"And now, for something completely different!"
This past January - at Quartzfest - there was a table covered with "junque" and taped to it was sign with the word "FREE" on it - and that's how I ended up with this device.
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| Figure 1: The front panel of the Ameco PCL-P preamp. The left-hand control tunes the front end of the preamp while the right-hand control selects the "band". The in/out switch is on the right. Click on the image for a larger version. |
The Ameco PCL-P
This device - which went on sale around 1965 - seems to have originally cost around $32.95 according to the RadioMuseum web page (link) - equivalent to around $300 today! Footnote 1.
But what's it for?
Back in 1965, many amateurs still used separate receivers and transmitters - and it was often the case that this gear would, itself, be at least a few years old - something that is likely true today, Similarly, shortwave listening was still in its heyday and it's likely that many of the receivers used by SWLs (ShortWave Listeners) were also likely to be "vintage".
In those days, tube (e.g. "valve") based gear was still the rule and this - particularly for older gear (from the mid-late 1950s) often meant several things were likely true about the receivers:
- Insensitivity on higher bands. On the higher bands - namely 15-6 meters - it was often a struggle to attain good sensitivity at these higher frequencies. Remedying this is surely the main purpose of this device.
- Image rejection may be marginal. Most receivers of this vintage were single conversion - that is, they converted from the receive frequency to a lower-frequency IF (Intermediate Frequency) - typically around 455. Some "fancier" receivers converted to something in the lower MHz range (often between 1.6 and 2 MHz) and then down-converted to something even lower.
A device like the PCL-P would be touted as an aid to mitigate both of the above: Its gain and low-noise amplification should help a "deaf" receiver and the fact that this device is somewhat selective may help the image problem as well - although that last point is debatable.
Whether or not a device like this was really helpful or not is irrelevant to our discussion - rather, this article is about the device itself.
Inside the device
Let's first take a look at the schematic diagram of PCL-P:
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| Figure 2: Schematic of the Ameco PCL-P preamplifier. Additional component annotates added to aid clarity of the description below. Click on the image for a larger version. |
Figure 2 shows the schematic diagram - some part numbers being added by me for clarity.
First, note S3: This allows the user to bypass the amplifier entirely - most useful when the unit is turned off. Immediately following S3a is S2a which is a rotary switch used to select the frequency range: As can be seen from Figure 1, above, this switch has four ranges: 1.8-4, 4-10, 10-23 and 23-54 Megacycles Footnote 2.
L1 is a coil - tapped at 50 ohms - that covers the lowest frequency range and is the large coil visible in Figure 3, below, but the higher bands' couplers - in the form of T1-T3 - are actually transformers clinging to the rotary switch, the turns ratios of the primary to secondary providing the impedance transformation: All of these, switched by S2b, connect to C1, an air variable tuning capacitor cross the grid of the first of two vacuum tubes (valves), V1.
There are two identical tubes here - 6DS4s in the case of my preamp - and these are Nuvistor tubes: About the size of a very large pencil eraser, these were some of the smallest vacuum tubes that were mass-produced - most of them being triodes, like V1 and V2, above. Being very small, they were well-suited for high frequency operation, finding their way into the UHF tuners of many contemporary televisions and since it was at about the same time as this unit was made that U.S. Federal law mandated the inclusion of UHF tuners on all new TVs, Nuvistors were widely available and comparatively inexpensive owing to the economy of mass production. (Wikipedia article about Nuvstors - link).
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| Figure 3: Top view of the Ameco PCL-P chassis, the variable capacitor visible near the upper-right, L1 the big coil in the center and the two Nuvistors visible just below/left of center. While this was originally equipped with RCA (phono) connectors, it has since been retrofitted with BNCs. Click on the image for a larger version. |
To some, the connection between V1 and V2 may look a bit odd, but the description on the front panel (seen in Figure 1) gives a clue: They are connected in cascode configuration - possibly an abbreviation for "cascaded triode/pentode" or similar. In this configuration the "bottom" tube (V1 in this case) gets its plate voltage via the cathode of the "upper" tube (V2) - but you might notice something else: The grid of V2 is at RF ground via C3 - being somewhat negatively biased by R2 which allowed current from V2's plate to get to V1's plate via V2's cathode.
This arrangement has a distinct advantage for higher frequencies: As the current through V2 is somewhat proportional to its grid-cathode voltage, when V1 conducts more - trying to pull the cathode of V2 lower. As V2's grid is "grounded" at RF via C3, pulling its cathode lower effectively increases the grid-to-cathode voltage: V2 also tries to counter this by conducting more, trying to pull the cathode back up. Because of this, the voltage change on V2's cathode (and, of course, V1's plate) changes relatively little compared to the change in current through it.
What this means it that the effect of Millier capacitance is minimized. Here we are concerned with Miller capacitance is that between the grid and plate of the tube - V1 in this case - and this capacitance couples the two together lightly, but this has the bad side effect of somewhat cancelling out the tube's amplification action: As the grid voltage tries to go up with the input signal, the plate voltage will go down - and the capacitance between the two will cancel out the signal on the grid to a degree: This is one of the reasons why it can be difficult to get a single-device vacuum tube RF amplifier to work well at high frequencies. Ff we prevent the plate voltage from changing as much and convey the signal more as current - as we are doing with the action of V2 - we can significantly reduce the Miller effect.
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| Figure 4: The underside of the PCL-P chassis prior to repair - the 2- section yellow capacitor and the diode on the left. Click on the image for a larger version. |
The rest of the circuit is a pretty straightforward power supply: The PCL-P used a silicon diode (D1) to half-wave rectify the plate supply - this being filtered first by C8, decoupled by resistor R4 and then filtered again by C9: The ultimate result is a nice, clean source of about 145-155 volts for tubes.
Construction quality
I'd say that the Ameco PCL-P is constructed "well enough": It looks as though a bit of thought and refinement occurred to assure stable operation at 6 meters - a frequency range that was above what the average amateur of the mid 1960's had equipment - while maintaining low cost and simplicity. A nice touch is the use of a feedthrough capacitor (C4) as a stand-off and bypass for the plate supply feeding the bottom of the output transformer, T4: This is surely the one place where the use of a comparatively expensive component was absolutely necessary as a lowly disc ceramic would probably not have sufficed.
From what I can tell, the PCL-P was originally fitted with "RCA" (phono) connectors on the input/output - a common practice on HF, VHF and even UHF amateur and commercial radios of this and later years - but they have been clearly been replaced with BNC types by a previous owner.
Refurbishing
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| Figure 5: This time, with a new diode and capacitors on the left. Output transformer T4 is visible near the right edge, supported by feedthrough capacitor C4. Click on the image for a larger version. |
Although I don't really have any intention to put this device into regular service, I did want to get it into operational condition.
Carefully powering it up on a current-limited mains supply, I noted that the power supply capacitors (C8/C9 - in the same yellow tube visible in Figure 4) was bad with about 10 volts ripple on the plate supply - but I was able to verify that the unit had good gain, indicating that both of the Nuvistor tubes were working properly despite receive signals being overlaid with 60 Hz "hum".
As the line cord was in very good shape, the only things I had to do were replace the yellow dual capacitor (C8/C9) with individual 22uF, 200 volt units - but I also replaced the diode (D1) with a more modern 1N4007 with a 1kV rating. Ultimately, the ripple on the power plate supply was well under a volt - as it should be!
Not surprisingly, I noted that the transformer in this amplifier "buzzed" quite a bit - but with a half-wave, capacitor-input rectifier conducting on every half-cycle, this isn't unexpected: The addition of a resistor (say, 100-470 ohms) in series with the diode (D1) would probably reduce this by limiting the peak current on the top of the AC waveform.
Performance
It's worth noting that any amateur receiver made since the 1980s - when it is working correctly - will very likely have more than adequate sensitivity on all bands so the PCL-P amplifier probaby has little place in the modern ham shack - but for a "deaf" radio from the 1950s and 1960s, of which there were many - particularly if they were in need of alignment - it would have likely been useful. The one place where this unit might be useful in the modern ham station - if only for nostalgic purposes - might be for a low-gain wire antenna (e.g. Beverage-On-Ground, Loop-On-Ground or Loop-Under-Ground) for the 160 and 80 meter bands.
Nevertheless, I decided to check the gain and selectivity of this device in the (non-WARC) amateur bands 160 through 6 meters: I have included these plots and comments below the conclusion of this article.
According to the official specifications of this amplifier, its gain is about 20dB - and my measurements - with 50 ohms in/out - corroborate this, more or less: At 10 and 6 meters it fell slightly short of this figure, but not dramatically so and this variance can be forgiven given the vagaries of manufacturing differences and age.
Unfortunately, I don't have a means of accurately measuring the noise figure, but testing with a "modern" radio (an FT-817) across HF and 6 meters indicates that this amplifier is NOT noisier than the FT-817 indicating that its noise figure is at least as good as it needs to be.
Above, I touched briefly on the idea of IF image rejection being slightly improved by a device like this that offers a bit of band-pass filtering: With a single-stage L/C filter, any improvements afforded by it are likely significant only at the lowest frequencies where the width of the peak is at its narrowest - but negligible on the higher bands as noted in the comments below the response plots.
Conclusion
As noted earlier, the PCL-P Nuvistor preamplifier is probably not a useful addition to a modern-day ham shack with radios: The problem that it solves - notably that of addressing the lack of sensitivity of some older radios on the higher bands - is simply a "non problem" these days. If you have some old "boat anchor" radios - particularly of the less-expensive variety - this sort of device may help pick up weak signals - particularly on a mostly "dead" band.
The noise floor of this preamplifier appears to rival that of a modern radio - but this doesn't mean that it would improve the sensitivity of a such a radio, but only that it would simply make the S-meter read higher without improving the signal-to-noise ratio: If your radio in question can already hear the noise floor on a given band when connected to your antenna, further amplification will not improve absolute sensitivity and will simply degrade receiver performance with too much signal!
As it is, this unit will sit on a shelf with some other "vintage" gear, always ready for some possible future use.
Footnotes:
- If you think about this for just a second, you can buy some really
nice accessories $300 these days such as an automatic antenna tuner, a
low-end laptop, or even one of several very nice QRP radios - some of
which are software-define radios. How times have changed!
- Until somewhere around 1970 or so, it was common - at least in the U.S. - to use "cycles" (e.g. Cycles per second) which is why older equipment may show "kc" (kilocycles) and "Mc" (Megacycles) rather than the modern "kHz" (kiloHertz) and "MHz" (MegaHertz), respectively. And no, you don't need a special "Mc to MHz" converter to use your old receivers!
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Frequency response plots of the Ameco PCL-P preamplifier/preselector
The following plots were taken using a DG8SAQ VNA with 20 dB of attenuation on the "Output" port and 6 dB of attenuation on the "input" port to prevent overload of both the preamplifier and the VNA as well as present a nice resistive 50 ohm source and load impedance. (Ignore the S11 and Smith plots as I forgot to turn them off).
For the response plots, there is a marker (#2) indicating the center frequency while other markers indicate the -10dB and -20dB responses (relative to the peak) - the numbers in the upper-left corner indicating the forward gains at those frequencies.
The final plot shows the insertion loss of the unit when the "in/out" switch is set to "out" (bypass).
Click on any of the plots below for larger version.
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| Tuned to 1.9 MHz on the 1.8-4.0 MHz position, the peak gain being about 23dB. The preselector does a decent job of rejecting a possible IF image (910 kHz above the center frequency for a 455 kHz IF). |
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| Tuned to 3.7 MHz in the 1.8-4.0 MHz position, the peak gain being a bit short of 28dB. On 80 meters and higher there is only minimal image rejection for 455 kHz IF radios. |
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| Tuned to 7.2 MHz in the 4-10 MHz position, the peak gain being just under 24dB. |
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| Tuned to 14.2 MHz in the 10-23 MHz position, the peak gain being just under 23dB. |
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| Tuned to 12.2 MHz in the 10-23 MHz position, the peak gain being just under 23dB. |
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| Tuned to 28.5 MHz in the 23-54 MHz position, the peak gain being just under 19dB. |
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| Tuned to 52 MHz in the 23-54 MHz position, the peak gain being just a bit more than 19dB. |
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| The "through" loss when switched to bypass ("out") mode. Loss is measured at 0.53dB at 53.5 MHz and 0.16dB at 28.1 MHz. |
This page stolen from ka7oei.blogspot.com
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