Eico HF30

Just recently restored a pair of Eico HF30 power amps. These amps use the same power transformer as the HF81 but a big output transformer to run 4 6BQ5 tubes to produce 30watts. The power supply is using 2 EZ81 tube rectifiers and a can cap with 2-20uf in parallel plus 2 10uf in parallel for the input stage. The first stage of filtering is used for the B+ centre taps and also the pin 9 on the power tubes.

Now there were 2 versions of this amp. The older version used a 6AV6 and a 6C4 tubes for the input gain stage and phase splitter, then the next version used a 12DW7 tube instead of the 2 7 pin tubes.

Restoration of these is amps is pretty straight forward. For these units they were built pretty bad with wiring laying all over, so I did a complete bare chassis removal of everything.

The chassis were cleaned above and the under chassis was cleaned to remove the heavy oxidization not allowing good ground connections.

The owner wanted to retain the screw speaker terminals and input jack.

The transformers were cleaned , tested and reinstalled. New Belton 9 pin sockets were installed, the 7 pins sockets were tightened and cleaned and reused. The wiring layout was retained as per the original build. There were no construction manuals available that I could find .

The filter caps were made by Hayseed Hamfest to their original specs and and additional 22uf was added to the second stage of filtering.

The .1uf/400 coupling caps were replaced with K40Y russian paper in oil. The small caps were replaced with silver mica, all plate resistors were 2 watt Kiwame and the rest of the resistors were carbon film. The resistor for the cathode supply was replaced,it calls for an 80ohm 5 watt,but I happened to have 2 new 82 ohm 10watt in stock so used those,  along with a new 330uf/50v.

A new 2 prong power cord was replaced.Note one side of the heaters for the power and input tubes off the green heater wires off power transformer gets grounded. Eico did it using the plug that was pushed in on the octal tube socket used to power mono preamps.So make sure you ground one side.

Power testing showed these amps produced just over 30 watts RMS at 1khz.

The amps were tested on the bench and then placed in my main system and they are absolutely amazing sounding amps. The produce lots of power to satisfy even inefficient speakers. If you happen to find a pair of these grab them as they are right up there with some of Eico's best power amplifiers. I think the EL84 amps have a bit sweeter mid range than any of the 6L6 or EL34 based amps.

Above pix show top 2 restored and the bottom 2 were the original builds are these were kits . Below pix of fully tubed amps.

Below is a copy of the schematic I found for the early version. The newer version is readily available.