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HT-40 Modification and Upgrades

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Steve - K4HX

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« on: April 22, 2016, 08:08:45 AM »

From Phil, AC0OB

Controlled-Carrier transmitters have always held a fascination for me so I try to improve the audio quality and modulation envelope.

I obtained this very clean HT-40 from a gentleman here on AMFONE and brought it up on a variac and to my surprise, with a 3.875 MHz crystal it output 15 Watts on AM and almost 40 Watts CW. On AM the modulation percentage was a max of 80% and the audio was decent.

The PDF below contains the mods for the Power Supply, the Speech Amplifier, the Modulator, and the additional -100VDC power supply.

All voltages with a "*" denotes the DC voltages taken on AM without modulation and with an input voltage of 120 VAC. Any component with an Asterisk* denotes either an additional component or an existing component with a modified value.

AM output unmodulated is a maximum of 30 Watts and CW is almost 55 Watts at 120 VAC. Modulation is now 100% positive and 98% negative. Not bad for a 6DQ5 sweep tube output circuit with screen grid modulation.

Power Supply:

The input is EMI filtered with L14 and L15 and C40 thru C43. These coils are also used as fuses so a new 3 Amp fuse on the input line was installed to protect those coils and subsequent circuitry. Since the chassis is grounded to the shack's system ground, I did not see it necessary to install a three-wire power cord at first, although I eventually replaced it.

The voltage multiplier Power Supply was upgraded with the components shown. With 120 VAC and modern components, the PS voltages were a bit high in some areas so the resistance values were changed to bring it down, especially for the 6CX8 crystal Osc. and the buffer amplifier. The Osc. was further filtered to keep a constant B+ on it for maximum stability.

Most transmitters switch the HV B+ rail. However, Hallicrafters for some reason decided to do HV switching via the negative supply rail. For AM or CW activation, the negative HV rail is connected to ground via the function switch. A snubber circuit composed of R24-C39 is there to minimize Function Switching arcing.

Otherwise, the negative HV rail is floating above ground so beware.  

Speech Amplifier:

The 12AX7A speech amplifier circuit was modified to change the audio response and to place the tube in the more linear portions of their curves. Negative feedback, in the form of an RC circuit, is sent from the Modulator cathode V4B pin 9 to the second stage of the speech amplifier V3B cathode 12AX7A pin 3. The feedback has to be capacitive coupled since we now have a bi-polar power supply.


In order to obtain 100% modulation, it is necessary to reference the cathode follower circuit to a negative voltage, otherwise the modulation will never go above 80%. This bi-polar supply arrangement converts the modulation system from a CC system to a true screen grid modulated system.

I used a modified K4TAX circuit. Since most CC mods are for the Heathkit DX-60 or Knight Kit circuits with 6146 finals, this unit required the circuit shown for the screen grid of the 6DQ5 tube. The 100k potentiometer in the grid bias voltage divider circuit sets the power level by biasing the grid of the last 6DE7/6DR7 stage and keeps the power dissipation within limits of the modulator tube.

Without a negative peak limiter, the modulation will go beyond 100% negative. With the circuit shown, the jumbo Red led will flash when negative modulation reaches about 95%. The Jumbo LED replaces the old neon modulation indicator on the right side of the front panel. Max negative modulation on the scope appears to be 98%.

The 12K R20, the 22k R11, the R21 100k, and the 0.47 uF C36 were removed. The new R20 of 22K is now connected to the negative supply voltage and the cathode of V4B.

Negative Voltage Supply:

The negative voltage supply is a small filament transformer in reverse. It was placed near the L12 choke at the rear of the chassis as that was the only real estate available. The primary winding of the transformer is placed across the filament string and connection was made at the terminal strip below the T1 power transformer.


There are two variations of the HT-40 circuit floating around, an earlier version and a later version. The later version can be identified by the 50 pF coupling caps in the OSC/Buffer circuits, a 10k value for the final grid metering resistor R10, and a switched modulator plate-to-grid circuit.

This HT-40 came with provisions for both circuits, so I modified the Function Switch-switching to conform to the earlier circuit. I did, however, keep the later RF circuit component values in the RF string.

The neutralization circuit is a simple (2-terminal strip) post located near the final tube with a wire going from that post to C15. This "gimmic" is not shown on either versions of schematics.


For AM operation, I set the Function Switch to AM, and ground pin 3 of the Auxiliary Control terminal strip with my PTT sequencer for transmitting.

The final does not have any plate current monitoring but the 6DQ5 does have a cathode resistor. The voltage across the cathode resistor shows 6.4 volts in AM which, when subtracting the screen current contribution, yields about 39 Watts DC input to the final. At 25 Watts out, this is an efficiency of about 65%.

Updated: 9/12/2014

Phil - AC0OB

AC0OB - A Place where Thermionic Emitters Rule!
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« Reply #1 on: December 31, 2016, 09:25:47 PM »

The complete Text, Schematics, and Parts List are now located at:


Phil - AC0OB

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
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