Using Elmac AF-68 with amplifier?

[kx9dk]

Anyone using an AF-68 with an amplifier?
I recently aquired an AF-68 and would like to run it with a Gonset GSB-201 Amplifier.
I'm looking for method(s) to reduce the ouput power of the AF-68. I need to get the power down to about 10-15 watts.

Thanks,
Dave
KX9DK

[K3ZS]

I am using an AF-67 with a Gonset GSB-101 amplifier.     I use a 110V light bulb in series with the output to the amp.   I just used an electrical octagon box with a lamp socket mounted to it.   Added two coax connectors and a switch to short out the light bulb when not used.   I use between a 40W and 150W light bulb.   On 75M a 40-60W seems to work the best.    You tune up the amp to the same loading you would use for SSB.    If you don't heavily load the amp you won't get the peaks on the sidebands.     THe AF-67 is tuned up to full power with the light bulb in series.    My amp runs about 275 mA on AM producing 175 W of carrier power on the output.    This is 100 mA on the AF-67 with about 500V plate voltage and a 40W light bulb in series.     The disadvantage is that you need diffent wattage bulbs on each band, but I use mainly 75M when using the linear amp.   

[flintstone mop]

Hello Dave
I'm using an AF67 driving an Ameritron 811 amplifier. I am not having any trouble....so far. I use it as my 40 M station. I tune the Elmac into a 50 ohm load and then start adjusting the AL811 for 200 watts carrier. I turn down the loading of the Elmac and adjust the AL811 to get to that magical 200 watts.
A LOT of modulation! 125 pos peaks easily
Fred

[k7yoo]

I have used many low power rigs to drive amps similar to yours. The easiest thing to do is build a 3dbT pad using 10 or 20 watt wirewound resistors (or non inductive if you can get them)--or use an EFJ power divider (hard to find). I build them in gutted LP filter cans. Don't worry about the fact that the resistors are not non-inductive--at least for the lower bands. Messing around with backing off on the loading IS NOT a good way to go. The PA in your AF 67 no longer presents a proper load to the modulator and the tank plate impedance is no longer correct for the component values --although the AF 67 tank isn't correct for a 50 ohm load on the low bands anyway--but that is another story. Tune the rig into the antenna (or dummy load) without the amp and then switch the rig to the amp/attenuator. I used to pretune the amp by driving it to max out with my 100watt tranceiver but ther are other ways--and there are other ways of dropping the power but this is the easiest

[Tim WA1HnyLR]

Hello Dave. The Elmac is a bit  light in the audio power department. You want to get all you can get. To reduce power drop the plate voltage to the final only. This can be accomplished two ways. Look at the pinout on the power socket, a Jones plug. There is a jumper that feeds the HV b+ to the bottom of the secondary of the modulation transformer. Remove that jumper. Run a length of three conductor power cable to a small box. Connect one lead to bottom of secondary of mod xfmer. one lead to the HV B + and the other to the LV B+ .Connect the low and HV supply line to each side of a suitable SP DT toggle switch. The arm of which connects to the bottom of the mod xfmr  secondary. Mount switch in small utility box.This method allows you to run full power or reduced power with the flip of a switch. This method also provides a proper match to the modulation transformer. In low power mode the modulator will have much headroom. The stock audio in the Elmac AF 67&68 is lackluster an d needs work. The mod xfmer is deliberately mismatched. the modulator has all it can do to modulate the transmitter at stock power levels. The only disadvantage to this method is that the outpoot power may be on the low side of the equasion.As I recall the Gonset GSB 201 uses 4-811A tubes. There may be a later model that uses 572-Bs If it uses 811A tubes you will be limited to about 100 watts out. The 572-B tubes perhaps 175 watts with 2Kv or more on the plates. Years ago a friend of mine had a GSB 201. He drove it with a Ranger that I did up the same way. Use of LV B+ for the final and full voltage on the modulator. He had the 572-B tubes and got about 175 watts out. He always had a 60Hz hum in his signal. The hum was not present on his signal from the Ranger. I looked at his amplifier. The stock plate transformer has the fillament winding on it as well. One side of the fillament winding was tied to the chassis ground. There was NO centertap on the fillament winding. To fix the hum the ground return MUST be from a centertapped fillament transformer. How do you fix the problem without adding a large fillament xfmr?. Simple. Take a small 6.3-12V 1 amp fillament transfomer.Lift main fillament winding from chassis ground. Connect secodary fillament winding of small transformer in parallel with the main fillament winding. Connect centertap of small fillament transformer to ground return.This will balance the fillament winding and eliminate the hum. cut off or tape up the unused primary winding.If more power outpoot is desired from the AF 68 in low power mode. annother power reduction method may be used. Rather than take from the low voltage supply place a power resistor in series with the  HV B + to the bottom of secondary of mod xfmr. MAKE SURE that you place a capacitor. such as a 20Mfd 450V cap in parallel with the resistor. you want to dissipate B + voltage. Not audio power. Typical values of resistor could range from 2Kto5K ohm. Make sure that the resistors have enough dissipation rating. Use a spst switch to short out the dropping resistor combination for full power tune up and operating mode. I friend of mine uses this method with a modified Eico 720/730 combination.He has several values of resistors selected by a rotary switch. He uses the combination to drive a 3-1000-z homebrew amp. As you can see this B+ reduction method is applicable to almost any transmitter.  The other suggested methods such as power output attenuation may facilitate the use of an amplifier but do not improve the qulity of signal. Lightly loading the final may reduce power but does not give you any more modulation headroom. Good luck,Tim WA1HnyLR

[W2VW]

That concept seems to be like MTV. Some people just don't get it. Nice description though.

[kx9dk]

Thanks for all the responses. You've given me a couple of things to try.
Just checked the junk box and found a nice 4k, 40 watt power resistor. Looks like I'll try this first to reduce the B+ to the final...then go from there....

Thanks,
Dave
KX9DK