Audio Modifications
for the
Eico 730 Modulator

by Steve - WB3HUZ

 Assuming you have a schematic, the mods are as follows, in order of importance. That is, if you only want to do one change, do #1. If you want to do two changes, do numbers 1 and 2. And so on....

1. Bypass the clipper filter stage. Break the circuit on the cold side of C3 and the output side of L1. Then connect the cold side of C3 to the grid of the pentode section of the 6AN8 (V3) or the top of R12.

2. Change C2 and C3 to 0.05 or 0.1 uF.

3. Change C8 and C9 to 0.1uF.

4. Change C16 to 330 pF.

5. Change C7 to 0.2 or 0.25 uF.

6. If you plan to use a D-104 mic with the 730, change R2 to 4.7 or 10 Meg Ohm.

7. There is no blocking cap on the mic input. I've never added one and never noticed a problem. But there is about -0.5 volts on the mic jack. If you want to remove this, add a 0.1 uF cap between the jack and the grid pin (2) of the 12AX7.

I have performed mods 1-5 on a 730 and it sounds great. The 730 circuit is nearly identical to many tube hi-fi amps of the early 1960s, except smaller coupling caps were used and a clipper filter stage was included. So, only simple mods are required to make it sound VERY good. The negative feedback loop makes the audio response very flat and clean. Very few ham rigs included feedback in the stock circuitry.


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12 June 2001