less is More?
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Eico HF-81 Wiring Shot

First of all, these notes were sent to me before I had an EICO HF-81 schematic which explains why they are rudimentary. Roger Stevens and I had exchanged several emails regarding mods to the EICO HF-81 and this the basic text from one of them along with a full-size shot of my HF-81 interior (which had a few replacements but was largely stock) with corresponding numbers to parts referred to in the text. To be honest, I now feel that it's hard to keep the original quality that makes the EICO HF-81 stand out if moving to some of these modern solutions.  I do know what of the EICO HF-81 I have heard and owned, my ORIGINAL unit you see above surpasses all others in the sonics department.  I have also noted that many that go the entire modern route cap replacement via Auricaps end up parting ways with the little EICO. 

Stock it's a 30mfd@350vdc electrolytic. I used a new axial electrolytic I obtained form AES (www.tubesandmore.com), and went to a 47mfd@450vdc cap. The make is probably a Nichicon or Xicon - They're equivalent. A step-up would be a Sprague Atom (see latest on this), also available from AES.

If it were me, I would go the extra route and at least put some Sprague Atoms (see at left) in here; This is a replacement you see often and is time tested in many a guitar amp showing good reliability.  Other more esoteric options have trouble fitting here.

In a head to head comparisons with the now virtually extinct
47uF ELNA Cerafine, the Cerafine won out but the ATOM is a good, solid choice (You cant go any higher than 50uf due to tube rectifiers).  Another cap option is the Panasonic TSHA series, which is getting good comments.  Regardless, like Stephen, I would tie these caps down in some fashion. The originals were held with clamps which can be modified to fit the ATOM.  Another choice is the glued plastic tie down option but not sure how this will handle the heat buildup over time.  Anyone with other options that have brought good results are welcome to drop me a mail as this is one of the first replacements done, after trying the "reforming the caps" option - kh.

#2 This is the power supply dropping resistor, probably a 5K. Leave it alone.

#3 These are the output tube cathode bias resistors, and are 165 ohms stock, at 5 watts. I'd make them a 180-ohms, 5-watt Xicon, available from Mouser (www.mouser.com) or DigiKey (www.digikey.com), you have to look. This will lower the temps a bit and more closely match modern voltages anyway. Get them as closely matched to each other as possible--order 5 and measure them and select the closest two.

Once again, I am not sure about this either. Would this cause the amp to be biased a bit colder and thus lose some of the warmth? His idea does make sense and is worth a try. I would probably elect to get new stock in both values and might just elect to keep the bias the same as stock.  The problem is, you cant even FIND the original value very easily.  Seems the prudent option is to replace with the ohm reading that would bring your EICO to stock readings with modern voltages.- kh

#4 These are the line stage coupling caps, and as stock are .1mfd@600vdc
if I recall correctly. I used 716P Orange Drops of the same value.

In the past, I suggested moving to the Auricaps at near the same value as these caps are supposed to be relatively neutral and not too harsh while also fitting within the chassis. This seems to be a preferred option to the Orange Drops (If going with the Orange Drops, make sure you get the 716P and not the 715p). I sure wish oils or some more could be used here and in the main coupling caps. I now believe these to be the smoothest but size is an issue.  There is also a lot of heat down below here.  Some have even gone as far as finding NOS Black Cats which is another option and one I might consider.  Again, I think the sound can be changed here if one is not careful - kh

#5 These are the output tube coupling caps. The stockers are ceramic and have to go. Space is, however extremely tight, and anything good would be tough to fit in there. I started out with no-name polyester film yellow jobs from AES, and used .1mfd@600vdc units. I later reduced them to .047mfd@600vdc Orange Drops (type 716P), also from AES, but they're physically a lot bigger, but usable. Sound great. 716P Orange Drops are the best overall compromise between price and performance for these amps. I haven't tried Illinois like they recommend up on the Vacuum Tube Valley web site (they sell the beasts).

Very important to sound of amp. Really can affect the smoothness and transparency. Worth the extra to put some quality here, although I am once again stuck as to what is the BEST option here. I have not made up my mind but most of the guys say the "Orange Drops" just aren't in the same league as other film/foil caps. As he said, room will be an issue in stock chassis. Others to consider included Hovland Musicaps and Oil Filled caps which can be costly but have the richest sound. NOTE: Probably the easiest option size wise would be the Auricaps I mentioned above. And at this stage, I am more inclined to not go that much bigger than stock. I would go with the 600v versions at .033 or even .027uf. The 400v are smaller and easier to fit, however (also cheaper).  The reason is that larger caps can actually slow the amp down and I believe the bass is already a strong suit in the EICO HF-81. 

Still, I am just wondering now if the Auricaps are the best route.  My FAVORITE HF-81 has the original CERAMIC DISCS still along with the Pyramids you see above.  Indeed, I have loved the sound of this amp so much, I have not changed a THING in coupling cap positions, either in the pre or output stage.  My Moondogs have Jensen  Copper Oils while ye olde lovable EICO is still festooned with the original Ceramic Discs in the output tube coupling stage - I'm afraid to mess with such a good thing!   kh

#6 These are the output tube cathode bypass caps. In stock form they are, I think, 25mfd@25vdc electrolytic caps. I used 100mfd@35vdc electrolytics from Radio Shack. A step-up would be 100mfd@25vdc Sprague Atoms from AES.

This is an interesting position.  The old geezer that was part of the part that originally sold the amp to me placed what appears to be some Tantalum Caps, something that my Quebec compatriot JFL believes to be a good choice and desired by much of the French DIY brigade.  One could go with some really high quality Black Gates here as well.  AGain, this is one of the changes in my amp that is different from any other EICO I have auditioned or owned.   I have to say, I prefer my original HF-81 to any I have heard to date and this is just ONE of the changes in mine.

#7   Phono Preamp Section - Stephen actually missed this via the email but I'll make a few comments.  As I noted on the mods page, there is real room for improvement in the phono as I think it ends up being somewhat of a weaker link via the HF-81 and it shouldn't be.  Many seem to find a bit of noise here, often in the left channel.  Moving to some better, quieter resistors and optimizing the RIAA and the components seems to be needed based on units I have heard.  The unit below had all Kiwame resistors in the phono stage with more Auricaps.  One thing that some have done is to move the RIAA section, which is on the selector switch assembly, and move it under the chassis near the rest of the phono/tubes.  See the phono RIAA pic in the mods section.  To be honest, I have never gotten the phono stage quite to my liking and end up using my other gear for the phono pre.  Still, I have heard of reported good sound via the HF-81 if optimized and checked out.

Click to see full Eico HF-81 Wiring Rebuild featuring Auricaps, Atoms, Kiwame Resistors
Fully Modified EICO HF-81 with Atoms/Auricaps/Kiwame Carbon Film Resistors
PS Atoms at top right should be tied down with stock clamps or plastic tie downs.
The jury is out on whether moving to all Auricaps is a good thing.  The mods to the phono
stage are promising.  Just don't forget the important RIAA located on the Source Selection rig!
- Click above for full view -

The Transformers and Other Odds and Ends

Ultimately, I still think the heart and soul of the EICO is the great output transformers which seem to do an excellent job with a great match with the HF-81 circuit.  I think copying the HF-81 circuit without the output iron brings something less than the HF-81.  Ditto for just the amplifier stage minus the preamp.  I think the goodness lies within the sum of the whole.  Bypassing the tone controls might bring you a different result than you originally thought.  Many have gone back to leaving the tone controls within the circuit as some of the bloom and warmth departs.  Bypassing the balance pot is sometimes done and might be worth the little effort.  Same with that pesky SWITCH that can divide the amplifier.  That seems to be nothing but a headache and best bypassed. 

While the Output iron is great, the POWER TRANSFORMER is another matter.  This unit appears to be slightly under-designed for the HF-81 despite what the factory said.  Often, it runs VERY HOT and if the power supply filtering is left to wayside, you will sometimes find overly stressed PT with buzzing and humming from the fact that the wax has melted.  Heyboer makes an EICO HF-81 power transformer upgrade that involves an overbuilt version, more able to handle the loads.  Although I have not tried the overbuilt unit in my own EICOS, I have heard good things from others.  They also supposedly make a copy of the output transformers but I've heard they might have thicker lams.   The jury on the sound is still out as I don't know anyone that has inserted them in the circuit. 

One last note of mention for the EICO, I don't know of many that have not had problems with that swine power switch located on the treble pot.  This will invariably wear out sooner or later.  I see moving it to the back near the power entrance where the extra socket would be.  Many have drilled holes into the side of the case to add a switch, certainly not the most aesthetically pleasing solutions.

Can I do this on my own?

Well, many have learned on these vintage amps.  Your success will depend.  You surely have to be very careful due to the voltage and patient as well.  Sometimes, debugging these beasts is more than a pain.   At the least, a good soldering implement should be a real soldering station, such as from Weller or Ungar  ie a Weller 971ZX. Any temperature-controlled pencil iron with about a 35-45 watt capacity will be fine; you will need a 140-watt gun (or bigger) for chassis ground work. Check the links page for a good tech to give you a hand here.  Hot Glass Audio can do some good work.  Tell Ryan that Kelly sent you and that I'm watching!  heh...  Craig at NOSValves does work on these as well but his philosophy is a bit different on parts replacement.  Both can potentially do good work.

Again, there is debate within my skull as to whether going to the Auricaps might change the sonic signature of the HF-81.  There seems to be something to the original sound, even with the much maligned Ceramic discs.


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