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Old 3rd May 2012, 03:10 PM   #21
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And an oscilloscope? Or a DC analog 1mA instrument plus a germanium diode? Or any other way to measure RF signals?
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Last edited by Osvaldo de Banfield; 3rd May 2012 at 03:13 PM.
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Old 3rd May 2012, 03:28 PM   #22
loopy is offline loopy  Australia
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Yep I do have a germanium diode and an oscilloscope, that measures up to 100MHz
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Old 4th May 2012, 07:45 AM   #23
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Ok I've been reading around but still having trouble fully understanding it. I understand how to use an oscilloscope to measure RF, but don't fully understand the "no input" part. It needs input switch to TX, so I dont exactly know what you mean.


Just in case you missed it, I did post a link of the schematic: http://www.rmitaly.com/download/manu...al_rel_210.pdf

From my understanding, a capacitor/s need adjusting. The only variable one in this circuit is Cv4.

Last edited by loopy; 4th May 2012 at 07:47 AM.
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Old 4th May 2012, 02:19 PM   #24
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Sorry, I misunderstand the circuit. This unit is a linear amplifier with grounded grid, and has no need to neutralize. In all cases, it will be important to check that the amplifier isn“t self oscillating. Try to measure RF output of the circuit, using a dummy load, and an oscilloscope, with the gnd clip to chassis, and nearing the tip of the oscilloscope probe to the output tank WITHOUT DOING ANY ELECTRICAL CONTACT, only a small capacitive pickup. Try to "PTT" the rig shorting the collector to emitter of the corresponding transistor to activate "ipso facto" such relay. BE EXTREMELY CAREFUL WITH THE HIGH VOLTAGES INSIDE THE SET. Tell me if you can see any RF output.

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Old 5th May 2012, 01:25 PM   #25
loopy is offline loopy  Australia
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Thanks.
I'm not seeing any RF output. With no input in "TX" mode, the oscilloscope isn't picking up anything.

This is very frustrating lol. The circuit, especially the transmit side of it, looks very simple. There aren't really many components involved, and I was hoping someone had come across something like this before. But I'm not having much luck at the moment.

Could it be a plate/screen voltage issue? Is there some way of checking? It looks like they connect to negative via L6, L7, L8, L9 & L13.

Also the plate voltage reads around 840v on TX, which seems ok.
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Old 5th May 2012, 10:15 PM   #26
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If the output were 300 Watts carrier power with proper headroom to handle 100% modulation undistorted (1200 Watts on the positive audio peak when r.f. voltage and current double), the key down dissipation with no audio will be about 600 Watts. That's 150 Watts per (output) tube, probably about 5 times the rated value. Television line-amp (a.k.a. sweep) tubes are very rugged and are often pushed well beyond the ratings in this type of amp. There should be a fan to help cool them, you should talk in short intervals allowing time for cooling, the amp circuit should cut off the cathode current completely for more cold-down during receive (the circuit shown doesn't), and you should be willing to accept much shorter life than seen in a television set. Allowing plenty of time for others to talk and some quiet time to listen for other signals on the channel helps give an amp time to cool down. Since the actual number of hours of transmit time won't add up nearly as fast as hours would in an audio or television application, the tubes may still provide service for years even with much shortened life.

I'd suggest measuring the total current into the output stage. (Be careful!) Multiply by the plate voltage to determine input power. If you measure the output power with a good meter into a 50 Ohm dummy load (I'd suggest a Bird meter, most made for c.b. are not accurate), subtracting that from the input power will give you a pretty close number for the dissipation.

If the efficiency is higher than about 33% at dead carrier, the amp will put out more and run cooler, but the modulation will be distorted and the signal will splatter more into other channels. The carrier power causes considerable heating with no audio, making the situation similar in performance to a class-A audio amp where in terms of the rated power output versus (idle) plate dissipation. The stress on the linear would be far less running single sideband than a.m.

Modulating the radio with a 500 or 1000 Hz or so triangular wave and looking at the r.f. output on a scope will tell you right away that it's saturating on positive peaks. The combination of bias, drive level, and plate loading all affect the peak power and performance. Output tuning should be done while looking at waveform peaks with modulation, not simply maximizing carrier power. Realistically, for someone that wants long tube life and a clean signal, power power output for a.m. should be down around 120 -160 Watts, running the tubes at only twice their ratings. The most important thing is the distortion, but the tubes will run hotter undistorted than when over driven. Hams are pretty good about avoiding distortion/splatter. Unfortunately the (usually illegal) use of these amps on c.b. invarably finds them grossly overdriven and mistuned. Using resistors at the input and perhaps interstage would bring down the gain and linearize the load the radio sees somewhat.

If there is no distortion, an average reading (r.f. rectifying) output meter would show no fall in power with modulation. For c.b. with no scope or function generator that usually means when you whistle... Another method of checking linearity is to feed audio from the transmitters modulator output (the modulated d.c. feeding the last r.f. amplifier) into the horizontal of a scope, while looking at the r.f. output on the vertical. You'll get a vertical line (set to about half screen height) showing r.f. power with no audio. With audio it'll swing left and right changing height to for a trapezoid or filled in triangle. The side where the height goes down approaching a point represents the modulation from the negative going audio, the side where it goes up is positive. Those upper and lower edges should form lines (linear!!!). No doubt you'll see when the positive modulation is degraded. If the amp is saturated, there won't be any positive modulation, and carrier efficiency will be way too high, perhaps approaching 70%. (again, compare d.c. input power to r.f. output power). Functioning properly for a.m., the amp tubes will be cooler with modulation than without.

With a thermocouple type r.f. ammeter between the output and load, the antenna current should rise just over 20% at 100% modulation. (The sideband or audio-containing power adds 50% to the carrier power at 100% modulation, with the average r.f. voltage or current going up by the square root of that 1.5 total) The plate current should not fall with modulation. Typically mistuned and overdriven, the performance of these amplifiers is usually terrible and the readings will fall. The schematic shows a little light bulb that glows when there's r.f. output. In c.b. terms, it should get brighter when you whistle.

In television sets, some line amp tubes actually had an overload rating. I believe it was the 6LQ6 that was rated to handle 200 Watts dissipation for 30 seconds. (that was not supposed to be repeatedly, but allow time for a circuit breaker to heat and trip under fault conditions). Airflow for cooling, and low/zero power cool-down periods are what allow high peak power. If you try to run hot continuously, you'll see terrible tube life for the same reasons some do with their audio gear.

Last edited by riccoryder; 5th May 2012 at 10:39 PM.
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Old 6th May 2012, 09:28 AM   #27
loopy is offline loopy  Australia
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That was a bit to soak in, riccoryder, but thanks for taking the time to read the circuit and get back.

Some of what you said, I was aware of, and totally agree with. Especially in regards to the carrier being ¼ of the PEP output to achieve 100%. The meter I have is a powered PEP meter, calibrated by N.I.S.T. equipment when they're made, so in terms of measuring what's coming out, I have that well covered.

The part about measuring the total current into the output stage, what is the best way of doing that? I suppose it's all guesswork unless I'm able to get some hard numbers by performing these sorts of measurements. Which I'm prepared to do, I just need a bit of advice as how to do it properly and understand what means what.


I just did a test after writing that, with the original tubes and have some results, which may be disappointing, unless your feedback says otherwise. I say disappointing because I've always aimed to have the carrier at ¼ of the 100% peak modulation, but it seems unlikely to work out with this amp.

Tuned for that light bulb at its brightest with 100% mod (and aiming for close to ¼ carrier):
Carrier = 250w
PEP @ 100% mod = 1080w
Red plating begins within seconds.


Tuned for no regard to where the carrier sits, just max PEP with 100% mod:
Carrier = 80w
PEP @ 100% mod = 1100w
Red plating barely noticable in the dark, if any. And only just noticable on one tube. Plus any red plating that does exist seems to disappear with modulation, which corresponds to what you said about things running cooler with modulation than without.

Will the signal still be just as good at 1100w PEP and 80w carrier compared with 250w carrier? I've always preferred having a decent carrier to go with the mod rather than a small carrier with a lot of mod.

And is it possible that asking this amp to deliver the carrier at ¼ the PEP simply too much to ask for, with these tubes? If so, I'll just have to accept that even though I can get the PEP to where I want, the carrier will just have to sit low.

Last edited by loopy; 6th May 2012 at 09:32 AM.
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