I came across my Eico 720 last year at the hamfest in Mendota IL.  After I acquired it, I recapped it and worked well on CW. But after while it ended up on the shelf for a while.  As I happened to have Heathkit W4-AM audio amplifier and small modulator transformer (LUX 7AM42 audio 40W modulation transformer), I have been thinking about using the Heathkit audio amp as a modulator.  But couple of weeks ago, I saw Eico 730 offered on E-bay and bought it from a husband and wife from PA (KB3GGC &KB3GGD).  It was unmodified unit with the case. I fabricated the patch wire cable, installed external TR switch (Dow Key Relay with 115V coil) and connected Eico 720 to 730.  It worked well... until. 

After testing on the air couple of times, Eico 720's power transformer started making a loud noise. I tightened the holding screws to ease vibration of laminated steel core, but still it kept vibrating and eventually the transformer was getting hot.  Few days later, during QSO with my AM buddies, the transformer made a fire work sound and the rig quit.  I checked the transformer winding continuity and they were all right.  But it produces the noise when high voltage was applied and excessive current blew the fuse.   I exchanged e mail with my friend Bob W9RAN who also happened to have Eico 720, seeking for some advice on the issue of failing power transformer replacement.  Bob told me Eico's transformer is made rather compact size considering for its handling high voltage.  To achieve 1400V high voltage, secondary winding of enamel wire had to be very thin in gauge. Paper insulation had to be thin to achieve its size but often it causes higher rate of failure.  The problem nowadays is to find its replacement of the original transformer. Bob suggested the use of commonly available low voltage B+ transformer of 330V - 380V range. I happened to have a spare parts transformer for Kenwood T-599D without center tap B+ winding. If I use voltage doubler circuit, it should work with 6146 well.  The only issue on this method is not to be able to switch off B+  with center tap that this transformer does not have.  So, according to Bob’s suggestion I added an additional relay to cut off and on B+.  Bob, observing my AM operation style always using Johnson Viking Thunderbolt linear amplifier with a small power AM transmitter as exciter,  suggested not to use the voltage doubler circuit on Eico 720, instead swap the tube from 6146 to 2E26 to get lower power to drive T-Bolt amp.   With reduced B+ voltage and the use of 2E26, Eico 720 will produce around 15W AM career, which is ideal power to drive Johnson Thunderbolt linear with resistive load input.  4-400A pair of T-Bolt amp does not need require haigh power to drive.  Higher power exciter will end up wasting most of the power with external attenuator... I was, at first, reluctant about downsizing the final power of Eico 720, but I tried it anyway. To my surprise, I was very pleased with the result. EL34 modulator Eico 730 provides more than enough modulator power into a single 2E26 and produce a distortion free very clean AM signal. So I will share the modifications I made on my Eico 720 and Eico 730 below.

Eico 720

(minor modifications)

Replaced RCA VFO input terminal to BNC female

Added RCA female for PTT operation (one hole on the back)

Added 2 DPDT relay and 24v AC CT transformer for PTT circuit

Changed 2 prong AC cord to 3 prong AC

Added inrush current limiter

(major modification)

Replaced power transformer (from the failing original to the parts unit Kenwood T-599D, the size is slightly taller, but the hole position is exactly the same)

Replaced 6146 with 2E26 (Exactly the same pin layout)

Solid state CZ34 full wave rectifier (made up solid state bridge rectifier on octal socket)

Added 22uf electrolytic cap to make capacitor input instead of choke input of the original filtering circuit. This rases B+ voltage to about 400V (PA is working as AB1, 2E26’s plate voltage max can be between 400V and 500V)

Eico 730

Solid state CZ34 full wave rectifier (made up solid state bridge rectifier on octal socket)

Changed the selenium rectifier to diode rectifier

Changed 2 prong AC cord to 3 prong AC

Added inrush current limiter

PS. If I planned for QRP mod from the beginning, independent 24V AC transformer was not necessary, as T-599D transformer already has a nice 14V AC secondary for relay operation. Also, the secondary has several screen bias voltage tap. So instead of voltage divider circuit for screen grid of 2E26 (originally 6146), I could use independent power supply for screen bias voltage. Well, I can use these additional secondary tap when I come up some other mod in the future…