Exploring the Ameco PCL-P Nuvistor cascode preamp/preselector

"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.  That's how I ended up with this box.

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

The PCL-P - 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}. The specifications say that it has about 20dB of gain and can be tuned for any frequency from 160 through 6 meters.

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 - likely WW2 surplus and/or gear from the 1950s.  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 and earlier) - 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.  This is particularly true on "simple" (e.g. inexpensive) where sensitivity would be fine on lower bands, but drop off precipitously with increasing frequency where signals were generally lower, anyway  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 kHz.  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 - often in the 40-100 kHz range - where the final band-pass filtering was done. 
  

A device like the PCL-P might 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 PCL-P

Let's first take a look at the schematic diagram of PCL-P:

Figure 2:
Schematic of the Ameco PCL-P preamplifier.
Additional component annotations added to aid clarity of the description below.
Click on the image for a larger version.

First, notice S3a and S3b on the input/output terminals:  This allows the user to bypass the amplifier entirely - most useful when the unit is turned off - but note that this switch does not power down the unit when set to "out" (bypass) mode.  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 overlapping frequency ranges:  1.8-4, 4-10, 10-23 and 23-54 Megacycles ^{Footnote 2}.

L1 is a coil (or autotransformer)  - tapped at 50 ohms - that covers the lowest frequency range (1.8-4 Mc) and is the large coil visible in Figure 3, below, but the higher bands' couplers - in the form of T1-T3 - are transformers (actually axially-wound coils with another winding over the top) clinging to the rotary switch, the turns ratios of the primary to secondary providing the impedance transformation from the 50 ohm input to the tuned grid circuit:  All of these, switched by S2b, connect to C1, an air variable tuning capacitor across the grid of the first of two vacuum tubes (valves), V1.

It's worth noting that the fact that this preamplifier is tunable is more of an artifact of the necessity of the technology used:  While it would, in theory, be possible to construct a "no tune" broadband amplifier, doing so - and maintaining equivalent performance over this wide frequency range - would have been a challenge.  One obvious advantage of making it tunable is that rather than amplifying the entire HF spectrum at once, its amplifying is limited to the vicinity of the frequency at which the input network is resonant meaning that by rejecting frequencies elsewhere, it's less likely to be overloaded by RF energy that is well away from the frequency of interest (e.g. strong shortwave broadcast stations on other bands).

There are two identical tubes here - 6DS4s in the case of my preamp  (other units may have been equipped with the similar 6CW4) 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 Nuvistors 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:  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 so Nuvistors were widely available and comparatively inexpensive owing to the economy of mass production.  (Wikipedia article about Nuvistors - link).

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)
 "Motorola" type connectors,
it has since been retrofitted with BNCs.
adsfadsf
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 a portmanteu 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 neutrally biased at DC by R2 which allowed current from V2's plate to get to V1's plate via V2's cathode.

This cascode circuit has a distinct advantage for higher frequencies:  As the current through V2 (effectively running in "grounded grid" configuration) is somewhat proportional to its grid-cathode voltage, when V1 conducts more - trying to pull the cathode of V2 lower - V2 conducts harder in response.  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 arrangement, the voltage 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 Miller capacitance is minimized ^{Footnote 3}.  Here we are concerned with the capacitance 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 would - in a typical single-tube circuit - go down by a comparatively large amount as the tube conducts more in response - 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.  If we prevent the plate voltage from changing as much and convey the signal more as current instead - as we are doing with the action of V2 in this cascode circuit- we can significantly reduce the Miller effect. 

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.

With the cascode configuration, the swing of the plate voltage of V1 is minimized - and so is the Miller effect resulting in better gain, flatter frequency response and potentially, lower amplifier noise overall.  As such, we get varying current on the plate of V2 which, via transformer T4 (visible on the far right in Figure 4 as several turns of enameled wire on what appears to be a threaded, ferrite transformer core) is coupled to the output.  Resistor R3 was likely added to help ensure stability of the amplifier both when it is being bypassed (the input and output having nothing connected to either) and also in the event that the input impedance of the receiver connected to the (un-tuned) amplifier output is a poor match at some frequencies.

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, filtered first by C8 - the neon power-on indicator (V3) is connected to this point via R5 - and then decoupled by 1k resistor R4 and filtered again by C9:  The ultimate result is a nice, clean source of about 145-155 volts for tubes when this is operated from a modern 123 volt U.S. mains source ^{Footnote 4}.

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 for equipment - while maintaining low cost and simplicity.  A nice touch is the use of a feedthrough capacitor (C4) as a component mounting point/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 somewhat expensive component was absolutely necessary as a lowly disc ceramic would probably not have sufficed owing to the comparatively high ESR and self-resonant properties that type of capacitor.

From what I can tell, the PCL-P was originally fitted with "RCA" (phono) "Motorola" type connectors (like those found on car radios) on the input/output - a somewhat common practice on HF, VHF and even UHF amateur and commercial radios - but they have been clearly been replaced with the more-common BNC types by a previous owner.

Refurbishing

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 (partly to accommodate the somewhat higher mains voltage these days) - but I also replaced the diode (D1) with a more modern 1N4007 with a 1kV rating.  Ultimately, the ripple on the plate supply was well under a volt - as it should be!  (Sharp-eyed readers may have noticed that the PCL-P is sitting atop the defunct filter capacitor in Figure 1.)

Not surprisingly, I noted that the transformer in this amplifier "buzzed" quite a bit - but with a half-wave, capacitor-input rectifier conducting on the peak of 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 by a major manufacturer since the 1980s - when it is working correctly - will very likely have more than adequate sensitivity on all bands to hear the local receive noise floor, so the PCL-P amplifier probably 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 band.

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.  It's worth noting that the 6DS4 triodes used in this copy have a very slightly lower rated gain than the nearly-identical 6CW4 triodes (an amplification factor 63 versus 65) that the schematic notes as an alternate, but the difference would likely be negligible in the real world as the in-circuit gains would surely be much lower - or in the case of this amplifier, it's around 20dB (e.g. voltage amplification factor of 10 and a power gain of 100).

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 implying that its noise figure is at least as good as it needs to be to be able to hear above the atmospheric noise level - even in an RF-quiet environment.  These Nuvistor tubes are capable of a noise figure of as low as 3dB on 6 meters, but mismatch and losses in the input (and, to a lesser extent, the output) networks would surely degrade this - but a noise figure of only about 9 dB  ^{Footnote 5} is likely to be sufficient in 6 meter work for anything other than, perhaps, EME (Earth-Moon-Earth).

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 made since at least the 1980s:  The issue 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 a 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 may simply degrade receiver performance by feeding it 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:

1. If you think about this for just a second, you can buy some really nice accessories for $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-defined radios.  How times have changed!
   
2. Until somewhere around 1970 or so, it was common - at least in the U.S. - to use "cycles" (e.g. Cycles per second) rather than Hz (Hertz) 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!
3. As noted, the Miller capacitance is often a limitation on the performance of high-frequency/high speed electronic components which is why the cascode configuration is used - and a similar reason why transimpedance amplifiers are the norm for interfacing with photodiodes in high-speed optical detectors  The Wikipedia article on the Miller effect is here:  link.   
4. When this unit was made the nominal residential mains voltage in the U.S. was closer to 110-115 volts and now it is more typically in the 120-125 volt range.  It's unclear when this (gradual) change occurred - and it didn't seem to happen everywhere in the U.S. all at once - but the shift from "about 115" to "around 125" likely happened over the period of the mid 1960s into the 1980s.  "Vintage" gear - that being from the 1960s or earlier - likely was designed to operate closer to 110 volts (especially devices from the 1940s and earlier) than 120 volts meaning that the supply voltages (filaments, B+, etc.) are going to be higher as will the magnetization current/losses in the transformers - something to consider if you routinely operate such gear:  The use of a Variac ^TM or a "buck" transformer in series (e.g. an out-of-phase 9-12 volt filament transformer wired to reduce the 120 volt mains) is suggested to prevent overvoltage of filaments, capacitors, transformers, etc. to maximize the lifetime of those components.
   
5. The article "Measurements on a Multiband R2Pro Low-Noise Amplifier System, Part 2" by Gary Johnson, WB9JPS, discusses the effects on noise figure on real-world performance and concludes that a receive system noise figure of 9dB is likely to be adequate for typical 6 meter operation:  Link (from the Web Archive)
   

 * * * * *

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 its "Output" port (connected to the input of the PCL-P) and 6 dB of attenuation on its "input" port (connected to the PCL-P's output) 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).  These plots cover the range from 1 through 80 MHz, overlapping all of the HF bands (plus 6 meters).  I did note that all of these bands overlap slightly, leaving no "gaps" in coverage and as expected, the gain and the "sharpness" of the filtering in these overlap areas (e.g. top end of the lower band with the tuning capacitor near minimum and the bottom end of the next higher band with the capacitor near maximum) were slightly different:  None of the amateur bands tested below fell  entirely within an "overlap" area.

For the response plots there is a marker (#2) indicating the center (peak) 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.

Tuned to 1.9 MHz (160 meters) in 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).  Note also that the input preselector does a decent job of attenuating much of the AM broadcast band - although it might still be overloaded by a local transmitter operating near the top end of that band.

Tuned to 3.7 MHz (80 meters) 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.

Tuned to 7.2 MHz (40 meters) in the 4-10 MHz position, the peak gain being just under 24dB.

Tuned to 14.2 MHz (20 meters) in the 10-23 MHz position, the peak gain being just under 23dB.

Tuned to 21.2 MHz (15 meters) in the 10-23 MHz position, the peak gain being just under 23dB.  At these higher bands the limitation of the simple, single-stage L/C filter starts to show up as an asymmetrical response - the filtering above the center frequency being less effective that below it.  Note also that at the marked 20dB point above the center frequency (marker #5) the gain of the amplifier is still about 2dB!

Tuned to 28.5 MHz (10 meters) in the 23-54 MHz position, the peak gain being just under 19dB.

Tuned to 52 MHz (6 meters) in the 23-54 MHz position, the peak gain being just a bit more than 19dB.  Its worth noting that the input network does appear to attenuate signals in the FM broadcast band by more than 20dB - something that may have been useful for receivers that suffered from ingress from a strong, local transmitter.

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 as indicated by the markers.

This page stolen from ka7oei.blogspot.com

 [END]