Another Henry C300C30R Amplifier Conversion to 6 meters

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Conversion to 6 meters
By John Haserick W1GPO

Background:

This amp was purchased new in the box on 47.XX MHz on a popular auction site at what seemed to be a good price. The serial number indicated it was a more recent version of the C300C30R I first converted. I should have noticed the maximum power was labeled 250W, not the 300W labeled on the amp first converted to 6M. Here are pictures of the interior before anything was done. Click on any photo for a larger image.

Here's a closer view of (left to right) the harmonic filter, the 4 dB input attenuator, and the power amplifier.

Differences from the Prior Amp:

The good news is that it looks like the harmonic filter was improved to four coils and five caps and was more compact. The bad news was that the harmonic filter did not roll off until higher than the second harmonic frequency! The first amp used six Toshiba 2SC2782A NPN power transistors while the second one used six M1104 power transistors, for which I could not find a data sheet. The first used a 1 dB hybrid attenuator pad while the second amp used a 4 dB hybrid attenuator pad. Both amps had rated input of 30-40 watts. The second amp was also missing the three 2.2 µH chokes on the input ferrite transformer secondary center taps that were present on the first amp. See the marked-up schematic diagram below. There are several errors in the schematic, such as the collector-to-collector short circuits.

Bob WA1MIK corrected and redrew the schematic. You can view it as a small PDF file here. You can find a manual for this amplifier on the Henry Radio page on this web site.

Fan Modifications:

As done on the first amp, the fans were disconnected from the temperature switch and actuated by repeater PTT with about a 10 second drop out delay to reduce initial heat buildup before the fans are actuated, and to save on unnecessary fan running time. Four 1/4 inch tuning access holes were drilled, so by shining a light through the top cover slits, you can peer through the front fan to see the insulated tuning stick mesh into the compression trimmers safely with the amp front cover kept tight. The quarter inch hole caps can be seen in the photo below.

The front fan copper mesh RF shield did not appear to be bonded as well as the RF shield on the older version, so four screws with washers were added around the perimeter. See the photo below.

Harmonic Filter Modifications:

Here's a spectrum of the original harmonic filter response. The sweep is centered at 50 MHz and goes plus and minus 100 MHz, so 150 MHz is at the right end. The low end of the sweep is truncated somewhere around several MHz.

To suppress the second harmonic, the harmonic filter required two more turns on the two inner coils, for a total of seven turns. #16 enameled wire was used to replace all four tinned coils. The two outer coils were left at five turns. See the photo below. The new coils were wound over a 9mm diameter tube, resulting in an outside diameter of 12mm due to spring back. The outside diameter of the original coils was 11mm but they had more space between turns so the turns would not short, and possibly stay cooler than if they were tightly compressed. After modifying the filter, the second harmonic is down 55 dB from the carrier, and the third harmonic is down about 90 dB from the carrier.

Amplifier Modifications:

The two 100 pF metal clad output fixed mica caps were replaced with 15-100 pF mica compression caps removed from Micor low-band mobile PAs, and three 2.4 µH chokes from those Micor mobile PAs were added to the input transformer's center taps to ground. A 22 pF metal clad mica was paralleled over the top of the existing 10 pF where the input RF comes in from the hybrid attenuator, because the existing parallel compression mica cap did not add enough capacitance when cranked down tight. All of these additional or changed components are indicated in pencil on the schematic drawing shown above.

The amp would not tune up on 6M without going into oscillation and spewing out RF spurs everywhere at over 200W out, probably because the M1104 transistors had more gain than the 2SC2782A transistors, so I decided to replace the M1104 transistors with three matched pairs of 2SC2782As from RF Parts. That looked like a very daunting project, but with a new 1/8 inch 700 degree tip on my Weller 100P iron, it just took a lot of time. The electrolytic capacitor on the harmonic filter side needed its ground lead disconnected to make more room, then the twelve 22 ohm 2W carbon resistors were unsoldered starting near the electrolytic and working toward the input connector side, then the six 10 ohm 1/2W carbon resistors were removed, and finally the M1104s were extracted using a lot of solder wick.

The new 2SC2782As were then flange trimmed, tinned underneath, inserted and soldered, then the six 10 ohm 1/2W resistors and twelve 12 ohm 2W 5% carbons were installed in reverse order from input side to harmonic filter side, then the six 0.022 µF caps were reused.

More Power, Scotty:

The result was over 300W out with 36W in and very clean and stable, well worth the effort! I later changed the 4 dB input hybrid attenuator to a 1 dB pad (that's a 3 dB improvement and thus requires half as much drive power) to match the original amp conversion, which gave me 270W out for 18W in, and that's where I left it so as not to melt down the duplexer again. So Henry's specs were exceeded. The first amp required 28W input for 270W output at 13.8V. The amp draws close to 33 amps from the power supply at this power level.

Henry Radio sells the hybrid attenuators in various values. The attenuator also provides a bit of isolation on the input of the amplifier and presents a better return loss to the input power source.

The compression mica trimmer capacitors, obtained from a 100W mobile amplifier, are only rated at 250 WV, not normally a problem. When the duplexer's first resonator melted from too much RF heat, the infinite SWR caused a flashover on the compression trimmer closest to the harmonic filter on the first amp that was converted. It actually may have saved the output transistors because the transistor current probably dropped instead of increasing, so no damage occurred to the amp, once the compression mica was replaced! Because of this, it is probably better to have these output compression micas operating more on the open side than closed tighter at resonance.

Here's a photo of the modified amp, showing the new harmonic filter coils and all the additional and replaced components on the amplifier circuit board.

Update: Since the first conversion gained efficiency when two of the mica compression caps were replaced with air variables, the compression caps were changed in the later model to the EF Johnson 194-18-5 or 194-8-5: 3-55 pF 750V air variable caps from Surplus Sales of Nebraska (CTA-194-18-5). (The changed middle number indicates the presence or absence of a second adjustment screw on the board side.) The rotor of the caps was connected to ground. This time there was no change in efficiency over the compression micas! The fixed metal clad mica on the coax side was changed from 100 pF to 75 pF and the metal clad mica on the other side was left at 50 pF. The reason for a lower value of capacitance on the newer model cap on the coax side was that the older version three-stage harmonic filter required more capacitance. My guess is the efficiency increase was due to the changed layout of the capacitors in the earlier filter. The picture below shows the amplifier after the air variables were added.

Credits and Acknowledgements:

All photos were taken by the author and are copyrighted by him.

Thanks to Bob WA1MIK for turning this into an article suitable for publication and for redrawing the schematic.

Contact Information:

The author can be contacted at: jhaserick84 [ at ] comcast [ dot ] net.

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This article created on Sunday 01-Nov-2020

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.