Vintage Tube Audio - Eico HF81 Full Restoration

Eico HF81 Full Restoration with new Heyboer Power Transformer

The Eico HF81 is an Integrated 14 watt per channel tube amplifier. In the last 5 years this amp has had so much hype that now the unrestored costs on Ebay have jumped to over $500. The output transformers are the reason behind the so called “magic” these amps produce. They have a super mid range which brings out vocals and they create an excellent soundstage. Below you can see a typical old unit completely torn down to the bare chassis and restored properly.

These amps in stock condition after 60 years ago when they were first intruduced, usually require a lot of work to get them into this magic class that everyone talks about. At least 90% of these amps were sold as kits like most Eico gear and were built by people that didn't have a lot of electronic skills. Solder connections are poor, wiring laid out tight like a piano string etc, and I have even found several connections were not even soldered!!

To get these amps up to a safe and reliable component , first the power supply capacitors should be replaced as the old ones were the paper type and leak, and the can cap should also be replaced, again these are over 60 years old. Reforming is a complete waste of time.

The next big item I have found are the badly corroded connections on the rca input jacks which cause hum and intermittent connections .The input selector control also can cause crackling and intermittent operation due to the wipers getting dirty on the switch, you can try dexoit and might get lucky for a while. I also label all the input rca jacks so you actually know what the inputs are. I have even made a custom jack board for some customers which require more line level inputs.

What I do for most of my customers is replace the rca plugs with new gold jacks, spaced farther apart and not put in unused inputs, new binding posts for the speaker connections and replace the input selector control with a new 6x2 switch. The phono RIAA filter components then have to be mounted elsewhere. I do this under the chassis on the terminal board by the 12AX7 phono tubes. Since the selector switch is being replaced, the tape head switch is then no longer required and the on/off switch can be installed there. No need to install it in the back as some have done. Don't continue to use the switch on the treble shaft, as they can fail and also everytime you switch the unit on and off it will cause wear to the carbon traces in the pot.

The rest of the electrical restoration involves either tightening, cleaning or replacing the tube sockets, replacing the coupling caps with orange drop , MIT, Auricap, PIO or which ever you wish depending on your budget. All the resistors should be checked for tolerances and replaced if over 10% from their original value.Here I prefer to use Kiwame 2W in the plate sections and Carbon film in the cathode and in the signal path. These are much closer to the original carbon resistors. Kiwame is slowly removing most of their values above about 47k in 2 watt. I have found Takman and Amtrans to sound very much the same and they are also carbon film.

Next I like to bypass the blend switch and disconnect the stereo/mono switch on the chassis.

The rest of the ceramic and electrolytic caps also get replaced . The biggest problem with this amp is the power transformer running very warm. With the increased AC wall voltages today and the underrated capacity of the transformer, it slowly will takes it's toll on these transformers. If they run hot and not vibrate then operate them on a variac down 5-7 volts, it will help increase their life. If any wax is dripping out the bottom, replace it.

Heyboer Transformers were the original Eico manufacturer of these units and can they build new ones which looks identical, can handle the higher AC voltages.

Even with all these mods it still is an Eico HF81. Just improved with today's modern parts that are available.

NEW-Testing Output transformers- Here are quick instructions on how to test if the output transformers have any shorts etc.

First remove power output tubes. Then take an alligator clip and attach to the 2 red primary leads coming from the output transformers and short the other end of the alligator or wire to ground to discharge the capacitors. Then attach your digital meter on the resistance scale, with one lead where the 2 red leads of the output transformers primary side connect to on the terminal board, then the other lead to pin 7 of one power tube and then move that lead to pin 7 of the other power tube socket for that transformer. Then repeat for other transformer. You should be seeing around 150 ohms on one pin and around 180 on the other. One lead should be blue the other brown each primary of the transformer.

For the secondary testing, attach one lead of meter to ground point of the black speaker wire. Then move other lead from the 4 ohm wires up to the 32 ohm wire. The 4 ohm should read around .7 ohm and go up about .2-.3 as you move up to the 32 ohm tap which should be around 1.5-1.7.

I have attached the pix out of the manual to see the primary spots to check. Hope this helps.

Here is a rusty chassis being sent in for a complete restoration

Here is the chassis all cleaned ready to rewire and install parts. You can never get these 100% as some have had rusts spots sitting for years and it's actually eaten away at the metal. But it looks a whole lot better than the original one.

Back of Chassis. Input jacks and speaker labels applied. Installed new gold RCA jacks which are now spread further apart so you can use some good quality interconnect cables. For the speaker binding posts, I usually connect the 4 and 8 ohm taps along with the common. As seen in the other pictures there is room to do 3 line inputs plus 1 phono on the back, plus line out.The 6x2 switch allows you to line up the first 3 on the front panel with the 3 input line jacks.

The speaker connections are replaced with binding posts, much better than the screw terminals. This way you can use banana jacks or spade terminals on your speaker wire.

Power Supply caps, new tube sockets being installed, All terminal strips completely stripped and cleaned . This one I left the orignal power transformer as the owner mentioned it didn't get hot or vibrate and he was using a variac to lower the AC voltage in. I highly recommend replacing the power transformer, not much sense putting all this time and money and leaving a 60+ year old transformer in the amp.

Top of amp showing new Belton sockets and shields , pots re-installed, new JJ filter cap,transformers cleaned and re-installed. Some of the JJ filter caps are no longer available and have been using F&T made in Germany. (see below-Hayseed Hamfest caps). The 2 centre sockets in this rebuild were ceramic 9 pin with shields. I use all Belton sockets now as long as supply is available.

Full rebuild does not include rebuilding volume/focus/bass and treble pots. A new Alps 250k pot can be installed along with a extender kit for the volume pot, which allows you to keep the original knob.(see below)

The rest of the pots are very hard to source parts for. I have sourced some original NOS parts for the treble/bass and focus control and could be repaired while the unit is here.

A fellow in Michigan also can rebuild the old pots if parts are available.

Bottom of Chassis, all new wiring and component installation. Resistors are carbon film, coupling caps are Russian PIO.

PIO availability changes all the time, check with me what I have available in stock. The quantity is starting to get scarce from sellers as audiophiles has realized how good these caps sound and they are a bargain for the costs.

Note to get the 165 ohm cathode resistors which are impossible to find that value, use a pair of 330 ohm 3 watt in parallel. DO NOT use a 175 or 180, it very important these amps have the proper 165 ohm cathode resistor. As per voltages in the manual, you should be measuring around 12.7 volts across the 165 ohm resistor, using ohms law this equates to approx 76 ma. total for each pair of EL84 tubes with no signal applied. It is nice to try and find a bias adapter and try and match up EL84 tubes the same in each channel. They should be in the 35-38ma for each tube.With a signal applied to the amp the voltage will jump up to approx 14.7v . Most amps I test are in the 14-14.5watt RMS full power before distortion.Make sure you have a load on each channel while performing these tests. Note the voltages in the manual were listed in around 1958-59 when AC voltages from your hydro was around 110v. I see 123-125 these days. So the B+ will be slightly higher than the manual shows.

Amp is completed here with new rca jacks, speaker binding posts etc.

2 pictures below is a recent update as one of my customers wanted to try an exotic coupling cap.

We installed 4 Mundorf Supreme Silver and Oil for the .1uf cap used in the preamp section of

the HF81. These caps are approx..10x the cost of the K40 PIO. These are rated at 1000v and just fit.

My initial comparision between this unit compared to another HF81 with K40 PIO is more detail and soundstage.

These caps require sometime to break in, they recommend 100-200 hours. I contacted the owner a few weeks later and he really liked

the sound of the Mundorf's..but like he mentioned they come at a cost. 2 pix below the Mundorf's are shown installed.

Above pix shows the upgraded Heyboer transformer and all the rca jacks, labelling and the speaker binding posts. Right now only the original HF81 transformers are available from Heyboer. The larger one has 1/4" more steel, but they are a custom build and take many months to get.

Below I have illustrated on how to add a better Volume pot like the ALPS 250K, which tracks perfectly channel to channel, as compared to the stock volume pots which are never close due to age. The ALPS installs right in the same hole in the chassis. Then you need to source a 20mm length coupling adapter with one side approx 1/4" for the alps pot, and and smaller hole for the original shaft diameter, then cut off the brass shaft off the old broken pot so you will be able to maintain using the same knob. You will have to trim some off the shaft of the volume pot so the front cover can go on the chassis.Then trim old brass shaft to fit length so the volume knob sticks out of the chassis as it did originally with the old pot. These couplers can be found in various sizes, the one side I ordered had to be drilled a big larger but works like a charm.

The present unit I am restoring the customer asked me to inquire from a new company I never heard of which is Hayseed Hamfest. They custom make a filter can cap to your size and specifications. This particular one is a 100uf/350v x2, 80uf/250v. The cost was about $37+20 shipping. The cost is about 3x that of a JJ 50/50 @500v and 68uf under the chassis. This unit looks great and it fit perfectly. A great upgrade if you are considering a replacement.Much cheaper than any of the CE products I have seen and none really have the proper sizes.Something to consider!!