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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th May 2020, 3:38 pm   #41
G0HZU_JMR
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Default Re: Building A VHF Amplifier For Valve Sets

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I'd think of something better than 750MHz Ft and a noise figure in the 2dB or better region. I wouldn't bust a gut trying to get much under 2dB.
Agreed. I have a bag of rather nice transistors here in the same plastic TO-92 as the BF199 but they are wasted on this application.

I couldn't resist trying one and it delivered just under 0.9dB noise figure and 22dB gain across the VHF FM band. This is beginning to stretch my test gear in terms of measurement uncertainty but it agrees with the gain and noise figure when tested in the Genesys simulator using the manufacturers model for the same transistor.
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Old 25th May 2020, 3:45 pm   #42
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Default Re: Building A VHF Amplifier For Valve Sets

With R2 removed 1v is measured at TP1. The BF199 must be bad. Although it tested good on the component tester.
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Old 25th May 2020, 3:50 pm   #43
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Default Re: Building A VHF Amplifier For Valve Sets

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With R2 removed 1v is measured at TP1. The BF199 must be bad. Although it tested good on the component tester.
Try testing the base to collector junction with a diode test function on a DMM. Is the junction OK? Is it backwards? I'm wondering if the base and collector legs are backwards on your transistor as this would deliver the same (strange) test voltages you saw earlier in the thread.
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Old 25th May 2020, 4:25 pm   #44
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Default Re: Building A VHF Amplifier For Valve Sets

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Originally Posted by OldTechFan96 View Post
With R2 removed 1v is measured at TP1. The BF199 must be bad. Although it tested good on the component tester.
Try testing the base to collector junction with a diode test function on a DMM. Is the junction OK? Is it backwards? I'm wondering if the base and collector legs are backwards on your transistor as this would deliver the same (strange) test voltages you saw earlier in the thread.
You are quite right! I doubled checked the pinout of the transistor with my DMM and component tester.

I thought the BF199 was C E B. The one in front of me is B E C. That's odd! The BF199 is marked 'PH11' if that is any help.

I will get things connected up the right way.
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Old 25th May 2020, 5:05 pm   #45
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Default Re: Building A VHF Amplifier For Valve Sets

I have put the transistor right and things are working!

B: 880mV

C: 3.8V

E: 112mV*

Ic: 6.1mA

I have been doing some testing with my 'scope and signal generator. With a 10mV input I get a 100mV output.

*I'm not too sure about the emitter voltage. Is it OK?

I've added a picture. The top waveform is the input and the bottom is the output. I'll hook it up to the Ekco soon.
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Old 25th May 2020, 5:20 pm   #46
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Default Re: Building A VHF Amplifier For Valve Sets

That all looks plausible to me.
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Old 25th May 2020, 7:08 pm   #47
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Default Re: Building A VHF Amplifier For Valve Sets

Looks good now! There is some risk that your transistor won't perform as well as a regular BF199 but it might be OK.

To give an idea as to the extremes of performance that can be achieved with different transistors I simulated a 2N3904 vs the ultra low noise NEC BJT transistor that I have in the circuit right now.

This showed that a 2N3904 can be expected to deliver about 7dB gain and about 6.5dB noise figure across 88-108MHz.

The NEC transistor (low cost in the 1980s, maybe 50p) can deliver about 22dB gain and a 0.9dB noise figure across the band. However, this amplifier has a very poor input match.


I simulated the NEC BJT circuit again with a simple L match at the input and also added the L match to the real circuit. The plots below show very good agreement with the manufacturer's model for the NEC transistor and my measurements on the real circuit. The match is now quite good with a VSWR better than 1.7:1 across the band. The noise figure is better than 1.3dB and this combination of good match and low noise figure is achieved using some emitter degeneration in the circuit.

The plots below show what this circuit is capable of with a low noise transistor and a basic input L match. The VNA analyser and the spectrum analyser and the Genesys software are about 15 years old and this gear was close to state of the art back when it was new. The spectrum analyser can control the noise source and generate the noise figure and gain plot and this shows very similar results to the simulation and the gain and return loss plots measured on the VNA.

I think that a 2SC2026 will offer performance quite close to this, the gain might dip to 20dB and the noise figure might be <1.8dB and this part will offer a good all round compromise I think. If your BF199 can't deliver good performance at VHF then I'd recommend the 2SC2026.

I'll also try testing a MPSH10 if I can find one.
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Old 25th May 2020, 7:15 pm   #48
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Arrow Re: Building A VHF Amplifier For Valve Sets

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I'm still here! I've been trying to get my head around all of these posts!
Well you're certainly getting a lot of advice - and most of it makes good sense, too.
The one thing that is possibly troubling you is that with this cct. you have entered the hallowed ground of VHF devices and associated circuitry. There are few - if any - 'guaranteed to work' circuits in this frequency region. Even time-served engineers and technicians can sometimes struggle in this area. A good deal of the performance at VHF and above is very dependent on what I call 'the invisible components' - stray L and C, in other words. Those 'components' can be more influential in determining a VHF amplifier's performance in all its respects than the specific discrete ones. And then on top of that there is the very relevant matter of measuring the performance of what has then been constructed.

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Old 25th May 2020, 9:08 pm   #49
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Default Re: Building A VHF Amplifier For Valve Sets

RF engineering for some reason seems to have acquired a reputation as the darkest of dark arts. Odd, that. It merely means that you have to learn a number of things that explain why non-RF people have a lot of trouble

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Old 25th May 2020, 9:14 pm   #50
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Default Re: Building A VHF Amplifier For Valve Sets

I had a good rummage and tidy up and found three bags of MMBTH10 transistors. These are all from Farnell and two of the bags have Farnell part number 2454030. I think these are obsolete now but I tested one in the LNA circuit. I think this part is the SMD version of the MPSH10.

http://www.farnell.com/datasheets/22...623.1571773262

If fitted straight into the circuit with no L match it managed a noise figure of 2.8dB and a gain of just over 16dB on the spectrum analyser.

With the L match fitted I could get about 17dB gain and a really good match with a VSWR less than 1.2:1 across the whole of the 88-108MHz band on the VNA and also a flat 2.5dB noise figure on the spectrum analyser. This is a good result. See below for the MMBTH10 (MPSH10) circuit with the L match added. I used 1nF ceramic caps for the dc blocking but the value isn't critical.

I think the 2SC2026 will probably manage a noise figure of 1.8dB with the L match and about 20dB gain based on the model. When used as a preamp in front of a FM receiver I'm not sure anyone would notice the improvement of the 2SC2026 over the MMBTH10 unless they used test gear to measure it on a test bench. I'd imagine that galactic band noise and man made noise would mask any difference once an antenna is fitted.
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Old 25th May 2020, 9:41 pm   #51
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Default Re: Building A VHF Amplifier For Valve Sets

I have tested the circuit with the Ekco and all I get is an odd electrical interference type noise which mostly wipes out any stations.

I do wonder if it is DC getting into the RF amp of the Ekco? That should be blocked by C2.

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Old 25th May 2020, 9:53 pm   #52
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Default Re: Building A VHF Amplifier For Valve Sets

What are you powering it with? Try it with a PP3 and see if the interference goes away.
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Old 25th May 2020, 9:57 pm   #53
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Default Re: Building A VHF Amplifier For Valve Sets

It might also be oscillating once an antenna is fitted and it is connected to a receiver. This might be happening up at UHF so it might not be obvious on your test gear.

The other reason could be that your BF199 isn't performing like a regular BF199 up at 100MHz. This wouldn't explain the funny noises though.
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Old 25th May 2020, 10:05 pm   #54
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Default Re: Building A VHF Amplifier For Valve Sets

A bench-top PSU. It's probably a SMPS. I'll try a battery in the morning.
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Old 26th May 2020, 10:56 am   #55
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Default Re: Building A VHF Amplifier For Valve Sets

I've just tried a 12V battery to power the circuit. Most stations are almost impossible to tune in to while Classic FM comes in better than it did before. I'm still having trouble with what sounds like interference. This goes away when the circuit is turned off. It might be the transistor oscillating?

When tuning around the FM band I can tell that some signals are coming in stronger so the circuit is working somewhat.
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Old 26th May 2020, 11:13 am   #56
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Default Re: Building A VHF Amplifier For Valve Sets

It does sound as if it's oscillation. At this stage it would make more sense to rebuild it on a ground plane as suggested earlier, or at least on a bit of Veroboard.
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Old 26th May 2020, 11:47 am   #57
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Default Re: Building A VHF Amplifier For Valve Sets

I agree that it is time for a rebuild which I would like to do in due course.

What is a ground plane?
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Old 26th May 2020, 1:01 pm   #58
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Default Re: Building A VHF Amplifier For Valve Sets

Sheet of conductive material, such as a piece of unetched PCB or any thin metal that will take solder. It forms the -ve circuit and you solder components directly to it. If necessary you can use 2M2 resistors as standoff pins.
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Old 26th May 2020, 1:19 pm   #59
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Default Re: Building A VHF Amplifier For Valve Sets

I'm currently on my lunch break but this evening I'll upload an image of my test board here. I deliberately built it using old school leaded parts rather than SMD and I did it over a sheet of unetched copper PCB.

I can show the outcome of a VNA stability analysis (of this amp built on the PCB) across LF through 4GHz and compare this with a simulation based analysis for a transistor similar to the MPSH10. I'll do this for the SMD version of the MPSH10.

I'm hoping that the outcome of the stability analysis for the MPSH10 will be that the output will just need a permanent shunt resistor at the output. This may have to be about 390R. This all assumes a tight layout over a copper groundplane. Otherwise, it might need some additional resistance somewhere else to prevent oscillation.

I've also managed to find some 2SC2026 devices plus some other stuff to test but this can wait for now.
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Old 26th May 2020, 1:28 pm   #60
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Default Re: Building A VHF Amplifier For Valve Sets

You can glue DIL ICs on, upside down with their legs in the air and solder components to the legs. I've glued TO-99 can opamps upside down as well, but their can is connected to the -ve rail so youneed to glue them down via an insulting washer.

It's amazing what you can build this way and the density you can achieve. A small pair of sidecutters, a good pair of tweezers and something to aid the eyeballs and you're away.

transistors have been getting faster and faster, even old type numbers are being 'improved' as they get moved onto newer process lines. I'm afraid large scale construction on PCBs with single sided copper has been left behind. Strays are too big for them to be stable. Veroboard runs into trouble quite frequently now as well.

Americans call it Ugly construction or Manhattan construction, but building trellises* over copperclad is an art form

* No relation to Mrs Trellis, but that's another radio programme.

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