Hygain Afterburner Plus 482 Grid Bias

[Dmans]

Nomad,
I haven't checked it yet. Just plugged it in to check bias voltage. Started "building" the coil and support structure. I probably should check the match before proceeding. If nothing else to see the difference or find out it may not be needed!

A step for the next "go" at it. Ran out of time and energy today.

73's
David

 

[Dmans]

OK, so after Nomad's note, my curiosity got the better of me. I tested "pass through" SWR of the Hygain without the input match components. The tests were done through the antenna. Shhhhh! (I don't have a dummy load)

The picture below is without theHygain in-line, transmitting on AM. The Pace tester is showing about a 1:7.1 SWR. The SWR meter on the radio is showing about 1:4 or 1:5.1. (To be as detailed as possible, after leaving the Pace meter, the signal goes through a 2 position coax switch, into another amp, into a TVI filter, into an MFJ meter then on to the beam.)

The picture below is through the Hygain amp with only the control grid modification performed. There is -7.11vdc on the control grids. The rest of the amp is "stock" (that doesn't mean too much since the amp was built around 1974).
As you can see the Pace meter is now above 3:1 as is the SWR meter on the radio.

It will definitely have to have the cathode tune modification to make this usable. At least I can now see the difference that the cathode/input tune modification will have.

My goal with this process was to clean up the SSB signal, allow the tubes to last a little longer, keep the amp a little cooler and not hurt my "like new" 139XLR.

Stay tuned.

73's
David

 

[Dmans]

Ok. It has been a busy 12 days since I last touched this unit. Today I finished the Cathode tune circuit and am quite pleased with the result.

A refresher of the schematic below. I concentrated my efforts today on the items in the upper left corner of the schematic. Looks like very little to do but more aggravating than I thought it would be.

Below is the circuit board supporting the relays and RF "sniffer" transistors. The pointer is on the coax that leads to pin 3 of the tubes. The Cathode tune circuit will interrupt this coax. Pulling the coax at this circuit board was easier than the tube socket circuit board.

The coax to pin 3 of the tubes is visible here-only after removing the front panel could it be re-routed to the location of the coil and capacitor.

Below is the preliminary fitting of the 6 turn coil. I used a couple 3 terminal boards (with 1 terminal removed) to support the coil and tuning cap.

Final installation of the cathode tuning circuit below. It is a tight squeeze.
The Arco 463 trimmer cap is attached by some12ga copper leads. It is positioned so that the trimmer can be adjusted through the holes in the chassis so that it can be set with the cover on.
Yes. Leaving the cover off and adjusting the trimmer is DANGEROUS and installation of the cover changes the match slightly. So why bother?

The 2 pictures are of the input match between the Cobra 139XLR and the Hygain when tuned with modulation. It took very little tweaking and no stretching or squeezing the coil to go from the 3:1+ before to 1:5.1 now. Perhaps it could be made a little lower but "the juice ain't worth the squeeze". The radio will be perfectly happy here.

Below is the peak modulated output power on AM. I get the same readings on SSB. (That is on the 200 Watt scale)

Below is the SWR match between the Hygain and the antenna with modulated peak power.

This has been a fun project for me. I have learned a lot and have more to learn. Even though I have now put a hair more than $75.00 in this unit, I believe it is a keeper!

Thanks to Nomadradio for his explanation of this circuit (sorry I couldn't do a video) and to Loosecannon's documentation on a Kris Power Pump rebuild-that got me thinking about this mod in the first place.

73's
David

 

[Shadetree Mechanic]

Great work, thanks for sharing. Yes tuning is dangerous while powered up with the cover off, I like your idea of adjusting the cap through a hole in the case. When I replaced the varicap in my Contex, I would power down, unplug, and then adjust. Then repeat as many times as necessary, what a pain. Some day I will do the bias mod to mine.
Chris

 

[kopcicle]

!!

 

[kopcicle]

Not sure where to find LC's diagram.

The factory schematic shows all three grids grounded on both tubes.

I recommend putting negative bias only on the control grid. That requires pulling pins 2 and 6 loose from ground on each socket. Leave the other two grids grounded.

A wire is added to join the control grid on one tube to the other. A .01uf disc cap goes from one grid pin on each tube to ground. We use a 1k 5 Watt resistor to ground from one socket's grid pin. The type of this resistor is not critical. A wirewound type is perfectly suitable, since this is a DC-only circuit with no RF voltage on it.

We use negative 8 Volts bias for this type tube. The two tubes' heaters are in series. Where the two heaters are connected together, you get six Volts AC. If you connect the cathode (banded) end of a rectifier diode like a 1N4002 to this point, you'll get negative 8 Volts DC by adding a 1000uf 25-Volt filter cap to the not-banded (anode) end, positive side of the cap to ground, negative side to the rectifier.

One additional rectifier gets connected, the cathode end to the filter cap negative and the anode end to the grid pins on the tube sockets.

But that's the easy part. If you put a SWR meter and a second coax jumper between the radio and the Afterburner * BEFORE* modifying it, key the radio with the amplifier tuned up normally and check the SWR feeding into the amplifier while the Afterburner is keyed.

You'll probably see a reading of 3-to-one, maybe more.

Oops.

Adding bias will usually just make this reading worse.

The fix is to add a coil between the relay's input side and the cathode pin (3) of the tube sockets. The coil just goes in place of the wire that's there now. I don't have a record of the best-size coil, but 6 turns of insulated solid hookup wire wound on a half-inch form is a good starting point. The coil alone usually won't bring down the SWR enough. You'll also need a capacitor around 100 pf from the relay side of the coil to ground. A mica compression trimmer cap like a Arco 464 will let you adjust for lowest input-side SWR. The coil might need to get squeezed or stretched for lowest input-side SWR reading.

But this is the non-advertised side effect of adding grid bias to this kind of amplifier. It will change the input impedance. When this amplifier was sold, most base radios had a tube in the final stage. Tubes are more forgiving about SWR than solid-state transmitters. Probaby didn't matter as much using a tube-type radio.

Biggest advantage to adding fixed bias to this kind of amplifier is that it prevents the tubes from overheating and blowing up when the radio's carrier is turned down to a reasonable level. The term "zero bias" means that the grids are connected to the same DC voltage as the tubes' cathodes.

A true zero-bias amplifier like this one depends on a high (4-Watt or so) carrier to control the tube current. Sounds backwards, but the lower you turn the radio's carrier, the higher the current drain the tube takes from the high voltage supply. This makes the tubes overheat if the radio's carrier is turned down. When this amplifier was sold, there was no such thing as a carrier-power knob on a base radio.

Adding fixed bias holds the tube current to a safe value when the radio's carrier is turned down to a level that makes it sound good.

You could feed the bias rectifier from 12 Volts instead of using the tubes' two heaters as a voltage divider down to 6 Volts. This would double the DC-bias to negative 16 Volts DC or so. My experience has been that this tends to be overkill.

Your mileage may vary, but adding bias will nearly always make the tubes last longer.

73

What is new is old
http://www.813am.qsl.br/artigos/moduladores/hileman/Class_K_Modulator_CQ_Oct_53.pdf

 

[743SEPA]

I have one of these. Works great,close to 100 watts on am,chatters badly on ssb no matter the drive.

 

[kopcicle]

"The standby-side voltage on the coil of K1 (+ side of C21) should be around 16 Volts DC. It will fall to 12 or 13 when the relay is keyed.

This keying circuit should be plenty sensitive, well below a half Watt. If R5 got burned by excessive drive power, that's worth checking. If R5's resistance is too high, that will reduce the keying circuit's sensitivity.

Getting the relay-circuit's power-supply voltage up where it belongs might not fix this, but it's important."

It's good advice as I've seen R5 high in a majority of Hygain Afterburners.

http://www.cbtricks.com/Amp/hygain/afterburner_plus_482/graphics/afterburner-plus_482.pdf

 

[Dmans]

I have one of these. Works great,close to 100 watts on am,chatters badly on ssb no matter the drive

743SEPA,
Check out the link below. I had a "Senior Moment" and caused some damage to my Hygain. The link details the corrections.

https://www.worldwidedx.com/threads/482-afterburner-plus-too-much-drive-power-damage.229275/

I also found a NOS set of 6KM6 tubes I have had for a couple years (Shack cleaning reveals some forgotten but amazing Gems!) and stuck them in the Hygain. Output power now on SSB is close to 200 watts while running cool, with a low input reflect and reports are it is clean to boot.

73's
David

 

[743SEPA]

Thanks for the info and the links but I don't have a clue how these work on the inside. I thought maybe the relay was just going bad. After all,how old are these things?

 

[Dmans]

After all,how old are these things?

The schematic I have for mine has a Copyright date of 1974.

73's
David

 

[kopcicle]

I don't have a clue how these work on the inside

does it still measure 330 ohms?

 

[BlowinSmoke]

Seems like an awful lot of work for an amp that makes barely 100 watts peak power. I would think getting an FT-101 radio would be a better idea and it can drive the balls off an SB-200 or SB-220 properly prepared for CB.

 

[743SEPA]

View attachment 27658

does it still measure 330 ohms?

I'd have to desolder at least one end to check it,correct?

 

[ExitThirteen]

Seems like an awful lot of work for an amp that makes barely 100 watts peak power. I would think getting an FT-101 radio would be a better idea and it can drive the balls off an SB-200 or SB-220 properly prepared for CB.

It's not always about the destination, it's about the journey. There's a lot of good info to be had here.


~Cheers~