The AM Forum

Knight T-150 Circuit Modification Description


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Very nice write up. Thanks!

I have done some further upgrades to my Knight T-150, mostly to the speech amplifier, modulation circuit and VFO. Modulation is really good now with modified feedback (purely DC now) from the screen modulator to the second stage speech amp. I can now get up to 50 Watts AM carrier if needed but usually run about 20 watts into the Henry 2K-4.
(NOTE 1: Decreasing the 4.7 Megohm resistor in the modulator circuit will get you slightly more positive cathode bias if your voltages don’t  get you enough output power. Do not go lower than 3.3  Megohm. 4.7 Megohms is usually sufficient).
(NOTE 2: Do NOT use a 6DR7 as the modulator since this tube’s first stage does not have the correct gain and current characteristics for this stage. Use only a 6EW7).  

In order to achieve at least 100% modulation without the Controlled Carrier time constant circuit, proper quiescent biasing of the screen grid voltage and sufficient audio is necessary to “swing” the screen grid sufficiently. The microphone gain potentiometer can now be set to about #4 or about 105 degrees of clockwise rotation for 100% modulation.

The VFO was further modified to improve spectral content and to prevent FMing. One change to prevent FMing was a change to an OA2 regulator tube and a change to the OD3 regulator resistor of 4.05k to reduce voltage variability during VFO operation. After tune-up, one still has to tweak the VFO frequency because of reflected loading on the VFO. After tweaking, the VFO remains stable and on frequency. Another improvement was the replacement of the 12BY7A grid resistor to a 15k 1W value since it had drifted up to 19k

Allied should have added a Cathode Follower (CF) stage between the VFO and the Buffer/Oscillator, IMHO. Maybe a further/future improvement? I am seriously thinking about placing a 6AH6 CF stage between the VFO and the 6CL6 Buffer/Oscillator. The proposed CF stage is on page 8.

Most tubes, except for the 6146 finals, are completely cutoff at around -10V. Instead of changing the value of R21, I elected to place the screen node at a low impedance of 1k to eliminate this since I had an additional relay contact with which to work.

The Relay circuit now does the switching for:

1)   The Keybias rail,
2)   The switching of the 545V to the Modulator plates and the Speech Amplifier in KeyDown. This switching only connects power to those circuits when needed and as such reduces standby heating of those tubes. This also keeps -55V from the modulator off the screens during KeyUp.

12/21/2020: Added Tune-Up procedure and further uggraded the HV PS with modern filter caps.

Phil - AC0OB

Thanks! Very detailed and thought out improvements!

I seem to recall that the T-150 has the ability to be plate modulated with an external modulator through a connector on the back, have you ever experimented with that? I think i remember hearing the power supply becomes the weak point when run that way, not being able to supply the load that the final draws when being plate modulated, you get a lot of flattopping with modulation.

Good question!

I have not considered that because it would require an external Modulator Chassis.

However, it could be done with another properly wired Accessory plug. See PDF file, upper right corner, where the Mod. transformer secondary is placed between Pins 7 and 8 of the Accessory plug.

Not sure about the original power supply transformer capability but if I did this I would replace the power transformer with an EDCOR XPWR220.

Also, an added 27k 10W resistor would have to be placed into the circuit to get modulated screen voltage. An added AM switch SPDT would allow AM Plate Modulation, AM Screen Modulation, and CW.

Who could ask for more? ;D


Revised 6/12/2021 to update specs on Reversed Audio Output Transformer


Phil

Regarding the beautiful schematic you produced for the external modulation source, what would you consider the mod transformer secondary to be?
Thanks Bob W4RFM

See schematic.

It appears the secondary of the Plate Modulation transformer will need an impedance of 1400 ohms@300mA. All of this is based on the modified Power supply.  

Updated for more stable bias on the SG Modulator Grid and the addition of optional Plate Modulation. Updated schematic for 7189 Buffer-Multiplier stage.

Phil