Amplifier Rework Project

I have an amplifier that was constructed by the real "Palomar Labs" back in the 70's, it is a Model #5300 with a low pass filter section on the output and, a band switch selecting the 80 through 10 meter low pass filters. It has a regulated B class bias voltage on the input transformer, the power switch now activates a relay to switch +12 volts to the amp and, the Antenna Switch relay failed and had to be replaced with a different model because the stock part is no longer attainable (and was too small for the output).

http://i111.photobucket.com/albums/n...psze4sgjnk.jpg

I also have an Input matching circuit board designed to match a 50 ohm transmitter to the cathode of a four sweep tube amplifier

http://i111.photobucket.com/albums/n...psaevm2xke.jpg

http://i111.photobucket.com/albums/n...psoty1aizr.jpg

So, i'm thinking that I can hook the original 50 impedance side to the input transformer and the output side to the MFJ 259 antenna analyzer and rework the caps and coils to work from 50 ohm radio input to transistor input matching impedance.

Does this sound like a feasible project?


BTW: also posted in Technical section of the swamp; I know i'm gonna get a bunch of Snarky-Chicken Band remarks but, I really thinking about doing this.

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First thing to look at is biasing, and how the design regulates it. Thermal stability is important here.

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Originally Posted by N8YX

First thing to look at is biasing, and how the design regulates it. Thermal stability is important here.

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Attachment 13276

The 3055 / regulator is bolted to heat sink

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Do the transistors all have been selected for the same bias setting? Motorola has a color dot for that, even then I build my amps with a bias circuit for each transistor with heat tracking as well.

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Originally Posted by PA5COR

Do the transistors all have been selected for the same bias setting? Motorola has a color dot for that, even then I build my amps with a bias circuit for each transistor with heat tracking as well.

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Nope, both pair of input transformers share the same bias supply.

Attachment 13279

I see, and no temperature control either per transistor, which can cause a thermal run away when one is not in the selected bias configuration.
Each of my transistors had a thermally coupled diode over the transistors each with it's own regulated bias from a low impedance source.
Might be overkill, but according the Motorola handbook Engineering Bulletins that was how they did it and I followed their design.
Transistors were quite expensive then.... ;)
The rest looks like the same AB Motorola design, wideband input and output coupling and transformers.
Even used it ( adapted) for 2 meter with 2 x the MRF 247 transistors.
That one didn't have much gain, 35 watts in for 160 out but that was satisfactory then.
Running that in 2 x 12 element beams was a hoot ;)

Look on the net for the schematic diagram of the HLA 300V amplifier from European make, there is a HAM version of it with band filtering etc reasonably clean too for 300 watt.
RM Italy does make a HLA-300V, 450 watt amplifier that has full auto band switch low pass filtering, with power meter and remote keying. I have tested the amplifier and it passes FCC certification but has not been certificated as of yet. You can buy them for about $360.
You do need a 50 to 60 amp power supply for it though...

http://pa0fri.home.xs4all.nl/Lineair.../schematic.pdf
Schematic diagram.

http://pa0fri.home.xs4all.nl/Lineair.../hla300eng.htm

Detailed description of that amplifier and test.

Cor nailed it - per-device thermal feedback to regulate bias or you'll get one or two that "hog" the load as die temps increase.

That bias circuit should be your first remediation point.

https://forums.hamisland.net/showthr...04-amp-project

Nice work here with a balderdash of info about such amp ;)

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Originally Posted by PA5COR

https://forums.hamisland.net/showthr...04-amp-project

Nice work here with a balderdash of info about such amp ;)

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I remember that thread, he documented his whole project but, there are a couple of major differences:

1. He started with a bare board, that was new construction; my circumstance is repair / retuning
2. His amp used control modules that are 40 years forward that the one I am working with.
3. His amp is designed to Fixed station use, mine is +12 vdc mobile
4. His design uses high input impedance devices that do not require toroid transformers thus allowing for dedicated bias controls for each transistor.

I digested the circuit at http://www.ab4oj.com/dl/eb104.pdf along with the write up on the operation of the amp. I found it interesting that the boiler-plate design of the now commonly accepted EB104 based amplifer has bias voltage ran through a chip that controlled voltage level and current load but, the bias voltages for each device was paralleled from a single output. The final voltage adjustments for the individual devices were from a voltage dividers behind the regulator chip; higher tech voltage control but, same circuit that was used in the 1980's device.

After some thought, looking at the Palomar TX-5300 circuit, I figured one way to install individual bias systems would be to

1. Connect the base of the transistor to the impedance matching transformer via large .001 uf capacitors
2. Isolate the impedance matching transformer from ground,
3. Apply bias voltage through chokes to each base, behind the now DC blocking capacitor in the base circuit.
4. I will leave the temperature compensation (wire a thermistor in) circuit for thermal tracking up to you guys to figure out.

Just some thoughts.

BTW: The .001 capacitors which are paralleled with the 15 ohms resistors in the base circuit is in error, they are not in the amplifier that I have; if they were there, RF drive would be divided / bypassed to ground.
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Don't forget to ground the B+

The cheap method of feeding bias to the transistors/fets from one source can be done if each transistor or fet has it's own temperature tracking and is selected for the gain, so that each transistor will work within that range with the same bias, see the Motorola color dots on the transistors to mark them for selection in your amplifier.

Certainly with fets I would incorporate temperature tracking for each fet because they are very liable for run away with changing temperature.

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Originally Posted by K7SGJ

Don't forget to ground the B+

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OK, I'll ground your tail.

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Originally Posted by K7SGJ

Don't forget to ground the B+

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Be careful: Intentional grounding can cause a loss of down and a 10 yard penalty.

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Originally Posted by wb5ydk

Be careful: Intentional grounding can cause a loss of down and a 10 yard penalty.

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Unless you do it within your own end zone.

Then it's 2 points for the other team, who get the ball back on a free kick.

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Originally Posted by W3WN

Unless you do it within your own end zone.

Then it's 2 points for the other team, who get the ball back on a free kick.

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And most of us know what a kick in the balls will do for you.

I wouldn't worry about biasing, Palomar wasn't one of those basement CB hack outfits, the transistors are matched. I've used plenty of their very high quality equipment and was perfectly satisfied with it, if you look up their history you'll see why. Just don't tell the FCC I used one of their 100W P-P broadband amps driven by a Rat Shack 23 channel rig in the Stereo mobile full time (no switch) and it was clean as the proverbial whistle. Another thing I used occasionally when I got my ham ticket was an FM adapter that plugged into the crystal socket on an AM rig, in this case a Gonset 2M Communicator. That BTW was an interesting adventure, zero beating the SSB stations with the matching VFO I heard them just fine and they didn't know I was transmitting AM until I told them.

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Originally Posted by wb5ydk

Be careful: Intentional grounding can cause a loss of down and a 10 yard penalty.

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Not if the defense is the Raiders. They will find them holding or something, count on it.

I say that as a fan.