#1 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
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
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
if I recall correctly. I used 716P Orange Drops of the same value.
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
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.
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.
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
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
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
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.
SEE EICO HF-81 REVIEW NOTES