The AM Forum

AMfone's Online AM Handbook => Transmitters => Topic started by: DMOD on August 14, 2013, 02:31:42 AM



Title: Knight T-150 Transmitter Modifications
Post by: DMOD on August 14, 2013, 02:31:42 AM
Knight T-150 Circuit Modification Description

Note: Components with an asterisk denote new components or original components with modified values.

Voltages marked with an asterisk are in AM mode, Keydown, with 20 Watts RF carrier, loaded into 50 ohm dummy load, and at 7.290 MHz.


High Voltage Power Supply:

The HV supply was improved with additional filtering on the Modulator +525 V supply rail. An additional 23.5 uF of capacitance was added to stabilize the Modulator supply during phone operation. A 50k 7 W resistor was added to equalize the C46B voltage.

The Modulator Supply Filter uses the third section of the can type PS capacitor and an extra 47 uF capacitor with equalizing resistors to provide a steady modulator supply voltage.


Direct Screen Grid Modulator:

Most screen grid modulated transmitters can only modulate to about 80%. This is due to its cathode follower circuitry being referenced to HV and ground. What is needed is a bipolar power supply to enable it to modulate the final at 100%. This converts the modulation system from a CC system to a true screen grid modulated system.

This is accomplished by providing a bias supply connected to R30*, and connecting the cathode of the final stage directly to the screens of the 6146s.

An additional, adjustable bias supply is provided for the last stage’s grid circuit to set the output power.

The first stage has an added cathode resistor to bias that stage and to reduce distortion. Coupling capacitors values were chosen to improve the low frequency response of the modulator.

Negative clipping is done with a steering diode and a zener to limit negative modulation to 98%.

Note: Due to the biasing and the increased current in the triode stages, a 6EW7 is used.


Final Amplifier Stage:

The 6146s have the screen grids driven in parallel by the cathode follower of the Modulator stage. In the T-150, the screens have a total of 0.002 uF of capacitance on them to bypass any RF and to reduce high frequency audio artifacts. A general complaint of the audio response is that of “muddiness” or lack of high frequency response. This is caused in part by the values of the C27 and C28 screen grid bypass capacitors.

Since we already had 47 pF of capacitance in the Modulator circuit for RF bypass, C27 and C28 were changed to 330 pF each at the screen pins. We thus have a total bypass capacitance of 707 pF in the screen bypass circuit.

No other changes were made to the Final RF circuit.


VFO, Osc., and Buff/Multiplier:

L4 was changed to 470 uH. C23 was changed to 100 pF 1kV. C17 was changed to 68 pF 1kV, and C12 was changed to 33 pF 1kV in order to better the spectral purity of the unit. Grid drive to the final was increased as was a more noticeable peaking of the oscillator RF voltage.



Speech Amplifier:

The 12AX7A speech amplifier circuit values were modified to flatten the overall frequency response. Feedback from the modulator in the form of a series RC circuit is important to reduce distortion and to not disturb the bias of the second stage. The speech amplifier B+ is now sourced from the relay-switched modulator supply and further filtered. There is no reason to activate the Speech Amplifier and Modulator when not needed. This modification also reduces tube stress when in KeyUp.


Bias Supply:

Unlike the Heathkit SG transmitters, the Hallicrafters HT-40 and this Knight T-150 does not have separate bias supplies. The negative 100 volt bias supply uses a Triad F-13X transformer in reverse. The transformer is mounted behind the meter and near the T1 power transformer. The filtering components are mounted on a single 6-pin terminal strip. Only 100 volts is available in this T-150 because the T1 filament winding only supplies about 6.0 volts.


Phone Operation:

A high impedance dynamic or crystal microphone with a source impedance of > 50k feeds the ¼” audio jack. A Shure 450 Series II set to High Impedance is a good alternative to the D-104. The Function switch is set to AM and the transmitter is keyed via the Key Jack.

Power is adjusted by the front panel potentiometer located above the meter. I normally set it to about 15 to 20 Watts output to drive my Henry 2K-4.

The T-150 as modified is capable of 45 Watts AM and 60 Watts CW on 40 meters.


Cooling Notes: . I removed the shield from the modulator tube, and have attached a separate 110V low-profile muffin fan to the grilled back cabinet. The fan is positioned directly behind the 6146s.


Phil - AC0OB

Updated 4/5/2018 to include tube templates and minor improvements to the Speech Amplifier and Modulator.


Title: Re: Knight T-150 Transmitter Modifications
Post by: Steve - K4HX on August 17, 2013, 01:12:24 PM
Very nice write up. Thanks!
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands