W1CKI AM GEAR
A Speech Amplifier For Everyone
The problem many of our old rigs have with audio can be found in the speech amp and driver section.
The preamp section, usually a 12AX7, should be set up according to the "Radiotron Designer's Handbook" and the RCA tube manual's "Resistance Coupled Amplifiers" charts or the 1963 ARRL Handbook, page 260, "Resistance - Coupled Voltage Amplifier Data".
The driver is usually a single-ended power amp and this can be a major defect. The driver transformer must carry the audio signal and the DC plate current of the driver. When a sufficient amount of audio is needed for 100% modulation the transformer saturates and distortion develops. The use of a push-pull driver circuit and a good driver transformer will eliminate saturation and provide plenty of audio for the modulator grids. The ideal transformer is a multi-tap interstage transformer sold by Antique Electronics. It is made by Hammond, part number is P-T124E. The price is about $25.00, and still sold by AES.
The P-T124E is used as a 3 to 1 turns ratio step down driver transformer and with 58 Hy per plate it has very good frequency response. This transformer will drive a modulator in AB1 and/or in class B.
An 18K 1-watt resistor parallels the full secondary and also by a 1000 pF silver mica and a 1.8K resistor in series.
The mic input at pin 2 of the 12AX7 can be 2.7M for a plain D-104 or 270K for an amplified mic. At pin 2 use a 470 micro-Henry choke and a 30 to 60 pF bypass cap to stop RF from getting into the audio input. The leads connected to the tube socket must be kept very short so they do not become antennas.
A 100K resistor is used to block RF at the audio gain wiper arm, again very short wire length is a must. If the old type shielded hookup wire is still in use in the gain pot circuit it should be replaced with RG-174U. Ground the shield at the tube socket end only, grounding both ends may cause ground loop induced hum.
The first cathode is bypassed with a large electrolytic to increase gain and to reduce hum. To decouple the first plate a 10 MF electrolytic will be large enough if the filter cap in the power supply is 80 to 100 MF or more.
In the audio shaping network, resistor Rh and cap Ch are used to cut the highs 6 db per octave from 4 kHz to 8 kHz and 8 kHz to 16 kHz. The network R1 and C1 is used to cut the lows 6 db per octave 120 Hz to 60 Hz and 60 Hz to 30 Hz, a 6 db cut. The audio shaping network from Radiotron Designer's Handbook is used to assure a stable speech amp, an amplifier that will not oscillate or produce ultrasonics. Oscillations and ultrasonics produce considerable side channel interference and can destroy the modulator section of your transmitter.
The plate and cathode resistors of the phase inverter must be matched pairs to prevent an unbalanced signal to the driver tube.
The driver tube cathode resister will be determined by the value of the B+ voltage. The resistor will have to be a value that will keep the plate dissipation less than the maximum value. The driver is a class A amp so the grid bias (voltage across Rk) can be calculated using your B+ voltage.
Class A bias = .66 Ep/mu (12AU7 mu approx. 14 when the total plate dissipation is 4.5 watts.) The grid bias, determined by the self-bias cathode resistor Rk, would be 16.5 volts (.66 x 350/ 14=16.5V.) The cathode current would be 4.5=(350-16.5)( I, 1=13.5 mA. The resistance of Rk is (16.5V=.0135 x R) or 1222 ohms; use a 1.2K ohm 1 watt resistor.
Feedback voltage is furnished by at least 3 resistors in series. Use at least three to withstand the high voltage from the modulator. Total value of 300K to 900K and a .047 MF @ 1600V cap. It is desirable to have as much feedback as possible. The signal must be in the proper phase or the amp will oscillate. If oscillation does occur then reverse the plate connections of the modulator tubes or increase the value of the feedback resistors. The feedback resistor and the 330-ohm resistor in the cathode form a voltage divider. With three 220K in the feedback line and 330 ohm in the cathode circuit, then about 1/2000 the modulator voltage is introduced in the cathode circuit as negative feed back.
I would suggest that if you are not familiar with feedback circuits you should consult someone that is. Wrong connections could destroy the speech amp and the modulator.
There is no secret to good audio, it is a matter of good design, proper coupling between stages and decoupling between B+ feed points and operating the tubes within their parameters. This circuit is being used in many Rangers, Valiants, Vikings, and DX-100's and can be used in any AM plate modulated rig. EB
John Shaw, K1DEU and Paul Frelick, W1ECO, designers and contributors to this article.
Parts supply sources are:
Antique Electronic Supply - (602) 8205411 Digi-Key Corp. - (401) 596-3080 Mouser Electronics - (800) 346-6873
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