help on design of a transistor amplifier

[george79]

Hello,
Im trying to modify one old transistored beacon with modern front end and keep the old final amplifier. The problem is like this way........................
 I have an old beacon amplifier with 4 transitstors, 2 in pairs. Max. output by the papers are 100w and pep 400w. Im including you the schematic,
My project is to remove everything from the final driver board and front and keep from T202 and rear, the finals, the output transformer etc.
Now to make some experiments, i made one new T202 made from 32 turns primary and the secondary is 7+7 turns. 32 turns is the original one but with a center tap. I was rewind it without the center tap. Im using one class A amplifier to drive the finals. BUT i get a very distorted signal in oscilloscope. im sending you some screenshots to get an idea, by the way, it is a AM carrier and with CW modulation. It is a beacon for direction finding. and working real low 400-500Khz, and thats the reason for the wattmeter.

here is the original schematic



remove everything from T202 and to the left
next is the driver that i made

[george79]

sorry, some problems on the attachments,
here is the original schematic, the first is the drivers that i used

you can see the coil that i wound, and that i have a good AM modulated signal on the output of the coil, but when i connect on the final transistors i get this ungly modulated signal, why this???

[george79]

any help please why i geto so much distorted signal on the output please?

[807]

Where exactly are you probing the output signal? Do you have a 100W 50Ω dummy load on the output?

Did you probe the output before you replaced the previous stage? I'm wondering what the reason was for you replacing a push-pull amplifier with a single ended one.

The output looks like it's overmodulated. Are you sure the original preamp was 4W output? You may be overdriving the final PA transistors.

What does the output look like with no modulation?

[Paul Price]

You've posted the wrong schematic. Your description of the device doesn't seem to be even close to what the schematic shows.

The circuit  you first posted using a low-freq TIP darlington transistor would likely feed a buoy audible-freq. device loudspeaker/transducer  for a fog horn.

[807]

You've posted the wrong schematic. Your description of the device doesn't seem to be even close to what the schematic shows.

The circuit  you first posted using a low-freq TIP darlington transistor would likely feed a buoy audible-freq. device loudspeaker/transducer  for a fog horn.

Although the Tip120 isn't an RF device, it still has enough gain at 475kHz to be useful as an amplifier.

[george79]

Hello, thanks for the answers,
Im probing at the output of the amplifier, with a 50 ohm dummy load,
The output was probe the output before replacing the driver and it was working OK, and i got good modulated signal in output.
After the replacement of the driver, i get this distorted signal, keep in mind that i used a transformer to split the signal for the push pull modeof the finals, and the input of thew finals is very good and undistorted.
The reason is for damaged board of the preamplifier in a non repairable stage, so i have to keep the finals and use new stage in the front end

Now for the driver, i got the schematic for this website, and it worked good on the frequency that i want, https://wb4jwm.com/630M%20200%20WATT%20LINEAR.pdf

[mag_therm]

The 470 uF and 0.33 uF decoupling capacitors on +24VDC rail need to have short leads ( < 25mm if possible , both sides) and need to be located close to the TIP120.
Did they get accidentally moved or disconnected in the changes?

[807]

Are you sure that the final PA stage is still amplifying the input? The driver is showing 20v p-p, which is 1W (assuming a 50Ω load), while the output is showing approx 10v p-p (it's hard to see the graticle), which is only 250mW.

Have you tried it without modulation to see if you are still getting the required CW power output?

I'm wondering if the transformer, T202, is still matching the 2 stages properly? The driver you have built has a 50Ω output impedance. Did the original push-pull driver also have a 50Ω output impedance? If not, then T202 will need to be modified to match the new driver.

[george79]

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??

[RFDx]

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??

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]

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??

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.