Electronics > RF, Microwave, Ham Radio

help on design of a transistor amplifier

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RFDx:

--- Quote from: george79 on May 18, 2024, 10:42:39 am ---im mthinking that the T202 is the problem. original it was primary 36+36 with a center tap for the output of the original preamplifier and the secondary was 7+7 with center tap for the input of the finals.

The green good AM signal is on the ouput of the transformer without load of the finals. But as a first experiment, i made a 36 turns only primary. So im thinking that i have to remove turns from the primary??? because on the secondary nothing changed. Im in the good side on this think? remove or add?????

--- End quote ---

Guess as good as you can the input impedance of the final stage. The schematic for the beacon amplifier doesn't show any component values, so it's impossible to tell how many turns for the primary/secondary of T202 are needed to get a good match for the new driver.

george79:
here is a more cleared schematic.


Original T202 had 36+36 on primary side and 7+7 on the secondary, but i dont know the mix of the toroid. I kept the 7+7 on the secondary, and i have to use one primary, because my new driver is Class A and not P-P, keep in mind that there is 4 finals coupled n two pairs, so the total input impedance will be the half i guess.
Please any starting point for winding a new transformer would be great. something to get close and then make experiments. I can guess that the 7+7 will be needed to have enough coupling to work, so i need a big primary to get the work done. is this correct??

george79:
ferrite is iron core green in color, but keep in mind the age, dont know if 40 years back the colors was standard.

RFDx:

--- Quote from: george79 on May 20, 2024, 09:44:03 pm ---Please any starting point for winding a new transformer would be great. something to get close and then make experiments. I can guess that the 7+7 will be needed to have enough coupling to work, so i need a big primary to get the work done. is this correct??

--- End quote ---

The driver (Uce=20V, Ic=0,4A) expects a 50 Ohm load. If we assume a rather modest current gain B = 50 for the four final transistors and account for the 1 Ohm emitter degeneration, the resistive component of the transistor input impedance is at least 50 Ohm. Transistor rBE can be ignored as it is very small in comparison. The 27 Ohm additional base resistance raises the input to at least 77 Ohm. With two push-pull stages in parallel, the differential input resistance is also at least 77 Ohm. With T202 as a 1:1 transformer (e.g. 20 turns primary / 10+10 turns secondary), the resistor R205 (original value 78 Ohm) that is in in parallel to the secondary would need to be about 150 Ohm to load the driver stage with ~50 Ohm. In reality the current gain B of the four finals is probably higher than 50 so R205 would have to be made smaller (e.g. ~82 Ohms for B = 100). Keep in mind that R205 dissipates quite some power and choose the number of turns for T202 to get XL > 250 at the lowest frequency.

Are you sure the power amplifier is designed for 400W PEP output? I found the manual for the SS-800AVS and it mentions a total DC current of only 6A for the power amplifier stage at max. output power. With a 50V DC supply the input power is merely 300W which would be enough for 200W but definitely not 400W PEP. The manual also talks about the beacon generating dual carriers and I don't see how that is possible with generic AM modulation.

MathWizard:
According to my sim of the 630m wattmeter, with a couple of 10uH inductors, the output would have a gain of around 20dB and a nicer roll-off than just the CE stage, but yeah it's power hungry (at up to IE=645mA if the TIP has a gain of 4k), I don't usually ever use more than 12V either.

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