Audio Mods for the Heathkit DX-60

by Bob - K4TAX


Earlier posts on the AM Window and discussions by me and others as applied to DX-60 carrier control modulation scheme led me to re-think the system. After a few cut and try efforts, a bit of pencil and paper work, I proceeded to change several components in my trusty Heathkit DX-60. This process closely follows the work previously done by KS3K and WC3K, but refines several values to enhance performance.

Explanation of Mods

Referencing the cathode (pin 9) and grid (pin2) of the 6DE7 modulator to the -140 volt internal bias supply, through a voltage divider and zener, provided the ability to modulate 100% positive and 100% negative.  The zener diode arrangement was found to be necessary and a nice improvement to earlier efforts.  It was found that during modulation, the bias voltage did vary, due to increased current demands within the power transformer, and thus distortion was higher.  The selection of resistor/voltage values established the no signal resting carrier about 20 to 30 watts output.  This target was chosen as most linear amps today exhibit some 10 to 12 dB of gain and thus 25 watts input should net some 250 watts output.  This is correct for a 1500 watt PEP amplifier under AM conditions.  

Changing the 6146A screen bypass capacitor to a smaller value (0.005uF to a 0.001uF) and eliminating other bypass capacitors in the audio section enhanced frequency response.  I found that by increasing coupling capacitors to larger values over the original design values  improved low frequency response.  

By adding an RC feedback network from the cathode (pin 9) of the modulator, 6DE7, to the input stage (pin 3) of the 12AX7 reduced distortion.  Raising the voltage amp cathode (pin 8) of the 12AX7 above ground, and on the 6DE7 (pin 8) adding a bypass capacitor further reduced distortion.  

The loading resistor (15k ohms) on the 6DE7 (pin 9) connected to the -140V bias was the critical element in getting the high frequency modulated distortion down.  This value can be reduced further to about 12k but I would not suggest going lower to 10k, as this will exceed the power dissipation of the 6DE7.   Elimination of the RC network (R32 & C35) between the 6DE7 cathode and the mode switch enhanced the low frequency response and improved modulation linearity.

The Results

Frequency response:  From mike input to about 100% modulation is +/- 2 dB from 35 Hz to 16 KHz*.  I observed that the 100% positive modulation power level is the same peak level as the CW tune power level or about 65 watts output.  Also, the positive and negative modulation peaks are equal and occur at the same percentage typical of conventional AM modulation.  I did not attempt to achieve more than 100% positive modulation.

Distortion:  At <100% modulation (95% typical): < 1% THD+n at 400 Hz., < 3% THD+n at 40 Hz and 10 KHz.

Carrier noise: Bandwidth limited 50 Hz - 7.5 KHz is some -55 dB below 100% modulation.   This was measured at the cathode of the 6DE7.  Demodulated RF waveform confirmed these values.

* This value seemed to vary noticeably with various 6146A tubes.  Some tubes were good to 14 kHz while others seemed to start a roll-off in the 7 to 10 kHz range.

Upper end frequency response appears to be limited by the screen bypass C (pin 3, 6146A) and by the coupling values of C and grid R's on the lower end.  There is a good bit of LF flicker noise generated at the input 12AX7 stage.  Efforts to push the low end down further in frequency by further increasing coupling R’s greatly adds to flicker noise.  Perhaps a 7025 in place of the 12AX7 might solve this.

The Mods/ Details

The following table shows the original values and the new values where applicable.


Original Value

New Value


R22 1 Meg 1 Meg
R23 4700 0.05 uF
R24 2.2 Meg 2.2 Meg
R25 470 k 470 k
R26 100 k 100 k Might be 500 k in some TX's which is OK.
R27 470 k 470 k
R28 3300 3300
R29 2.2 Meg Removed
R30 1 Meg 220 k 2 W
R31 33 k 12 K 10 W Caution: The value has a major effect on positive modulation. Lower values increases dissipation of the 6DE7 triode.
R32 10 k Removed
R33 33 k 33 k
C29 100 pf 100 pf
C30 0.001 uF Removed
C31 0.005 uF 0.05 mF
C32 20uF@350V 20uF@350V
C33 0.005 uF 0.05 uF
C34 100 pf Removed
C35 0.1 uF Removed
C42 0.005 uF 0.001 uF Screen bypass
C43 0.001 uF Removed

Table 1 - Component Info


See the schematic for the changes and values of new added components.  Modifications to the transmitter are strictly wiring and component changes with a couple of terminal strips added using existing screws at the tube sockets. No holes and no added things to deface or detract from the value of the DX-60 were required.   The zener value can be made up of anything available to arrive at a reference of 120 V.  In my case I used a 100 V and a 20V in series.  A pair of 60 V _ watt zeners will work as well.

The PA operating parameters of the 6146A in CW tune are 650 VDC at 175 mA and 2.5 mA of grid current. In AM they are 700 VDC at 70 mA (no modulation) and 2.8 mA of grid current. The Plate meter just twitches upward on modulation.  All in all this is pure screen modulation with PA efficiency at 68% thus about what is expected with a class C amp.

Voltage values DCV:  AM mode, no signal


Pin 1


Pin 2


Pin 3


Pin 6


Pin 7


Pin 8



Pin 1


Pin 2, 3


Pin 6


Pin 7


Pin 8


Pin 9


Table 2 - Voltage Info





1 October 2006