Hygain Afterburner Plus 482 Grid Bias

[Dmans]

I'd like to add Grid Bias to my Hygain Amp (using LooseCannon's Schematic) to extend tube life (6JU6's currently) and clean up the signal a bit.
What Bias voltage should I aim for to enable the best result? Tube datasheet shows -55vdc maximum for Grid 1.
Is there any advantage to putting max voltage on the grid? Is a happy medium (maybe -25vdc) better for the result I hope to get?

Any and all suggestions appreciated.

73's
David

 

[nomadradio]

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

 

[Dmans]

Thanks for the info Nomad. (LC's diagram attached)

I do not use AM mode but my thinking was that "zero bias" would be beneficial to the tubes on SSB between words to cut off the tubes. I do understand the need to add "input tune" when performing this mod and am trying to figure out how to physically do this as this chassis is very tight.
Knowing my negative voltages are limited (either 6vac or 12vac) before rectification, I guess my question is, as in most other things, If a little is good, is more better?

Again, thanks for the education!

73's
David

 

[nomadradio]

If you plan to use sideband, you definitely need bias on the tubes in this amplifier. This amplifier and LC's Kris amplifier are similar, but his effort was to remove the driver stage from his, and drive the final tubes directly from the radio. The Afterburner doesn't have a driver stage, only the single two-tube stage. He is using, more or less the bias circuit I described.

The schematic of the model 482 one is posted at http://www.cbtricks.com/Amp/hygain/afterburner_482/index.htm

The higher the negative bias voltage, the lower the tube's anode current. Make the bias voltage high enough and you reach a level called cutoff, where the tube's current is reduced to zero.

Doing that will make a SSB signal sound choppy. AM signals don't get distorted this way since the carrier will turn on the tube even if the bias cuts off the tube current with no drive.

But too much bias on SSB causes the voice signal to switch the tube on and off and gets a raspy sound. To get smooth SSB audio you need a small steady "idle" current through the tube. Negative 8 Volts DC will do that, and hold the tubes' current to a safe level that .

The problem with having no bias at all is that the tube's current with no drive power is excessive. The original hookup was built to depend on the radio's drive carrier to hold the tube current down to a safe value.

A SSB signal with no carrier at all will nearly always overheat the tubes and lead to premature failure.

The "SSB" switch on the front panel is a borderline fraud. Using it for sideband, with the factory-stock zero bias will blow out tubes.

73

 

[Dmans]

If those Stackpole switches were easier to find, I would change the AM-FM/SSB switch to add the Grid bias on sideband only, but then again I don't use AM any way. nThe juice ain't worth the squeeze.

Negative 8vdc will be my target for bias then. More is not always better! Adding the few components for the grid bias voltage will be easy on the back side of the circuit board containing the tube socket traces. (See photo)

It looks like my only option for the input tune circuit will be tapping into the back side of the relay circuit board trace(See pic with pointer)-interrupting the coax from the relay to the tubes cathode (pin 3).

If the cut and try coil and trimmer cap are tucked in behind the rear tube and in front of the transformer, is it likely there will be any interaction/interference RF wise from the transformer windings and/or proximity to the high plate voltage?

 

[Dmans]

Update/delay to Grid Bias & Input tune mod on this little unit.

A little background on this unit first. I picked this up for next to nothing a couple years ago (advertised as working with normal output-YEAH RIGHT!) I went through the unit and replaced the filter capacitors, the rectifier diodes, bypass capacitors in the power supply, RF chokes, SO239 connectors (Lightning damaged I think) all internal coax jumpers, cleaned the switches and replaced the pilot lamp and Tubes. It has worked flawlessly(150w peak on SSB) since then.
It seems I inadvertently (Senior Moment!) left this unit inline while using my Kenwood on sideband and caused some issues. The amp was turned off at the time but RF was passing through. (Note to self-GET A COAX SWITCH!)
After a few minutes I smelled the familiar electrical burning smell. On shutdown I found resistor R5-330 ohm burned. Replacement yielded no relay engagement of the amp when driven with 3 watts dead key. Modulating the 3 watts, the relay engaged but buzzed and did not stay engaged. Sideband was the same result. When engaged, amp output was normal. Further attempts at repair included testing Q1 (Tested BAD!) and replacement of Q1-Ditto on Q2. (Replacements were NTE 159 for Q1 and NTE 123A for Q2) Replacement of C20, R6 and R7. C21,C22 and C26 were also replaced with same results. New relays were tried with same results.
Currently the amp will power up, the Pre-Amp works and K1 relay energizes with SSB input but does not hold (Tried larger value C22 with same result). K1 relay will engage with AM input but only when modulated. It seems that input wattage is not enough to make Q1 and/or Q2 switch (double checked value of C20-all is per original design)but holding K1 engaged is a separate issue I think? I am measuring 17vdc at the + side of C21 at rest and about 10Vdc when relay is energized. Just to be sure current draw was not falling, I replaced D3 with a 1N5054 I had on hand, results were the same.
Is it possible RFC 3 is damaged from high RF? (I don't currently have an extra for replacement)
I am running out of ideas with this one. Any and all help is appreciated.
73's
David

 

[Onelasttime]

Update/delay to Grid Bias & Input tune mod on this little unit.

A little background on this unit first. I picked this up for next to nothing a couple years ago (advertised as working with normal output-YEAH RIGHT!) I went through the unit and replaced the filter capacitors, the rectifier diodes, bypass capacitors in the power supply, RF chokes, SO239 connectors (Lightning damaged I think) all internal coax jumpers, cleaned the switches and replaced the pilot lamp and Tubes. It has worked flawlessly(150w peak on SSB) since then.
It seems I inadvertently (Senior Moment!) left this unit inline while using my Kenwood on sideband and caused some issues. The amp was turned off at the time but RF was passing through. (Note to self-GET A COAX SWITCH!)
After a few minutes I smelled the familiar electrical burning smell. On shutdown I found resistor R5-330 ohm burned. Replacement yielded no relay engagement of the amp when driven with 3 watts dead key. Modulating the 3 watts, the relay engaged but buzzed and did not stay engaged. Sideband was the same result. When engaged, amp output was normal. Further attempts at repair included testing Q1 (Tested BAD!) and replacement of Q1-Ditto on Q2. (Replacements were NTE 159 for Q1 and NTE 123A for Q2) Replacement of C20, R6 and R7. C21,C22 and C26 were also replaced with same results. New relays were tried with same results.
Currently the amp will power up, the Pre-Amp works and K1 relay energizes with SSB input but does not hold (Tried larger value C22 with same result). K1 relay will engage with AM input but only when modulated. It seems that input wattage is not enough to make Q1 and/or Q2 switch (double checked value of C20-all is per original design)but holding K1 engaged is a separate issue I think? I am measuring 17vdc at the + side of C21 at rest and about 10Vdc when relay is energized. Just to be sure current draw was not falling, I replaced D3 with a 1N5054 I had on hand, results were the same.
Is it possible RFC 3 is damaged from high RF? (I don't currently have an extra for replacement)
I am running out of ideas with this one. Any and all help is appreciated.
73's
David

Very interesting! Yes you actual need to ground grids and add bias if you are an SSB enthusiast! It will not "clean things up" but it should not be that dirty to start with. The fact that it has TV Sweep Tubes in it does not make it dirty how the unit is operated in use though and aging components can make it dirty. No on SSB the bias scheme makes a bigger difference. While you can operate a tube amp in class 6 on SSB and not kill it quick it will be dirty and sound as it should for obvious reasons. That is why people went to AB1. A tube is much more forgiving that a transistor in this regard. If you where operating on AM it would not matter much.

Life cycle between TBO should drasticly increase. Today life cycle is huge for use sense they tubes that in the 1980's where often had new good pulls for well under $10 are now $30-$65 a tube(not just your specific tubes mentions but sweep tubes in general). No one is really making new sweep tubes the only tubes still being produced are those popular with audio and guitar amps and those that where dedicated RF tubes. That said the new ones are Chinese and no where near as good as the old tubes made in USA, Europe and Russia back in the hey day of tubes.

 

[nomadradio]

Since C26 is new, I would have a look at R9. It's a surge-limit resistor feeding into D3.

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.

73

 

[Onelasttime]

Very interesting! Yes you actual need to ground grids and add bias if you are an SSB enthusiast! It will not "clean things up" but it should not be that dirty to start with. The fact that it has TV Sweep Tubes in it does not make it dirty how the unit is operated in use though and aging components can make it dirty. No on SSB the bias scheme makes a bigger difference. While you can operate a tube amp in class 6 on SSB and not kill it quick it will be dirty and sound as it should for obvious reasons. That is why people went to AB1. A tube is much more forgiving that a transistor in this regard. If you where operating on AM it would not matter much.

Life cycle between TBO should drasticly increase. Today life cycle is huge for use sense they tubes that in the 1980's where often had new good pulls for well under $10 are now $30-$65 a tube(not just your specific tubes mentions but sweep tubes in general). No one is really making new sweep tubes the only tubes still being produced are those popular with audio and guitar amps and those that where dedicated RF tubes. That said the new ones are Chinese and no where near as good as the old tubes made in USA, Europe and Russia back in the hey day of tubes.

This is why you should not type when you have not slept in 3+ days! LOL...Insomnia does nothing for my proof reading. LOL Sorry!

 

[Dmans]

OLT,
No worries. I have had a bout of insomnia lately (the last month or so) while recovering from some surgery. Looks like I have a couple weeks more. In the meantime, all I can do is read and study schematics to make my brain tired and hope that sleep will come.
73's
David

 

[Dmans]

All,
After 3 months, 2 hand surgeries, lots of physical therapy (some mental as well) and a couple of deliveries from "brown", I have gathered all the pieces/parts to add tuned input and control grid bias to this little gem.

Now to find the time! (And a crowbar to fit it all in this little chassis)

Stay tuned!

73's
David

 

[Dmans]

I was reminded of some advice I gave to Klondike Mike last week ("Inch by inch is a cinch-Yard by yard is very hard!") so I decided to start this input tune and control grid bias modification. Doubtful I will finish it today but I will never finish it if I don't get started.
Nomadradio had suggested that "someone else" make a video showing how to do this, I do not have the capability to do this but will try my best to take plenty of pictures and explain the steps (and mistakes) along the way.

Below is the "subject" of this mod.

Subject with the cover off. (Note extra room is limited.)

I'm hoping to squeeze the coil and input tune capacitor in between the rear tube and the transformer.

Back side of the tube sockets where the control grid circuit will be attached.

And finally, my hand drawn schematic for the modifications. (Thanks Nomadradio!)

Back to work!
73's
David

 

[Dmans]

Step 1 was to isolate tube pins 2 & 6 on each tube. I did number the pins to prevent any confusion on my part. Hope I was paying attention!

I removed the solder on each pin, bent the pins up off the circuit board and cut and removed the trace around the pins.

I started loading the components for the control grid voltage. Below, the current limiting resistor, the jumper between the 2 tubes and a couple of the .01uF capacitors.

Below, 2 more capacitors, the rectifiers and electrolytic making up the negative grid bias.

As you can see below, there is just enough clearance for the cover.

On to the tuned input circuit and components. Stay tuned.

73's
David

 

[Dmans]

Voltage test of negative control grid bias.

So far so good!

Now to find that shoehorn or small crowbar for the tuned input components!

73's
David

 

[nomadradio]

Way, way cool!

Looks very sharp.

How bad is the input match with it this way?

73