ANOTHER GK MODULATOR UPDATE

[W8UJX]

Hi Guys,

I’m also reworking the modulator in my GK500A.

The mods I have made so far include removing the couplates and installing conventional R/C circuits.  All the electrolytic caps in the power supply have been replaced except the HV cap.  The speech amp was completly rewired using new components.  I also removed all the cathode bypass caps to add some negative feedbackto reduce distortion.  This modulator has two 6C5’s so I have plenty of gain.  I also included a negative feedback loop around the 6L6 similar to that shown in the ARRL handbook.

Tests so far show a flat frequency response (+/- 1 dB) 20 Hz to 20 KHz from the mic connector to the 6L6 grid.

I’m thinking of moving the final plate current meter from the cathode circuit to the pate circuit of the 4-400A.  That way I can connect one side of the 4-400A filament directly to ground.   

When I modulate the 4-400A, about what % mod and distortion level should I expect to see?  Later I plan to boost the Vp on the 811-A’s to 1500 VDC.

I would also like to see the mods others have used for the R/C coupling
between various stages.

JKdotASHLEYatEARTHLINKdotNET

Comments?
Tnx,
Jerry/W8UJX/CA   

 

[ke7trp]

Nice work Jerry. I am shocked you are getting the high end out of the mod iron.  Mine has no Couplates(roberts kit), Bigger caps, larger HV cap, 1500+ V on the 811s and I drive the 6C5 with outboard audio gear. It will do the 20hz but its dead by 10,000 on the high end.  Maybe I missed something. 

I need to reconfigure my Modulator plate meter.  It wacks the side of the plastic meter case at levels not even close to 100% on the scope. Its going to just kill the meter at some point.

Did you change the Bypass caps on the 4-400??

Clark

[W8UJX]

Hi Clark,

Well my output measurement was taken at the 6L6 grid.  I have not looked
at the output of the mod xmfr yet.  My next measurement will be to fire up
the rig on AM and do some tests on the detected audio.  That's what's important.

I’m thinking of moving the final plate current meter from the cathode circuit to the pate circuit of the 4-400A.  That way I can connect one side of the 4-400A filament directly to ground.  Only one bypass cap required.

I wonder what is the size of the new bypass caps you used?

Jerry

[ke7trp]

Oh.. That makes sense.. I missed that..   Expect the top end to be chopped off with the stock iron.. But its not really that bad!   

Robert advised me to use .01 large disk caps on the filiment to ground. They used 005s on it and they where tiny and burned. 

He also suggested TWO 500pf micas from screen to ground on each side..   I used one 1000PF from the CENTER of the cross bar to the Ground scew on the chassis. I did this because thats what I had on hand.

This made a big difference in the low end!

http://amfone.net/Amforum/index.php?topic=19389.0

Clark

[WD5JKO]

Jerry, Clark, and All;

 I have a GK, and although it is a hulk now in dis-repair and in storage, I am familiar with what you guys are doing.

One weakness of that modulator is the single ended 6L6 audio driver. Adding NFB as you did is good as it lowers the output impedance of that stage to better handle the 811 class B grid loading. That said, leaving the cathode bypass capacitor out raises the output impedance which will work against what you want to do. I would reconsider and bypass that cathode (6L6) with a beefy cap, maybe 100 uf or more. The increased gain can then be taken away with more NFB.

The older versions of this modulator (GK 275 and GK400) used a push pull driver with triode connected 6F6's. I believe this to be a superior design. So while looking at the GK500 modulator schematic, it occurred to me that going to a push pull driver might be very easy. I base this since my 500B had a P-P driver transformer even though it used a single ended 6L6. If yours is the same (The center tap wire is there taped off), then your all set to proceed to P-P driver.

My idea is to replace the 6L6 with a 6DZ7. The 6DZ7 has an Octal base, and contains dual 6BQ5's or EL-84's with a common cathode and screen grid terminal. Wire in the upper tube much like the 6L6 is now, and then take the lower tube g1 and ground it. Leave the cathode resistor NOT bypassed so that we cathode couple to the lower tube which operates in grounded grid mode. Then rewire the driver transformer primary for push pull with the B+ going to the center tap.

If your driver transformer is truly P-P like mine was, then all you need is a 6DZ7 and to rewire. The cathode resistance might need to be adjusted, but that is it.

The 6DZ7 will have about twice the gain of a 6L6, so you can use that extra gain as an excuse to increase the NFB from an 811 grid to the P-P driver grid 1 resistor bottom end via voltage divider, or go back to the 2nd 6C5 unbypassed cathode via voltage divider. If the phasing is off, then pull from the other 811 grid.

Comments?

Jim
WD5JKO

[W8UJX]

Thanks a lot Jim. 
This is the kind of response that really helps.  You included enough information to help someone implement the new design.

I will add the bypass cap to the 6L6. 

I knew a push pull driver for the 811-A's was a better design but I did not want to add another tube.  Now that I think about it, since my 500A uses two 6C5's I probably could rewire one 6C5 socket to take another 6L6.

I like the idea of the 6DZ7 much better.  I can use the second 6C5 to get the out-of-phase signals I need to drive the 6DZ7.   

My driver xmfr is center tapped so the hunt for a 6DZ7 is on.

I have two questions Jim.

What do you think of the idea of going from choke input to capacitor input in the modulator HV supply to raise the HV on the modulator tubes?  My goal is to be able to modulate the rig 100%.

Second, do you have any feel for the maximum distortion of the modulated signal I should shoot for?  I have test equipment to test for distortion in the modulator and the transmitted signal.

Any ideas for other mods to the rig will be appreciated.

Jerry/W8UJX/CA 

[WD5JKO]

Jerry,

I'd consider using the second half of the 6DZ7 as a grounded grid amp, cathode coupled to the other side so you don't need a phase inverter. The conversion will be fast and nearly as good. You could make it better if the cathode resistor was replaced with a solid state current sink, somewhere around 30-40 ma. This will balance the P-P output almost perfectly, but using a unbypassed resistor should work well enough.

If you want to use one 6C5 as a hot cathode phase inverter, then consider a dual triode in the other location such as a 6SN7. 

You asked what % of modulation can you achieve and at what distortion. I bet there are folks here on AM Fone that will claim they get 100% modulation with the stock iron, and stock voltages. Maybe they are correct. Remember that mod transformer is old and fragile; pushing it harder might turn your GK into a CW only rig. 

Case in point; I once beefed up a GK 400 for Ozona Bob, W5PYT. This GK had the 6L6 audio driver like yours. It was about 200 miles drive, and this was back when 10M was wide open. I copied Bob all the way over there 10M mobile via backscatter from his 20 element Sterba Curtain, but his audio was low relative to the carrier (> S9 w/flutter). I changed that 6L6 to an EL-34, added NFB, and beefed up the audio driver power supply filter capacitor to a higher value. The audio was much fuller on the scope, and the average level could be maintained higher. When I drove back I copied him Q5 with much better audio recovery. Then a week or two later the Rf final flashed over on a modulation peak, and a nice pair of V70D's proceeded to melt.

My point is that WRL equipment, although highly regarded by many, is in my opinion a little light on the components, so if you beef up the modulator, expect something else to flash over and die.


If I wanted more modulation from a near stock GK, I might replace the 866's in the RF high voltage supply with 1616's. This will sound strange, but the vacuum rectifier will lower the 4-250 B+ a couple of hundred volts such that the modulator will be able to modulate the carrier to a higher level.

I'm not sure about the distortion capabilities, but I'd wager you would need to do a fair amount of work to get the THD to < 5% @ say 85% modulation at 1 Khz. Since you have some test equipment see what it does now stage by stage. I would not be surprised to see the low level 6C5 stuff at about 2% per stage, and the 6L6 class A power stage at 10% or higher. The figures are not always additive because the non linearity of one stage can be offset by another to some extent.

Here is an excellent reference that might help you:

http://www.turneraudio.com.au/education+diy.htm

Good Luck,
Jim
WD5JKO

[W8UJX]

Thanks Jim,

I will definitely go with the 6DZ7 grounded grid circuit.  Looks easy. 

I’ve heard guys claim 20 Hz to 20KHz with the stock iron.  I have doubts about that.  I wonder how flat the response was and what percent modulation and distortion they were getting.  If I remember correctly the spec for a commercial AM BC station is only 100 HZ to 5000 Hz (+/-) 3 dB.

I don’t think WRL gave much thought to component ratings or how accurate their schematics were.   

You have given me a lot to think about.  That’s good.  I love to tinker.

I will be back to get your comments on the final circuit as soon as I
finish the design. 

Jerry/W8UJX/CA