hot roding a gonset g-50

[RidgeRunner]

I have a fairly stock g-50 other than replaced old/failing parts.  I was thinking about "hot rodding" it.  I heard somewhere that you could get 100% modulation out of it by using a PA amplifier with a hammond single ended audio outpoot xformer.  Just curious if anyone has done this.

Also on mic selection I was looking at an amplified d-104.  My question is I have been told and I also think so that I have a nasaliy voice.  So what would be a good mic selection?  I am not hung up on name brands nor am I opposed to any homebrew solutions.

I would just like to hear suggestions from the group.

[WA1HZK]

Nothing beats a decent mic. and a good voice processor then the proper compressor/limiter. You will need to hunt around unless you are ready to throw some serious bucks at it. I usually tell people to pick up a: http://www.zzounds.com/item--BEHB1
with a: http://www.amazon.com/exec/obidos/ASIN/B0006HBP2O/interactiveda8258-20
That's a B1 with a 528E or the older 528.
Both are on E-Bay all the time.
If you want a better mic look at the Electrovoice RE20 or RE27. These can be found new & used all over the place.
Between this stuff goes the negative peak limiter/compressor just before the transmitter. There's lots of used stuff kicking around E-Bay. Those are big bucks new.
There are better audio guys here that I'm sure will give you their 2 cents also.
Keith
WA1HZK

[Pete, WA2CWA]

This is 6 meters. 6 meter AM gets good when the band opens up. Under those types of conditions, sounding great (broadcast quality) is not a priority nor is it necessary. I also never had a G-50 that didn't modulate almost to 100%. A non-amplified D-104 works great on it.  Forget about limiters, compressors, expensive mikes etc. for working 6 meters. Put your money in the antenna and get it high in the air.

[RidgeRunner]

Ya I guess I'm not so concerned about band openings anyways.  I just thought it would be nice to do something different with a g-50, since DX isnt really something that "blows my skirt up" I guess I'd like to see if anyone has any suggestions and if not I'll just let this thread sink .

[W1AEX]

Check and see if the G-50 uses the same transformer for modulation and receive audio output. If the mod transformer does have an 8 ohm winding, it's really easy to use a relay to switch that 8 ohm winding over to the output of an audio amp when in transmit mode, and you can modulate it externally that way. I have done this by using a dynaco amplifier with a Clegg 99'er, a Clegg 66'er, an HA-460, and a Hallicrafters SR-34. Works great, sounds as good as the audio you feed into it, and it doesn't butcher the rig.

[WD5JKO]

I have a G-50, and I went through it several years ago looking at how it worked, and what it needs.

The G-50 modulator runs two 6L6 type tubes as a Class A single ended Heising modulator. The same modulator is used to drive the speaker during receive. Here is what Gonset says in the manual:

"Transmitter AUDIO system It will be noted that class A single- ended bean tetrodes are used in preference to a Class B modulator. The reason for this is that when “square wave” audio is involved, as when heavy  speech clipping is employed at high modulation percentages, the former type modulator compares very favorably with the latter, with the advantages of more constant plate current drain and elimination of a driver stage and its transformer. It also facilitates designing the modulator for integral speech clipping, making the incorporation of a separate speech clip- per unnecessary (as well as adjustment thereof).
The speech system of the communicator is designed so that to obtain maximum practical speech clipping one need only talk closer to or louder into the microphone, up to the point where the maximum tolerable distortion is obtained.
With voice waveforms and sufficient audio input to produce heavy speech clipping, the percentage modulation is held to approximately 90 per cents and under no condition is it possible to exceed this modulation percentage. This means that ''splatter'' from negative peak clipping is avoided, and no critical adjustments are involved.
The audio characteristics of the transmitter, from microphone input through the modulator, having been engineered to provide maximum utilization of the carrier power from the standpoint of intelligibility under favorably receiving conditions."

   In my opinion there is tremendous room for improvement here. I feel that I overcame a lot of the weaknesses of this rig. If anybody cares, I will dig out my G-50 and go over the details.

73,
Jim
WD5JKO

[RidgeRunner]

Thanks for all of the info this gives me some good starting points!

[WD5JKO]

This post got me to do another round on my G-50.....

    I have uploaded the schematic, some pictures, and scope plots. See link at the end of this post. The schematic is hand drawn, but the information is there.
 
I decided to go a little further with my G-50, so I replaced the single ended modulation transformer with a push pull transformer. This is still single ended class A Heising. This made a huge difference. From the Mod adj pot onward, the modulator is flat from 50-5000 Hz with little distortion up to 85% (sine wave), and capable of 100% positive peaks on voice. See scope plot of YEA_AM.jpg where it hits +100% modulation.

http://pages.prodigy.net/jcandela/G-50/YEA_AM.JPG
 
I used the P-P transformer in a unusual way where the RF class C plate current (120ma), and the TX Modulator current (120ma) balance out the DC modulation transformer magnetic field (120 - 120 = 0) such that I get around the saturation issue common with Heising modulation. This allows the use of full transformer lamination interleaving of the E & I lamination pieces such that the primary inductance is at maximum. The way it was, the currents were additive, and the E & I laminations were grouped together and the core gaped. Getting good modulation below 300 hz at full depth was a big problem with the stock iron.

  I added a parallel RC in the modulated B+ line such that it drops the B+ about 50 volts to the 6146, but the capacitor bypasses the resistor for AC such that instantaneous voice peaks at + 100% modulation are possible. The reduced B+ is offset by solid stating the 5U4G, so the Modulated B+ cut and Power supply B+ gain pretty much cancel each other out.
 
With sine wave drive, the cathode bias rises as you approach 100% modulation, and the screen voltage goes down. This makes the modulator with over-driven voice audio somewhat limiting, or compressed as in the Gonset G50 tradition, yet with these modifications, the audio is very clean when not overdriven up to 100% (+) modulation. Another part of this is where I limited the (-) downward modulation with a diode clamp on the 6146 G2 grid such that the G2 over the audio swing cannot dip below about +30 volts. This makes the rig incapable of over modulating downward and splattering. The +30v is from the modulator cathode bias. If I tapped down the Rk resistor, then it would be possible to Taylor the maximum (-) modulation percentage to to 90 or 95%.
 
In order to get linear modulation out of the 6146 over the audio range, I had to do a lot of work. The work included increasing the grid1 leak resistor, increasing the G2 dropping resistor, and making a capacitive AC voltage divider for G2 to set the % of G2 modulation to a value less than before, and a value that was more constant over the audio bandwidth. This is an area for improvement in many HF plate modulated rigs using 6146 RF tubes. I was modulating with triangle waves to look for RF linearity; alternatively I could have configured scope in X:Y mode for trapezoid waveform.
 
The frequency response is Tailored in the first audio stage (12AT7) to suit your voice, microphone, and individual preferences. I increased R17 from 270K to 1 M to boost the base a bit (D-104), and I added a R-C preemphasis network going to the Mod adj pot too boost the midrange in the hope of boosting weak signal intelligibility. Depending on your voice, and chosen microphone, preemphasis can be a very useful tool. Do you really want to sound like a "Woofer" on the receive end?

Notice I did change out the 12AX7 for a 12AT7. I cannot recall why, but The 12AT7 does a great job with plenty of reserve gain when using a Crystal Mic D-104..
 
Let me know what you think...
 
Here is the link:
http://pages.prodigy.net/jcandela/G-50/

 
Jim
WD5JKO