2SC1971 amplifier

[SOUNDTX]

Hey Guys,

Got a 1w PLL that I would like to amplify to give me a few more watts.

I came across the 2s1971 amplifier design on the internet. One design had the output connected on the wrong pad but found others that looked right. In the end, I settled on one by "Juggler" that I retrieved from the Wayback Machine, so I built his version.

The issue I have found with this 2SC1971 amplifier is that when it's connected to a dummy load, it gives 5-6watts fine and low SWR, but when it's plugged into the tuned antenna 1/4wave vertical on the VSWR meter, the SWR shows 2.1

What could be the issue? wondering if the output matching isn't right? I did try a trimmer instead of the 10pf, but it has the same issue.

Any thoughts or suggestions...

Thanks

[teckniqs]

Problem might just be with your antenna, but looking at that circuit I'd definitey change that 10pF cap to a 27 or 33 and also change the 470pF to the RF socket to a 1 or 10nF. I'd also change that 470pF on the input to a yellow 5-65 trimmer or 1nF.

....I've managed to squeeze 18w out of one of these long in the past, but highly not recommended.

[SOUNDTX]

Thanks Teckniqs,

I have tried both a Tuned dipole like the NRG one and a 1/4wave vertical. Both show a good result when testing on a nanoVNA.

The antenna is fed with 10m of RG-213, and with the 1W tx on its own, the SWR shows good, but when the amp is connected it shows alot higher?

I'll try what you have suggested.

[teckniqs]

What frequency are you testing it on? That circuit looks better for top of band say 100 to 108MHz, anything lower I'd probably also change the 2 turn coil to 3 turns and maybe also the 4 turns to a 5.
Problem is probably just the output matching. See how you get on.

[SOUNDTX]

Been testing the 1watt on 87.7 for now but will be moving top of the band.

I swapped the 470pf on the input to the trimmer and changed the 10pf to 27pf and used a 1n on the output and that has resolved the SWR issue between dummy load and antenna.

Getting 8watts out now.

Thanks for your advice

[EFR]

Is your 2SC1971 old andgenuine or new and fake?

[teckniqs]

Is your 2SC1971 old andgenuine or new and fake?

Sounds like it's most probably real, but even if it's not it doesn't really matter as it's working.

[Krakatoa]

1W into the 2SC1971 is too much input for this device, given the high gain it has.
The SWR going up maybe effect of the trash that the amplifier is giving because of this overdrive, thus, lots of harmonics and spurious signals on other frequencies hit the antenna and get back to the amp, increasing the SWR.
Since your dummy load is equally 50 ohm on all freqs. the swr is low, but when connected to the antenna it acts like a filter, reflecting the trash that's not in its resonant frequency.

[radionortheast]

yeah I think its half a watt drive, you don't want to over drive these things for sure, they do use these in those 8066 kits, tecknigs is right the biasing capacitor at the output should between 20-30pf, you would loose a lot of the signal with 10pf, I have 27pf in mine. You usually have 3 coils on an output filter too, I can only see 2 in the diagram, more of the harmonics may also come through. I guess there will be an output filter on the output from the 1w pll, it will be still better to have 3 coils in the output after the 2s.

[King Croccy]

Yes I agree, I think 1w too much power. If you cant lower the power of the pll, you could try making a simple resistive pi attenuator to knock the power down.

[SOUNDTX]

Is your 2SC1971 old andgenuine or new and fake?

The 2SC1971 was bought from Enigma

The "1W" is producing 900mW.
I have made a roughly 3dB attenuator with the resistors I have, and it's dropped the driver down to just under 500mW.

Added the attenuator into the system and still producing 8watts?

[SOUNDTX]

yeah I think its half a watt drive, you don't want to over drive these things for sure, they do use these in those 8066 kits, tecknigs is right the biasing capacitor at the output should between 20-30pf, you would loose a lot of the signal with 10pf, I have 27pf in mine. You usually have 3 coils on an output filter too, I can only see 2 in the diagram, more of the harmonics may also come through. I guess there will be an output filter on the output from the 1w pll, it will be still better to have 3 coils in the output after the 2s.

Can you provide a picture of your 2SC1971 amplifier, so can cross-reference it to the one i've built?

[King Croccy]

Is your 2SC1971 old andgenuine or new and fake?

The 2SC1971 was bought from Enigma

The "1W" is producing 900mW.
I have made a roughly 3dB attenuator with the resistors I have, and it's dropped the driver down to just under 500mW.

Added the attenuator into the system and still producing 8watts?

That sounds about right to me, the 2sc1971 doesn't need a lot for full power out. Probably 500mW is fine and wont damage the input of the device.

[jvok]

Is your 2SC1971 old andgenuine or new and fake?

Added the attenuator into the system and still producing 8watts?

You were definitely overdriving it then. You want the smallest amount of input power that will still give 8w out

[Shedbuilt]

yeah I think its half a watt drive, you don't want to over drive these things for sure, they do use these in those 8066 kits, tecknigs is right the biasing capacitor at the output should between 20-30pf, you would loose a lot of the signal with 10pf, I have 27pf in mine. You usually have 3 coils on an output filter too, I can only see 2 in the diagram, more of the harmonics may also come through. I guess there will be an output filter on the output from the 1w pll, it will be still better to have 3 coils in the output after the 2s.

There's no filter per-se in that schematic. The output network is just output impedance matching (although it will give some low pass filtration). Lack of output filter obviously increases harmonics. Overdriving increases spurious and harmonics. Both will potentially increase SWR via antenna.

[Albert H]

Exactly. It would be worth adding a couple of 3.5-turn coils in series and capacitors at 27p at each end and 56p in the middle to ground to get rid of any harmonic crud. The predominant harmonic with a single-ended stage is the second, so you need the filter to turn over well before 150MHz, so that by the time you get to twice the carrier frequency, the attenuation is sufficient. Assuming your amplifier is matching 50Ω well, the filter can be just a couple of cascaded pi-filters (since they're symmetrical).

Take a look at the old NRG lab pages for ways of constructing filters. I always prefer to screen the filter from the rest of the circuit to keep things clean.

[SOUNDTX]

Take a look at the old NRG lab pages for ways of constructing filters. I always prefer to screen the filter from the rest of the circuit to keep things clean.

I have made this filter before and copied it to the same specs.

Measuring it on my nanoVNA it's good up to 89MHz. So either I've made it wrong (though it looks like the same as NRG one) or the components need changing to the top end of the band.

[Albert H]

As a first fix, try stretching the coils a bit to reduce their inductance, and try it again.

[radionortheast]

Can you provide a picture of your 2SC1971 amplifier, so can cross-reference it to the one i've built?

Hi, sorry I was meaning in the filter i’d made, (below) I got rid of the amp, it had a black transistor before the 1971, without headsink so would of been alot less than a watt, as you do need a heatsink for that.

I’m guessing with your old nrg filter you’d likely need to change values of c to get a match on a higher frequency

funnyweefilter.jpg

This is an lpf, I borrowed from a pll kit, were you change the values of capacitance using ic sockets, you could maybe use two 65pf trimmers in place of the capacitors. it is made from 3 5 turn coils wrapped around a 4mm drill bit, onto a small strip board, its abit crude. I have 2, 30pf capactors soldered underneath, either side of the middle coil, there is a 27pf at the output end, you’ve got 3 sockets were you could puts values between, 20-30pf, or zero capantence.. it can depend on the frequency its used at, it works for afew mhz before the values have to change, at the moment 88-95, with different values it can work equally well at any frequency.

It can also be connected on the back of low power transmitter without messing having to mess about with the filtering inside a transmitter, useful if its 75ohms, the aerial is not quite tuned. You maybe want to use thicker wire for higher power, obviously want to have some screening.
this is the original circuit I borrowed from, below, ignore nc, I wished i’d though of it sooner as I may of been able to keep the amp. Its a neat thing to have, if that helps anyone I don't know.

lpffrom1wpll.png