Eico HF87 Restoration

 The Eico HF87 is a 35 watt per channel tube power amplifier. Using 2 diodes for the  rectifier and a voltage doubling circuit to develop the B+ voltages.It has cathode bias meaning no adjustment in the bias of the power tubes. A single 10 watt resistor controls the bias per pair of output tubes. Thus a matched pair of tubes is required for each channel to minimize distortion. The original schematic uses a 235 ohm 10watt for one pair and then a 165 ohm 5watt and the ground of that resistor then feeds the heaters on the 12ax7 tube. I have found no real need for this. Some have noted the power transformers 6.3 v heater winding is maxed out, thus adding in the extra 12ax7 will make it run hotter. In my experience just hook up the 12ax7 to the 6.3v heater wires with pins 4 and 5 together on one side and pin 9 on the other. The rating of the 6.3v taps are rated at 7.6A. 4 EL34 heaters =1.5Ax4=6 amps   2  6SN7 =.6A x 2=1.2  12ax7=.3 amps . Total 6+1.2+.3=7.5A...so the 6.3v taps will easily handle the additional 12ax7 tube. 

The restoration involves replacing the filter can caps first. You require 2 seperate 300uf @300v can caps. You can go higher if you wish on the capacitance size. The latest one I just restored , I used 2 JJ 350uf/385v. The next section required is 80uf at 500v, here I used 1 JJ dual 50uf/500v and joined the 2 sections together to make 100uf at 500v, last section calls for 40uf and 20 uf at 500v, here I used another 50uf/50uf/500v JJ. Check all the resistors going between all the sections, check for cracks in the old ceramic ones, I used Mills and Welwyn 5W and 10W resistors.Replace the diodes also, I used 3A 1N5408. The rest of the restore was done with new tube sockets, .1uf /600 PIO K40 caps, Kiwame 2 W resistors and some Amtrans 2W carbon film.120 ohm 2 w resistors were put in on the screens. To do this, install some small terminal boards under each screw of the socket so this will give you a spot to solder from the screen tap, and then place the resistor to pin 4. 

What is very important is to have good clean tight grounds.This may require to wire brush the bottom of the chassis, as lot of these eico amps oxidized over the years.

These amps use  EL34 based power tubes fed by 6SN7 phase splitter and a single 12AX7 predriver for both channels. If you look at the orignal specs for the cathode resistor it is listed at 235 ohms. The manual lists you having about 34 volts across the 235 resistor. Using ohms law this would equate to both pairs of tubes drawing about 144ma or 77ma each!!! This is way too high.Back when these amps were designed the AC line voltages were close to 110v, where we are in the 120+ area today. Most EL34 cannot handle that much current flowing through the tube.The spec for max plate dissipation is about 24 watts. That is calculated by the .077 amps x plate voltage, which on the last amp I did was about 450v.That works out to 34 watts. Way too high.This will cause red plating. I found using a pair of 300ohm 10watt, which actually measured 291 ohms. I ended up having 34volts across the 300ohm which works out to 58ma per tube. That is a perfect spot for the EL34. Make sure you have matched tubes. You can buy or make a simple bias plug socket tester, which goes between the socket on the amp and the tube.You cut the pin 8 on both the bias plug and the top socket and your milliameter goes between the pin 8's and thus you can measure the exact bias of each tube. Try for a match of a max of 2-4ma between tubes to keep distortion down to a min. All this biasing info is available on many sites.

The top pix was one which I purchased many years ago and was stock. Below are some pix of the recently restored unit . The pix below shows the output transformers reinstalled , new gold ceramic tube sockets and the 4 can caps for the power supply. Below that shows a pix with the power transformer re-installed and also the speaker terminals changed to binding posts.