Class AB1 AM Modulator Project

[KC9KEP]

Hello,

I am building a circa 1953 AM Modulator to go along with my 6146B that I had posted
questions about earlier.

I think that it's working fairly well, judging by my power meter and scope trace display.  I have
yet to make a QSO or to get on the air yet (I've just got it operating today).  I have the modulator
connected to my 6146B transmitter and am measuring about 60 Watts output.  
(The modulator is connected to both the Plate Supply to the 6146B and Screen of the 6146B
via a "dropping" resistor.)

One concern that I have; The B+ in the construction article is stated to be around 600VDC,
but my power supply is about 80VDC higher.  I was thinking that perhaps I should have increased
the battery bias supply, but it seems to be working well as it is.
There is a 50 Ohm cathode resistor included in the design that allows the operator to monitor
the cathode current (but I have yet to do so).

Anyway, here are some images.  I am curious to see what others think of this design :-)

http://www.bignick.net/Mod/before.jpg
http://www.bignick.net/Mod/rear_1.jpg
http://www.bignick.net/Mod/under.jpg
http://www.bignick.net/Mod/Temp_Front.jpg
http://www.bignick.net/Mod/front_cab.jpg
http://www.bignick.net/Mod/rear_cab.jpg
http://www.bignick.net/Mod/top_cab.jpg

Scope Trace:

http://www.bignick.net/Mod/scope.jpg

Schematic wiring diagram:

http://www.bignick.net/Mod/Mod_2.jpg

73!

--KC9KEP

[W9GT]

Hello,

I am building a circa 1953 AM Modulator to go along with my 6146B that I had posted
questions about earlier.

I think that it's working fairly well, judging by my power meter and scope trace display.  I have
yet to make a QSO or to get on the air yet (I've just got it operating today).  I have the modulator
connected to my 6146B transmitter and am measuring about 60 Watts output.  
(The modulator is connected to both the Plate Supply to the 6146B and Screen of the 6146B
via a "dropping" resistor.)

One concern that I have; The B+ in the construction article is stated to be around 600VDC,
but my power supply is about 80VDC higher.  I was thinking that perhaps I should have increased
the battery bias supply, but it seems to be working well as it is.
There is a 50 Ohm cathode resistor included in the design that allows the operator to monitor
the cathode current (but I have yet to do so).

Anyway, here are some images.  I am curious to see what others think of this design :-)

http://www.bignick.net/Mod/before.jpg
http://www.bignick.net/Mod/rear_1.jpg
http://www.bignick.net/Mod/under.jpg
http://www.bignick.net/Mod/Temp_Front.jpg
http://www.bignick.net/Mod/front_cab.jpg
http://www.bignick.net/Mod/rear_cab.jpg
http://www.bignick.net/Mod/top_cab.jpg

Scope Trace:

http://www.bignick.net/Mod/scope.jpg

Schematic wiring diagram:

http://www.bignick.net/Mod/Mod_2.jpg

73!

--KC9KEP

Very nice work! 

73,  Jack, W9GT

[ke7trp]

Very nice!  Love it!  Did you think about adding a window to the modulator ?

C

Hello,

I am building a circa 1953 AM Modulator to go along with my 6146B that I had posted
questions about earlier.

I think that it's working fairly well, judging by my power meter and scope trace display.  I have
yet to make a QSO or to get on the air yet (I've just got it operating today).  I have the modulator
connected to my 6146B transmitter and am measuring about 60 Watts output.  
(The modulator is connected to both the Plate Supply to the 6146B and Screen of the 6146B
via a "dropping" resistor.)

One concern that I have; The B+ in the construction article is stated to be around 600VDC,
but my power supply is about 80VDC higher.  I was thinking that perhaps I should have increased
the battery bias supply, but it seems to be working well as it is.
There is a 50 Ohm cathode resistor included in the design that allows the operator to monitor
the cathode current (but I have yet to do so).

Anyway, here are some images.  I am curious to see what others think of this design :-)

http://www.bignick.net/Mod/before.jpg
http://www.bignick.net/Mod/rear_1.jpg
http://www.bignick.net/Mod/under.jpg
http://www.bignick.net/Mod/Temp_Front.jpg
http://www.bignick.net/Mod/front_cab.jpg
http://www.bignick.net/Mod/rear_cab.jpg
http://www.bignick.net/Mod/top_cab.jpg

Scope Trace:

http://www.bignick.net/Mod/scope.jpg

Schematic wiring diagram:

http://www.bignick.net/Mod/Mod_2.jpg

73!

--KC9KEP

[W7TFO]

FB metalwork.

73DG

[N0BST]

FB metalwork.

73DG

Ditto that.  Glad you didn't use the tube rectifiers.  Can't see how you could possibly use 6X5s without using separate filament windings.  That common winding would be taxing their H-K breakdown voltages for sure.

[WU2D]

6146's will not notice the extra 80V, in fact they are presented with much higher plate voltages routinely.

[KC9KEP]

Thanks everybody!

Well, I did a little more measuring today.

B+ Supply (Modulator off) = 810VDC
B+ Supply (Modulator on) = 732VDC

Low V Supply (Modulator off) = 316VDC
Low V Supply (Modulator on) = 285VDC

(The original schematic says B+ should be 600VDC and L.V supply 230VDC.)

Voltage across 50 Ohm Cathode resistor:
Cathode Current (Modulator on) = 6VDC/50 Ohms = 120mA

The text says that the modulator should idle at 50 mA and peak at 100 mA
(no more than 65 mA for speech.)
Also ..one of the plates is getting a touch red.

I was considering subbing in a bench supply for bias, adjusting the bias for 50 mA
cathode current, then measuring the bias and upgrading the batteries for a more
negative bias (which I'm guessing will be necessary.)

Funny .. the modulator seems to work quite well as is ..

73

--KC9KEP

[W2PFY]

Beautiful, perhaps consider zeners for the biasing in the cathodes of the 807"s?

[ssbothwell KJ6RSG]

really nice mechanical construction! how did punch that square hole for the iec power socket? i use a nibbler but never get such a clean square hole.

[VE3LYX]

"There is a 50 Ohm cathode resistor included in the design that allows the operator to monitor
the cathode current (but I have yet to do so)."
After measuring mine and finding a bit of grid current flowing I calculated the cathode resistance I needed to stop it since if there is no grid current then a resistor there does little at the moment of that condition. The figure I came up with for my HB amp was quite small and I have no small value resistors here so I went through some ideas. I came up with using a #40 lamp to keep my tube parked at reasonable idle  plate current. I can use a #52 but  it is a bit bright. Now my amp is not running as hard as yours. I am only about 1/2 voltage since that is all I needed and my tube is a 6293 which is a hd pluse transmitter tube very similar to a 6146B.(Even looks the same) What I discovered doing this was the lamp worked great for tuning  as well and in the event I did domething stupid which I have from time to time the bulb works as a fuse and prevented a catastrophic disaster. I have come to like the idea so much I added it to two other HB rigs and they both already had cathode resistors.  Another very workable alternative is B- metering instead of B +. The total curent would be included but it would not be that much extra. The shunt would work to park the tube in the safety zone or would have in mine anyway as it was the same value as the bulb I choose. Your circuit may be a bit diffferent in that you are using  yours to plate modulate and I am driving mine as a linear with about 250 MW but a PA is a PA pretty well whether AF or RF. 
If you want to do an on air test  a time and freq would get you a sig report from this neck of the woods if I hear you.
Really nice neat work. All the best
Don

[KC9KEP]

Thanks for all the responses, ideas and kind words .. I will be experimenting again tomorrow.

I did want to answer a question about how I made the chassis holes ..

Most round holes were cut with Greenlee punches.  The large circular hole for the modulation
transformer was cut with an adjustable circle hole cutter.
(It has an arbor and adjustable "arm" and cutting bit.)

The rectangular hole was cut with a sabre saw with a metal blade, then filed to finish it nicely.

I cover the entire chassis with masking tape before marking and cutting.  After all the holes
are cut, I remove the protective masking tape.

73

-- KC9KEP

[KR4WI]

Very nice, its like a work of art. It makes you just want to stare at it.  Thanks for sharing it. Matthew KR4WI

[N0WEK]

Beautiful work!

What month in 1953 is that QST article?

I like the power supply design, did you solid state the rectifiers?

Greg

[KC9KEP]

Thanks for the help everyone :-)

The original article is in the May 1952 QST - "Class AB1 Modulator for the Small Transmitter".

(It's in the 1954 ARRL handbook too.)

Yes, I used silicon rectifiers.  I don't know he could have fit an ore tubes into that design,
and he used a smaller footprint than I!

I think I used 1N4007's; 1000VDC @ 1 Amp if I recall ..

73

KC9KEP